Investigations of the 

New Madrid, Missouri, 

Earthquake Region

GEOLOGICAL SURVEY 

D0OFESSIONAL PAPER 1236

Investigations Of The New Madrid, 

Missouri, Earthquake Region

Edited by F. A. MCKEOWN and L. C. PAKISER

A. Overview and discussion, by F. A. McKeown

B. Damaging earthquakes of the central Mississippi valley, by O. W. Nuttli

C. Present-day seismicity and identification of active faults in the New Madrid seismic zone, by William Stauder

D. Microearthquakes and faulting in the area of New Madrid, Missouri Reelfoot Lake, Tennessee, by D. O'Connell, C. G. Bufe, and M. D. Zoback 

E. Magnetic basement in the upper Mississippi embayment region A preliminary report, by T. G. Hildenbrand,

M. F. Kane, andj. D. Hendricks 

F. Tectonic features of the New Madrid seismic zone from seismic reflection profiles, by R. M. Hamilton, and M.

D. Zoback

G. A crustal structure study of the Northern Mississippi embayment, by C. B. Austin, and G. R. Keller 

H. Style and significance of surface deformation in the vicinity of New Madrid, Missouri, by D. P. Russ 

I. Configuration and deformation of the Paleozoic bedrock surface in the New Madrid seismic zone, by A. J.

Crone, and S. R. Brockman 

J. High resolution seismic reflection surveying on Reelfoot scarp, northwestern Tennessee, by J.L. Sexton, Frey,

and Malicki

K. Stratigraphy arid structure of sediments above the Newport pluton of northeastern Arkansas, by E. E. Glick 

L. The northeastern extension of the New Madrid seismic zone, by L. W. Braile, W. J. Hinze, G. R. Keller, E. G.

Lidiak 

M. Heat flow in the upper Mississippi embayment, by C. A. Swanberg, B. J. Mitchell, R. L. Lohse, and D. D.

Blackwell 

N. Radon emanation in the New Madrid seismic zone, by S. R. Steele, W. C. Hood, J. L. Sexton

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1236

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

First Printing 1982 

Second Printing 1983

Library of Congress Cataloging in Publication Data

Main entry under title:

Investigations of the New Madrid, Missouri, Earthquake Region.

(Geological Survey Professional Paper 1236)

Bibliography: 201 p.

1. Earthquakes Missouri New Madrid region.

I. McKeown, Francis A. II. Pakiser, L. C. (Louis Charles), 1919-. III. United States. Geological Survey. IV. Series.

QE535.2.U6I58 551.2'2'09778985 81-607888 AACR2

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402
PREFACE

The sequence of earthquakes during 1811-12 in the New Madrid region has been a 

seismological enigma, and the potential damage and loss of life that a repetition of these 

earthquakes might cause has been ignored by most people. The earthquakes have been 

an enigma because they occurred in a part of the country generally described as the stable 

interior of a continent. The potential hazards of a repetition of them are commonly not 

realized because much of the country was undeveloped and unsettled in 1811-12; consequently, the earthquakes did not cause the massive amount of damage possible should a 

similar sequence occur in the near future.

However, recent increased public, Federal, and State awareness of the potential earthquake hazard in the New Madrid, Mo., and contiguous regions has prompted initiation 

of a variety of geological, geophysical, and geochemical studies at a scale never before attempted in the Central United States. Prior to about 1973, most investigations related to 

earthquakes in the region were made by faculty and students of St. Louis University using 

only small financial resources. In 1973, new regional gravity and aeromagnetic surveys 

were started by the USGS (U.S. Geological Survey). In 1974, the USGS funded St. Louis 

University to establish a seismographic network to record and locate small earthquakes in 

the New Madrid region, and about a year later the USGS started 

geologic investigations

In 1976, the NRC (Nuclear Regulatory Commission) funded a six-State cooperative 

project involving both State and university communities as principal investigators. The 

principal objective of the cooperative project is to conduct investigations that will contribute to the seismic-hazard assessment of nuclear power plants that might be affected by 

a large earthquake in the New Madrid region.

Because a wealth of data has been acquired in the past few years, it seems propitious 

and timely to publish a collection of this information, rather than having it dispersed in 

many journals. The intent of this Professional Paper, therefore, is to publish, in one 

volume, reports of both the USGS and other investigations that represent the present state 

of knowledge of the seismicity and geology of the New Madrid region. Some of the results 

of earthquake investigations in the New Madrid region have provided valuable information for other geologic interests. For example, the inferred rift described in this volume, 

and reports that preceded the present study, is of interest to petroleum companies 

because of the possibility that thick sections of sedimentary rocks may occur within the 

rift. Plutons inferred in the subsurface and chemical and mineralogical data from igneous rocks penetrated by drill holes suggest the possibility of mineralization more 

widespread than outcrops indicate.

We believe that the studies reported in this volume are major advances toward understanding the nature of earthquakes and earthquake hazards in the Midcontinent region. 

Earthquake investigations in this region have in the past been assigned a low priority as 

compared with those in the West. Now at last, as indicated by the following papers, the 

importance of potentially destructive earthquakes in a region of high population density 

has been acknowledged and a much higher priority has been assigned to the study of 

earthquakes in this region.

We also believe that advances which have been and are continuing to be made in the 

New Madrid region are due as much to the manner in which the investigators have 

cooperated and shared their knowledge as to the talents of individual investigators. Most 

complex geological problems require a multidisciplinary approach for their efficient and 

rational resolution. The results of the investigations of the New Madrid region are a good 

example of an integrated multidisciplinary effort to resolve a long-standing, complex 

geological and seismological problem.

in
Wood cut of scene of the great earthquake in the West (p.220 m Our First Century: One Hundred Great and Memorable Events, 1877). Published with permission of Historical Society of Missouri, Columbia.

IV
CONTENTS

Page

(A) Overview and discussion, by F. A. McKeown .............................................. 1

(B) Damaging earthquakes of the central Mississippi valley, byO. W. Nuttli ....................... 15

(C) Present-day seismicity and identification of active faults in the New Madrid seismic zone,

by William Stauder .............................................................. 21

(D) Microearthquakes and faulting in the area of New Madrid, Missouri Reelfoot Lake, Tennessee,

by Daniel O'Connell, C. G. Bufe, andM. D. Zoback .................................... 31

(E) Magnetic basement in the upper Mississippi embayment region A preliminary report,

byT. G. Hildenbrand, M. F. Kane, andj. D. Hendricks ................................. 39

(F) Tectonic features of the New Madrid seismic zone from seismic-reflection profiles,

by R. M. Hamilton, and M. D. Zoback ............................................... 55

(G) A crustal structure study of the Mississippi embayment, byC. B. AustinandG. R. Keller .......... 83

(H) Style and significance of surface deformation in the vicintiy of New Madrid, Missouri, by D. P. Russ . 95 

(I) Configuration and deformation of the Paleozoic bedrock surface in the New Madrid seismic zone,

by A. J. Crone and S. R. Brockman ................................................... 115

(J) High-resolution seismic-reflection surveying on Reelfoot scarp, northwestern Tennessee,

byj. L. Sexton, E. P. Frey, and Dave Malicki .......................................... 137

(K) Stratigraphy and structure of Post-Paleozoic sediments above the Newport pluton of northeastern

Arkansas, by E. E. Click........................................................... 151

(L) The northeastern extension of the New Madrid fault seismic zone, by L. W. Braile, W. J. Hinze,

G. R. Keller, andE. G. Lidiak...................................................... 175

(M) Heat flow in the upper Mississippi embayment, by C. A. Swanberg, B. J. Mitchell, R. L. Lohse,

D. D. Blackwell. ................................................................. 185

(N) Radon emanation in the New Madrid seismic zone, by S. R. Steele, W. C. Hood, andj. L. Sexton . . . 191

Overview and Discussion

By F. A. MCKEOWN

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-A
CONTENTS

Page

Abstract ............................................................................... 1

Introduction............................................................................ 1

Geologic history ......................................................................... 4

Faults ................................................................................. 5

Surface faults ....................................................................... 5

Subsurface faults .................................................................... 7

Inferred characteristics of Seismicity associated with rifts ........................................ 8

Characteristics of faults in rifts ......................................................... 9

Rates ofdeformation ................................................................. 9

Chemical and physical characteristics of rocks in rifts ....................................... 10

Possible significance of alkalic-rock characteristics to seismicity .................................. 10

Summary .............................................................................. 12

References cited ......................................................................... 12

ILLUSTRATIONS

Page

FIGURE 1. Modified Mercalli intensity isoseismal zones for the San Francisco (1906) and the New Madrid (December 16, 1811) earthquakes superimposed on a 1970 population density map ............................................................................... 2

2. Distribution of sand blows produced by the 1811-12 earthquakes ............................................................... 3

3. Generalized fault map of the New Madrid region showing plutons, approximate rift boundaries, and epicenters of earthquakes detected from

July 1974 toJune 1977 .............................................................................................. 6

4. Graph showing rate of deposition of sediments during subsidence of embayment based upon thickness of sediments deposited during indicated

periods of geologic time but not the loss of record of deposition at diastems and unconformities ................................... 7
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

OVERVIEW AND DISCUSSION

By F.A. MCKEOWN

ABSTRACT

The 1811-12 sequence of earthquakes commonly referred to as the New 

Madrid earthquake was felt and produced damaging intensities at far greater 

distances than any other historical earthquake in the conterminous United 

States. This fact and the current population within the Modified Mercalli intensity VII isoseismal of the 1811-12 sequence force the conclusion that, 

should the sequence be repeated today, widespread destruction of property 

and loss of life would very likely occurred. Magnitude-frequency studies and 

sand-blow relationships indicate a recurrence interval of 600-700 years for 

large earthquakes in the New Madrid region. The geological structures on 

which the earthquakes occurred and the forces that drove them have been an 

enigma. However, since seismological, geological, and geophysical studies 

were started in 1974, enough information has been acquired to propose 

reasonable explanations of the locations of the earthquakes, but the driving 

forces are still unexplained. In addition to the great distances at which the New 

Madrid earthquakes were felt and caused damage, another preeminent 

characteristic of them was the great number of large aftershocks and the long 

period over which they occurred.

Earthquakes in the New Madrid region occur in a geologically complex environment. Geophysical and petrologic data indicate that intracontinental rifting occurred in late Precambrian or Early Cambrian time. This tectonic event 

was followed by deposition of clastic and carbonate sediments during the 

Paleozoic and uplift to form the Pascola arch in the late Paleozoic or Mesozoic. 

During the emergence of the Pascola arch, more than 2.5 km of sediments 

were eroded. Subsidence of the region started in Late Cretaceous time and probably continued until at least the beginning of the Holocene, resulting in formation of the present-day southward-plunging syncline in the Mississippi Embayment. Mapping in the region is too incomplete to make comprehensive tectonic analyses from the distribution and character of faults exposed at the surface. Recent seismic-reflection profiling shows a number of faults in the subsurface, some of which coincide with epicentral trends, particularly in the axial 

region of the rift. In addition to the evidence of faults in the profiles, a gross 

perspective of the characteristics of the subsurface structure and rocks may be 

had by assuming that the buried rift is analogous to well-exposed rifts in east 

Africa. Interpretation of geophysical data, drill-hole information, and outcrops in the Magnet Cove area of Arkansas show that a number of mafic intrusive masses occur along the boundaries of the rift and that several occur 

along the axis of the rift. All available information shows that the plutons comprise complexes of mafic to felsic alkalic rocks. Most of the seismic activity in 

the region occurs within the inferred boundaries of the rift, particularly along 

the axis and where apparent crosscutting structures occur in the vicinity of New 

Madrid, Mo. Despite the present seismicity, which indicates deformation at 

depth, rates of deformation by faulting or uplift at the surface and at shallow 

depths are more than an order of magnitude less than rates of deformation in 

the seismically active areas of Western United States. This profound difference 

may be part of the explanation for the apparent lack of surface rupturing that 

would be expected for some of the New Madrid earthquakes. The importance

of recognizing that earthquakes are associated with rifts, regardless of their age 

and the current state of stress on them, is that rifts are major breaks in the 

Earth's crust which have properties more favorable for the occurrence of earthquakes than adjacent, less faulted parts of the crust. Given the facts that rifts 

are highly faulted and are associated with an abundance of alkalic rocks that 

had a high content of volatiles, range from mafic to felsic, and have a wide 

variation in physical properties, a qualitative physical explanation of earthquakes occurring in rifts is proposed. In brief, the explanation is that the high 

volatile content has resulted in rocks that have higher porosity and probably 

higher pore pressure within or associated with a rift in comparison to most 

rocks adjacent to a rift. This condition, in addition to the abundance of faults, 

is conducive to the generation of earthquakes. Also, the contrasts in physical 

properties of the rocks provide conditions for stress amplification and therefore 

local control of the distribution of earthquakes.

INTRODUCTION

Comparison of any isoseismal map (for example, Nuttli, 

1973a; Stearns and Wilson, 1972) of the principal shocks of 

the New Madrid sequence of earthquakes with a current 

population map of the United States makes one realize that 

such a series of earthquakes today would be likely to cause very 

widespread destruction of property and loss of life. Figure 1 

shows the isoseismal map of Nuttli (1973a) superimposed on a 

population density map. The intensity contours to the west of 

the epicentral area, for which no data are available, have been 

arbitrarily made symmetrical to those east of the epicentral 

area. According to the 1975 census information (M. McGrath, 

written commun., 1980), the population within the Modified 

Mercalli intensity VII or greater zone, which corresponds with 

damage threshold, for the New Madrid sequence is about 12.6 

million people. In contrast, the 1975 population within the intensity VII zone for the 1906 San Francisco earthquake is only 

about 4.0 million people. This comparison must be tempered 

with the fact that earthquakes occur more frequently on the 

San Andreas fault in California, on which the 1906 earthquake occurred, than in the New Madrid region. New 

Madrid-type earthquakes may only occur every several 

thousands of years. If so, the short-term probability of much 

damage and loss of life is very low. The best estimates, 

however, of the recurrence interval for large (mfc ^ 6.2) 

earthquakes in the New Madrid region is on the order of every
bO

OJH 

Q

5

O§ O

 p»-.:-.2.:»»

,» «iu>«Emtf

FIGURE 1. Modified Mercalli intensity isoseismal zones for the SanFrancisco (1906) and the New Madrid (December 16, 1811) earthquakes superimposed on a 1970

population density map
OVERVIEW AND DISCUSSION

600-700 years (Nuttli, 1974; Russ, 1979; Russ, this volume) 

compared with about 100 years for large earthquakes on the 

San Andreas fault. The 500- to 600-year difference in recurrence intervals for an exposure time of 100 years reduces the 

probability of exceeding a given level of ground motion by a 

factor of two to three (K. Shedlock, oral commun., 1980).

Another disturbing aspect of earthquakes in the New 

Madrid region is that their causes and the geologic structures 

on which they occur are not understood. Furthermore, much 

of the knowledge available from study of earthquakes in 

Western United States does not seem applicable to study of 

earthquakes in the New Madrid region. One case of inapplicability is the profound difference between rates of tectonic deformation in seismically active regions of Western United 

States and rates of deformation in the New Madrid region. For 

example, the rate of movement in historic time along the San 

Andreas fault zone is 2-3 cm per year. In contrast, the rate of 

movement during the last 100 m.y.(million years) across a 

fault zone that appears to be associated with some earthquakes 

in the New Madrid region is less than 0.008 mm per year. Most 

areas of high seismicity and large earthquakes around the 

world are found where movement between oceanic and continental plates is occurring, and rational explanations can, 

therefore, be given for the occurrence of the earthquakes. 

Within continental plates of the United States, the cause of 

localization and the crustal or mantle driving forces of earthquakes are still largely an enigma. However, seismologic, geophysical, and geologic data acquired since about 1974 have 

begun to provide earth scientists with enough information to 

advance reasonable explanations of at least the localization of 

earthquakes in the New Madrid region. Explanation of the 

driving forces however, is speculative.

The history of earthquakes in the New Madrid region has 

been adequately described by a number of investigators. (See, 

for example, Fuller, 1912; Nuttli, 1973a.)

Fuller's account is the most descriptive and best documented. It has served as the basic document for subsequent investigators' studies of the New Madrid region. Restating much of 

the detail that is already in the literature is unnecessary. A very 

brief description of some of the effects is appropriate, 

however, and emphasis on the effects and features of the New 

Madrid sequence of earthquakes that now appear to be more 

significant than early investigators recognized is warranted.

According to Fuller's account, three major shocks occurred: 

December 16, 1811; January 23, 1812; and February 7, 1812 

(Fuller, 1912). Nuttli (1973a) provided evidence from a study 

of intensity information which indicates that the first shock 

was located about 100 km southwest of New Madrid near 

Marked Tree, Ark. This location is supported in part by the 

distribution of sand blows (Heyl and McKeown, 1978), as indicated on figure 2. The last large shock, on February 7,1812, 

is presumed to have been in the vicinity of New Madrid. This 

shock is reported by Nuttli to have been the largest of the three 

principal shocks. Sand blows, however, are not nearly as abun90°45' 90°30' 90°15' 90°00' 89°45' 89°30' 89°15' 36°45' I I I I I I 

36°30'

36°15'

36°06'

35°45'

35°30'

35°15'

New Madrid.

20 KILOMETERS _i

I

FIGURE 2. Distribution of sand blows produced by the 1811-12 earthquakes

dant or as large in the vicinity of New Madrid as they are to the 

southwest. During the time between the three principal 

shocks, many other smaller shocks occurred, some of which 

were felt in places as distant as Washington, D.C.; Baltimore; 

and other cities along the East Coast. It seems likely, therefore, 

that all of the large and most of the smaller shocks occurred 

under the area of most abundant sand blows. Most of the current seismicity, as detected by the St. Louis University network 

which has been in operation since 1974, is also largely confined 

to the area of sand blows. This may be further confirmation of 

the source zones of the 1811-12 sequence.

Three facts are preeminent about the New Madrid sequence of shocks: (1) the great distances at which they were felt 

and caused damage, (2) the great number of shocks that apparently were of large size (as indicated by reports of effects 

and the great distances at which they were felt), and (3) the 

long period of time over which they occurred.

The great distance at which earthquakes were felt and caused damages has been explained by low attenuation of seismic 

waves (for example, Nuttli, 1973b; Evernden, 1975). The 

reason for low attenuation in regions of Central and Eastern 

United States has not been explained rigorously by physical 

models but is well recognized and makes estimation of the size 

of earthquakes in these regions a subject of controversy.

The large number and long aftershock sequence of large 

shocks seem well documented, but no explanations have been 

offered in the literature for these effects. Not only did many
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

relatively large shocks occur for several months in the New 

Madrid area, but the seismic activity apparently migrated northward and continued for at least 2 years. For example, small 

shocks were felt almost daily at Saline Mines, formerly called 

United States Saline, 111., for 2 years (Fuller, 1912). Some of 

the most severe of these shocks were felt in Kaskaskia, 111., and 

along the Wabash River. Whether these shocks were related to 

the shocks near and south of New Madrid cannot be proved. 

No similar long sequence in southern Illinois has occurred, 

however, since the United States Saline shocks in 1812-14.

The detailed accounts of historic seismicity (Nuttli, this 

volume) and present day seismicity (Stauder, this volume) provide more precise descriptions of the effects, size, and distribution of earthquakes in the New Madrid region than earlier 

reports. Their work indicates that damaging earthquakes in 

the region are commmon. That more damage was not caused 

by the pre-1900 shocks is probably attributable to the fact 

that much of the region did not have large towns or cities close 

to the epicentral areas. Even today the areas of most intense 

seismic activity are rural. However, the increasing size and 

number of moderate-sized cities such as Memphis, Tenn., Little Rock, Ark., and Paducah, Ky., and of towns serving farming communities such as Sikeston, Mo., Dexter, Mo., 

Paragould, Ark. and Forrest City, Ark., make the potential 

damage much greater than it was 50-150 year ago.

The recording and analysis of present-day seismicity, as 

reported by Stauder, have provided, for the first time, an indication of specific zones of seismic activity which are 

presumably related to specific fault zones. More detailed locations of zones are given in the study of seismicity in the New 

Madrid, Mo. Reelfoot Lake, Tenn., area conducted by 

O'Connell and others (this volume). Their results are based on 

data from a net of 19 seismometers spaced less than 5 km 

apart. The results indicate that the seismicity is occurring on 

several short north- and northwest-striking fault zones. The 

geologic characteristics of the zones of seismicity and their extent, however, must be resolved by a variety of earth science 

disciplines.

The first subsurface information indicating that the seismic 

activity is associated with a rift about 70 km wide and more 

than 200 km long is provided by the gravity and aeromagnetic 

studies of Hildenbrand and others (1977), and Hildenbrand 

and others (this volume). The extent of the rift to the 

southwest is not yet known. A northeastern extension of it to 

the Wabash Valley is postulated by Braile and others (this 

volume). The deep-crustal and upper- mantle properties of 

the rift have been described by Austin and Keller (this 

volume); their model of the structure is consistent with models 

of other intraplate rifts throughout the world. Swanberg and 

others (this volume) identified a thermal anomaly in the New 

Madrid region, but the limited spatial extent of the anomaly 

indicates that it is not related to an upper-mantle source as 

might be expected in an active rift zone. The lack of evidence

of active rifting is one of several enigmas that compound the 

difficulty in explaining the driving forces of the earthquakes.

Studies of deformation are reported in this volume. Russ 

demonstrated, using geomorphic methods, that an elongate 

composite uplift of the surface occurs over the zone of most frequent seismic activity in the vicinity of New Madrid, Mo. 

Most of the uplift, known as the Lake County uplift, is younger 

than 6000 years B.P. and parts of it formed between 200 and 

2000 years B.P. Crone and Brockman (this volume), using 

results of seismic-reflection profiling as reported by Hamilton 

and Zoback (this volume), determined that the deformation of 

the buried Paleozoic surface in the Lake County uplift area is 

concordant with the surface uplift. No serious efforts have yet 

been made to attempt prediction of earthquakes in the New 

Madrid region. However, the study of radon-isotope emanation by Steele and others (this volume) is a first step in assessing 

the usefulness of the emanation method for predicting earthquakes.

The reader may find some differences in interpretation of 

the same or similar data among the various investigators who 

have contributed to this volume. This is to be expected when 

the geologic complexity and imperfect knowledge of the region 

are considered.

In the following parts of this introductory chapter, an attempt will be made to integrate much of the recent earthscience information reported in subsequent chapters with 

earlier information and to present a generalized description of 

the tectonic environment of New Madrid seismicity. In addition, the implications of earthquake source zones associated 

with intraplate rifts will be discussed.

GEOLOGIC HISTORY

The geologic history of the New Madrid region or parts of it 

has been described by a number of investigators from a variety 

of perspectives. (See, for examples, Fuller, 1912; Stearns and 

Marcher, 1962; Caplan, 1954; Grohskopf, 1955; Ervin and 

McGinnis, 1955; and Heyl and Brock, 1961.) Until the past 

few years when studies related to earthquakes were started by a 

number of investigators, most previous work, except that of 

Fuller, was related to mineral deposits in the Ozark Mountains, southern Illinois, and western Kentucky; water 

resources in the Mississippi Valley; and flood control in the 

valley. The monumental work on the Mississippi Valley by 

Fisk (1944) and later mapping of the upper part of the valley by 

Saucier (1964,1974) and Smith and Saucier (1971) give details 

of the Quaternary history of the embayment. More recently, 

Russ (this volume) has started detailed studies of river terraces 

and geomorphic features to search for evidence of Holocene 

regional deformation.

On cursory examination, the geology (fig. 3) of the central 

and lower Mississippi Valley area appears relatively simple. 

Actually it has been and still is an area with very active and
OVERVIEW AND DISCUSSION

complex geological processes that probably started as early as 

the late Precambrian and have continued intermittently to the 

present. Knowledge of Precambrian events is inferred mostly 

from rocks in the St. Francois Mountains of the Ozark uplift, 

where thick extensive subalkaline to peralkaline rhyolitic tuffs 

were deposited about 1.2 b.y. (billion years) ago 

(Anderson,1970; Bickford and Mose, 1975; Kisvarsanyi, 1976; 

and Kisvarsanyi, 1980). Alkaline granites were emplaced 

about the same time. Precambrian igneous activity ended in 

the area with intrusion of gabbro and diabase dikes. This igneous activity is believed to have been accompanied by epeirogenic uplift and the formation of the "Reelfoot Rift" (Ervin 

and McGinnis, 1975).

The area of the present Mississippi Embayment subsided in 

early Paleozoic time, and a basin called the Reelfoot Basin 

(Schwalb, 1971) formed with a center of deposition in the 

vicinity of western Kentucky and southeastern Missouri. 

Clastics followed by carbonates were deposited in Middle and 

Late Cambrian time. By Silurian time the center of deposition 

of the Reelfoot Basin had migrated northward and merged 

with the southward-migrating Illinois Basin. Late Paleozoic 

and Mesozoic uplift of the Reelfoot Basin resulted in formation and later erosion of the Pascola arch between the Ozark 

Dome and Nashville Dome. The uplift apparently continued 

until Late Cretaceous time as no younger Mesozoic sediments 

have been found, and the arch apparently was emergent until 

the Early Cretaceous. On the basis of reconstruction of the 

stratigraphic section, Stearns and Marcher (1962) estimated 

that at least 2.5 km and as much as 4.5 km of rocks were eroded from the arch before subsidence started in the Late 

Cretaceous. This subsidence and accompanying deposition of 

predominantly clastic rocks continued into the Pleistocene 

and probably the Holocene. Subsidence may have been occurring about 6000 years B.P. when the Mississippi and Ohio 

River systems changed from a braided to a meandering system 

in the northern part of the Mississippi Embayment (Saucier, 

1974). Certainly much of the alluvium that fills the eastern 

lowlands of the Mississippi Valley was deposited during the interval from about 20,000 to 6000 years B.P. (Saucier, 1974). 

Interpretation of this deposition being related to subsidence 

becomes a problem beyond the scope of this paper, however, 

because it requires detailed consideration of climatic changes 

and of the effect of sea-level changes on stream gradients and 

loads. A rough comparison of rates of deposition since Late 

Cretaceous time is shown in figure 4.

Deposition of sediments, largely of marine origin (Stearns, 

1957), in the Mississippi Embayment was clearly most rapid 

during the early Tertiary, which presumably indicates the 

greatest rate of subsidence. The high rate of deposition of continental sediments calculated for the Quaternary Period may 

indicate an increase in subsidence, but, as stated above, sealevel and other changes may have had major effects on rates 

and amount of deposition of alluvium in the valley of the 

Mississippi and Ohio rivers.

FAULTS

This discussion of faults is separated from geologic history 

because of their direct importance to earthquakes (fig. 3). The 

fundamental questions to be answered in any study of the location and cause of earthquakes in a particular region are (1) 

where are the seismically active faults, and (2) what is the past 

and present rate of movement of them. Extensions of the questions include consideration of the structural regime in which 

the faults occur, their length, and their mode of displacement.

SURFACE FAULTS

The amount and history of faulting are not well known 

because of cover by Quaternary sediments and because detailed geologic mapping has not been done in much of the area. 

The faults shown in the generalized map of the region (fig. 3) 

are inadequate for comprehensive tectonic analysis. Even 

more detailed compilations have limitations. As Heyl and 

McKeown (1978) stated, for a more detailed map than is 

shown in figure 3, most of the eastern Ozark Mountains, most 

of western Tennessee, and most of southwestern Illinois have 

not been mapped in detail, whereas western Kentucky and the 

Fluorspar district of southern Illinois have been. Comparison 

of the geologic structure among the areas is, therefore, not 

justified. The New Madrid fault zone of Heyl and Brock 

(1961) is suggested in part on the basis of the zone of northeasttrending faults in southeastern Illinois. The trend, distribution, and abundance of faulting in southwestern Illinois, 

however, are not known owing to lack of enough detailed 

mapping.

Some gross inferences of major structural trends can be 

made, particularly from a regional perspective. Major faults 

such as the Palmer and Sims Mountain faults in the Precambrian and Paleozoic rocks of the St. Francois Mountains trend 

predominantly west and northwest. The largest fault or fault 

zone in the northeastern part of the Ozark uplift is the Ste. 

Genevieve fault zone, which is part of the 38th parallel lineament of Heyl (1972). The fault apparently had post-Devonian 

movement and possibly post-Mississippian movement on it. In 

contrast to the northwest strike of major faults, the trend of 

diabase dikes in the St. Francois Mountains is predominantly 

north (Gibbons, 1972). As noted by McKeown (1978), this 

trend is coincident with the strike of most nodal planes in 

focal-mechanism solutions of Street and others (1974). 

Whether or not this coincidence in trends is causally related is 

controversial, but the trend of diabase dikes is certainly indicative of a structural grain related to major deep-seated 

crustal stresses during the late Precambrian.

The northeast-trending faults in southern Illinois and 

western Kentucky offset Mississippian and Pennsylvanian 

rocks. They form a broad zone but, as mentioned above, their 

western extent is unknown. Displacement on them is as much 

as about 150 m, but none of the faults appears to be a major
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

92W 91°00' 90°00' 89°00' 88°00'

38°00

37W

ILLINOIS

INDIANA

FAULT

v

OZARK 

UPLIFT

\u

BLOOMFIELD" 

o PLUTON °

/ X7V

-4^\"

MISSOURI ____ ^^ n w,

ARKANSAS" .^ MAIDEN1 

<£/ PLUTON

$y PARAGOULD ) . 

y PLUTON^^CARUTHEI

^a

JONESBORO

PLUTON^ j/

,yoc / 

 - _/

-^o-^^ (^

NEWPORT^ 

PLUTON m Marked 

4 Tree

xCOVINGTON 

^PLUTON

l^

V -v

i? r~>

.c^-Jgj Memphis________ TENNESSEE 

^ ") 0 1020304050 KILOMETERS MISSISSIPPI

'_______j_______|_______|______I

^r\ Northern limit of Mississippi Emboyment 

^^ coastal-ploin material

A Mafic or ultramafic intrusion within the 

Mississippi Embayment-ldentified in 

drill cores

EXPLANATION 

, Mofic or ultramafic intrusion within the 

Mississippi Embayment interpreted from 

the magnetic field-- Approximate boundary 

of intrusion determined from zero contour of associated onomoly on the secondverticol-derivative magnetic map

Fault

Earthquake epicenter

Boundary of inferred 

subsurface rift

FIGURE 3. - Generalized fault map of the New Madrid region showing plutons, approximate rift boundaries, and epicenters of earthquakes detected from

July 1974 to June 1977.

structural feature. As noted by Heyl and McKeown (1978), minate at shallow depths (Bristol, 1975). More recent interthey appear to be tensional faults related to anticlines, and pretations of drill-hole data by Bristol and Treworgy (1979)

some of them in the Wabash Valley fault system may ter- show that the major faults in the system probably extend to
OVERVIEW AND DISCUSSION

0.03i

o

0) 

X

L_

<D

a 0.02 :

= 0.01 T

40 60 80 100 

Million years

120 140

FIGURE 4. Graph showing rate of deposition of sediments during subsidence 

of embayment based upon thickness of sediments deposited during indicated periods of geologic time but not the loss of record of deposition 

at diastems and unconformities

basement, which is 3.6-4.3 km below sea level. They also 

show, as Bristol did earlier, that some faults die out with 

depth.

Most of the youngest faults strike northeast and offset Tertiary gravels (Lafayette Formation of former usage), whose age 

is controversial. These gravels are generally considered to be 

Pliocene (Potter, 1955), but some evidence suggests a Miocene 

age (W. W. Olive, written commun., 1978). Recently, Russ 

(1979) reported more than 3 m of offset of strata in Holocene 

alluvium near the base of a scarp on the west side of Reelfoot 

Lake. The tectonic significance of this fault is not clear, 

however, and the reader is referred to Russ for details.

SUBSURFACE FAULTS

Recent exploration of the New Madrid region by gravity, 

magnetic, and seismic-reflection-profile methods has tremendously advanced knowledge of the tectonic framework of the 

region. The first significant information on deeply buried 

geologic structure has resulted from the gravity and aeromagnetic surveys started under the leadership of Martin Kane, of 

the USGS, in 1973. Maps (Hildenbrand and others, 1977) and 

reports (Hildenbrand and others, this volume) show and 

describe a major subsurface rift extending from the vicinity of 

New Madrid, Mo., southwestward at least as far as the latitude 

of Memphis, Tenn. The northeastern extent is not known. By 

analogy with rifts in other parts of the world, particularly east 

Africa, an abundance of faults may be inferred within the rift. 

Available reflection profiles, however, do not show an abundance of faults that offset post-Paleozoic rocks (Zoback, 1979; 

Zoback and others, 1980; Hamilton and Zoback, this volume). 

Very few reflections from below the surface of the Paleozoic 

rocks occur in these profiles, however, and so the amount of

faulting in Paleozoic rocks is not well known. Only two of the 

profiles have reflections below the Paleozoic surface. These 

profiles were obtained along lines in the southern part of the 

current zone of seismicity and cross the zone in two places. 

Zoback and others (1980) interpreted the profiles as showing a 

fault zone with a vertical offset of about 1 km in rocks of 

Paleozoic age. In addition, they interpreted several small faults 

that offset the Paleozoic surface with a cumulative vertical 

displacement of about 50 m over a distance of 9 km. The zone 

of small faults overlies the large fault zone, and both zones are 

coincident with the northeast-trending zone of seismicity. 

Because of the differences in the character of the seismic 

signature of the deep reflectors on both sides of the inferred 

large vertical offset and the relatively small vertical displacement of the Paleozoic surface, an interpretation of some lateral 

movement across the zone is permissible. Such an interpretation is compatible with focal- mechanism studies of Hermann 

and Canas (1978). No unambiguous geologic evidence of 

lateral movement is known; however, large (tens of kilometers) 

movement can be precluded from stratigraphic relationships in 

the Pascola arch area. Some investigators (Grohskopf, 1955; 

Marcher and Stearns, 1962) have constructed pre-Cretaceous 

subsurface geologic maps of the area. The drill-hole control for 

these maps is poor, but it is adequate to define the gross 

distribution of Paleozoic rock units. The continuity of the units 

across the inferred rift does not suggest lateral movement.

Many small faults that offset the Paleozoic surface and rocks 

of Paleocene and Eocene age occur locally in the Reelfoot Lake 

area. Most of the faults cannot be correlated from one profile 

line to another. A notable exception is the Cottonwood Grove 

fault, defined and described by Zoback and others (1980). This 

fault and subparallel faults are part of a small northeasttrending fault system that appears to be on a projection of the 

zone of seismicity along the axis of the rift. The system is about 

3 km wide and 12 km long. However, neither epicenters nor 

nodal planes of seismic events in the vicinity of the fault system 

parallel the northeast trend of the system. Any relation between structural and seismic trends is at best equivocal.

Prior to the inferred delineations of subsurface faults from 

the reflection-profile data, the trends of epicenters reported by 

Stauder and others (1976) and Stauder and others (this 

volume) indicated the direction of seismically active faults or 

fault zones. The longest trend extends for about 100 km from 

Marked Tree, Ark., to about Caruthersville, Mo. Whether the 

epicenters that define this trend are all related to a single long 

fault or a zone of subparallel short faults along the axis of the 

rift cannot be determined from any available data. The fact 

that this trend is the longest, is along the axis of the rift, and is 

coincident with the linear zone of sand blows caused by earthquakes in the New Madrid sequence of 1811-12 suggests that 

it is related to a fault zone with the potential of having the 

largest earthquakes in the region. This trend is not, however, 

indicative of the structural complexity in the region. The
8 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

seismological and geophysical data northeast of Caruthersville 

in the vicinity of Reelfoot Lake provide insight into some of 

this complexity. The trend of epicenters (Stauder and others, 

this volume) indicates a north-northwest structural zone that 

extends from near Dyersburg, Tenn., to near New Madrid, 

Mo., where it bifurcates into an east-west branch and a northeast branch. The branches occur approximately on edges of 

the Bloomfield pluton (Hildenbrand and others, this volume). 

Some of the more recent locations of earthquakes between 

Dyersburg and New Madrid suggest that the north-northwest 

zone may comprise an en echelon northwest- trending zone of 

faults. Recent work by O'Connell and others (this volume) 

shows northwest and north trends in seismic data acquired 

from a portable network of 19 stations in the area.

In summary, even though more geophysical, seismological, 

and geologic data are available for the area of currently most 

frequent seismic activity than for any other area in the region, 

exact delineations of seismogenic faults cannot yet be made. 

The reader is referred to references cited earlier in this section 

for more detailed information on subsurface structure as inferred from geophysical and seismological data. Many of these 

data indicate the presence of many relatively small bodies of intrusive rocks. As these bodies of rock appear to have some relation to the location of some seismic activity and are probably 

genetically related to repeated faulting and minor doming, 

they may be of considerable significance to understanding the 

seismic activity. Details of the relation of structure to intrusive 

masses are giveri in Zoback and others (1980) and Hamilton 

and Zoback (this volume).

INFERRED CHARACTERISTICS OF 

SEISMICITY ASSOCIATED WITH RIFTS

As the available evidence strongly indicates that the seismicity in the New Madrid region is associated with an ancient intraplate rift, a brief review of the evidence and an analysis of 

the possible significance of the rift to characteristics of seismicity are appropriate. It is unnecessary here to review the possible 

causes and timing of the rift and its relation to other possible 

rifts and tectonic events in Central and Eastern United States. 

An excellent overview of these topics has been given by Hinze 

and others (1980). In addition, the reader is referred to Burke 

and Dewey (1973), Ervin and McGinnis (1975), Hoffman and 

others (1974), Lidiak and Zietz (1976), and Rankin (1975) for 

extensive discussion on and inferences about the location and 

development of rifts. Perhaps more relevant to discussion of 

the relation of seismicity to rifts are the data and concepts given 

by Kumarapeli and Saull (1966). They argued convincingly 

that the St. Lawrence Valley is a rift zone initiated in the 

Mesozoic Era that may extend to the midcontinent region. 

Further, they attributed current seismicity to the rift system. In 

a later paper, however, Kumarapeli (1976) modified some of 

the concepts presented in 1966. One of the chief criticisms of

the 1966 paper was that the St. Lawrence rift system is not currently an active extensional feature (Voight, 1969). Results of 

in situ stress measurements presented by Voight indicate that 

the region is under high horizontal compressive stress. This 

observation of the current state of stress does not invalidate the 

importance of Kumarapeli's concepts of the distribution and 

ages of the St. Lawrence rift system, or of the mineral 

assemblages and seismicity associated with it.

That the rifts tend to be located along old structural 

elements is widely recognized. (See Sykes, 1979, for a review of 

evidence of reactivation of rifts.) It is significant to note that 

the east coastal area of the United States, which was involved 

with continental rifting in the Mesozoic, is today under compression (Zoback and Zoback, 1980). Therefore, arguments 

about whether a rift is currently in a state of compression or 

extension are not necessarily relevant to explanations of why 

earthquakes occur in rifts. What is relevant is that rifts are major breaks in the crust that have properties more favorable for 

the occurrence of earthquakes than adjacent less broken parts 

of the crust.

Recognition of young or old rifts is of value to earthquake 

studies because it helps to differentiate faults in a region that 

extends to hypocentral depths of earthquakes from faults that 

may or do not extend to depths of more than a few kilometers. 

Recognition of rifts and an association of earthquakes with 

rifts are important steps in attempts to understand the location 

of earthquakes within continental plates. However, in concert 

with this knowledge, efforts must be made to explain physically why earthquakes are more likely to occur in rifts than in 

adjacent areas. Rigorous physical explanations cannot be 

given yet, but several qualitative geologic explanations will be 

given in subsequent paragraphs. Explanation of earthquakes 

occurring in rifts may not only provide a basis as to why earthquakes occur where they do but also provide a possible basis to 

estimate limits both on location and on source dimensions of 

earthquakes in rifts. Explanations of earthquakes associated 

with plate boundaries in Western United States, particularly 

California, can be made; and these, along with empirical 

data, are used to estimate source dimensions, size of earthquakes, zones of potential seismic activity, and so forth. The 

tectonic environment of intraplate earthquakes in Central and 

Eastern United States, however, is poorly known, and some 

knowledge commonly used to describe and evaluate earthquakes in Western United States does not appear to be relevant to studies of earthquakes in Central and Eastern United 

States. Discussion of earthquake sources and dimensions in 

this chapter, therefore, will be limited to the tectonic regime 

of the New Madrid region and comparison with similar 

regimes elsewhere in the world. The features of the regime 

that need to be considered as a basis for estimating limits on 

earthquake source dimensions and their physical 

characteristics are (1) abundance, dimensions, and mode of 

displacement of faults in and adjacent to intraplate rifts; (2)
OVERVIEW AND DISCUSSION

rates and kind of deformation occurring now or in the recent 

past; and (3) physical characteristics and distribution of rocks 

in rifts.

CHARACTERISTICS OF FAULTS IN RIFTS

The abundance of, dimensions of, and mode of displacement on faults in the buried rift of the New Madrid region may 

never be known in detail but may be postulated by analogy 

with exposed rifts. According to interpretations of the aeromagnetic and gravity data presented in this volume, major rifting occurred in the late Precambrian or Early Cambrian. As 

delimited by offsets of the aeromagnetic basement, the rift is 

about 70 km wide and more than 200 km long. Relief on the 

aeromagnetic basement is about 2 km. The bounding fault 

zones have not been observed on the seismic-reflection profiles; 

however, the profile lines did not extend far enough towards 

the boundaries of the rift to detect the zones for certain. An axial zone of faults and probably intrusives has been identified in 

reflection profiles southwest of Blytheville, Ark. (Zoback and 

others, 1980). The northeastern and southwestern extent of the 

zone is not known from available geophysical data. As a northeastern trend of seismicity that extends from Marked Tree, 

Ark., to about Caruthersville, Mo., seems to be associated 

with the axial zone, the zone may extend equally as far. 

Reflection-profile data show faults of the Paleozoic surface, but 

they are not particularly abundant and offsets are small. For 

example, a cumulative offset of the Paleozoic surface of only 50 

m occurs within a distance of about 9 km across the axial zone.

Studies of exposed active or recently active rifts in east Africa 

(for example, Baker and others, 1972; Bailey, 1974; Barberi 

and others, 1974; and Ring, 1970) provide examples of the 

characteristics of faults and igneous rocks in rifts that may be 

similar to the rift in the New Madrid region. These studies also 

show, however, that rifts are quite complex and are varied 

enough so that generalizations could be misleading. Nevertheless, it is reasonable to cite features of the East African rifts as 

models for the New Madrid region. Where exposures are 

good, detailed mapping shows an abundance of faults within 

rifts (for example, Baker and others, 1972, figs. 11 and 12). 

King (1970, p. 203) noted for the East African Rift System the 

"...extraordinary intensity of faulting (ranging from Tertiary 

to Recent) within their limits and the almost complete absence 

of faults outside." The rift in the New Madrid region, 

therefore, may have an abundance of faults at a depth below 

the top of the aeromagnetic basement, which presumably 

represents Precambrian or Lower Cambrian igneous rocks, 

that would have been involved in rifting. By analogy, if east 

African-rift-type faults are present, they will typically be segmented, intersecting, and en echelon; their lengths will commonly range from a few to about 15 km. In addition, King 

(1970) has observed that the tectonic and igneous activity 

migrates with time towards the center of rifts. It is reasonable 

to expect, therefore, that the axial zone of a rift may be the

most disrupted and long-lasting zone of weakness. If so, it is 

not surprising that some of the current seismicity in the New 

Madrid region is along an inferred axial zone of the rift.

Rifts are the result of extensional tectonics. Causes of the extension are controversial, and the reader is referred to Bailey 

(1974) for review of some of the controversy. It is not controversial that faults within an active rift predominantly have normal dip-slip displacement. As mentioned earlier in this 

chapter, however, the mode of displacement of faults initially 

formed in a rift may change in accordance with changes in the 

state of stress. The strike-slip and reverse fault mechanisms 

reported by Herrman and Canas (1978) for modern earthquakes in the New Madrid region reflect the current state of 

stress on faults in an ancient rift.

RATES OF DEFORMATION

As was briefly stated in the introduction to this chapter, the 

rate of movement on faults in the New Madrid region is profoundly less than the rate of movement on faults in an active 

plate boundary system or the rate of movement of oceanic rifts. 

A wide range in rate of fault slip can be calculated in the New 

Madrid region, but the rates are all low relative to plateboundary motion. If the 50-m cumulative vertical displacement of the Paleozoic surface across a width of 9 km, reported 

by Zoback and others (1980), is assumed to have occurred in 

the last 50 m.y., the rate of displacement is 0.001 mm/yr. 

Another example is the 80-m vertical displacement on the Cottonwood Grove fault, reported by Zoback and others (1980). 

The total offset on this fault likely occurred in the last 50 m.y. 

However, even if it is assumed to have occurred in the last 10 

m.y., the rate is only 0.008 mm/yr. The largest rate of 

displacement may be deduced from the work of Russ (1979). 

He reported at least a 3-m vertical offset of alluvium exposed in 

a trench near Reelfoot Lake in the last 2,000 years. This is a 

rate of 1.5 mm/yr. All of these calculated rates of displacement 

are more than an order of magnitude smaller than the 2-3 

cm/yr horizontal movement measured along the San Andreas 

fault. Another comparison of rates of fault slip can be made 

using information from the Basin and Range province provided by R. C. Bucknam (oral commun., 1980). Rates of structural relief that may be presumed to have occurred on faults 

range from 0.2 to 0.4 mm/year in a time range of 105 -107 

years. On a shorter time scale, movement on the Wasatch 

fault ranges from 1.1 to 1.7 mm/yr during the last 104 years.

Deformation by warping (uplift or subsidence) of large 

areas of the land surface, however, is also an order of 

magnitude different for the New Madrid region when compared with the southern California uplift. Russ reports a 10-m 

vertical displacement of the Lake County uplift area in the last 

6,000 years, which is 1.6 mm/yr. Most of the uplift could have 

occurred in the last 2000 years, which would be a maximum 

rate of 4.8 mm/yr. In contrast, uplift and subsidence rates of 

the southern California uplift are on the order of 1-3 cm/yr
10 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

(Castle and others, 1976).

Another comparison of rate of tectonic deformation within 

rift zones is given by Bailey (1974) and Baker and others 

(1972). Bailey, citing other authors, reported extension rates 

of 0.1 and 0.5 mm/yr for the Rhine and Kenya rifts, respectively. Similarly, Baker and others reported maximum extension rates of 0.4-1 mm/yr for rifts in Ethiopia, Kenya, 

Turkana, and Tanzania. As emphasized by Bailey, the rate of 

extension of intracontinental rifts is more than an order of 

magnitude less than the rate of extension of oceanic rifts, 

which suggests that they are fundamentally different structures.

In view of the above examples of rates of various kinds of 

tectonic deformation, it is not surprising that surficial 

evidence of deformation associated with historic earthquakes 

in the New Madrid region is lacking.

A deduction from the above data may be that the large 

earthquakes (the 1811-12 sequence) were deeper than 

postulated on the basis of attenuation studies (Nuttli, 1973b) 

or that the source dimensions are smaller (Evernden, 1975). 

Either circumstance would very likely preclude rupturing of 

the surface or even offset of deeply buried Paleozoic rocks and 

explain the lack of surface rupturing during the 1811-12 sequence of earthquakes.

Covington pluton about 80 km north of Memphis, Tenn. 

Lamprophyre and mica peridotite were penetrated in drill 

holes in southwestern Tennessee and southeastern Missouri. 

As noted by Hildenbrand and others (this volume), these occurrences of alkalic rocks range from Permian to Cretaceous in 

age. Interpretations of seismic-reflection profiles (Zoback and 

others, 1980; Click, this volume) suggest Tertiary intrusive activity. This range in age of alkalic intrusive rocks along the 

boundaries of a Precambrian or Early Cambrian rift is indicative of renewed rift tectonics.

The characteristics of alkalic rocks that may affect the location of seismicity are their origin in the upper mantle or lower 

crust; their original, very high content of volatiles; and their 

wide range in mineralogic composition. Convincing arguments and data support the view that alkalic rocks are derived 

from the upper mantle or lower crust (for example, Bailey, 

1974). Bailey also stated (p. 155), "Abnormal enrichment in 

volatiles and alkalies is the keynote of rift magnetism." The 

wide range in mineralogic composition is well documented in 

the study of the Magnet Cove area by Erickson and Blade 

(1963); it requires the interpretation that the elastic moduli 

and density of the rocks also range widely. These characteristics, as they may relate to earthquakes, are discussed in the 

next section.

CHEMICAL AND PHYSICAL 

CHARACTERISTICS OF ROCKS IN RIFTS

The great abundance of alkalic rocks in or associated with 

rifts is well documented. The fact that assemblages of these 

rocks have unique chemical and physical characteristics and 

originate in the upper mantle or lower crust makes any zone 

that has an abundance of them of particular interest to a variety 

of earth-science specialists. Interests include questions related 

to the fields of economic geology and mineralogy and, of importance to this discussion, the possible effect that assemblages 

of alkalic rocks may have on the location of earthquakes. The 

abundance of such rocks in or associated with the rift in the 

New Madrid region can only be inferred. As all outcrops and 

samples of rock from drill holes immediately adjacent to or 

within the Reelfoot rift are alkalic, it is reasonable to infer that 

an abundance of such rocks occur in the subsurface. The best 

known occurrences of alkalic suites of rock in the region occur 

at Magnet Cove, Ark. (Erickson and Blade, 1963), and in the 

Little Rock, Ark., bauxite district (Gordon and others, 1958). 

Earlier Williams (1891) described all of the known occurrences 

of igneous rocks in Arkansas, all of which are alkalic. These 

alkalic rocks occur in petrologically complex plutons that are 

part of the northwest side of the Reelfoot rift; they range from 

felsic to mafic varieties of alkalic rocks. The Magnet Cove 

Complex includes carbonatite, which is common in areas of 

rifting. Exploration drill holes for oil in Tennesse and Missouri 

have penetrated syenite, lamprophyre, and mica peridotite 

(Kidwell, 1951). Syenite was penetrated by a drill hole over the

POSSIBLE SIGNIFICANCE OF

ALKALIC-ROCK 

CHARACTERISTICS TO SEISMICITY

In addition to the fact that alkalic rocks have deep-seated 

origins and therefore must have penetrated through deep fractures in the crust to reach shallow crustal levels, they prompt 

suggesting a speculative hypothesis that is relevant to seismicity 

in rifts. Briefly, the hypothesis presented in this section is that 

the volatile content of rocks in the rift of the New Madrid 

region was or is still high enough to make them particularly 

weak because of increased porosity and (or) high pore pressure 

compared with rocks not associated with the rift. Furthermore, the large contrasts in rock types, ranging from very 

dense mafic rock to much less dense felsic rock, provide differences in elastic moduli that may cause local concentrations 

of stress. Arguments to support this hypothesis rest principally 

on the assumption that a variety of alkalic rocks are present in 

the rift, as available evidence previously given indicates, and 

that extrapolation of the amount of volatiles and their effects 

on the strength of materials are as given in the literature.

The volatile content of alkalic rocks, particularly those that 

never reached the surface, is difficult to document. Roedder 

(1965) noted in his study of CO2 inclusions in olivine-bearing 

nodules that the volume percent of CO2 ranged widely, from a 

few parts CO2 per trillion of rock to as much as 3 percent in individual grains of olivine. Studies of phase relationships of 

compounds in the mantle and lower crust provide some insight
OVERVIEW AND DISCUSSION 11

into the limits of the volatile content of magmas and rocks. As 

mantle magma cannot dissolve more than 5 percent CO2 at 

depths above 80 km but may contain 40 percent CO2 below 80 

km, a large amount of CO2 is presumably exsolved as the 

magma ascends. This potentially large exsolution of CO2 may 

be an explanation of the explosive emplacement of kimberlites 

(Wyllie and Huang, 1976). The many occurrences of 

diatremes (Heyl and McKeown, 1978) are evidence that exsolution of CO2 has occurred on a large scale in the New 

Madrid region. A further argument that the alkalic rocks in 

the region contained a high volatile content is that CO2 and 

H2O must be present to explain alkalic magmas (Wyllie,

1978).

The best quantitative information on the volatile content of 

alkalic rocks similar to those penetrated by drill holes in the 

New Madrid region is in the studies of the Magnet Cove area, 

Ark., by Ericksori and Blade (1963). In table 46 of their 

report, the computed values of volatiles in Magnet Cove rocks 

and average igneous rocks are as follows:

Rocks i

COo

r,i

Fl

S"3

S

af Magnet Cove Complex 

(percent)

........... .2.3

............ .14

............ .23

............ .11

............ .54

Total .... .3.32

Average igneous rocks 

(percent)

0.10

.05

.03

.05

0.23

They also calculated that the end fraction of magma, which 

was 9 percent of the original Magnet Cove magma, contained 

more than 10 percent CO2 . It is assumed that this excess CO2 

combined with CaO to form carbonatite bodies. As the exact 

quantities of compounds and elements in the end fraction of 

magma are not known, it is impossible to compare excesses 

and deficiencies in them with stoichiometric calculations. 

Considering the generally known fact that all volcanic activity 

includes discharges of large amounts of volatiles, it is 

reasonable to assume that an excess of those that do not combine to form crystalline products is present in all magmas. 

This is not to say that all excess volatiles remain trapped in the 

solidified magmas of intrusive and extrusive bodies of igneous 

rock. But it is certainly reasonable to assume that rocks derived from highly volatile magma will have a greater residual uncombined, trapped volatile content than rock derived from 

magma with a low volatile content.

The above information and arguments are given simply to 

support the concept that even though no quantitative information on the porosity or volatile content of deeply buried 

alkalic igneous rocks in the region is available, the probability 

is high that they should still contain more volatiles or have 

higher porosity than adjacent metamorphic or granitic rocks 

whose source zones are at much shallower crustal depths than 

those of alkalic rocks and that did not evolve from a magma 

with an extremely high content of volatiles.

Given the preceding hypotheses about the physical

characteristics of deeply buried rocks in the rift zone in the 

New Madrid region, one can conclude that (1) trapped 

volatiles in phenocrysts, miarolitic cavities, or other bubbles 

are likely to be under high pressure; and (2) whether or not the 

volatiles have reacted with minerals or escaped through fractures or other permeable channels, the alkalic rocks are likely 

to have a higher porosity than adjacent country rock.

The first conclusion is based on the supposition that the 

magma, which persumably starts exsolving CO2 at a depth of 

about 80 km would trap ( 25 kb), CO2 or other volatiles at 

high pressure as crystallization of the magma occurs during its 

upward ascent. This process, therefore, would result in a rock 

having high pore pressure, an environment conducive to generation of earthquakes.

The second conclusion permits suggesting a similar environment but one in which the pore pressure would not be greater 

than the lithostacic load, or the hydrostatic load if the pores 

were interconnected to a hydrologic system. As demonstrated 

by Hubbert and Rubey (1959) and suggested earlier by Yoder 

(1955), pressure in rocks with very low permeability and with 

pores essentially isolated approximates the weight of the overburden. Similar to the environment derived from the first conclusion, this environment of high, though not abnormal, pore 

pressure would be conducive to generation of earthquakes. 

The normal stress across any shear plane in rocks in either environment would be reduced by the pore pressure (Hubbert 

and Rubey, 1959).

Pore-pressure effect, as hypothesized above, may be the 

largest factor contributing to the localization of earthquakes 

in the New Madrid region, but porosity alone could contribute 

to weakening of the rock, particularly if the porosity is greater 

than a few percent. Experiments to determine the empirical 

relationship of the stress required to cause brittle fracture of 

rocks as a function of porosity have been made by Dunn and 

others (1973) ori sandstone and quartzite. Hoshino (1974) 

reported results of similar experiments on argillaceous and 

arenaceous rocks. Prior to the experiments on rocks, experiments on the properties of ceramics had determined a 

relationship of strength to porosity (for example, Ryshkewitch, 

1953; Coble and Kingery, 1956; Knudsen, 1959; and Friedman, 1976). The empirical data from these experiments for 

both rocks and ceramics indicate large changes in strength for 

small changes in porosity at low values of porosity. As examples of the range in percentage change in strength as a 

function of porosity, data from Hoshino (1974) and Dunn and 

others (1973) are cited. Hoshino (1974) determined about a 10 

percent reduction in compressive strength of argillaceous rocks 

for a change in porosity from 1 to 5 percent at a confirming 

pressure of 100 bars. Data on sandstone and quartzite, 

reported by Dunn and others (1973), show a 60 percent reduction in compressive strength for a change in porosity from 1 to 

5 percent at a confining pressure of 1000 bars. According to 

the experimental data on rocks, at higher confining pressures 

the reductions in strength are larger. These relatively large
12 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

changes in strength of rocks as a function of porosity indicate 

that the small difference in porosity inferred from the 

petrologic characteristics and origin of alkalic rocks compared 

to other igneous types of rocks may be very significant to the 

interpretation of the relationship of seismicity to type of igneous rock.

The actual porosity of rocks at the depths of earthquakes in 

the New Madrid region is of course unknown and may never 

be known. Some insight on the porosities can be gained from 

measured porosities of igneous rocks reported in the literature. 

Farbeov and Levykin (1971) noted that the differences in 

elasticity among some rocks could not be explained by differences in mineral computations. The relatively low elasticity 

of some ultrabasic xenoliths was due to microporosity of as 

much as 5 percent. Measurement of intragranular porosity 

calculated from bulk and grain densities of a variety of syenites 

ranges from 0.6 to 0.9 percent (Wuerker, 1956). Additional 

perspective on the range in porosities of igneous rocks can be 

gained from some of the abundant physical-property 

measurements of igneous rocks at the NTS (Nevada Test Site), 

even though these rocks are not alkalic. Porosities of quartz 

monzonite and granodiorite from the Climax stock at the NTS 

average 0.71 percent, 1.08 percent, and 1.55 percent for finegrained quartz monzonite, medium-grained quartz monzonite, and granodiorite respectively (Maldonado, 1977). It 

should be noted, perhaps emphasized, that the porosities cited 

above are for a type of rock that probably did not have as high 

a volatile content as alkalic rocks. Alkalic rocks that have 

higher volatile content and that were emplaced under ex tensional tectonic conditions of a rift may certainly be expected to 

have a total in situ porosity higher than that of the intrusive 

rocks at NTS and rocks of similar composition elsewhere. A 

higher porosity of only a few percent would result in alkalic 

rocks in a rift being significantly weaker than nonalkalic rocks 

outside of a rift.

The concept of stress amplification around masses of rock 

surrounded by other rocks with different elastic moduli has 

been proposed by Kane (1977), Long (1976), and McKeown 

(1978). In the author's opinion, this concept must be considered as an explanation for local control on the distribution of 

earthquakes in the New Madrid region because of the 

divergence in trends of epicenters near the Bloomfield pluton. 

Geophysical, outcrop, and drill-hole information strongly indicates an abundance of mafic intrusive masses ranging in size 

probably from a few tens of meters to several tens of kilometers 

in their greatest horizontal dimensions. That intrusives having 

different elastic moduli than the host rock cause stress concentrations is well documented (for example, Oudenhoven and 

others, 1972). It seems reasonable to expect that some if not all 

of the intrusive masses in and associated with the rift will cause 

local perturbations and concentrations in the regional stress 

field and, therefore, have an effect on the location of earthquakes.

SUMMARY

The environment of the source zones of earthquakes in or 

associated with the rift in the New Madrid region can be summarized as follows:

1. A large number of faults are likely present, such as those 

mapped in rifts of east Africa. Individual faults probably 

have lengths less than 15 km.

2. The felsic-to-mafic range in composition of igneous rocks 

results in large contrasts in density and elastic module.

3. The pore pressure and (or) porosity is likely to be higher in 

rocks within or immedately adjacent to the rift than 

elsewhere in the region.

4. As the alkalic rocks originated in the lower crust or upper 

mantle, some faults must extend essentially through the 

crust.

The above four factors suggest that earthquakes in the rift 

may have the following characteristics:

1. Source dimensions should be less than 15 km in length.

2. Multiple rupturing may occur because of close spacing and 

interconnection of faults.

3. Local control of the distribution of earthquakes should 

occur because of stress concentrations around bodies of 

rock that have grossly different physical properties, as 

postulated by Long (1976), Kane (1977), and McKeown 

(1978).

4. Because of the high pore pressure and (or) porosity, rupturing may occur on relatively low stress levels.

5. As some faults may extend through the crust, some earthquakes may have deep hypocenters.

REFERENCES CITED

Anderson, R. E., 1970, Ash-flow tuffs of Precambrian age in southeast

Missouri (Contribution to Precambrian Geology, no. 2): Missouri

Geological Survey and Water Resources Report of Investigations 46, 50 p. 

Baker, B. H., Mohr, P. A., and Williams, L. A. J., 1972, Geology of the

eastern rift system of Africa: Geological Society of America Special

Paper 136, 67 p.

Barberi, F., Bonatti, E., Marinelli, G., andVaret, J., 1974, Transverse tectonics during the split of a continent Data from the afar rift: Tectonophysics, v. 23, p. 17-29. 

Bailey, D. K., 1974, Continental rifting and alkaline magnetism, in

Sorensen, H., ed., The alkaline rocks: New York, N.Y., John Wiley and

Sons, p. 148-159. 

Bickford, M. E., and Mose, D. G., 1975, Geochronology of Precambrian

rocks in the St. Francois Mountains, southeastern Missouri: Geological

Society of America Special Paper 165, 48 p. 

Bristol, H. M., 1975, Structural geology and oil production of northern

Gallatin County and southernmost White County, Illinois: Illinois State

Geological Survey, Illinois Petroleum 105, 20 p. 

Bristol, H. M., andTreworgy, J. D., 1979, The Wabash Valley fault system

in southeastern Illinois: Illinois State Geological Survey Circular 509,

19 p.
OVERVIEW AND DISCUSSION 13

Burke, Kevin, and Dewey, J. F., 1973, Plume-generated triple junctions 

Key indicators in applying plate tectonics to old rocks: Journal of 

Geology, v. 81, p. 406-433.

Caplan, W. M., 1954, Subsurface geology and related oil and gas possibilities of northeastern Arkansas: Arkansas Resources and Development 

Commission, Division of Geology Bulletin 20, 124 p.

Castle, R. O., Church, J. P., and Elliott, M. R., 1976, Aseismic uplift in 

southern California: Science, v. 192, p. 251-253.

Coble, R. L., and Kingery, W. D., 1956, Effect of porosity on physical properties of sintered alumina: American Ceramic Society Journal, v. 39, 

p. 377-385.

Dunn, P. E., LeFountain, L. T., and Jackson, R. E., 1973, Porosity dependence and mechanism of brittle fracture in sandstones: Journal of Geophysical Research, v. 78, p. 2403-2417.

Erickson, R. L., and Blade, L. V., 1963, Geochemistry and petrology of the 

alkalic igneous complex at Magnet Cove, Arkansas: U.S. Geological 

Survey Professional Paper 425, 95 p.

Ervin, C. P., and McGinnis, L. D., 1975, Reelfoot rift Reactivated 

precursor to the Mississippi Embayment: Geological Society of America 

Bulletin, v. 86, p. 1287-1295.

Evernden, J. F., 1975, Seismic intensities, "size" of earthquakes and 

related parameters: Seismological Society of America Bulletin, v. 65, p. 

1287-1313.

Fabareov, A. I., and Levykin, A. I., 1971, Elastic properties of Kamchatka 

rocks at pressures up to 30kb in connection with problems of deepseated structures of volcanic areas Preliminary results: Volcanology 

Bulletin, v. 35, p. 183-197.

Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower

Mississippi River: Vicksburg, U.S. Corps of Engineers Mississippi River

Commission, 78 p. 

Friedman, M,, 1976, Porosity, permeability and rock mechanics A review:

Symposium on Rock Mechanics, 17th, Proceedings, Snowbird, Utah,

p. 1-16. 

Fuller, M. L., 1912, The New Madrid earthquake: U.S. Geological Survey

Bulletin 494, 119 p. 

Gibbons, J. F., Ill, 1972, Tectonics of the eastern Ozarks area, southeastern

Missouri: Syracuse University Ph. D. thesis, 70 p. 

Gordon, M., Jr., Tracey, T. I., Jr., and Ellis, M. W., 1958, Geology of the

Arkansas bauxite region: U.S. Geological Survey Professional Paper

299, 268 p. 

Grohskopf, J. G., 1955, Subsurface geology of the Mississippi Embayment

of southeastern Missouri: Missouri Geological Survey and Water

Resources, v. 37, ser. 5, 133 p. 

Herrmann, R. B., andCanas.J., 1978, Focal mechanism studies in the New

Madrid seismic zone: Seismological Society of America Bulletin, v. 68,

no, 4, p. 1095-1102. 

Heyl, A. V., Jr., 1972, The 38th parallel lineament and its relationship to

ore deposits: Economic Geology, v. 67, p. 879-894. 

Heyl, A. V., Jr., and Brock, M. R., 1961, Structural framework of the

Illinois - Kentucky mining district and its relation to mineral deposits,

in Short papers in the geologic and hydrologic sciences: U.S. Geological

Survey Professional Paper 424-D, p. D3-D6. 

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map

of central Mississippi Valley and environs: U.S. Geological Survey Miscellaneous Field Studies Map MF-1011. 

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic

and gravity anomalies in the northern Mississippi Embayment and their

spatial relation to seismicity: U.S. Geological Survey Miscellaneous

Field Studies Map MF-914. 

Hinze, W. J., Braile, L. W., Keller, G. R., andLidiak, E. G., 1980, Models

for Midcontinent tectonism: National Academy of Science Monograph

on Continental Tectonics [in press].

Hoffman, P., Dewey, J. F., and Burke, K., 1974, Aulacogens and their 

genetic relation to geosynclines with a Proterozoic example from Great 

Slave Lake, Canada, in Dott, R. H., Jr., and Shaver, R. H., eds., 

Modern and ancient geosynclinal sedimentation: Society of Economic 

Paleontologists and Mineralogists Special Publication 19, p. 38-55.

Hoshino, K., 1974, Effect of porosity on the strength of clastic sedimentary 

rocks, in Advances in rock mechanics: International Congress on Rock 

Mechanics, 3d, Proceedings, v. 2A, p. 511-516.

Hubbert, M. K., and Rubey, W. W., 1959, Role of fluid pressure in 

mechanics of overthrust faulting: Geological Society of America 

Bulletin, v. 70, p. 115-166.

Kane, M. F., 1977, Correlation of major eastern earthquake centers with 

mafic/ultramafic basement masses: U.S. Geological Survey Professional Paper 1028-0, p. 199-204.

Kidwell, A. L., 1951, Mesozoic igneous activity in the northern gulf coastal 

plain: Transactions Gulf Coast Association of Geological Societies, Annual Meeting, 1st, New Orleans, La., p. 182-199.

King, B.C., 1970, Vulcanicity and rift tectonics in East Africa, in Clifford, 

T. N., andGass, I. G., eds., Africanmagmatism and tectonics: Darien, 

Conn., Hafner Publishing Co., p. 263-283.

Kisvarsanyi, E. B., 1980, Granitic ring complexes and pre-Cambrian hotspot activity in the St. Francois Terrane, Midcontinent region, United 

States: Geology, v. 8, p. 43-47.

Kisvarsanyi, Geza, 1976, Precambrian metallogenics in the St. Francois 

Mountains igneous province, southeastern Missouri, in Kisvarsanyi, E. 

B., ed., Studies in Precambrian geology: Missouri Department of 

Natural Resources Report of Investigations 61, p. 60-76.

Knudsen, F. P., 1959, Dependence of mechanical strength of brittle polycrystalline specimens on porosity and grain size: American Ceramic Society Journal, p. 376-387.

Kumarapeli, P. S., 1976, The St. Lawrence rift system, related 

metallogeny, and plate tectonic models of Appalachian evolution: 

Geological Association of Canada Special Paper 14, p. 301-320.

Kumarapeli, P. S., andSaull, V. A., 1966, The St. Lawrence Valley system 

 A North American equivalent of the east African rift system: Canadian Journal of Earth Sciences, v. 3, p. 639-658.

Lidiak, E. G., and Zietz, Isidore, 1976, Interpretation of aeromagnetic 

anomalies between latitudes 37N. and 38N. in Eastern and Central 

United States: Geological Society of America Special Paper 167, 37 p.

Long, L. T. 1976 Speculations concerning southeastern earthquakes, mafic 

intrusions, gravity anomalies, and stress amplifications: Earthquake 

Notes, v. 47, p. 29-35.

Maldonado, Florian, 1977, Summary of the geology and physical properties 

of the Climax Stock, Nevada Test Site: U.S. Geological Survey OpenFile Report 77-356, 25 p.

Marcher, M. V., and Stearns, R. B., 1962, Tuscaloosa Formation in Tennessee: Geological Society of America Bulletin, v. 73, p. 1365-1386.

McKeown, F. A., 1978, Hypothesis Many earthquakes in the Central and 

Southeastern United States are causally related to mafic intrusive 

bodies: U.S. Geological Survey Journal of Research, v. 6, no. 1, p. 

41-50.

Nuttli, O. W., 1973a, The Mississippi Valley earthquakes of 1811 and 

1812 Intensities, ground motion, and magnitudes: Seismological 

Society of America Bulletin, v. 63, p. 227-248.

____1973b, Seismic wave attenuation and magnitude relations for eastern 

North America: Journal of Geophysical Research, v. 78, p. 876-885.

____1974, Magnitude-recurrence relation for central Mississippi Valley

earthquakes: Seismological Society of America Bulletin, v. 64, p. 

1189 1207.

Oudenhoven, M. S., Babcock, C. O., and Blake, Wilson, 1972, A Method 

for the prediction of stresses in an isotropic inclusion or orebody of irregular shape: U.S. Bureau of Mines Report of Investigations 7645, 36 p.
14 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Potter, P. E., 1955, The petrology and origin of the Lafayette Gravel: Journal of Geology, v. 63, no. 2, p. 115-132.

Rankin, D. W., 1975, The Continental margin of eastern North America in 

the southern Appalachians The opening and closing of the ProtoAtlantic Ocean: American Journal of Science, v. 275-A, p. 298-336.

Roedder, E., 1965, Liquid CO2 inclusions in olivine-bearing nodules and 

phenocrysts from basalts: American Mineralogist, v. 50, p. 1746-1782.

Russ, D. P., 1979, Late Holocene faulting and earthquake recurrence in the 

Reelfoot Lake area, northwestern Tennessee: Geological Society of 

America Bulletin, v. 90, pt. 1, p. 1013 1018.

Ryshkewitch E., 1953 Compression strength of porous sintered alumina and 

zirconia: American Ceramic Society Journal, p. 65 68.

Saucier, R. T., 1964, Geological investigations of the St. Francis basin, 

lower Mississippi Valley: U.S. Army Corps of Engineers, Waterways 

Experiment Station Technical Report 3 659.

____1974, Quaternary geology of the lower Mississippi Valley: Arkansas 

Archeological Survey, Publications on Archeology, Research Series 6, 

26 p.

Schwalb, H. R., 1971, The northern Mississippi Embayment A latent 

Paleozoic oil province, in Proceedings of Symposium on Future 

Petroleum Potential of NPC region 9 (Illinois basin, Cincinnati arch, 

and northern part of Mississippi Embayment): Illinois State Geological 

Survey, Illinois Petroleum 95, p. 44-49.

Smith, F. L., and Saucier, R. T., 1971, Geological investigation of the 

western lowlands area, lower Mississippi Valley: U.S. Army Corps of 

Engineers, Waterways Experiment Station Technical Report S-71-5.

Stearns, R. G., 1957, Cretaceous, Paleocene, and Lower Eocene geologic 

history of the northern Mississippi Embayment: Geological Society of 

America Bulletin, v. 68, p. 1077-1100.

Stearns, R. G., and Marcher, M. V., 1962, Late Cretaceous and subsequent structural development of the northern Mississippi Embayment 

area: Geological Society of America Bulletin, v. 73, p. 1387-1394.

Stearns, R. G., and Wilson, C. W.,Jr., 1972, Relationships of earthquakes 

and geology in west Tennessee and adjacent areas: Report for Tennessee Valley Authority, Office Tributary Area Development Research 

Paper, Nashville, Tenn., Vanderbilt Univ., 344 p.

Street, R. L., Herrmann, R. B., and Nuttli, O. W., 1974, Earthquake 

mechanics in the Central United States: Science, v. 184, p. 1285-1287.

Stauder, William, Kramer, M., Fischer, G., Schaefer, Stephen, and, Morrissey, S. T., 1976, Seismic characteristics of southeast Missouri as indicated by a regional telemetered microearthquake array: Seismological Society of America Bulletin, v. 66, no. 6, p. 1953-1964.

Sykes, L. R., 1979, Intraplate seisrnicity, reactivation of preexisting zones of 

weakness, alkaline microzonatism, and other tectonism post-dating 

continental fragmentation: Geophysics and Space Physics Review, v. 

16, p. 621-688.

U.S. Geological Survey, 1974, National Atlas of the United States of 

America: Washington, B.C., sheet 418.

Voight, Barry, 1969, Evolution of North Atlantic Ocean Relevance of 

rock-pressure measurements, in Kay, M., ed., North Atlantic geology 

and continental drift: American Association of Petroleum Geology 

Memoirs 12, p. 955-962.

Williams, J. F., 1891, The igneous rocks of Arkansas: Arkansas Geological 

Survey Annual Report 1890, v. 2, 391 p.

Wuerker, R. G., 1956, Annotated tables of strength and elastic properties of 

rocks: American Institute of Mining and Petroleum Engineers, 

December, p. 12, table 5.

Wyllie, P. J., and Huang, Wuu-Liang, 1976, Carbonation and melting 

reactions in the system CaO-MgO-Si02 -C02 at mantle pressures with 

geophysical and petrological applications: Contributions to Mineralogy 

and Petrology, v. 54, p. 79-107.

Wyllie, P. J., 1978, Mantle fluid compositions buffered in peridotite 

C02 -H2 O by carbonates, amphibole, and phlogopite: Journal of 

Geology, v. 86, p. 687-713.

Yoder, H. S., Jr., 1955, Role of water in metamorphism, in Poldervaart, 

Arie, ed., Crust of the earth: Geological Society of America Special 

Paper 62, p. 505-523.

Zoback, M. D., 1979, Recurrent faulting in the vicinity of Reelfoot Lake, 

northwestern Tennessee: Geological Society of America Bulletin, v. 90, 

pt. 1, p. 1019-1024

Zoback, M. D., Hamilton, R. M., Crone, A. J., Russ, D. P., McKeown, F. 

A., and Brockman, S. R., 1980, Recurrent intraplate tectonism in the 

New Madrid seismic zone: Science, v. 209, no. 4460, p. 971-976.

Zoback, M. L., and Zoback, M. D., 1980, State of stress in the conterminous United States: Journal of Geophysical Research, v. 85, p. 

6113-6156.
Damaging Earthquakes of the 

Central Mississippi Valley

By O. W. NUTTLI

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-B
CONTENTS

Page

Abstract .................................................... 15

Introduction................................................. 15

The 1811-12 earthquakes ...................................... 15

Earthquake of December 16, 1811 ........................... 15

Earthquake ofJanuary 23,1812 ............................. 16

Earthquake of February 7, 1812 ............................. 16

Aftershocks of the 1811-12 sequence ......................... 16

Other damaging earthquakes ................................... 16

Earthquake ofJune 9,1838 ................................. 16

Earthquake of January 4, 1843 .............................. 17

Earthquake of October 8,1857 .............................. 17

Earthquake of August 17, 1865 .............................. 17

Earthquake of April 12,1883 ............................... 17

Earthquake of October 31, 1895 ............................. 17

Earthquake of April 29, 1899 ............................... 17

Earthquake of November 4, 1903 ............................ 17

Earthquake of August 21, 1905 .............................. 17

Earthquake of May 26,1909 ................................ 18

Earthquake of July 18, 1909................................. 18

Earthquake of September 27,1909 ........................... 18

Earthquake of November 26, 1922 ........................... 18

Earthquake of October 28, 1923 ............................. 18

Earthquake of April 26, 1925 ............................... 18

Earthquake of May 7, 1927 ................................. 18

Earthquake of December 16, 1931 ........................... 18

Earthquake of August 14, 1965 .............................. 18

Earthquake of November 9, 1968 ............................ 18

Earthquake of March 24, 1976 .............................. 19

Geographic distribution of damaging earthquakes .................. 19

References cited .............................................. 19

ILLUSTRATIONS

Page

FIGURE 1. Magnitude-recurrence curves for 3-month interval of New Madrid earthquakes in 1811-12 and for 41-year interval of southern California

earthquakes from 1932 through 1972 .................................................................................. 16

2. Epicenters of damaging earthquakes in the central Mississippi valley ............................................................. 19
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

DAMAGING EARTHQUAKES OF THE 

CENTRAL MISSISSIPPI VALLEY

By OTTO W. NuTTLi 1

ABSTRACT

The history of damaging earthquakes in the central Mississippi valley 

began on December 16, 1811, with the initiation of the great series of earthquakes of the winter of 1811-12. Three major earthquakes, plus 203 damaging aftershocks, occurred in the 3-month interval from December 16, 1811, 

through March 15, 1812. The body-wave magnitudes of the major earthquakes were 7.35, 7.2, and?.5. The body-wave magnitudes of the damaging 

aftershocks ranged from 5.0 to 6.7.

Since 1812, 20 damaging earthquakes have occurred in the central 

Mississippi valley of epicentral intensity VI-VII to IX and of body-wave 

magnitude 5.0 to 6.2. This seismic activity is greater than that of any other 

region in the Central or Eastern United States.

The three major earthquakes of 1811-12 occurred in the central portion 

of the New Madrid fault zone. All 20 later damaging Earthquakes occurred 

at the two ends of the fault zone, suggesting that the 1811-12 series released 

the accumulated stress in the central part. Microearthquakes have occurred, 

however, in large numbers in the central portion.

INTRODUCTION

Much has been written about the great series of earthquakes 

that occurred in the central Mississippi valley in the winter of 

1811-1812. But it is not as well known that, since that time, 

the region has experienced 20 damaging earthquakes; that is, 

earthquakes of epicentral intensity greater than VI. On the 

basis of these earthquakes alone, the region would be considered to be the most seismically active area of the Central 

and Eastern United States. This paper will give a brief account 

of each of the earthquakes, including location, epicentral intensity, magnitude, and a description of the damage.

THE 1811-12 EARTHQUAKES

Little information exists about the seismicity of the central 

Mississippi valley prior to the nineteenth century. Five earthquakes are known to have occurred between 1776 and 1804 

(Coffman and von Hake, 1973), but it is likely that many were 

not reported during that time interval. The seismic history of 

the region actually began with the activity that took place in 

the winter of 1811-12.

' Department of Earth and Atmospheric Sciences Saint Louis University, St. Louis, Mo. 63156.

EARTHQUAKE OF DECEMBER 16, 1811

At 2:15 a.m. (local time) the entire Central United States 

and much of the Eastern United States was shaken by a major 

earthquake, the first of a series of events that continued 

strongly through the spring of 1812 and to a lesser degree for 

more than 5 years after that. Fuller (1912) gave a description 

of these earthquakes, particularly of their physiographic effects, some of which were still visible upon his inspection of the 

area almost 100 years after the occurrence of the Earthquakes. 

Nuttli (1973a) prepared an isoseismal map, based on contemporary newspaper reports, and gave the epicentral intensity 

and body-wave magnitude as X-XI and 7.2, respectively. In 

subsequent studies (Gupta and Nuttli, 1976; Nuttli and 

others, 1979), Nuttli revised the value of epicentral intensity 

upwards to XI and of the body-wave magnitude to 7.35.

Copies of the newspaper reports on the effects of the earthquake are contained in Nuttli (1973a). The earthquake was 

noteworthy for the large damage area (radius of approximately 600-700 km) and the large felt area (5,000,000 km2). These 

large areas are a consequence of both the low attenuation of 

highfrequency wave energy in the Central United States (Nuttli, 1973b, 1978) and the size of the earthquake. Practically 

speaking, the upper limit of the body-wave-magnitude, m^, 

scale is about 7.5. The greatest earthquakes occurring 

anywhere in the world will not have mfc values exceeding 7.5. 

Thus this earthquake was near the upper limit of the bodywave-magnitude scale. However, although it was a very large 

earthquake, the implication is not that it was the greatest to 

occur in the world. Unlike the body- and surf ace-wavemagnitude scales, the seismic-moment scale does not get 

saturated and thus is a better measure of the strength of great 

earthquakes. Herrmann and others (1978) estimated the total 

seismic moment for the three 1811-12 earthquakes to be 

6 X 1027 dyne-cm. By comparison, the moment of the 1906 San 

Francisco earthquake has been estimated both by Thatcher 

(1975) and by Sieh (1977) to be 4 X 1027 dyne-cm. The 1952 

Kamchatka earthquake had a moment of 350 X 10" dyne-cm; 

and the 1960 Chilean earthquake, the greatest in the last two 

centuries, had a moment of 2000 X 10" dyne-cm (Kanamori, 

1977).

15
16 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

From a description of the damage accounts of the earthquake, Nuttli (1973a) found the epicenter to be southwest of 

the town of New Madrid, Mo. Although a precise epicenter 

could not be determined, it almost certainly lies on the 

southwest branch of the New Madrid fault zone, as delineated 

by microearthquakes (Stauder and others, 1976). Tentative 

coordinates of lat 36 ° N., long 90 ° W. were assigned by Nuttli 

(1973a).

EARTHQUAKE OF JANUARY 23, 1812

The December 16, 1811, earthquake was followed by a 

series of aftershocks, some of which were large, destructive 

events. The aftershock series was interrupted by the occurrence of a second major earthquake at 9 a.m. (local time) on 

January 23, 1812. This earthquake also was felt as far away as 

the Atlantic and gulf coasts. The revised value of epicentral intensity is X-XI (Gupta and Nuttli, 1976) and of body-wave 

magnitude is 7.2 (Nuttli and others, 1979). The felt area was 

approximately 5,000,000 km2 . The epicenter probably was at 

the northern end of southwest branch of the New Madrid fault 

zone. Its epicentral coordinates thus would be lat 36.3° N., 

long 89.6° W. (Nuttli, 1973a).

EARTHQUAKE OF FEBRUARY 7, 1812

The largest of the so-called New Madrid earthquakes occurred on February 7, 1812, at 3:45 a.m. (local time). It completely destroyed the town of New Madrid, which was apparently near the epicenter. Nuttli (1973b) assigned epicentral 

coordinates of lat 36.5° N., long 89.6° W. Newspaper and 

other reports of the 1811-12 earthquakes often did not 

distinguish among the events, so there were too few reliable 

data to construct isoseismal maps for this or the January 23 

earthquake. The revised value of epicentral intensity is XII 

(Gupta and Nuttli, 1976) and of body-wave magnitude is 7.5 

(Nuttli and others, 1979). The felt area was at least 5,000,000 

km2 .

AFTERSHOCKS OF THE 1811-1812 SEQUENCE

The aftershock activity of the 1811-1812 earthquakes was 

noteworthy. Contemporary observers remarked that the earth 

seemed to be in a continuous state of unrest during the entire 

winter of 1811-1812. One particularly astute observer, Jared 

Brooks' of Louisville, Ky., set up horizontal pendulums and 

vertical mass-spring systems to detect and measure the size of 

the movement produced by the earthquakes. He also classified 

them as to their degree of violence at Louisville, some 400-500 

km from the epicenter. Fuller (1912) reproduced Brooks' classification of earthquake intensities, from which Nuttli (1973a) 

was able to assign body-wave magnitudes for the earthquakes 

reported by Brooks from December 16, 1811, through March 

15, 1812. There were 62 damaging aftershocks from 

December 16 to January 23, 30 from January 23 to February 7, 

and 111 from February 7 through March 15.

A plot comparing the cumulative number of earthquakes in 

the New Madrid area and in southern California as a function 

of their magnitude (mfc for the Mississippi valley, ML for 

southern California) was prepared (fig. 1). The two straightline curves are almost coincident, which indicates that the 

number and the magnitude of earthquakes that occurred in 

the New Madrid fault zone in 3 months equaled that which occurred in southern California in 41 years. This is a remarkable 

phenomenon, which further attests to the large energy release 

accompanying the 1811-12 earthquakes.

10000

1000

CD

z

100

10

-SOUTHERN CALIFORNIA EARTHQUAKES 

40 YEARS 

1932-1972

NEW MADRID AFTERSHOCKS

3 MONTHS 

DEC. 16,1811-MAR. 15,1812

45678 

MAGNITUDE

FIGURE 1. Magnitude-recurrence curves for 3-month interval of New 

Madrid earthquakes in 1811-12 and for 41-year interval of southern 

California earthquakes from 1932 through 1972. Crosshatched area 

of index map shows southern California source area considered. 

Crosses refer to New Madrid data (m^), circles to southern California 

data (ML) (Hileman and others, 1973).

OTHER DAMAGING EARTHQUAKES

Although activity subsequent to the 1811-12 earthquakes 

does not compare in strengh or in number to those events, it 

nevertheless is large enough on its own to justify the classification of the region as a seismically active one. A description of 

the 20 damaging earthquakes follows.

EARTHQUAKE OF JUNE 9, 1838

The earthquake of June 9, 1838, had its epicenter in southcentral Illinois, about 100 km east of St. Louis. Nuttli (1974) 

gave the coordinates as lat 38.5° N., long 89° W., the epicentral intensity as VII-VIII, the felt area as 500,000 km2 , and 

the body-wave magnitude as 5.7. Part of a chimney was 

thrown down in St. Louis (Coffman and von Hake, 1973).
DAMAGING EARTHQUAKES OF THE CENTRAL MISSISSIPPI VALLEY 17

EARTHQUAKE OF JANUARY 4, 1843

Nuttli (1974) prepared an isoseismal map of the January 4, 

1843, earthquake on the basis of contemporary newspaper accounts. The earthquake occurred at 8:45 p.m. (local time) 

and was felt over an area of 1,500,000 km2 . The epicentral intensity was estimated to be VIII, and the body-wave 

magnitude 6.0. The epicenter was placed at approximately lat 

35.5° N., long 90.5° W., at the center of the intensity-VII 

isoseism. This location is compatible with the observations 

that at Memphis the shocks appeared to come from the northwest and that at Helena, Ark., where some chimneys were 

thrown down, the shock was more severe to the north. 

Chimneys also were thrown down at Mills Point, Tenn. Coffman and von Hake (1973) reported that walls were cracked, 

chimneys fell, and windows were broken in Memphis and that 

one chimney fell in St. Louis.

EARTHQUAKE OF OCTOBER 8, 1857

The epicenter of the October 8, 1857, earthquake was near 

that of the June 9, 1838, event. Nuttli (1974) gave the epicentral coordinates as lat 38.7 ° N., long 89.2 ° W., with a felt area 

of 200,000 km2 , an epicentral intensity of VII, and a bodywave magnitude of 5.4. He also gave an isoseismal map for the 

earthquake. At Centralia, 111., near the epicenter, chimneys 

were demolished. At Belleville, 111., to the west, some plaster 

fell and bricks were dislodged from an old chimney. Coffman 

and von Hake (1973) reported that plaster fell and bricks were 

dislocated at St. Louis, about 100 km west of the epicenter.

EARTHQUAKE OF AUGUST 17, 1865

From newspaper accounts of the August 17, 1865, earthquake, Nuttli (1974) placed the epicenter north of Memphis at 

lat 35.5° N., long 90.5° W. Its felt area was 250,000 km2 , 

epicentral intensity VII, and body-wave magnitude 5.3. 

Damage at Memphis consisted of bricks knocked out of 

parapet walls, tops knocked off chimneys, and broken windows. Coffman and von Hake (1973) reported that objects fell 

from shelves and chimneys were damaged at New Madrid, 

Mo.

EARTHQUAKE OF APRIL 12, 1883

The effects of the earthquake of April 12, 1883, were confined to the epicentral region near Cairo, 111., where one old 

frame house was reported shaken down (Coffman and von 

Hake, 1973). Nuttli (1979) gave its epicentral coordinates as 

lat37.0°N., long 89.2° W., its epicentral intensity as VI-VII, 

and its body-wave magnitude as 4.0.

EARTHQUAKE OF OCTOBER 31, 1895

The October 31, 1895, earthquake was the largest to occur 

in the central Mississippi valley after 1812. Newspaper

coverage was unusually thorough, allowing Nuttli (1974) to 

construct a fairly detailed isoseismal map. The earthquake 

was felt as far away as the gulf and Atlantic coasts. The felt 

area was 2,500,000 km2 . The epicenter was near the town of 

Charleston, Mo., with coordinates of lat 37.0° N., long 89.4° 

W. Epicentral intensity was IX and body-wave magnitude 6.2. 

Coffman and von Hake (1973) reported that near Charleston 

16m2 of ground sank and a lake was formed. At Cairo, 111., 

church steeples were twisted and chimneys were cracked and 

demolished. Near Bertrand, Mo., hundreds of mounds of 

sand were piled up, ranging from 30 cm to 3 m in diameter.

EARTHQUAKE OF APRIL 29, 1899

On the evening of April 29,1899, southwestern Indiana and 

southeastern Illinois were shaken by an earthquake. The 

epicentral coordinates were lat 38.8° N., long 87.0° W., the 

felt area was 100,000 km2 , the epicentral intensity VI-VII, 

and the body-wave magnitude 5.0 (Nuttli, 1979). Coffman 

and von Hake (1973) stated that the earthquake was strongest 

atjeffersonville and Shelbyville, Ind., and that a few chimneys 

were thrown down and brick walls cracked at Vincennes, Ind.

EARTHQUAKE OF NOVEMBER 4, 1903

The earthquake of November 4, 1903, was unusual in that 

it was accompanied by a foreshock and two aftershocks (Coffman and von Hake, 1973). Building walls were cracked at 

New Madrid, Mo., and Cairo, 111., and chimneys fell at Cape 

Girardeau, Mo. Nuttli (1979) gave the epicentral coordinates 

as lat 36.9° N., long 89.3° W., the felt area as 340,000 km2 , 

the epicentral intensity as VII, and the body-wave magnitude 

as 5.3.

EARTHQUAKE OF AUGUST 21, 1905

The August 21, 1905, earthquake had epicentral coordinates of lat 36.8° N., long 89.6° W., a felt area of 325,000 

km2 , an epicentral intensity of VI-VII, and a body-wave 

magnitude of 5.0 (Nuttli, 1979). Coffman and von Hake 

(1973) reported that chimneys were thrown down and a wall 

cracked at Sikeston, Mo.

EARTHQUAKE OF MAY 26, 1909

The earthquake of May 26, 1909, occurred in northern 

Illinois. Its epicentral coordinates were lat 42.5° N., long 

89.0° W., its felt area 800,000 km2 , its epicentral intensity 

VII, and its body-wave magnitude 5.3 (Nuttli, 1979). Docekal 

(1970) reported that damage was centered at Aurora, 111., 

where many chimneys fell and a stove was overturned. He also 

reported that chimneys fell in Waukegan, 111., and in the 

suburbs of Chicago. Brick walls cracked at Bloomington, 111., 

sidewalks at Freeport, 111., and an old building at Platteville, 

Wis. Houses were thrown out of plumb in Beloit, Wis.
18 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

EARTHQUAKE OF JULY 18, 1909

Central Illinois was shaken by an earthquake on the evening 

of July 18, 1909. Coffman and von Hake (1973) reported that 

many chimneys were thrown down at Petersburg, 111., Hannibal, Mo., and Davenport, Iowa. More than 20 windows 

broke, brick was pushed out over the doorway arches, and 

plaster cracked near Petersburg, 111. Nuttli (1979) gave the 

epicentral coordinates as lat 40.2° N., long 90.0° W., the felt 

area as 100,000 km2 , the epicentral intensity as VII, and the 

body-wave magnitude as 5.3.

EARTHQUAKE OF SEPTEMBER 27, 1909

The epicentral coordinates of the September 27, 1909, 

earthquake were lat 39.5° N., long 87.4° W. (Nuttli, 1979). 

The felt area was 250,000 km2 , the epicentral intensity VII, 

and the body-wave magnitude 5.3. Docekal (1970) reported 

that Terre Haute, Ind., suffered two downed chimneys, cracked plaster, and severed light connections. A few windows were 

broken and chimneys thrown down in Covington, Ind.

EARTHQUAKE OF NOVEMBER 26, 1922

Docekal (1970) reported that the earthquake which occurred on the evening of November 26, 1922, broke windows and 

overturned chimneys at Eldorado, 111. Its epicentral coordinates were lat 37.5° N., long 88.5° W., its felt area 130,000 

km, its epicentral intensity VI-VII, and its body-wave 

magnitude 5.0 (Nuttli, 1979).

EARTHQUAKE OF OCTOBER 28, 1923

The earthquake of October 28, 1923, occurred near the 

southern limit of the New Madrid fault zone. At Marked Tree, 

Ark., windows were shattered, several old chimneys were razed, and walls were cracked (Coffman and von Hake, 1973). 

The epicentral coordinates were lat 35.5° N., long 90.4° W., 

the felt area 120,000 km2 the epicentral intensity VII, and the 

body-wave magnitude 5.3 (Nuttli, 1979).

EARTHQUAKE OF APRIL 26, 1923

On the evening of April 26, 1925, an earthquake in the 

Wabash valley near Princeton, Ind., caused chimneys to fall 

(Docekal, 1970). The epicentral coordinates were lat 38.3° 

N., long 87.6° W., the felt area 250,000 km2 , the epicentral 

intensity VI-VII, and the body-wave magnitude 5.0 (Nuttli, 

1979).

EARTHQUAKE OF MAY 7, 1927

The earthquake of May 7, 1927, had epicentral coordinates 

of lat 35.7° N., long 90.6° W., a felt area of 300,000 km2 , an 

epicentral intensity of VII, and a body-wave magnitude of 5.3 

(Nuttli, 1979). Chimneys fell at North Jonesboro, Ark. (Coffman and von Hake, 1973). Docekal (1970) reported broken 

windows and dishes at Memphis.

EARTHQUAKE OF DECEMBER 16, 1931

The December 16, 1931, earthquake occurred in northern 

Mississippi and was felt over an area of 220,000 km2 . The 

epicentral coordinates were lat 34.1° N., long 89.8° W., its 

epicentral intensity VI-VII, and its body-wave magnitude 5.0 

(Nuttli, 1979). Chimneys were thrown down, and walls and 

foundations cracked in Charleston, Miss. (Coffman and von 

Hake, 1973). Chimneys also were damaged at Tillahota and 

Water Valley, Miss., and buildings were damaged and plaster 

fell at Belzoni, Miss.

EARTHQUAKE OF AUGUST 14, 1965

The earthquake of August 14, 1965, is an example of a 

small-magnitude event that can produce damage because of 

shallow focal depth. Herrmann (1979), from a study of the 

spectra of Rayleigh waves, determined the focal depth to be 

only 1.5 km. Herrmann and Nuttli (1975) showed that for 

such shallow focal depths the high-frequency, fundamentalmode surface waves are strongly excited, by one or two orders 

of magnitude more than for depths of 5 km or greater. Their 

attenuation is large, because it is controlled by the anelastic 

properties of the surficial layer. Thus their damage is confined 

to a small epicentral area, in contrast to the more normal Central and Eastern U.S. earthquakes which have relatively large 

damage and felt areas. Their magnitude is determined by the 

excitation of the higher mode waves, which does not vary with 

depth in the crust.

The epicentral coordinates were lat 37.1° N., long 89.2° 

W., the felt area was 700 km2 , the epicentral intensity VII, 

and the body-wave magnitude 3.8 (Nuttli, 1979). Chimneys 

were knocked down, walls cracked, water muddied, and 

groceries shaken from shelves at Tamms, 111. (Coffman and 

von Hake, 1973). In epicentral location and damage effects, it 

is similar to the earthquake of April 12, 1883, which had a very 

shallow focal depth.

EARTHQUAKE OF NOVEMBER 9, 1968

The earthquake of November 9, 1968, was the largest in the 

central Mississippi valley after 1895. Its epicentral coordinates 

were lat 38.0° N., long 88.5° W. It was felt over an area of 

1,600,000 km2 , had an epicentral intensity of VII, and had a 

body-wave magnitude of 5.5 (Nuttli, 1979). In the epicentral 

area there were downed chimneys, cracks in foundations, 

overturned tombstones, and a few instances of collapsed 

parapets. In south-central Illinois bricks were thrown from 

chimneys, windows were broken, television aerials were toppled, and plaster broke and fell. Moderate damage to chimneys 

also occurred in southwestern Indiana, northwestern Kentucky, and at Herrmann, St. Charles, Sikeston, and St. Louis,
DAMAGING EARTHQUAKES OF THE CENTRAL MISSISSIPPI VALLEY 19

Mo. (Coffman and von Hake, 1973).

EARTHQUAKE OF MARCH 24, 1976

The earthquake of March 24, 1976, occurred during the 

early evening hours (local time). Its epicentral coordinates 

were lat 35.59° N., long 90.48° W., and its body-wave 

magnitude 5.0 (Simon and others, 1978). The epicentral intensity was VI, so strictly speaking this earthquake should not 

be included in the present list of damaging events. However, 

its body-wave magnitude of 5.0 usually corresponds to an 

epicentral intensity of VI-VII. Furthermore, the felt area of 

280,000 km2 and the character of the damage argue in support of including it in the present list.

Simon and others (1978) reported telephone lines downed 

and a power blackout in Jonesboro, Ark.; broken windows in 

Paragould and Walnut Ridge, Ark.; cracked masonry in 

Tupelo, Miss.; roof damage and fallen ceiling tiles in Decatur, 

Ark.; and damaged telephone circuits in Union City, Tenn.

The earthquake triggered strong-motion accelerographs at 

seven sites in Mississippi, Missouri, and Tennessee, at distances 

of 100-150 km. The maximum recorded acceleration was 

0.041 g, at a distance of 100 km (Herrmann, 1977).

GEOGRAPHIC DISTRIBUTION 

OF DAMAGING EARTHQUAKES

The three major earthquakes of 1811-12 plus 11 of the 20 

other damaging earthquakes are associated with the New 

Madrid fault zone, which extends from about lat 35.5° N. to 

37.0° N., and long 90.5° W. to 89.5° W. (fig. 2).

The three main earthquakes occurred in the center of the 

region whereas all the damaging earthquakes subsequent to 

those occurred at the southern and northern edges of the fault 

zone (fig. 2). The great 1811-12 sequence appears to have 

released much of the strain energy in the central region, but 

not at the ends of the fault zone. Microearthquake studies 

(Stauder and others, 1976), however, indicate a concentration 

of small earthquakes in the vicinity of the epicenters of the 

January 23 and February 7, 1812, earthquakes. Thus the 

whole New Madrid fault zone must be considered to have the 

potential for producing damaging earthquakes.

Five of the damaging earthquakes can be associated with 

the Wabash valley fault zone of southeastern Illinois and 

southwestern Indiana. The two earthquakes near lat 38.5 ° N. 

and long 89° W. may be associated with the Centralia fault, 

although no evidence exists except for proximity of the 

epicenters to the fault. The same is true of an association of the 

epicenter of the earthquake at the Illinois-Wisconsin border 

with the Sandwich fault. The Illinois earthquake of 1909, with 

epicenter near lat 40° N., long 90° W., remains an enigma. 

The Mississippi earthquake near lat 34° N., long 90° W., 

could possibly be associated with the buried Ouachita mountain front, as minor seismic activity has occurred along that 

front in Arkansas and Oklahoma.

On the basis of the available historical record, which is short 

compared to the time scale of the geologic processes related to 

earthquake activity, we can conclude that most of the future 

damaging earthquakes in the central Mississippi valley will 

have their epicenters in the New Madrid and the Wabash 

valley fault zones. However, we cannot exclude the possibility 

that a damaging earthquake could occur anywhere in the central Mississippi valley.

92° 90° 88

IOWA

 V ILLINOIS

MISSOURI

INDIANA Jt

WABASH 

VALLEY

^) FAULT ^ o 

/ 0 1 ZONE

V£ENTRALIA / t 

.FAULT 0^r

V

3 KENTUCKY

ARKANSAS

0 50 100 KM

j MISSISSIPPI I ALABAMA

FIGURE 2. Epicenters of damaging earthquakes in the central Mississippi 

valley. Large circles refer to epicenters of the three major earthquakes 

of 1811-12. Epicenters of the 203 damaging aftershocks are not plotted. Small circles refer to damaging earthquakes subsequent to 1812. 

The number 2 indicates two earthquakes with the same epicenter.

REFERENCES CITED

Coffman, J.L. and von Hake, C.A., eds., 1973, Earthquake history of the 

United States Revised edition (through 1971): U.S. Environmental 

Data Service Publication 41-1, 208 p.

Docekal, Jerry, 1970, Earthquakes of the stable interior, with emphasis on 

the Midcontinent, v. 2, Catalogue: University of Nebraska Ph. D. dissertation, 332 p.
20 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Fuller, M. L., 1912, The New Madrid earthquake: U.S. Geological Survey

Bulletin 494,119 p. 

Gupta, I. N., andNuttli, O. W., 1976, Spatial attenuation of intensities for

central U.S. earthquakes: Seismological Society of America Bulletin, v.

66, no. 3, p. 743-751. 

Herrmann, R. B., 1977, Analysis of strong motion data from the New

Madrid seismic zone 1975-1976: Saint Louis University, Department

of Earth and Atmospheric Sciences, 144 p. 

____1979, Surface wave focal mechanisms for eastern North American

earthquakes with tectonic implications: Journal of Geophysical

Research, v. 84, p. 3543-3552. 

Herrmann, R. B., and Nuttli, O. W., 1975, Ground-motion modelling at

regional distances for earthquakes in a continental interior, II, Effect of

focal depths, azimuth and attenuation: Earthquake Engineering and

Structural Dynamics, v. 4, no. 1, p. 59-72. 

Herrmann, R. B., Cheng, S. H., Nuttli, O. W., 1978, Archeoseismology

applied to the New Madrid earthquakes of 1811-12: Seismological

Society of America Bulletin, v. 68, no. 6, p. 1751-1759. 

Hileman, J. A., Alien, C. R., and Nordquist, J. M., 1973, Seismicity of the

southern California region, 1 January 1932 to 31 December 1972:

California Institute of Technology, Seismological Laboratory, 83 p.,

404 tables.

Kanamori, H., 1977, The energy release of great earthquakes: Journal of

Geophysical Research, v. 82, p. 2981-2987. 

Nuttli, O. W., 1973a, The Mississippi Valley earthquakes of 1811 and 1812;

intensities, ground motion and magnitudes: Seismological Society of

America Bulletin, v. 63, no. 1, p. 227-248.

__1973b, Seismic wave attenaation and magnitude relations for eastern

North America: Journal of Geophysical Research, v. 78, no. 5, p.

876-885. 

__1974, Magnitude-recurrence relation for central Mississippi Valley

earthquakes: Seismological Society of America Bulletin, v. 64, no. 4, p.

1189-1207. 

__1978, A time-domain study of the attenuation of 10-Hz waves in the

New Madrid seismic zone: Seismological Society of American Bulletin, 

v. 68, no. 2, p. 343-355.

Nuttli, O. W., 1979, The seismicity of the Central United States, in Geology 

in the siting of nuclear power plants: Geological Society of America, 

Reviews in Engineering Geology, v. 4, p. 67-93.

Nuttli, O. W., Bollinger, G. A., and Griffiths, D. W., 1979, On the relation 

between MM intensity and body-wave magnitude: Seismological Society of America Bulletin, v. 69 no. 3, p. 893-909.

Sieh, K. E., 1977, A study of Holocene displacement history along the 

south-central reach of the San Andreas fault: Stanford University Ph. 

D. dissertation, 219 p.

Simon, R. B., Stover, C. W., Person, W. J., and Minsch, J. H., 1978, 

Earthquakes in the United States, January-March 1976: U.S. 

Geological Survey Circular 776-A, p. A18-A20.

Stauder, William, Kramer, Mark, Fischer, Gerard, Schaefer, Stephen, and 

Morrissey, S. T., 1976, Seismic characteristics of southeast Missouri as 

indicated by a regional telemetered microearthquake array: Seismological Society of America Bulletin, v. 66, no. 6, p. 1953-1964.

Thatcher, W., 1975, Strain accumulation and release mechanism of the 

1906 San Francisco earthquake: Journal of Geophysical Research, v. 

80, no. 35, p. 4862- 4880.
Present-Day Seismicity and 

Identification of Active Faults 

in the New Madrid Seismic Zone

By WILLIAM STAUDER

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-C
CONTENTS

Page

Abstract ............................................................................... 21

Introduction............................................................................ 21

Characteristics of the network .............................................................. 22

Seismicitystudies ........................................................................ 23

Recurrence ratio ........................................................................ 25

Focal-mechanism studies.................................................................. 26

Three-D velocity model ................................................................... 28

Conclusion and discussion ................................................................. 29

Acknowledgments ....................................................................... 30

References cited ......................................................................... 30

ILLUSTRATIONS

Page

FIGURE 1. Epicentersof 488 earthquakes of magnitude mjj 3 occurring in the central Mississippi Valley from 1811 tomid-1974. ..................... 22

2. Location of the Saint Louis University microearthquake network stations in southeast Missouri and adjacent areas, and index map of cities

mentioned in the text ............................................................................................... 22

3. Relative operating characteristics for stations of the southeast Missouri network. ................................................... 23

4. Epicentersof 731 earthquakes located in the southeast Missouri region in the 4-year period, 1 July 1974 to 30 June 1978 ................... 23

5. Epicenters of 626 earthquakes located within 1°30' X 1°30' area of the network from 1 July 1974 to 30 June 1978 ........................ 24

6. Epicenters of earthquakes located in the 30' X 30 ' region from New Madrid, Mo., to Ridgley, Tenn., near the station CRT (Gratio, Tenn.) . . 24

7. Cumulative number of earthquakes, Nc vs. im-,, for earthquakes occurring within the network during the 4-year period from 1 July 1974 to

SOJune1978 ...................................................................................................... 26

8. Focal-mechanism diagrams of the five earthquakes identified in Table 3 for which focal mechanisms have been determined by Herrmann and

Canas(1978), superposed on the seismicity map of the area ................................................................ 27

9. Composite focal-mechanism diagrams ..................................................................................... 28

10. Model derived from an inversion using blocks having lateral dimensions of 30 km and thicknesses given in table 3 ......................... 29

TABLES

Page

TABLE 1. Number of earthquakes during each quarter, Qi, versus rajj located within the 1.5° X 1.5° area of the network during the 4-year period

1July 1974 to SOJune 1978 ........................................................................................... 25

2. Earthquake source parameters ............................................................................................ 27

3. Initial model parameters for the crust and upper mantle beneath the New Madrid seismic zone ........................................ 29
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

PRESENT-DAY SEISMICITY AND IDENTIFICATION OF 

ACTIVE FAULTS IN THE NEW MADRID SEISMIC ZONE

By WILLIAM STAUDER'

ABSTRACT

A regional seismic network was established in the New Madrid seismic 

zone in the summer of 1974 in order to study the seismicity of the region in 

detail. During the first 4 years of operation of the network, 731 earthquakes 

were located. These earthquakes are reported in quarterly bulletins, entitled 

Southeastern Missouri Regional Seismic Network Quarterly Bulletin.

The main features of the seismicity include a diffuse seismic activity over 

the regional area. Within the network are northeast-southwest trends of 

faulting, varying in length from 25 km to 120 km, and extending from approximately Cairo, 111., into northeastern Arkansas. There is a major offset 

in these trends in the region between New Madrid, Mo., and Ridgley, Tenn. 

The region of offset is that of greatest seismic activity and is the feature least 

well defined at the present time. The b value, determined by the recurrence 

relation of Nc vs. m^ for low-magnitude earthquakes, is 0.78.

Focal-mechanism studies indicate right-lateral, predominantly strike-slip 

motion on the major seismic trend extending from Ridgley, Tenn., into 

northeast Arkansas. A composite focal mechanism indicates reverse motion 

on a transverse trend extending from Ridgley southeastward into Tennessee. 

Application of the stochastic inversion procedures of Aki and others (1976, 

1977) to the relative residuals of teleseismic P waves yields a threedimensional velocity model in which a 5 percent decrease in velocity is found 

to exist in the region of the crust and upper mantle directly beneath the region of greatest seismic activity. The low-velocity zone extends at least 150 

km into the upper mantle.

INTRODUCTION

In the preceding paper Nuttli has given a brief account of 

the larger earthquakes that have occurred in the central Mississippi valley, beginning with the three devastating earthquakes of the winter of 1811-12 and then considering 20 other 

damaging earthquakes that have occurred in the region from 

that date down to the present time. These 23 earthquakes are 

not isolated events, but a part of a continuing pattern of seismic activity. Yet while the region of southeast Missouri in particular has been recognized as one of potentially hazardous 

earthquakes, little has been known until recently of the seismotectonic character of the region. This paper briefly presents

'Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, Mo, 63156.

the seismic history of the region and then reviews what has 

been learned of its present-day seismic activity through the operation of a regional microearthquake array.

In addition to his paper in this volume, Nuttli (1979) recently completed another study dealing with the seismicity of the 

Central United States. He has drawn together into a single paper all that he could discover from published and unpublished 

sources about the seismic history of the Central States. The 

result includes a listing of 1,128 earthquakes and a consideration of the inferences that can be drawn from the distribution 

and magnitudes of these events. The study is based on intensity 

data. Through a relationship established between intensity 

and magnitude (Nuttli, 1974), however, Nuttli has attempted 

to include all known earthquakes of mfc > 3.0.

The central Mississippi valley region is bounded by lat 35 ° 

and 39° N. and by long 87°30' and 91 °30' W. Nuttli's catalogue, which excludes aftershocks of the 1811-12 earthquakes, 

contains 488 earthquakes occurring within this 4° X 4° region 

from the year 1811 to mid-19 74. The epicenters of these events 

are shown in figure 1, which indicates not only the presence of 

a few isolated larger or damaging events, but also the existence 

of a regular, more OK less continuous seismic activity of lesser 

magnitude. A concentration of these events is found in the 

southeast Missouri region, extending along the Mississippi 

River from Cairo to Memphis, but earthquakes as large as mfc 

= 5 are apt to occur almost anywhere throughout the region.

While Nuttli's study has enhanced the knowledge of the seismic history of the region, thus emphasizing the potential seismic hazard present there, it leaves questions about the detail 

of the seismotectonic activity of the region unanswered. 

Knowledge of faults is rudimentary at best, and prior to 1974 

no active faults had been mapped in the area. In order to study 

the seismicity of the region in detail, then, a microearthquake 

network was established in the New Madrid seismic zone in the 

summer of 1974. The network has now been in operation con*

21
22 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

39°

91° 90° 89° 88°

38°

37£

36C

35C

yoo

goo o ,

03OO

O /

MISSOURI ILLINOIS

INDIAN A

o KENTUCKY

"TENNESSEE

EXPLANATION 

3.0 < mb < 3.9 _ 

4.0 < mb < 4.9 

5.0 < mb < 5.9 

O 6.0 < mb < 6.9 

 7.0 < mb < 7.9 

0 50 KILOMETERS

______I

FIGURE 1. Epicenters of 488 earthquakes of magnitude raj, = 3 occurring in 

the central Mississippi valley from 1811 to mid-1974.

tinuously since that date. The remainder of this paper will be 

devoted to updating an earlier report on the network (Stauder 

and others, 1976) and to presenting the findings that have resulted from its operation.

CHARACTERISTICS OF THE NETWORK

The configuration of the microearthquake network is 

shown in figure 2. The network itself consists of 16 stations. 

The instrumentation is of the standard design developed and 

supplied by the U.S. Geological Survey for regional microearthquake networks. Each remote station consists of a single 

package containing a short-period seismometer, a preamplifier/VCO, and a telemetry package. Signals are telemetered 

by radio link to a collection point, where they are retransmitted via phone line to the Seismic Data Center in the Department of Earth and Atmospheric Sciences of Saint Louis 

University. Recording is by Develocorder by monitor pen-andink recorders, and by slow-speed 16-channel magnetic tape 

recorders.

The selection of station sites was guided by an effort to cover 

the entire southeast Missouri region. In addition, the desirability of locating stations on the more competent rock on 

the edges of the Mississippi Embayment to the west and on the 

higher ground above the banks of the river to the east, off the 

seismically noisy Quaternary sediments of the alluvial plain, 

was considered. As a consequence the aperture of the network 

is relatively large; it is 120 km in width and 240 km in length, 

and has a station spacing of approximately 50 km.

Before installation, each station was calibrated by placing 

the seismometer package on a precision, Geo-Techmanufactured shake table. The relative operating characteristic so obtained for a typical station is indicated in figure 3. The 

maximum gain at 10 Hz for traces displayed on the Develocorder screen for stations on the Paleozoic carbonate rocks 

bordering the embayment is 1500 k. For stations located within the embayment, the gain at 10 Hz is 100-200 k. The network was designed to have a location capability for events occurring within the network down to a threshold of m^ = 1.

92° 91 90° 89°

39°

38° ~

37°

36°

MISSOURI

>

/ ILLINOIS 

\N

\ 

i

New Madridp, -, _,'

^KENTUCKY

35C

FIGURE 2. Location of the Saint Louis University microearthquake network 

stations (triangles) in southeast Missouri and adjacent areas, and index 

map of cities mentioned in the text.
SEISMICITY AND IDENTIFICATION OF ACTIVE FAULTS, NEW MADRID SEISMIC ZONE 23

CD

UJ

HERTZ

10 100

FIGURE 3. Relative operating characteristics for stations of the southeast 

Missouri network, as recorded on Develocorder.

SEISMICITY STUDIES

Figure 4 presents the epicenter locations for the period 1 

July 1974 to 30 June 1978, the first 4 years of operation of the 

network. Epicenters have been located by means of the HYPO 

71 program (Lee and Lahr, 1972) with adaptations to local 

crustal models. A network bulletin, the Southeastern Missouri 

Regional Seismic Network Quarterly Bulletin, lists the events 

located during each 3-month period. During the four-year 

period in question, 731 events were located.

Two observations can be made on examination of the distribution of epicenters in figure 4: first, epicenters are concentrated in the New Madrid region. In fact, 626 or approximately 85 percent of the events occur in this region of southeast Missouri, within the dimensions of the array. But second, epicenters are generally dispersed throughout the map area D. 

More than 100, or 15 percent of the events, occur outside the 

array. This latter feature is important for understanding the 

overall seismicity of the region. Earthquakes may occur almost 

anywhere throughout the region. Indeed, some of the larger, 

more recent events in Nuttli's catalogue (Nuttli, 1979) include 

the Centerville, Mo., earthquake (m^ = 4.9) of 16 October 

1965 and the southeast Illinois earthquake (mfc = 5.5) of 9 

November 1968, which occurred in the Ozark uplift region

and in the Illinois basin, respectively. Nuttli estimates the 

maximum magnitude for these regions to be mfc = 6.0 in the 

St. Francois zone and mfc = 6.5 in the Wabash valley zone.

Figure 5 presents the epicenter locations occurring inside a 

1 °30' X 1 °30' region within the array. As noted previously, 

626 earthquakes were located within this region during the 

first 4 years of operation of the array. The spatial distribution 

of epicenters indicated on the figure has been noted in a previous publication (Stauder and others, 1976) and in the quarterly network bulletins. The pattern is coming to be a familiar 

one referred to by other authors in publications on the area. 

The familiarity of the pattern should not, however, detract 

from the recognition of its significance. While as yet incomplete, and hence in need of further documentation, the 

distribution of epicenters in figure 5 has identified the active 

seismic features of the New Madrid seismic zone and is providing a mapping of the active faults for the first time.

The features of figure 5, worthy of comment, are as follows:

91°30' 

39°00'

90°30' 89°30' 88°30' 87°30'

38°00'

37W

36°00' -

35°00'

FIGURE 4. - Epicenters of 731 earthquakes located in the southeast Missouri 

region in the 4-year period, 1 July 1974 to 30 June 1978. Triangles, station locations; dots, locations of epicenters.
24 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

90°30' 

37°00' 90W 89°30' 89°00'

36°30'

36°00'

35°30'

RMB

New ModridJ CRU^ 

^ " KENTUCKY

TENNESSEE

0 10 20 30KILOMETERS

, .ARKANSAS

JHP

FIGURE 5. Epicenters of 626 earthquakes located within 1° 30' X 1°30' 

area of the network from July 1974 to 30 June 1978. Triangles, station 

locations; dots, locations of epicenters. Station code: RMB, Rombauer, 

Mo.; DWM, Dogwood, Mo.; LST, Lone Star, Mo.; ECO, Elk Chute 

Ditch, Mo.; CRU, Crutchfield, Ky.; CRT, Gratio, Tenn.; LTN, 

Lennox, Tenn.; NKT, Nankipoo, Tenn.; OKG, Oak Grove, Tenn.; 

JHP, Judd Hill Plantation, Ark. Ridgley, Tenn., is located just west of 

the station GRT.

1. A major linear seismic trend about 120 km in length extends in a northeast-southwest direction from Ridgley, 

Tenn., near the station GRT, into north-eastern Arkansas.

2. To the north a lesser, parallel trend about 40 km in 

length extends from the station DWM toward the station LST.

3. A region of increased seismic activity, though as yet not 

well defined, exists in the offset region between the two 

trends just mentioned, extending from the New Madrid 

bend of the Mississippi River to the station GRT.

4. An apparent east-west trend extends from the New 

Madrid bend westward past the station LST.

5. Another, less active zone, parallel to the one mentioned 

in item 1 above and about 40 km to the east, passes 

through the line of the stations NKT- OKG.

6. A northwest-southeast trend extends from the Mississippi 

River, near GRT, into Tennessee.

7. A gap in the seismicity separates the trend just noted 

from the northern extremity of the principal 

northeast-southwest trend of item 1.

8. The earthquake activity terminates abruptly at either 

end, north and south, of the trends indicated, implying that the seismic regime of southeast Missouri is selfcontained within the latitude limits of 35°00' N. and 

37°00' N.

While not noted on figure 5, the region of offset between 

New Madrid and the station GRT (near Ridgley, Tenn.) is 

marked by a greater than average microearthquake activity. 

Each quarter, 40-80 earthquakes are detected at the stations 

GRT and LST that are not large enough to record at other stations, or at a sufficient number of stations to permit location. 

Nevertheless, their detection and number, even without location, indicate a high level of seismic activity in the offset region 

and direct attention to this region as one which merits further 

study.

Figure 6 presents the earthquake activity of this New 

Madrid-Ridgley region on a 30' X 30' map. While some patterns might be pointed out (a general north-south distribu89°48' 

36°42'

36°36' -

36°30' -

39042' 89°36' 89°30' 89°24' 89°18'

36°24' -

36°18' -

36°12'

* - ' ' '.' "%\\ . I )

-'"./. -j ' -" /UPLANDS

FIGURE 6. - Epicenters of earthquakes located in the 30°x 30° region from 

New Madrid, Mo., to Ridgley, Tenn., near the station GRT (Gratio, 

Tenn.). Triangles, station locations; dots, locations of epicenters; 

square, location of USGS explosion; dashed line, edge of alluval plain.
SEISMICITY AND IDENTIFICATION OF ACTIVE FAULTS, NEW MADRID SEISMIC ZONE 25

tion? or a series of echelon northwest-southeast trends?), the 

dispersion of the epicenters does not allow the identification of 

any distinct features at this time.

Two factors affect the interpretation of the distribution of 

the epicenters indicated in the figure. First, there is the accuracy of epicenter location. The standard deviation in the 

positiion of the epicenters is of the order of 0.5-1 km (see quarterly bulletins). Second, the station spacing of the array does 

not permit accuracy in the determination of the depth of focus 

of the earthquakes. Depth is routinely fixed at 5 km or at 10 

km. Even if the depth of focus is allowed to be determined 

freely by the HYPO 71 program, the resultant value is no better, statistically, than the fixed-depth solutions. While the pattern of epicenters in figure 6 is somewhat diffuse, it is not 

different from what might be expected for foci distributed on 

a dipping fault or on a system of dipping faults.

An understanding of the seismic activity in this offset region 

depends, then, upon an improvement in the capability of determining the location and depth of focus of the numerous 

small earthquakes that occur here. For this purpose the number of stations in the network is presently being increased and 

the station separation improved from approximately 50 km to 

10-15 km throughout the region of seismic activity shown in 

figure 6.

Some further information about the recent faulting and the 

seismicity of this region is known. Russ (1979) has conducted a 

trenching experiment across the Reelfoot scarp just west of 

Reelfoot Lake and has found evidence of three earthquakes 

near the trench site strong enough to liquefy sediments and 

generate surface faulting in the last 2,000 years. Zoback (1979) 

has reported the results of 32 km of seismic-reflection profiling 

in this same vicinity. This profiling has revealed numerous 

faults involving vertical motion in late Cenozoic and 

Cretaceous time; coincidence of surficial indicators of faulting 

with several of the subsurface offsets implies that Holocene activity probably has occurred as well. Jordan and others (1978)

have reported results from a Vibroseis survey conducted in the 

vicinity of this same scarp and from a 4-week field study 

conducted using a dense micronet of temporary seismic stations deployed in the Reelfoot Lake area. Each of these projects was sponsored by the U.S. Geological Survey. The latter 

study included a calibration explosion detonated at the point 

indicated on the map of figure 6. Signals from this explosion 

were detected by the Saint Louis University network, and the 

event was located as if it were an earthquake. With no prior 

knowledge of the location of the event itself, the computer 

solution yielded an epicenter distant 390 m to the east of the 

shot point for a solution constrained to 0 focal depth. An epicenter 75 m to the west of the shot point was obtained for a 

focus restrained to 5-km depth, one 750 m to the south for a 

10-km focal depth, and one 550 m south for a freely determined depth of 8.6 km. These calculations give empirical evidence of the accuracy of epicenter determination, noted 

above, of 0.5 to 1.0 km. Finally, independent of the USGS 

group, a second micronet field experiment was conducted by 

Saint Louis University personnel during the spring of 1978 in 

the vicinity of Ridgley, Tenn. (Nicholson and Singh, 1979).

RECURRENCE RATIO

Table 1 lists the frequency of occurrence of the earthquakes 

that have been located during the 4-year period from 1 July 

1974 to 30 June 1978 and whose epicenters lie within the 

1 °30 ' X 1 °30 ' area of the network, as listed in the quarterly 

bulletins. The table gives the number of earthquakes by halfmagnitude units and by each of the 16 quarters of network 

operation.

The bottom line of the table, which gives the total number 

of earthquakes located each quarter, indicates a variation in 

the rate of occurrence of the smaller earthquakes in the region. Over the 4-year period in question, the number of earthquakes located per quarter varies from a low of 23 to a high of 

65.

TABLE 1. Number of earthquakes during each quarter, Q^, vs. m^ located within the 1.5° x 1.5° area of the network during the 4-year period from 1 July

1974 to 3 June 1978

[N, total number per m^ range; Nc , cumulation number of events in the given m^ range]

mb

0.0-0.4

0.5-0.9

1.0-1.4

1.5-1.9

2.0-2.4

2.5-2.9

3.0-3.4

3.5-3.9

4.0-4.4

4.5-4.9

5.0-5.5

Ql

4

26

20

6

6

2

0

1

0

0

0

Q2

2

18

22

3

5

2

1

0

0

0

0

Q3

0

9

19

7

5

0

1

0

0

0

0

Q4

1

10

13

5

8

0

1

0

1

0

0

Q5

0

12

13

6

3

2

1

0

0

0

0

Q6

0

21

21

6

4

1

0

0

0

0

0

Q7

0

14

17

8

1

0

0

0

0

1

1

Q8

1

9

21

7

2

0

1

0

0

0

0

Q9

0

10

15

7

3

0

0

1

0

0

0

Q10

0

8

17

4

1

0

0

0

0

0

0

Qll

0

0

19

6

2

2

0

0

0

0

0

Q12

0

3

20

4

2

0

0

0

0

0

0

Q13

0

4

12

8

1

0

0

0

0

0

0

Q14

0

6

10

3

8

1

1

0

0

0

0

Q15

0

6

15

7

1

2

0

0

0

0

0

Q16

0

13

30

2

1

5

1

0

0

0

0

N

4

163

284

89

53

17

7

2

1

1

1

\626

618

455

171

82

29

12

5

3

2

1

Total 65 53 41 39 37 53 42 41 36 30 29 29 25 23 31 52 626
26 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

The last column of the table gives the cumulative frequency 

of earthquake occurrence by half-magnitude units. The same 

information is given in figure 7 in a semilogarithmic plot of Nc 

vs. mb- Examination of this figure shows, first, that the sensitivity of the network does indeed extend down to a location 

capability of mb = 1. Second, during the 4-year period some 

500 events occurred having mb > 1, while three events occurred having mb > 4. These frequencies are similar to those 

found by Nuttli (1979) using the data from his historical catalogue of earthquakes for the New Madrid region.

The recurrence relation obtained by determining the leastsquares best fit to a straight line for the points for which 

limb < 3.5 is

logNc =3.437±0.032-(0.783±0.013)mb.

It should be noted, however, that the b value, b = 0.783, of this 

relation is dependent on a magnitude scale developed for 

small local earthquakes and may not be applicable to larger 

earthquakes. Although the values of Nc vs. mb seem to depart 

from the straight line at the lower end of the curve, this is attributable to a pair of earthquakes. These two earthquakes had 

mb = 5.0 and mb = 4.5; they occurred near the southern extremity of the active zone on 25 March 1976, during the 

seventh quarter of network operation. The period of observation has not been sufficiently long to establish the frequency of 

occurrence of events of this magnitude.

1000

100

10

1.0 2.0 3.0 4.0 5.0

m b

FIGURE 7. - Cumulative number of earthquakes, Nc vs. m^, for earthquakes 

occurring within the network during the 4-year period from 1 July 1974 

to 30 June 1978.

FOCAL-MECHANISM STUDIES

Using the data both from long-period surface waves and 

from P-wave first motion, Herrmann and Canas (1978) have 

determined the focal-mechanism solutions for five earthquakes that occurred within the region of the network. Table 2 

lists the date of occurrence, location, origin time, and mb for 

each event, together with the orientation of the tension and 

compression axes of the focal mechanism.

The relation of all five earthquakes to the linear seismic 

trends identified above is indicated in figure 8, in which the 

lower hemisphere focal-mechanism symbols are superimposed 

on the seismicity map of figure 5. The solid quadrants of the 

mechanism diagrams correspond to a compression first motion of the P wave, the open quadrants to a rarefaction first 

motion. Given that events 1 and 2 occurred prior to the 

installation of the network, at a time when location accuracy 

of the earthquakes in the region was probably not better than 

6 ', the correspondence between the focal mechanisms and the 

linear seismic trends is quite satisfactory.

The occurrence of events 1 and 3 near a point of convergence of three seismic trends makes it difficult to relate the

nodal planes unambiguously to the fault motion at the source 

for these two earthquakes. The most likely identfication would 

seem to be a selection of the nodal plane striking east-west like 

the fault plane. In this case the mechanism diagrams indicate 

predominantly left-lateral strike-slip faulting along a fault 

dipping steeply to the south. The solutions for the two events 

are quite similar to one another.

Earthquakes 2, 4, and 5 occurred along the 120-km-long 

northeast- southwest seismic trend, which extends from near 

Ridgley, Tenn., into northeastern Arkansas. The first two of 

these earthquakes have almost identical focal mechanisms. On 

the basis of the seismicity pattern, the nodal plane striking 

northeast would be the fault plane. Earthquake number 5, 

which is an aftershock of earthquake number 4, differs in its 

focal mechanism from that of the main event in the direction 

of dip of the northeast-trending nodal plane. An interpretation of right-lateral motion on a northeast-trending fault is 

common, however, to all three of these events occurring on 

this same seismicity trend.
SEISMICITY AND IDENTIFICATION OF ACTIVE FAULTS, NEW MADRID SEISMIC ZONE 27

90°30' 

37W

90W 89°30' 89°00'

36°30'

36°00' - -

35°30'

FIGURE 8. Focal-mechanism diagrams (beach balls) of the five earthquakes 

(circles) identified in table 3 for which focal mechanisms have been 

determined by Herrmann and Canas (1978), superposed on the 

seismicity map of the area. Mechanism diagrams represent lower half of 

focal sphere; solid quadrants correspond to a compression first motion 

of the P wave, open quadrants to a rarefaction first motion. Triangles, 

station locations; dots, locations of epicenters. Dashed lines enclose 

zones E and F, used for composite focal-mechanism solutions in figure 9.

Herrmann and Canas (1978) included a determination of 

composite focal mechanisms in their study. For this they first 

delineated six possible linear segments or zones within the seismicity pattern indicated in figure 5. The first four of these 

zones were in the region north of the station GRT. The fifth 

segment, which they designated zone E, corresponds to the 

northeast- striking Tennessee-Arkansas trend. The sixth segment, their zone F, corresponds to the northwest-striking 

trend extending from GRT into Tennessee. Each of these latter zones is shown in figure 8. In the determination of the composite focal mechanisms, only stations with clear unambiguous directions of first motion were used. For the four northern 

zones, either because of the small number of usable data for 

each zone or because of the lack of definition of quadrants of 

compression and rarefaction first motion, the composite focal 

mechanisms that resulted were not satisfactory. More data are 

required, or a more appropriate definition of zones.

For zones E and F, however, composite solutions were indicated that can be interpreted without too much difficulty. 

The composite solutions for these two zones are given in figures 

9A and 9B. For zone E, as is evident in figure 9A, the composite solution may be either a simple vertical strike-slip 

mechanism or an oblique fault that has a significant component of right-lateral motion along a northeast-striking fault. 

This composite solution is in agreement with the combined 

surface wave-body wave focal-mechanism solutions by Herrmann and Canas (1978) for the three individual events along 

this trend, as discussed previously.

The correspondence between the individual focal mechanism of the three larger events and the composite focal mechanism obtained for events distributed along the entire length 

of the zone supports the hypothesis that this trend identifies a 

single fault feature that is active along its entire length.

The data of zone F, in figure 9B, are interpreted as a northwest-trending reverse fault which agrees with the northwestsoutheast seismicity trend. The agreement between the composite focal-mechanism solution and the trend in seismicity 

identified visually by inspection of figure 5 would seem to confirm that the trend corresponds to a physical feature in the 

crust. The reverse motion on this presumed fault is consistent 

with strike-slip motion on the northeast trends.

TABLE 2. Earthquake source parameters

[OT, origin time (BMT); h, focal depth; Tr, trend; PI, plunge]

T-axis

Event

1

2

3

4

5

Date

2 Feb

17 Nov

13 Jun

25 Mar

25 Mar

1962

1970

1975

1976

1976

Lat °N

36.5

35.9

36.5

35.6

35.6

Long °W

89.5

89.9

89.7

90.5

90.5

OT

06

02

22

00

01

43

13

40

41

00

mb

4.3

4.4

4.2

5.0

4.5

h (km)

8

16

9

12

16

Tr PI

301

176

313

181

174

28

32

8

38

13

P-axis

Tr PI

43 19

272 9

49 34

272 1

271 28
28 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

W

FIGURE 9. Composite focal-mechanism diagrams. Circles, compression 

first motions of P waves; triangles, rarefaction first motions. A, for Zone 

E, from Ridgley, Tenn., into northeastern Arkansas; B, for Zone F, 

from Ridgley southeastward into Tennessee.

THREE-D VELOCITY MODEL

Although the stations of the New Madrid network were 

designed to record seismic waves from local earthquakes and, 

hence, have a peak response at about 10 Hz, the stations do 

operate at sufficiently high magnification that teleseimic P 

waves for larger earthquakes are well recorded. Travel-time 

residuals relative to a selected reference station of the network 

varied over a range of about 1.5 seconds, from -0.5 seconds to 

+ 1.0 seconds, and a number of stations were found to be 

azimuth-dependent. This suggests a three-dimensional velocity structure in the crust and possibly in the upper mantle 

under the network.

Aki and others (1976, 1977) developed stochastic and generalized inversion procedures, using observations of this 

nature, for inferring lateral velocity changes within each of 

several layers. The procedures begin by assuming a simple 

plane-layered model, each layer of which is characterized by a 

constant velocity and is divided into a number of blocks, the 

number and size of which are determined by the area covered 

by the seismographic network and by the density of the stations. The method is restricted to obtaining velocity perturbations within individual layers; independent data are required 

to determine the average velocities in each layer.

Mitchell and others (1977) have applied the method of Aki 

and others to determine the three-dimensional velocity structure beneath the New Madrid network. Since the aperture of 

the network is moderately large and the station density is not 

great, blocks of relatively large lateral dimensions had to be 

used. A number of configurations were tried. The best results 

were found using a four-layer model consisting of blocks in 

each layer that are 30 km in horizontal dimension, cover the 

area of figure 5, and extend northward another 70 km in a five 

by eight configuration.

The starting model was taken from Nuttli and others (1969) 

and is given in table 3. The resultant model is shown in figure 

10. Positive numbers (percentages) indicate velocities that are 

less than the mean velocity in each layer and vice versa. The 

variation between highest and lowest values within each layer 

amounts to about 5 percent. The standard errors for this inversion are consistently between 0.2 and 0.3 for every block. 

The resolution was found to be poor for the crustal layers but 

good for both mantle layers.

The most interesting result of this inversion is the indication 

that a zone of low velocity exists below the region of most intensive seismic activity and that this zone extends from near 

the surface to depths of at least 150 km. The velocities, as 

noted above, are about 5 percent lower than the velocities in 

the surrounding rock for each layer. The low-velocity zone appears to dip to the west and to become broader and more diffuse with increasing depth in the mantle.
SEISMICITY AND IDENTIFICATION OF ACTIVE FAULTS, NEW MADRID SEISMIC ZONE 29

TABLE 3. Initial model parameters for the crust and upper mantle 

beneath the New Madrid seismic zone

[From Nuttli and others, 1969]

Layer

1

2

3

A

Thickness 

(km)

20

20

57

57

P-wave velocity 

(km/sec)

6.15

6.70

8.18

8.37

LAYER 2

TO.2

5.8 W 0.2 0.9 01 

LAYER 3

-3.010.9 0.0 0.2^0.5

-2.d/0.3 0.0-0.1 ,-{.1 

-1.7

LAYER 4

FIGURE 10. Model derived from an inversion using blocks having lateral 

dimensions of 30 km and thicknesses given in table 3. The numbers are 

percent velocity perturbations from the average layer velocity; positive 

values (and solid contours) indicate lower than average velocities; 

negative numbers (and dashed contours) indicate higher than average 

velocities. Dashes indicate blocks for which no values are obtainable. 

Contours are drawn at 1 percent intervals. Shaded areas denote 

velocities that are more than 1 percent than average (from Mitchell and 

others, 1977).

CONCLUSION AND DISCUSSION

The operation of the microearthquake network in the New 

Madrid seismic zone has made it possible for the first time to 

map linear trends in the New Madrid seismic zone. Presumably these correspond to active faults in the region. The 

main features of the seismicity indicate two principal 

northeast- southwest-trending zones extending from Cairo, 

111., into northeastern Arkansas. These two zones are offset 

from one another in the region between New Madrid, Mo., 

and Ridgley, Tenn. The b value, determined by a plot of Nc 

vs. mb for 626 earthquakes that have occurred in the region 

over a 4-year period and using the scale developed for smallmagnitude local earthquakes, is 0.78.

While the strikes of the linear features are fairly well determined, the density of stations of the network has not been 

appropriate to the present time to determine accurately the 

depth of focus of events that are located. Hence, threedimensional mapping of foci along the seismically active zones 

has not been possible. This stage of the seismicity studies will 

be pursued as soon as data are available from additional stations of the network that are presently being installed. A region of particular interest in this regard is between New Madrid, Mo., and Ridgley, Tenn. This is the region of greatest 

seismic activity and the one whose features are least resolved by 

the distribution of epicenters determined thus far.

The most prominent single feature is the northeast-southwest seismic trend extending from Ridgley, Tenn., into northeastern Arkansas. Individual focal-mechanism solutions for 

three earthquakes located two at the southern extremity and 

one in the middle of this trend; along with a composite focalmechanism solution from P-wave first motion for earthquakes 

occurring along the whole extent of this feature, these individual focal-mechanism solutions indicate uniform rightlateral, predominantly strike-slip motion. The dip of the fault 

is probably to the northwest, though verification of this 

conclusion awaits the study of additional events and the 

evidence which will be forthcoming relative to depth of focus 

of events occurring on this trend.

A composite focal mechanism for earthquakes occurring on 

a northwest-southeast trend extending from Ridgley, Tenn., 

into the interior of the State affirms a visual identification of 

this trend on the seismicity map and indicates that the motion 

on the fault is reverse in character.
30 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Finally, three-dimensional modeling, using data from the 

relative residuals of teleseismic P waves and applying the inversion procedures of Aki and others (1976, 1977), indicates the 

existence of a low-velocity zone under the region of greatest 

seismic activity. This zone extends at least 150 km into the upper mantle. A question arises, naturally, concerning the 

nature of this zone and its possible relationship to the seismic 

activity that occurs in the crust above it. Mitchell and others 

(1977) remarked that a generic relationship may possibly exist 

between the low-velocity material in the lithosphere of this region and the occurrence of earthquakes there. If so, this discovery bears an important practical implication concerning 

the determination of earthquake risk in the Central and Eastern United States, and perhaps in seismic regions more 

generally.

ACKNOWLEDGMENTS

The research reported in this paper was supported by the 

Department of the Interior, U.S. Geological Survey, under 

Contracts 14-08-0001-14555, 15867, and 16708. The substance of the paper is drawn from the previously published papers of Stauder and others (1976), Herrmann and Canas 

(1978), and Mitchell and others (1977).

REFERENCES CITED

Aki, K., Christoffersson, A., and Husebye, E. S., 1976, Three-dimensional 

seismic structure of the lithosphere under Montana LAS A: Seismological Society of America Bulletin, v. 66, p. 501-524.

________1977, Determination of the three-dimensional seismic structure

of the lithosphere: Journal of Geophysical Research, v. 82, p. 277-296.

Herrmann, R.B., and Canas, J., 1978, Focal mechanism studies in the New 

Madrid seismic zone: Seismological Society of America Bulletin, v. 68, 

p. 1095-1102.

Jordan, D., O'Connell, D., Coakley, J., Zoback, M. D., and Bufe, C. G., 

1978, Microearthquakes near Reelfoot Lake, Tennessee (abs.): American Geophysical Union, 4th Annual Midwest Meeting, Program and 

Abstracts, Saint Louis University, p. 11.

Lee, W. K., and Lahr, J. C., 1972.HYPO71, a computer program for determining hypocenters: U.S. Geological Survey Open-file report, 100 p.

Mitchell, B. J., Cheng, C. C., and Stauder, William, 1977, A threedimensional velocity model of the lithosphere beneath the New Madrid 

seismic zone: Seismological Society of America Bulletin, v. 67, p. 

1061-1074.

Nicholson, C., andSingh, S., 1979, A detailed microearthquake study of the 

New Madrid fault system's western Tennessee segment (abs.): Earthquake Notes, v. 49, p. 19.

Nuttli, O. W., 1974, Magnitude-recurrence relation for central Mississippi 

Valley earthquakes: Seismological Society of America Bulletin, v. 64, 

p. 1189-1207.

Nuttli, O. W., 1979, The seismicity of the Central United States, in geology 

in the siting of nuclear power plants: Geological Society of America, 

Reviews in Engineering Geology, v. 4, p. 67 93.

Nuttli, O. W., Stauder, William, and Kisslinger, C., 1969, Travel time 

tables for earthquakes in the Central United States: Earthquake Notes, 

v. 40, p. 19-28.

Russ, D. P., 1979 Late Holocene faulting and earthquake recurrence in the 

Reelfoot Lake area, northwestern Tennessee: Geological Society of 

America Bulletin, v. 90, pt. 1, p. 1013-1018.

Stauder, William, Kramer, M., Fischer, G., Schaeffer, S., and Morrissey, S. 

T., 1976, Seismic characteristics of southeast Missouri as indicated by a 

regional telemetered microearthquake array: Seismological Society of 

America Bulletin, v. 66, p. 1953-1964.

Zoback. M. D., 1979, Reactivation of Late Cretaceous faulting in the vicinity of Reelfoot Lake, northwestern Tennessee: Geological Society of 

America Bulletin, v. 90, no. pt. 1, p. 1019-1024.
Microearthquakes and Faulting in the 

Area of New Madrid, Missouri 

Reelfoot Lake, Tennessee

By DANIEL R. O'CONNELL, C. G. BUFE, and M. D. ZOBACK

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-D
CONTENTS

Page

Abstract ............................................................................... 31

Introduction............................................................................ 31

Determination ofhypocenters.............................................................. 31

Composite focal mechanisms .............................................................. 34

Discussion.............................................................................. 36

Conclusion ............................................................................. 38

Acknowledgments ....................................................................... 38

References cited ......................................................................... 38

ILLUSTRATIONS

Page 

FIGURE 1. Map of nothern Mississippi Embayment region showing earthquake epicenters, plutons and rift boundaries, and faults ................... 32

2. Seismicity near Reelfoot Lake, Tenn., for the period from March 20, 1978, to May 7, 1978, and distribution of seismic stations used in this

study ............................................................................................................ 33

3. Travel times from a calibration explosion fired near Reelfoot Lake on April 7, 1978 ................................................ 34

4. Details of the composite mechanisms shown in figure 2 ........................................................................ 35

5. Interpretive map of structures relating to seismicity intheNewMadrid-Ridgley area 5 .............................................. 37

TABLES

Page 

TABLE 1. Hypocenter data for the USGS array, March-April 1978 ....................................................................... 34

2. Focal-mechanism parameters ............................................................................................ 36
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

MICROEARTHQUAKES AND FAULTING IN THE 

AREA OF NEW MADRID, MISSOURI REELFOOT LAKE,

TENNESSEE

By DANIEL R. O'CONNELL, CHARLES G. BUFE, and MARK D. ZOBACK

ABSTRACT

A part of an ongoing investigation of the tectonic setting of the 1811-12 

New Madrid earthquakes, the U.S. Geological Survey operated a dense network of 20 portable seismographs in the vicinity of Reelfoot Lake, Tenn., 

during a 6-week period in March-May 1978. Using readings from these instruments and from the Gratio Station of Saint Louis University, 23 earthquakes with magnitudes between - 1 and 4- 3 were located relative to a calibration shot fired near the lake. Most of the epicenters lie to the west of 

Reelfoot Lake in an 8-km-wide, north-northwest- trending strip extending 

from Ridgely, Tenn., to New Madrid, Mo.; focal depths range from 3 to 18 

km and have a median depth of 7 km. Composite focal mechanisms are 

varied, with the compressional axis shifting from east-west near Ridgely to 

northeast-southwest near New Madrid. The northeasterly compression may 

result from the left-stepping offset of the northeast-striking, right-lateral 

fault system near New Madrid.

INTRODUCTION

The southeastern Missouri earthquakes ofl811-12 were felt 

over a wide area of the Central and Eastern United States, Major deformation was induced in the surficial blanket of unconsolidated sediments in the epicentral region, but little is known 

of the orientation or sense of movement of the basement faults 

that produced these earthquakes.

The New Madrid seismic zone encompasses northeastern 

Arkansas, southeastern Missouri, southwestern Kentucky, and 

western Tennessee. Earthquakes in this region (fig. 1) are concentrated along three principal zones: (1) a 100-km-long 

linear zone that extends northeast from Marked Tree, Ark., to 

Caruthersville, Mo., approximately along the center of the rift 

structure defined by Hildenbrand and others (1977); (2) a 

zone of relatively more intense activity that extends from near 

Ridgely northward to west of New Madrid; and (3) a relatively 

short, northeast-trending zone that passes west of New 

Madrid. A recent microearthquake study (Herrmann and 

Canas, 1978) indicates a short east-west trend of seismicity 

starting just south of New Madrid and extending to the west. 

Two of the large 1811-12 earthquakes may have occurred in

the northeast-trending zone along the rift axis, but the third 

and largest earthquake (February 7, 1812) probably occurred 

near New Madrid (Nuttli, 1973).

Hermann and Canas (1978) obtained both single-event and 

composite focal-plane mechanisms for earthquakes along the 

center of the rift southwest of New Madrid. These show 

primarily right- lateral strike-slip motion on a northeaststriking fault plane and thus correlate well with the axis of the 

rift structure. It is clear, however, that the zone of seismicity 

extending from near Ridgely to west of New Madrid is geologically and seismologically more complex.

Three of the linear zones of seismicity intersect immediately 

west of New Madrid. Efforts to obtain coherant composite 

focal mechanisms for microearthquakes in this zone, as well as 

in the zone of seismicity extending to Ridgely, have been largely unsuccessful owing to the complex distribution of P-wave 

first motions (Herrmann and Canas, 1978) and sparse station 

density.

In an effort to reduce the uncertainty in hypocenter locations and determine the focal mechanisms for earthquakes in 

the zone extending from near Ridgely to New Madrid, the 

U.S. Geological Survey operated a dense network of 20 shortperiod seismic stations from March 20, 1978, to May 7, 1978, 

within the regional seismic network of Saint Louis University. 

Our objective was to study the processes related to currently 

active faulting in the New Madrid epicentral region by examining the distribution and focal mechanisms of microearthquakes occurring there today.

DETERMINATION OF HYPOCENTERS

Seismic data were recorded by 12 three-component, 5-day 

analog-magnetic-tape systems and 8 vertical-component 

smoked-drum seismographs. Station distribution is shown in 

figure 2. P and S arrivals were read from playbacks to 0.01 second, but the arrival-time uncertainty is often larger due to 

unclear onsets in the presence of variable, high-amplitude

31
32

92°00'

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

91°00' 90°00' 89°00' 88°00' 1 '-> '-

1 \ \

, I

V ILLINOIS

\ ^ X v.

^ \

/ '&'

'' «?/ 1

38°00'

37°00

36WI

35°00'

MISSOURI

ARKANSAS

CARUTHERSVILLE

____TENNESSEE 

MISSISSIPPI

FIGURE 1. - Map of northern Mississippi Embayment region showing earthquake epicenters (open circles) (Stauder and others, 1979), plutons (line 

pattern) and rift boundaries (heavy solid lines) (Hildenbrand, 1979), and faults (fine solid lines) (Heyl and McKeown, 1978). Modified from 

oback and others (1980).

background noise. Hypocentral coordinates, origin time, 

depth, and first-motion plots were determined using the 

HYPOELLIPSE location program (Lahr, 1979). The earthquakes were located using a crustal model in which velocity increases linearly with depth. The velocity- depth relation is based on travel times (fig. 3) for a calibration explosion near 

Reelfoot Lake (location of calibration shot shown in fig. 2). 

Station corrections are based on residuals from the calibration 

shot, but have been modified slightly to reduce average station

residuals. Coordinates of the shot point and its location as 

determined by the above model are listed in table 1.

We located 23 earthquakes with magnitudes ranging from 

about -1 to 3. Magnitudes were determined from coda-length 

(T), using the relationship, M = 2.54 log (T)-2.25. The 

calculated depths range from 4 to 18 km and have a median 

depth of 7 km (table 1). The earthquake locations and composite focal-mechanism solutions for four groups of earthquakes are shown in figure 2.
MICROEARTHQUAKES AND FAULTING, NEW MADRID, MISSOURI-REELPOOT LAKE, TENNESSEE 33

89°45' 

36°45l 89°00'

Composite 

MECHANISM 1

A

A

TENNESSEE

Composite" lo© 

MECHANISM 2 .., '

A

A

CALIBRATION

A

Composite 

MECHANISM 3

i

MAGNITUDE 

 2 to 3

O I to 2 

® 0 to I 

0-1 to !

Composite 

MECHANISM 4

10 15 KM

36°00'

Figure 2. Seismicity near Reelfoot Lake, Tenn., for the period from March 20, 1978, to May 7, 1978, and distribution of seismic stations used in this 

study. Open triangles represent temporary seismic stations operated between March and May. Star is the calibration-shot site. Numbers indicate 

depth in kilometers. Composite focal mechanisms are shown for four groups of events. Shaded areas on the spheres represent zones of compressional 

first motion; dashed lines in spheres indicate limits of position of the nodal planes. Details of the composite focal mechanisms are shown in figure 4.
34 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

TABLE 1. Hypocenter data for the USGS array, March-April 1978

[MAG, magnitude; NO, number of P and S readings used in locating the earthquake; GAP, largest azimuthal separation in degrees between stations, in degrees; DMIN, Distance in kilometers to the 

third-closest station; RMS, root mean square of P and S time residuals in seconds; ERH, standard error of the epicenter in kilometers; ERZ, standard eror of the focal depth in kilometers; 

QS, solution quality of the hypocenter: A, excellent, B, good; C, average; D, bad; and MEC, composite focal mechanism number that includes this event; leaders ( ) indicate no data. 

Location of the USGS calibration shot is 36° 24.71 'N., 89° 25.23 'W., depth 0.02 km; shot occurred on April 7, 1978, at 7:35:37.11]

Date

Origin

h min

time

s

Lat

degree

N.

rain

Long W.

degree rain

Free solution

04/07/78 7 35 37.61 36 24.72 89 25.31

Depth

km

for the

0.

MAG NO

USGS calibration

00 0.38 17

GAP

shot

49

DMIN

4.9

RMS ERH

0.13 0.5

ERZ

6.1

QS

B"

MEC

 

Hypocentral data

03/21/78

03/21/78

03/22/78

03/27/78

03/30/78

03/30/78

03/31/78

04/01/78

04/05/78

04/07/78

04/10/78

04/16/78

04/20/78

04/20/78

04/23/78

04/29/78

04/29/78

04/30/78

05/03/78

05/04/78

05/05/78

05/05/78

05/05/89

10 10

15 54

11 45

8 18

7 33

23 2

6 25

23 34

15 43

5 56

7 40

1 57

4 20

6 32

20 26

10 22

15 26

8 32

3 37

23 29

4 1

7 13

23 50

29.74

29.51

21. 17

16.78

12.45

38.03

22.97

18.03

33.48

5.95

16.89

31.06

24.57

16.73

56.36

11.64

16.95

30.20

48.96

14.59

54.88

13.29

45.45

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

36

31.74

13.60

13.76

26.02

31.66

15.24

32.90

32.75

21.73

15.07

29.12

19.37

14.22

18.22

30.41

16.32

27.66

16.14

20.98

14.72

28.09

13.15

17.18

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

89

27.64

25.33

25.79

31.95

39.42

27.22

34.13

33.66

33.98

26.95

33.47

32.06

30.61

30.89

34.18

27.35

13.21

27.91

28.26

27.14

31.24

24.75

31.40

9

5

12

11

5

5

10

9

18

5

7

11

4

6

7

5

16

7

5

5

10

4

6

1.4

.1

1.3

1.2

1.4

3.0

1.7

2.1

1.3

1.4

1.2

.9

.6

.4

.8

.8

2.4

.2

1.0

.6

-1.2

-.5

.5

9

7

12

18

14

20

30

19

14

19

15

23

10

25

12

20

9

14

16

14

9

6

11

255

225

154

112

310

122

259

270

209

120

186

158

151

146

235

70

290

71

173

160

214

285

194

16.0

10.4

8.9

10.2

23.0

5.5

15.9

16.6

14.0

6.0

13.9

9.0

5.6

7.6

13.5

4.4

14.0

6.6

10.4

8.2

12.5

11.7

9.0

0.08 1.3

.06 1.5

.19 1.9

.13 .9

.13 2.1

.14 .9

.16 1.1

.17 1.6

.15 2.1

.05 .3

.13 1.0

.14 1.0

.07 1.3

.11 .5

.06 .8

.06 .2

.14 6.0

.10 .6

.11 .8

.07 .4

.05 1.0

.13 9.1

.15 3.5

2.2

3.4

3.4

1.4

1.4

.9

1.3

2.0

1.8

.4

2.1

1.0

1.3

1.0

1.2

.4

2.2

1.2

1.5

1.0

1.2

11.8

5.7

C

C

C

B

D

B

C

C

D

B

C

C

C

B

C

A

D

A

6

B

C

D

D

I

4

4

2

1

3

1

1

2

3

 

2

2

 

 

3

 

3

 

4

 

4

2

Model showing velocity changes with depth for 

V,, V,,andV,

20 40 60 80 

Distance from calibration shot (km)

100

Figure 3. Travel times from a calibration explosion fired near Reelfoot 

Lake on April 7, 1978. Layered-model interpretation of the travel-time 

data is shown. The 1.70 km/s surface-layer velocity (V^ is based on well 

logs and reflection data. Earthquakes were located using the velocitydepth relation V = 5.6 + 0.1 Z, where Z represents depth in kilometers.

COMPOSITE FOCAL MECHANISMS

The absence of sufficent first motions to obtain a unique 

focal mechanism for any one event forced us to use composite 

focal mechanisms. Only those records having unambiguous

first motions were used. The first-motion data were plotted on 

a lower hemisphere, equal-area projection. The composite 

focal-mechanism solutions are shown in figure 4, and the solution parameters are given in table 2. The events used in each 

composite focal-mechanism solution have corresponding 

mechanism numbers in table 1.

Initially we attempted to obtain a composite focal 

mechanism using all the earthquakes located. It was impossible to obtain a focal-mechanism solution without numerous 

contradictory points. We therefore attempted a composite 

solution using various groups of adjacent events. It is important to approach composite fault-plane solutions with caution 

because they are not unique. However, each of the composite 

focal-mechanism solutions in figure 4 contains only events that 

can be located on a plane nearly parallel to one of the nodal 

planes. All of the mechanisms are composed of four or more 

events, and no event in any mechanism was farther than 7 km 

from the nearest of the other events used in the same 

mechanism. Although several other combinations of events 

are possible, the four composite mechanisms shown in figure 4 

are the most reasonable using the criteria described above. 

The three events located between those in mechanism 1 and 2 

cannot be composited and were not included in either 

mechanism because of contradictory first motions. For the 

same reason the two events north of Ridgely (with depths of 4 

and 6 km) were not used in either mechanism 2 or 4.
MICROEARTHQUAKES AND FAULTING, NEW MADRID, MISSOURI-REELFOOT LAKE, TENNESSEE 35

Composite 

MECHANISM 1

Composite 

MECHANISM 3

Composite 

MECHANISM 2

Composite 

MECHANISM 4

- E W

Figure 4. - Details of the composite mechanisms shown in figure 2. Closed circles are compressional arrivals, open circles are dilatational arrivals, and the 

smaller circles are weak arrivals. These are lower hemisphere equal-area projections. Solid lines are the nodal planes. Dashed lines delineate the limits of 

position of a nodal plane. P is the position of the compressional axis, and T is the position of the tensional axis.

Mechanism 1 contains tightly constrained nodal planes and because the events in mechanism 1 lie on or near a plane that

P- and T-axes. The orientations (azimuth, plunge) of the P- has this orientation. Components of inferred slip on this plane

and T-axes are (227 °, 6 °) and (133 °, 33 °) respectively. The N. consist of left-lateral strike-slip and thrust with the north side

80° W., 62° N. nodal plane is designated as the fault plane, up. Mechanism 1 is similar to the focal-mechanism solutions
36 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

TABLE 2. Focal-mechanism parameters

[*, all values reported in degrees represent an approximation of the orientation of the nodal plane, P-axis, or T-axis. 

The variability is described in the te»t]

Composite 

mechanism

Nodal Planes Pressure axis Tension axis

Strike Dip Strike Dtp Trend Plunge Trend Plunge

133 

267 

218 

34*

33 

29* 

42* 

42*

for the two earthquakes that Herrmann and Canas (1978) 

located south and southwest of New Madrid.

The events in mechanism 2 yield a solution that has one 

well-constrained nodal plane trending north and dipping 

steeply to the west. It is assumed to be the fault plane because 

it coincides with the locations of the events in the group. The 

indicated motion is down on the west side. A second nodal 

plane is poorly constrained; its possible strike and dip range 

from N. 65° W. (31 ° NE.) to N. 42° E. (21 ° SE.). The orientation of P varies from (80°, 44°) to (114°, 39°), and T varies 

from (286°, 44°) to (388°, 40°).

Mechanism 3 contains one tightly constrained nodal plane 

striking N. 4° W. and a second nodal plane that is not well 

constrained (a range of orientation of about 22° is possible). 

The orientation of P varies from (111°, 14°), to (131 V4°), 

and T varies from (220 °, 55°) to (220°, 28°). Although the 

selection of the fault plane is slightly ambiguous, the choice of 

the north-striking nodal plane is the more reasonable, and the 

indicated motion on this plane has components of thrust (east 

side up) and left-lateral strike-slip.

Each nodal plane in mechanism 4 can vary over a range of 

± 12 °. This variation alters the nature of the solution very little, with the result that the P- and T-axes change by less than 

5°. The orientation of P varies from (279°, 21°) to (285°, 

18°), and T varies from (30°, 38°) to (34°, 40°). The steeply 

dipping nodal plane trending N. 20° W. is assumed to be the 

correct fault plane because it contains both the shallow and 

deep events and matches the trend of seismicity of the events. 

The indicated motion has components of thrust (southwest 

side up) and left-lateral strike-slip.

One of the clearest trends of the seismicity pattern is north 

to northwest through Ridgely; it includes the composite events 

that led to mechanisms 2,3, and 4. It was not possible to combine any events used in mechanism 3 with the events used in 

mechanism 4 without producing one or more points of contradiction (and vice versa). Also, none of the events used in 

mechanism 3 was compatible with those in mechanism 2.

DISCUSSION

Figure 5 is an interpretative structure map of this region 

that presents various types of data. We suggest that deformation in this region generally results from right-lateral strikeslip motion on two northeast-southwest-striking faults. The 

major fault is located southwest of Ridgely and trends down 

the center of the basement rift; the second fault is located 

north of New Madrid and trends northeast toward Charleston, 

Mo. The area between New Madrid and Ridgely is a zone of 

complex deformation in which both lateral and vertical motion occur on a variety of fault planes.

Figure 5 presents faults interpreted on the basis of the earthquakes discussed above, seismic-reflection profiling (Zoback 

and others, 1980), and the focal-mechanism studies of Herrmann (1979) and Herrmann and Canas (1978). In terms of the 

interpretation just discussed, note that mechanism 1, with its 

left-lateral strike-slip component of motion, appears to be a 

conjugate fault to the right-lateral, northeast-trending fault 

north of New Madrid. Similarly, mechanisms 3 and 4 seem to 

define subparallel, left-lateral faults conjugate to the major 

rift fault. Figure 5 also compares these faults with local topographic features described by Stearns (1979). The Tiptonville 

dome lies between the Reelfoot Fault and the surface projection of the fault plane defined by mechanism 2. Thus, this 

uplift may result from vertical movements on these two faults. 

Stearns (1979) has estimated that Tiptonville dome has been 

uplifted about 6 km in the last 1400 years. The topographic 

high known as the Ridgely Ridge also may reflect vertical fault 

motion, as it lies on the upthrown block between the Ridgely 

and Cottonwood Grove faults.

The focal mechanisms of Herrmann and Canas (1978) and 

the surf ace-wave focal mechanisms of Herrmann (1979) indicate that the New Madrid seismic zone is located in an area 

of regional east-west to northeast-southwest compression. The 

orientations of the P-axes of our composite focal mechanisms 

southwest are generally consistent with relative east-west or 

northeast- southwest compression. Composite focal 

mechanism 1 indicates northeasterly compression near New 

Madrid, which is in agreement with the focal mechanisms of 

two earthquakes located just south and southwest of New 

Madrid by Herrmann and Canas (1978). Composite focal 

mechanisms 3 and 4 are indicative of relative east-west compression near Ridgely, and earthquakes located along the 

100-km-long rift southeast of Ridgely all have focal-plane 

solutions that indicate east-west compression (Herrmann, 

1979). This suggests that from Ridgely to the southwest the 

maximum compressive stress is fairly uniform and oriented 

nearly east-west. In the area north of New Madrid, east-west 

compression is also indicated. Both the south-central Illinois
MICROEARTHQUAKES AND FAULTING, NEW MADRID, MISSOURI-REELFOOT LAKE, TENNESSEE 37

89°55'00" 890 15*00"

36°00'00H

Figure 5. - Interpretive map of structures relating to seismicity in the New Madrid-Ridgely area. Long dashed lines represent faults inferred from 

focal-plane solutions, and long and short dashed lines represent faults inferred from seismic-reflection profiles; U, upthrown side; D, 

downthrown side; arrows indicate relative motion. A short dash perpendicular to the line indicates the direction of dip. Surface projections of the 

designated fault planes: *, from mechanisms 1,2,3, and 4;^, inferred from Herrmann and Canas (1978); X, inferred from the seismic-reflection 

profiles of Zoback and others (1980) and; + , inferred from the seismic-relection profiles of Zoback (1979).
38 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

earthquake of November 9, 1968 (Stauder and Nuttli, 1970), 

and composite focal mechanisms from earthquakes just north 

of New Madrid (Herrmann and Canas, 1978) indicate eastwest compression. Thus, it appears that the northeastsouthwest compressive-stress direction near New Madrid is a 

localized change in the stress field resulting from the leftstepping offset of the major northeast- trending, right lateral 

strike-slip fault system.

CONCLUSION

This work demonstrates that the seismicity in the New 

Madrid-Reelfoot Lake region is quite complex and is the result 

of the interaction of several major structures. The seismicity 

will be understood only through gathering precise information 

about those structures. Our report is a preliminary step in that 

direction, and future work will surely clarify both the structures and processes we describe.

ACKNOWLEDGMENTS

We thank Dan Jordan, Christi Scofield, and John Coakley 

of the U.S. Geological Survey for their efforts in data gathering and analysis. Our colleagues at Saint Louis University graciously provided copies of seismograms from their Gratio, 

Tenn., Station on a timely basis and were very supportive of 

our study.

REFERENCES CITED

Herrmann, R. B., 1979, Surface wave focal mechanisms for eastern North 

American earthquakes with tectonic implications: Journal of 

Geophysical Research, v. 84, no. 37, p. 3543-3552.

Herrmann, R. B., and Canas, J. A., 1978, Focal mechanism studies in the 

New Madrid seismic zone: Seismological Society of America Bulletin, v. 

68, no. 4, p. 1095-1102.

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map 

of central Mississippi Valley and environs: U.S. Geological Survey Miscellaneous Field Studies Map MF-1011.

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic 

and gravity anomalies in the northern Mississippi Embayment and their 

spatial relation to seismicity: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-914.

Lahr, J. C., 1979, HYPOELLIPSE- A computer program for determining 

local earthquake hypocentral parameters, magnitude, and first motion 

pattern: U.S. Geological Survey Open-File Report 79-431, 57 p.

Nuttli, O. W., 1973, The Mississippi Valley earthquakes of 1811- 

12--Intensities, ground motion and magnitude: Seismological Society 

of America Bulletin, v. 63, p. 227-248.

Stauder, William, and Nuttli, O. W., 1979, Seismic studies South central 

Illinois earthquakes of November 9, 1968: Seismological Society of 

America Bulletin, v. 54, p. 1257-1269.

Stauder, William, and others, 1979, Central Mississippi Valley Earthquake 

Bulletin: Saint Louis University, 19 p.

Stearns, R. G., 1979, Recent vertical movement of the land surface in the 

Lake County uplift and Reelfoot Basin areas, Tennessee, Missouri, and 

Kentucky: U.S. Nuclear Regulatory Commission NUREG/CR-0874, 

37 p.

Zoback, M. D., Hamilton, R. M., Crone, A. J., Russ, D. P., McKeown, 

F.A., and Brockman, S. R., 1980, Recurrent intraplate tectonism in 

the New Madrid seismic zone: Science, v. 209, p. 971-976.
Magnetic Basement in the

Upper Mississippi Embayment Region 

A Preliminary Report

By T. G. HILDENBRAND, M. F. KANE, and J. D. HENDRICKS

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-E
CONTENTS

Page

Abstract ............................................................................... 39

Introduction............................................................................ 39

Aeromagneticsurveys .................................................................... 39

Magnetic basement ...................................................................... 40

Maximum depth to magnetic basement...................................................... 42

Interpretation of aeromagnetic data ........................................................ 44

Area I ............................................................................. 44

Area II ............................................................................ 48

Area III............................................................................ 48

Area IV ............................................................................ 49

Magnetic structures spatially related to seismicity .............................................. 50

Discussion and conclusions ................................................................ 50

References cited ......................................................................... 52

ILLUSTRATIONS

Page

FIGURE 1. Outlines of aeromagnetic surveys in the upper Mississippi Embayment ........................................................... 40

2. Residual aeromagnetic map of the upper Mississippi Embayment and surrounding regions .......................................... 41

3. Gray-shaded aeromagnetic map showing interpreted lineaments and anomalies related to igneous intrusions. ........................... 42

4. Seismotectonic map showing inferred magnetic structures in the northern Mississippi Embayment and surrounding regions. ............... 43

5. Gray-shaded map of maximum depth to magnetic basement ................................................................... 45

6. Gray-shaded map of the second vertical derivative of the total magnetic field ...................................................... 46

7. Maximum depth of magnetic basement along X-X', Y-Y', and Z-Z ' ............................................................ 47
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI 

EMBAYMENT REGION A PRELIMINARY REPORT

ByT. G. HILDENBRAND, M. F. KANE, andj. D. HENDRICKS

ABSTRACT

Analysis of aeromagnetic data from the upper Mississippi Embayment 

region indicates a prominent northeast-trending graben that developed on 

the Precambrian surface and has structural relief of 1.6-2.6 km. This 

graben, probably associated with a pre-Late Cambrian rift, may have 

developed along a boundary or suture separating contrasting basement-rock 

types. The graben formed where tensional stresses caused the crust to fail by 

brittle fracture and shear. Subsequent crustal thinning and partial melting 

of the crust led to plutonism and possibly volcanism. Mafic or ultramafic 

plutons intruded along both rift- border and rift-axial faults. However, this 

magmatism occurred hundreds of millions of years after formation of the 

structural graben. The available magnetic data suggest that the rift zone terminates northward in western Kentucky and southern Illinois, a region characterized by intense Paleozoic igneous activity and faulting.

Our interpretation of the geophysical data requires the upper Mississippi 

Embayment region to have experienced a long and complex tectonic history. 

The location of this ancient rift axis is presently one of the most active seismic 

regions in the United States. Geometries of some of the interpreted magnetic 

structures suggest that they are tectonic stress guides and that they determine 

the occurrence of release of present-day seismic energy.

INTRODUCTION

In 1974 aeromagnetic surveys were initiated by the U.S. 

Geological Survey in the upper Mississippi Embayment and 

surrounding regions. The purpose of these surveys is to delineate major geologic or tectonic features that may be responsible 

for the generation or control of seismic energy. To date, 

analysis of the magnetic data has been primarily qualitative, 

although some quantitative aspects of the data have been considered.

The qualitative approach has included close inspection of 

the aeromagnetic map and related filtered maps and subjective conclusions about the anomalous behavior of the 

magnetic signatures that were reached on the basis of contour 

closures and localized changes of gradients. A map of the 

magnetic second vertical derivative has been compiled to 

facilitate the identification of magnetic sources. Quantitative 

aspects of the magnetic interpretations have involved 

calculating the geometry of major geophysical structures. For

this purpose, a generalized map of maximum depth to 

magnetic basement has been constructed using depth 

estimates determined from the horizontal extent of the 

steepest gradient (Vacquier and others, 1951).

Prominent structures inferred from the analyses of the 

magnetic data include (1) mafic-ultramafic plutonic masses of 

large areal extent; (2) a northeast-trending, parallel-sided 

depression of crystalline basement interpreted as a late Precambrian-early Paleozoic graben, probably associated with a 

 continental rift; (3) several magnetic lineaments indicating 

lithologic contrasts in basement, probably along deep-seated 

fault zones; and (4) a block-faulted region characterized by 

appreciable vertical and lateral offsets. Geometries of some of 

these structures suggest that they are tectonic stress guides and 

that they determine the occurrence of release of seismic 

energy.

AEROMAGNETIC SURVEYS

The map of the residual total magnetic intensity was compiled from a synthesis of data acquired from several 

aeromagnetic surveys (fig. 1). These surveys, encompassing a 

total area of about 126,000 km2 , were flown at elevations ranging from 900 ft (0.27 km) to 2,000 ft (0.61 km) barometric and 

with flight-line spacings of 0.5 mi (0.85 km) or 1 mi (1.61 km). 

Flight-line directions were east-west. All surveys were corrected for the diurnal variations of the Earth's field and for 

changes in flight elevation. The residual aeromagnetic field 

was obtained by removing the international geomagnetic 

reference field after updating to the years in which the surveys 

were flown.

To merge the various surveys, an elevation of 1,000 ft (0.305 

km) barometric was selected as the reduction datum level. 

Surveys flown above or below this datum level were analytically continued upward or downward so that the data are compatible. The surveys were merged utilizing one-dimensional 

splining techniques described by Bhattacharyya and others 

(1979).

39
40 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

91° 90° 89° 88°

TENNESSEE VALLEY AUTHORITY 

(UNPUBLISHED DATA, 1976-77)

HENDRICKSAND

HILDENBRAND0979)

MISSOURI DIVISION OF GEOLOGICAL

SURVEY AND WATER RESOURCES

(1963,1968 a,b) «V^*-

HILDENBRANDAND

JOHNSON (1977)

JOHNSON AND OTHERS 

(1978)

TENNESSEE

FIGURE 1. Outlines of aeromagnetic surveys in the upper Mississippi Embayment.

MAGNETIC BASEMENT

The resulting residual aeromagnetic map (figs. 2, 3) reflects 

structure and lithologic contrasts in the rocks of magnetic 

basement. Magnetic basement is defined as any lithologic unit 

having a susceptibility or remanent magnetization of sufficient 

magnitude to produce distortion in the magnetic field. In the 

Mississippi Embayment region, magnetic basement is interpreted as Precambrian crystalline rocks together with any igneous rocks of younger age. Phanerozoic sedimentary units 

observed in this region generally lack magnetic properties and, 

therefore, produce little or no effect on the magnetic field. On 

the other hand, Precambrian basement has a substantial effect on the magnetic field. Precambrian volcanic and related 

epizonal intrusive rocks represented in outcrops in the St. 

Francois Mountains (fig. 4) have average magnetic susceptibilities ranging from 0.6 to 2.4X 10~3 cgs (Phelan, 1969).

Because of the thick sequence of sedimentary strata, the 

structure and geology of Precambrian basement are known 

largely from the effects of Precambrian structures on younger

rocks, from drill records, and from geophysical surveys. In 

general, the buried Precambrian surface is undulating. It rises 

on the west toward the crest of the Ozark uplift (fig. 4) and 

descends deeply to the north and south beneath the Illinois 

Basin and Mississippi Embayment, respectively. Localized 

Precambrian structural deformations such as faults and folds 

can be observed by their effect on the total magnetic field intensity. For example, a linear and steep magnetic gradient 

may reflect the juxtaposition of different rock types along a 

buried fault or contact. Folded and tilted structures may be 

represented as regions of either low or high magnetic intensity, 

depending on the depth to the anomaly-producing rocks constituting magnetic basement.

Highly magnetic Phanerozoic dikes, diatremes, and plugs 

intrude Precambrian basement and younger formations along 

numerous faults within the Mississippi Embayment and surrounding regions. For instance, fracturing associated with the 

Ste. Genevieve fault (fig. 4) may have accommodated 

emplacement of intrusions in Early to Middle Devonian time 

(Zartman and others, 1967). In the Illinois-Kentucky
87°

FIGURE 2. Residual aeromagnetic map of the upper Mississippi Embayment and surrounding regions. Roman numerals denote generalized areas of differing magnetic character as discussed in the text. Contour interval is 50 *Y, Contours are dashed between index contours. Hachures indicate magnetic lows. 

The magnetic signature (Arkansas Geological Commission, 1967) associated with the Magnetic Cove ring-dike complex (lat 34°30 ' N. and long 92 °50 ' W.) has been added to this map owing to Magnet Cove's relevant geographic location with respect to structures discussed in this paper.
42 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

90° 89o

FIGURE 3. Gray-shaded aeromagnetic map showing interpreted lineaments and anomalies related to igneous intrusions. Upper case letters denote locations 

of magnetic anomalies associated with highly magnetic intrusions. Dashed lines with lower case letters designate interpreted magnetic lineaments indicating structural or lithologic boundaries.

fluorspar district near lat 37°30' N. and long 88°15' W. 

(fig. 4), kimberlite dikes and diatremes intruded during Late 

Pennsylvanian and Permian time are restricted to northwesttrending fractures (Watson, 1967). Lamprophyric dikes have 

been encountered in drill holes (fig. 4) in rocks of Cambrian 

and Ordovician age on the Pascola arch (Kidwell, 1951; 

Grohskopf, 1955), a structure of major folding and faulting. 

Late Cretaceous lamprophyres, syenites, and nepheline 

syenites (Moody, 1949; Kidwell, 1951) have been detected in 

wells in southwest Tennessee. Bond and others (1971) suggested that these intrusions were emplaced along faults over a 

large area paralleling the Mississippi River in southwest Tennessee. In central Arkansas, the Magnet Cove ring dike complex (Erickson and Blade, 1963) and the syenite bodies near 

Little Rock (Gordon and others, 1958) may be alined along a 

zone of weakness associated with the Ouachita orogeny.

These alkalic intrusions are highly magnetic and produce 

intense anomalies. The Magnet Cove Complex is represented 

by a circular anomaly of 8,500 7 near lat 34°30 ' N. and long

93° W. (fig. 2). Large-amplitude and circular or elongated 

magnetic highs associated with inferred mafic or ultramafic 

intrusions will be considered here as reflecting magnetic basement. In those regions not intruded by Phanerozoic mafic 

rocks, magnetic basement will be assigned to the anomalyproducing Precambrian igneous or metamorphic basement.

MAXIMUM DEPTH TO 

MAGNETIC BASEMENT

An interpretational method developed by Vacquier and 

others (1951) has been employed to estimate the maximum 

depth to magnetic basement. In this method, an observed 

anomaly is compared with theoretical anomalies produced by 

vertical prisms of various dimensions and by different orientations of the Earth's ambient field. One generally assumes that 

the magnetic source is uniformly polarized in the same direction as the Earth's present field. When the areal shape of a the-
MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI EMBAYMENT REGION 43

EXPLANATION

Northern limit of coastal-plain material of 

the Mississippi Embayment

Mafic or ultramafic intrusion within the 

Mississippi Embayment Identified in a 

drill-hole core

Mafic or ultramafic intrusion within the 

Mississippi Embayment interpreted from 

the magnetic field Approximate 

boundary of intrusion determined from 

zero contour of associated anomaly on the 

second vertical derivative magnetic map

Principal magnetic lineaments reflecting

faulting and lithologic contrasts in magnetic

basement 

Fault Bar and ball on downthrown side.

Dashed where inferred 

Possible or hypothetical fault - Based upon

subsurface data or exceptionally strong

lineaments from aerial photos 

Approximate margins of the Mississippi

Valley graben Shown by sections

A - A ' and B - B' 

Earthquake epicenter

FIGURE 4. Seismotectonic map showing inferred magnetic structures in the northern Mississippi Embayment and surrounding regions. This map has been 

modified from the "Tectonic Map of the United States" (Cohee and others, 1962) and the Seismotectonic map of the Mississippi valley (Heyl and 

McKeown, 1978). It is provided as a reference for the accompanying magnetic maps, not as a revision of the sources mentioned. Epicenters of earthquakes detected by the southeast Missouri regional seismic network from July 1974 to June 1977 (W. Stauder, R. Herrman, R. Perry, S. Singh, M. 

Woods, and S. Morrissey, written commun., 1974-77) and major structures indicated in the magnetic field have been added to the map.
44 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

oretical anomaly closely matches that of the observed anomaly, maximum depth to the magnetic source is determined by 

comparing lengths of horizontal gradients associated with the 

two anomalies. The method yields maximum depths to 

magnetic sources because the theoretical models used in the 

curve-matching process are assumed to have vertical sides. In 

real geologic situations, the causative body generally has sloping sides; and, therefore, its actual depth of burial is more 

shallow than the computed depth. The estimated average error of depth to a magnetic rock body is 10 percent (Vacquier 

and others, 1951).

The accuracy of depth estimates greatly depends on the 

validity of the geometric and magnetization assumptions. If 

these assumptions are reasonably valid, the method yields 

suitable depth estimates. Large errors occur, however, when 

the magnetic source is characterized by gently dipping sides, 

or nonuniform magnetization. The probable error associated 

with computed depths to magnetic basement underlying the 

Mississippi Embayment is difficult to assess owing to sparse 

drill-hole information. The depth estimates reported here 

must, therefore, be regarded as generalized. The computed 

depths are, however, in excellent agreement in those regions 

where depth to Precambrian basement is known. Four hundred and fifty estimates of maximum depth to magnetic basement have been determined by analyzing the anomalies of 

figure 2. The resulting generalized map of depth to magnetic 

basement is shown in figure 5.

INTERPRETATION OF 

AEROMAGNETIC DATA

The second vertical derivative map (fig. 6) and generalized 

map of maximum depth to magnetic basement (fig. 5) are used in addition to the residual aeromagnetic map (fig. 2) in 

delineating and describing magnetic structures in the 

Mississippi Embayment and surrounding regions. Derivative 

maps resolve or sharpen anomalies of small areal extent. The 

zero contours of the second derivative map of the totalmagnetic-field anomalies can be used to outline the approximate boundaries of causative bodies (Vacquier and others, 

1951).

Close inspection of the magnetic data reveals four generalized areas, shown in figure 2, which contain different magnetic 

patterns. Area I is characterized by a prominent, northeasttrending, grabenlike feature. Gradients and intense anomalies 

of small areal extent trend predominantly northwest in Area II 

and northeast in Area III. The magnetic pattern within Area 

IV contains pronounced northwest-trending magnetic features. The following discussions of the resulting interpretations of the magnetic data are separated into individual sections dealing with the various lithologic contrasts and structures indicated within each area.

AREA I

The most striking magnetic feature within the surveyed area 

is the broad zone of subdued magnetic expression within Area 

I. This zone has been interpreted by Hildenbrand and others 

(1977) as an expression of graben, probably associated with a 

rift that has been active since late Precambrian or early 

Paleozoic time. The graben, producing the quiet magnetic 

zone, trends between lat N. 45° E. and long N. 50° E. and has 

a width of approximately 70 km. The average maximum 

depth to magnetic basement is 4.3 km within Area I (fig. 5).

The aeromagnetic and derivative maps delineate probable 

mafic intrusions in four localized regions in the graben. One 

circular magnetic anomaly (A, fig. 3) within the graben indicates as many as eight dikelike bodies (fig. 6), which is suggestive of a ring-dike complex. Four wells (fig. 4) within the 

areal extent of this magnetic high have encountered mica 

peridotite sills within rocks of Cambrian to Ordovician age. 

Peridotite from one of these wells, in Lake County, Tenn., has 

been dated at 267 m.y. (Permian) by the K-Ar method (Zartman, 1977). This magnetic high is geographically located over 

the crest of the late Paleozoic Pascola arch. The correspondence in age and location of the eight intrusive bodies and the 

arch suggests that they intruded along faults evolved during 

regional tectonism which formed the Pascola arch.

Three inferred mafic intrusions, represented by magnetic 

highs J, K, and L in figure 3, were emplaced in the center of 

the graben floor. The geographic relationship between intrusive emplacement and graben axis suggests that faults 

which formed during rifting provided channelways for 

magma. The ages and compositions of these intrusions are not 

known. The derivative (fig. 6) and depth-to-magneticbasement (fig. 5) maps do, however, reveal the shapes and 

depth of burial, respectively. Anomaly J, located primarily in 

western Kentucky, is elongate along the axis of the graben. 

The associated source is delineated as a series of mafic dikes, 

sills, and plugs which intruded along 60 km of deep-seated 

faults that parallel the graben's axis. These inferred igneous 

bodies, lying at depths of about 3 km, produce a structural 

high in magnetic basement and may, therefore, penetrate 

Phanerozoic sedimentary strata. The second vertical 

derivative of anomaly K reveals a circular pattern of emplacement of basic rocks. This anomaly may reflect a ring-dike 

complex lying at a depth of about 4.8 km. Anomaly L is interpreted as a deeply buried (4.7 km) plutonic mass approximately 12 km in diameter.

A prominent magnetic gradient denoted as lineament a-a 

on figure 3 flanks anomaly A to the northeast. Although there 

are no associated surface faults, a linear trend of contemporary earthquake epicenters (fig. 4) is nearly alined with this 

gradient. The gradient presumably reflects faulting and will 

be discussed later in more detail.

Maximum depth of magnetic basement along profiles 

(X-X ', Y-Y ', and Z-Z', fig. 5) is shown in Figure 7. Magnetic
92°

MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI EMBAYMENT REGION 

91° go0 89° 88°

45

BELOW SEA LEVEL

FIGURE 5. Gray-shaded map of maximum depth to magnetic basement. Open circles denote locations of 

generalized depth estimates. Depths along profiles X-X', Y-Y ', and Z-Z' are shown in figure 7.

basement along profile X-X, approximately oriented along 

the axis of the graben, appears to rise gently from 5.2 km at 

point X (on the southwest) to about 4 km at the crest of the 

Pascola arch (P). This 1.2 km of relief in magnetic basement 

may be related to uplift of the Pascola arch. Northeast of the 

Pascola arch, magnetic basement ascends to about 3 km. This 

probably reflects, however, shallow igneous bodies rather than 

regional Precambrian basement. Basement then descends 

deeply into the Moorman syncline within Area IV. Depth profiles Y-Y ' and Z-Z', oriented perpendicular to the graben, 

depict the pronounced depression of magnetic basement of 

this feature. Basement noticeably deepens near the southeast 

margin of the graben. The implied relief of 1 km within the 

basin indicates a greater vertical displacement on faults 

paralleling the axis of the graben.

The margins of the graben (g-g' and h-h', fig. 3) are characterized by a series of pronounced magnetic highs (B through

G, fig. 3). These anomalies are presumably caused by shallow 

mafic or ultramafic intrusions. The range in amplitudes is 

from 100 to 1100 7 , and the sources are at depths of between 

1.3 and 2.1 km. The locations and shapes of these highly 

magnetic plutons (Hildenbrand and others, 1977) are shown 

in figure 4. Less prominent magnetic highs (H and I, fig. 3) 

bordering the southeast margin of the graben are also believed 

to represent shallow igneous intrusions. This interpretation is 

supported by drill-hole information that reveals the presence 

of lamprophyric and syenitic dikes and sills (fig. 4) within the 

vicinity of these elongated magnetic highs. Although there is 

no surface expression of faults, the plutons, dikes, and sills 

must have intruded along northeast-trending faults that 

border the graben. The presence of these faults supports the 

hypothesis that Area I is underlain by a graben. The faults 

probably formed during the initial phases of rifting in late 

Precambrian or early Paleozoic time. On the other hand, 

drill-hole information and recent magnetic interpretations
46 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

91" 90o

FIGURE 6. Gray-shaded map of the second vertical derivative of the total magnetic field.

suggest that the large mafic or ultramafic masses were emplaced along graben-border faults subsequent to early Mesozoic 

time. The Pure Oil Company McGregor well, located near the 

Covington pluton (fig. 4), encountered syenite directly 

beneath Upper Cretaceous sedimentary rocks (Caplan, 1954). 

The absence of stratigraphic or radiometric ages for the remaining plutons required development of an interpretation 

method utilizing the magnetic field intensity to obain approximate ages of these plutons (Hildenbrand, 1978). In this 

method, the age of a pluton is estimated by calculating the 

direction of remanent magnetization and comparing the corresponding paleomagnetic pole position with the polar 

wandering path for North America (Van Alstine, 1978). 

Remanent direction is chained by first calculating the total 

magnetization direction from the moments of the anomaly 

and then subtracting an induced component in the direction 

of the Earth's field. Because the magnitude of the induced 

component cannot be determined, the remanent direction is 

estimated within a range of possible values. If appreciable

care is taken in isolating the anomaly and removing noise, this 

method indicates age accurate to, perhaps, part of a geologic 

era, although geologic periods cannot be resolved.

Owing to appreciable noise effects associated with neighboring magnetic sources, age determinations were not obtained for the Newport, Jonesboro, or Maiden plutons (fig. 4), expressed as anomalies F, E, and C in figure 3, respectively. The 

computed ages of the Bloomfield, Paragould, and Covington 

plutons (fig. 4), represented by anomalies B, D, and G in 

figure 3, respectively, are post-early Mesozoic. This igneous 

period suggests reactivation of the rift, in the sense that faults 

bordering the graben provided channelways for the magma. 

The absence of major volcanism within the northern Mississippi Embayment indicates that magna ascended only to a 

depth at which the magmatic pressure balanced the lithostatic pressure.

The nepheline syenite bodies in western Tennessee (Kidwell, 1951; Moody, 1949) may have formed as the magma 

cooled by fractionation and differentiation of the more basic
MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI EMBAYMENT REGION 47

sw NE

o: iu ,

I- 3

O

d 4

0 30

A

DISTANCE,IN KILOMETERS

390

NW SE

o

d 3

Q. 

UJ0 5

B

20

DISTANCE,IN KILOMETERS

160

FIGURE 7. Maximum depth of magnetic basement along X-X', Y-Y ', and 

Z-Z '. Profiles shown in figure b. A, parallel to graben axis; B, perpendicular to graben axis.

parent. Gravity modeling (Hildenbrand and others, 1977) of 

the Covington anomaly (G, fig. 3) indicates that the associated pluton is very dense (contrast of +0.22 g/cm3). The average density of nepheline syenite is 2.52 g/cm3 (Thoenen and 

others, 1945), suggesting a density contrast of 0.08 g/cm3 

for an assumed density of 2.6 g/cm3 for the host rock. The 

discrepancy between the observed and computed density contrasts (-0.08 vs. +0.22 g/cm3) implies that only the upper 

top of the pluton is syenite. The middle and lower sections of 

the intrusions must consist of denser rocks such as mafic or 

ultramafic alkalic masses similar to those exposed at the Magnet Cove Complex in central Arkansas.

The Magnet Cove ring-dike complex produces a pronounced magnetic high at lat 34°30' N. and long 92°50 ' W. (fig. 

2), along the projection of the northwest margin (g-g', fig. 3) 

of the graben. This igneous complex, of Late Cretaceous age 

(Scharon and Hsu, 1969) may also have been emplaced along 

faults flanking the graben. The Magnet Cove Complex consists of alkalic rocks that include nepheline syenite, ijolite, 

jacupirangite, pyroxenite, and trachyte (Erickson and Blade, 

1963). The more mafic components produce the large 

magnetic amplitude of 8500 7 associated with the complex. 

This amplitude is approximately an order of magnitude

greater than those corresponding to the intrusive plutons 

situated along the margins of the graben. Most of these plutons are embedded entirely within Precambrian basement, 

whereas the Magnet Cove Complex intrudes Paleozoic rocks. 

Difference in depth of emplacement can, however, only partially explain the appreciable contrast in magnetic intensities. 

An obvious explanation for this is that the original melt composition of Magnet Cove was different from that of the other 

bordering plutonic masses. Magnetic Cove rocks developed 

from magma rich in the iron-titanium-oxygen compounds. In 

addition, the fine-grained alkalic rocks belonging to the Magnet Cove extrusive suite cooled faster than the intrusive alkalic 

rocks associated with the plutons. In general, fine-grained 

materials formed by rapid cooling have higher susceptibilities 

than coarse-grained crystals (Grant and West, 1965). The 

suggested difference in cooling rates may account, in part, for 

the substantial contrast between the magnetic amplitude of 

Magnet Cove and amplitudes of the buried plutons.

The proposed graben is clearly defined at the southern 

boundary of the surveyed area and probably extends somewhat farther southwest beyond the limits of the data. The 

northern terminus of the graben is difficult to determine owing to strong northwest-trending magnetic features that lie 

approximately normal to the graben's axis. These magnetic 

features mask the more subdued magnetic gradients associated with the graben. Consequently, the northwest margin of 

the graben, which is presently shown to end near the Bloomfield pluton (fig. 4), may extend farther northeast into Kentucky and Indiana. The derivative map (fig. 6), however, suggests that the trend of this boundary changes from northeast 

to north. A gradient transition with a northerly trend flanks 

the Bloomfield anomaly on its eastern side. Additional magnetic data north of the surveyed area will aid in defining the 

northern limits of the graben.

These magnetic structures indicate that Area I is underlain 

by a graben, probably associated with a rift. A graben is defined as an elongate depression of crustal rocks bordered by 

faults. The depth-to-magnetic-basement map clearly shows a 

crustal depression within Area I. The linear trend of igneous 

intrusions that border this depression strongly suggests the 

presence of faults that formed in conjunction with the graben. 

A rift can be defined as a large graben; generally in a rift, 

however, secondary faulting occurs along the floor, producing 

a series of smaller scale horsts and grabens. Magnetic basement along profiles perpendicular to the graben's axis (fig. 7) 

suggests that secondary faulting occurred within the graben 

floor. The area of principal seismicity in the upper Mississippi 

Embayment (fig. 4) lies within the graben structure and, thus, 

reveals the presence of active fault zones associated with this 

structure. These observations have prompted us to interpret 

this feature as an expression of an ancient rift.
48 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

AREA II

The magnetic signature within Area II is characterized by 

intense anomalies of relatively small areal extent; it is similar 

to the magnetic signature of the St. Francois Mountains (U.S. 

Geological Survey, 1978). Unmetamorphosed volcanic and 

related epizonal intrusive rocks of late Precambrian age (Kisvarsanyi, 1974) make up the majority of the St. Francois 

region. However, included in the St. Francois Mountains Intrusive Suite (Kisvarsanyi, 1976) are highly magnetic mafic 

rocks such as pyroxene gabbro, hornblende diorite, olivine 

gabbro, and troctolite. An average susceptibility of 0.0024 cgs 

units has been determined for diabasic rocks in the St. Francois Mountains, although susceptibilities as high as 0.014 cgs 

units may be associated with some mafic intrusions (Phelan, 

1969).

Magnetic basement within Area II is interpreted as containing rocks similar to those encountered in the nearby St. Francois terrane. These rocks, formed about 1.5 b.y. ago (Bickford 

and Mose, 1975), predominantly include rhyolitic ash-flow 

tuffs, intrusive granites, and diabasic and basaltic dikes and 

sills. The mafic plutonic rocks presumably produce the observed intense highs of small areal extent (fig. 2). Precambrian 

basement (fig. 5) underlying Area II appears to be eroded into 

isolated hills and knobs. Drill-hole information indicates that 

the Paleozoic sedimentary rocks within Area II are relatively 

flat lying. The development of relief on Precambrian basement must, therefore, have occurred prior to the deposition of 

the oldest Paleozoic sedimentary unit. Similar erosional 

features are observed in the St. Francois Mountains and other 

Precambrian outcrops in southeast Missouri.

Five prominent northwest-trending magnetic lineaments 

(b-b' through f-f, fig. 3) are also present within magnetic 

basement underlying Area II. Four of these lineaments appear to terminate near the northwest boundary of the graben 

(g-g', fig. 3) in Area I. The northwest-trending magnetic 

features may represent deep- seated fracture systems in an 

older crustal terrane along which highly magnetic igneous 

material intruded. These mafic intrusions are probably 

similar in age (1.5 b.y.) and composition to those observed in 

the St. Francois terrane.

Magnetic lineament b-b' (fig. 3) trends N. 52° W. along 

the southern boundary of the Bloomfield anomaly (B, fig. 3) 

to the edge of the surveyed area. If extended about 50 km to 

the northwest, the lineament alines with the Black fault (fig. 

4) and may delineate the extension of this fault into the embayment. This interpreted fault may have also provided a 

channelway for magma which formed the Bloomfield pluton 

(%. 4).

Magnetic basement (fig. 5) is relatively shallow within Area 

II, where computed depths of less than 2 km are common. 

More significantly, magnetic basement often rises to a kilometer or less below sea level, a depth suitable for mining ore 

deposits. The St. Francois Mountains are well known for ore 

deposits of iron-copper-cobalt, iron-apatite-rare earth,

tungsten-lead-silver, and manganese (G. Kisvarsanyi, 1976). 

Within the southeast portion of Area II, magnetic basement 

descends to depths greater than 2 km. The eastern terminus 

of the Arkoma basin (fig. 4) is located in this region and probably causes a deepening of the basement surface.

AREA III

Magnetic highs of small areal extent are observed in the 

vicinity of the southern and eastern boundaries of Area III. 

The average depth of the associated magnetic sources is 2.5 

km. This 2.5-km depth is approximately 1 km deeper than 

that of the inferred Precambrian mafic intrusions located 

within Area II. Magnetic signatures of Areas II and III 

became comparable after compensating for the difference in 

emplacement depth. The high-frequency anomalies extensively distributed east of the graben are, therefore, interpreted 

as Precambrian mafic dikelike or sill-like bodies analogous to 

those west of the graben. An area characterized by nondescript magnetic signatures lies between the southeast margin of 

graben h-h' (fig. 3) and the regions of Area III intruded by 

mafic masses. Magnetic basement within this relatively quiet 

magnetic zone evidently lacks highly magnetic mafic intrusions.

Distinct magnetic lineaments within Area III (j-j ' through 

n-n', fig. 3) strike predominantly northeast. The hypothesis 

that the Mississippi Embayment contains a tensional structure, namely a rift, is further supported by the correlation in 

trends of these lineaments and the margins of the graben. As 

stated previously, magnetic lineaments west of the graben are 

strongly oriented to the northwest. This contrast in the direction of magnetic lineaments on either side of the graben suggests that the magnetic basements flanking the graben underwent different tectonic evolutions.

Magnetic basement underlying Area III may also be 

younger than that within Area II. A well in St. Francois County, Ark., lat 35° N. and long 91 ° W. (Howard Schwalb, written commun., 1979), and one in Giles County, Tenn., lat 

35°45 ' N. and long 87° W. (Wasserburg and others, 1962), 

have encountered granites dated at about 1.1 b.y. old by the 

K-Ar method. Granite from a well in north-central Mississippi 

has been dated as 0.79 b.y. by the K-Ar method. This latter 

granite, however, may be metamorphosed and, thus, 

somewhat older (Riggs, 1976). The indicated differences in 

age and structural trends in magnetic basements on either side 

of the graben suggest that the graben developed along a juxtaposition of contrasting lithologic basements. The age of basement rock east of the graben prompts comparison with Grenville terrane. According to Baer (1976), a Grenville event involved reactivation of an ancient crust by continental collision 

1.0 to 1.1 b.y. ago. Metamorphism associated with the Grenville orogeny occurred over much of the eastern part of the 

continent (Lidiak and others, 1966). The extension of the 

Grenville Front into western Tennessee (Area III) will,
MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI EMBAYMENT REGION 49

however, require closer examination when additional data 

become available.

Magnetic lineaments e-e', k-k', and n-n' (fig. 3) flank 

magnetic highs associated with mafic intrusions. These lineaments may, therefore, reflect boundaries between different 

basement-rock types. Because of the intense highs lying along 

lineament j-j', the associated structure is delineated as a deepseated fault along which dikelike and sill-like masses intruded. 

Lineament m-m' parallels a hypothetical fault shown in 

figure 4. Owing to the proximity of the Pascola arch, the interpreted fault may represent partial decoupling associated with 

the late Paleozoic tectonism that formed the arch.

Substantial structural relief in magnetic basement is 

noticeable within Area III (fig. 5). Near its northern boundary, the basement appears to deepen along a troughlike 

feature trending east-west at an average depth of 3.9 km along 

lat 36° N. This feature is flanked to the south by an east-westtrending basement high lying at an average depth of about 2.2 

km. The tectonic evolution of these two basement features is 

difficult to formulate because they trend obliquely to 

magnetic lineaments (fig. 3) and known Phanerozoic structures in the embayment (fig. 4). On the other hand, the Moorman syncline (fig 4) underlying Area IV is described as an eastwest-trending trough of early Paleozoic age. The upwarp and 

adjacent basin may, therefore, represent Phanerozoic readjustments to tectonic forces analogous to those that formed the 

Moorman syncline but unrelated to those associated with the 

formation of the northeast-trending graben within Area I.

AREA IV

Regional structures and lithologies of magnetic basement 

underlying most of Area IV were previously studied by Lidiak 

and Zietz (1976). Their major observation pertaining to this 

region, characterized by strongly oriented west-northwest lineaments (fig. 3), concerns the good correspondence between 

surface faults and magnetic lineaments. Excellent correlation 

exists between magnetic anomalies and the surface faults associated with the Rough Creek fault zone, which, in turn, coincide with magnetic lineament s-s'. Lineament o-o' alines with 

the buried extension (fig. 4) of the Ste. Genevieve fault in 

western Kentucky (Freeman, 1950). Also, strong gravity gradients alining with the Ste. Genevieve fault in Illinois appear 

to extend into western Kentucky along lineament o-o' (Hildenbrand and others, 1977). Lidiak and Zietz (1976) have, 

however, interpreted a magnetic lineament, approximately 

coinciding with lineament p-p', as an expression of the 

southern part of the Ste. Genevieve fault. Because the 

southeast projection of the fault lies between lineaments 

o-o'and p-p', it is difficult to assess which lineament 

represents the buried extension of the fault in western Kentucky. The Ste. Genevieve fault may bifurcate into two faults 

expressed as lineaments o-o' and p-p'. Nevertheless, the correlation of magnetic lineaments and major fault zones suggests 

that the faults are basement controlled and are related to

Precambrian structural trends (Lidiak and Zietz, 1976).

Alternating magnetic highs and lows of Area II are bounded by the five magnetic lineaments oriented to the westnorthwest (fig. 3). This magnetic pattern may in part, reflect 

block faulting. For instance, the magnetic high situated between lineaments o-o ' and p-p' may represent an uplifted 

block. Depths to magnetic basement, to some degree, support 

this hypothesis. Because substantial distortion in the southeast 

margin of the graben (h-h', fig. 3) occurs in the region between lineaments o-o' and p-p', this uplifted block may have 

also experienced lateral movement to the southeast. The 

resulting sense of movement would have been right-lateral 

along faults expressed as lineaments o-o' and left-lateral along 

lineament p-p'. The estimated amount of offset is about 12 

km. A depression in magnetic basement located between 

lineaments p-p' and s-s' apparently reflects the geographic extent of the Moorman syncline. A series of prominent 

northwest-trending magnetic highs bounded by lineaments 

q-q' and r-r' lie within this downfaulted block. Syenite to 

peridotite dikes and sills of varying composition, together with 

explosion breccias (Clegg and Bradbury, 1956; Koeing, 1956; 

Heyl and others, 1965), underlie this magnetic feature in the 

vicinity of the Kentucky-Illinois State boundary. Hicks dome, 

a structural feature of the Kentucky-Illinois fluorspar mineral 

district near lat 37°30' N. and long 88°15' W., is located a 

small distance beyond the surveyed area and along a northwest 

projection of this series of magnetic highs. This dome is an 

alkalic cryptovolcanic structure that consists of mineralized 

breccia and kimberlite dikes (Heyl and others, 1965). 

Kimberlite and peridotite dike rocks encountered within these 

regions have been dated as approximately 260 m.y. (Permian) 

by the Rb-Sr and K-Ar methods (Zartman, 1977). This 

northwest-trending magnetic feature evidently represents a 

region intruded by mafic igneous bodies that produce a 

lithologic contrast in magnetic basement.

An intense circular magnetic high (M, fig. 3) is interpreted 

as a large plutonic mass: the Madisonville pluton which lies 

within the Moorman syncline. The approximate age of the 

Madisonville pluton has been determined using the total-field 

dating method referred to previously. The resulting early 

Paleozoic age obtained for this feature coincides with the estimated time of formation of the Moorman syncline. An early 

Paleozoic igneous event apparently accompanied crustal 

warping, from which the Moorman syncline evolved.

The geographic extent of the graben, clearly defined in 

Area I, is somewhat ambiguous within Area IV. Westnorthwest-trending magnetic features characterizing Area IV 

(fig. 3) appear to mask the more subdued gradients related to 

the rift. The close correspondence in trend of the New Madrid 

fault system and the axis of the graben provides indirect 

evidence that the graben continues somewhat farther to the 

northeast. This fault system is described by Heyl and 

McKeown (1978) as a complex zone of horsts and grabens having an appreciable width (more than 60 km). The northeast-
50 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

trending faults associated with the New Madrid system may 

reflect the presence of the rift in northwest Kentucky and 

southeast Illinois. The derivative map (fig.3) suggests that the 

southeast margin of the graben (h-h', fig. 3) extends northeastward across perpendicular-trending features and then bends 

easterly where it merges with lineament r-r'. The implied 

easterly trend of the graben's margin occurs geographically 

near the eastern boundary of the New Madrid fault system. 

Faults in this area also curve to the east in western Kentucky. 

As previously discussed, a marked change on the trend of the 

northwest margin of the graben is also indicated on the 

derivative map near the Bloomfield anomaly. The implied 

widening of the graben boundaries in the northern regions of 

the surveyed area may reflect a transition from a rift structure 

to a block- faulted region. In other words, the intense faulting 

and igneous activity within Area IV may represent structural 

features that formed within the northern terminus of the inferred rift zone. Without detailed analyses of additional 

magnetic data north of the surveyed area, the authors cannot 

firmly conclude that the northern terminus of the graben lies 

within the surveyed area, although this is suggested by interpretations of the available data.

MAGNETIC STRUCTURES SPATIALLY 

RELATED TO SEISMICITY

The graben structure noticeably contains the area of principal seismicity (fig. 4) in the upper Mississippi Embayment. 

Also, the epicentral line of the 1811-12 New Madrid earthquake series (Fuller, 1912) lies within the geographic limits of 

the graben. In particular, the present-day seismicity pattern 

suggests the existence of linear active zones (Stauder and 

others, 1976; Herrmann and Canas, 1978). A linear trend of 

epicenters strikes northeast along the graben's axis. Inferred 

vertical displacements, associated with deep-seated faults 

paralleling the graben's axis, are consistent with crustal fracturing of known rifts. The close correspondence between the 

interpreted fault zone and the linear northeast trend of 

present-day seismicity suggests that the associated rift is 

presently active. The axial seismicity pattern of known 

spreading rifts (Sykes, 1967) is very similar to that of the proposed rift. On the other hand, focal-mechanism studies by 

Herrmann and Canas (1978) revealed significant components 

of right-lateral motion occurring along the northeast-trending 

seismicity zone. This sense of movement is incompatible with 

that of extensional features such as rifts. Another explanation 

for right-lateral faulting along the graben's axis may be that 

past rifting activity resulted in highly fractured and folded 

crustal material within this region. The axis may represent a 

zone of crustal weakness along which strain, produced by tectonic forces different from that which formed the graben, is 

being relieved (Hildenbrand and others, 1977). Detailed investigations pertaining to the dynamics of rifting in the embayment, as well as additional numerical ground-motion

studies, are needed to better understand the relationship between the graben's axis and contemporary faulting.

Seismicity trends change abruptly from northeast to slightly 

west of north at the intersection of the graben with the Pascola 

arch. This trend change may be intimately related to inferred 

igneous masses emplaced along the axis of the graben (fig. 4). 

These intrusions, if present, may have cemented the associated 

faults in such a manner as to influence the distribution of 

stress. Strain related to present-day earthquakes would then 

follow structural features of less resistance, such as the juncture of the Pascola arch and the graben. The resulting northnorthwest seismicity trend roughly coincides with the interpreted fault expressed as lineament a-a' in figure 3. The 

magnetic low flanking this fault suggests that the fault is 

downthrown to the north.

Another change in direction of seismicity occurs near the 

Bloomfield pluton. Along the edge of the pluton, linear 

seismic zones corresponding to active faults appear to strike 

both in a westerly and northeasterly direction. The northeast 

component of seismicity may follow border faults associated 

with the graben's margin. Apparently, the Bloomfield pluton 

influences the distribution of stress in this region. Strong correlations between stress accumulation and mafic or ultramafic 

intrusions in the eastern United States have been previously 

discussed by Kane (1977), Long (1976), andMcKeown(1975). 

Kane (1977) observed that extensive magnetic highs are present in or near seven major eastern North American earthquake areas as defined by Hadley and Devine (1974). He suggested that stresses are concentrated around the periphery of 

plutonic masses in much the same manner as stress concentrations occur near holes in plates under stress. The precise 

nature of the relationship between igneous intrusions and 

faulting in the embayment certainly requires closer examination.

Earthquakes are much less frequent southeast of the graben 

than northwest of it. This may indicate that active uplift or 

subsidence is presently occurring in the Ozarks and that the 

region east of the graben has essentially readjusted to presentday tectonic forces.

DISCUSSION AND CONCLUSIONS

Interpretation of the aeromagnetic data suggests a complex 

tectonic evolution of the upper Mississippi Embayment region. 

A major tectonic event involving the development of a large 

graben or rift zone occurred in late Precambrian or early 

Paleozoic time. The average maximum depth to magnetic 

basement is 4.3 km within the graben and 1.7 km and 2.7 km 

within the regions flanking the graben to the northwest and 

southeast, respectively. The suggested 1.6-2.6 km of relief between basement inside and outside the graben does not, 

however, fully account for the contrast in magnetic signatures. 

Magnetic data within those regions flanking the graben were 

continued upward the appropriate distance to compensate for
MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI EMBAYMENT REGION 51

the depression of basement associated with the graben. 

Magnetic signatures of the graben feature remained inherently different from those outside the graben. This difference 

diminished when data outside the graben were continued upward an additional kilometer or more. This simple exercise indicates that an intrinsic contrast exists in basement-rock type. 

Magnetic basement flanking the graben to the northwest may 

consist of metamorphic and granitic rocks intruded by mafic 

plutonic rocks. A somewhat similar composition is proposed 

for basement lying southeast of the graben. The widespread 

distribution of bodies of mafic rock within these two regions 

flanking the graben generates intense magnetic anomalies of 

small areal extent. Sparse distribution of similar mafic bodies 

within the graben may account for the observed contrast in 

magnetic signatures. Another explanation for this contrast in 

magnetic signatures is that magnetic properties of basement 

related to the graben have been appreciably reduced due to 

extensive fracturing or shearing. Whatever the precise cause, 

this feature is obviously of tectonic origin and must be intimately related to the formation of the Mississippi Embayment.

Available geologic information suggests that the sedimentary graben-fill is pre-Late Cambrian in age (E. Click, oral 

commun., 1979). Development of the graben must have, 

therefore, occurred prior to Late Cambrian time. The 1.2- to 

1.5-b.y.-old volcanic and shallow intrusive rocks related to the 

Central Province give evidence of a thermotectonic event during which continental rifting in central North America may 

have taken place. The extensive terrane of volcanic and 

shallow intrusive rocks associated with the Central Province 

extends from Michigan and Ohio to as far southwest as 

Arizona (Seyfert and Sirkin, 1973). Formation of the 

Oklahoma aulacogen (Wickham, 1978) also demonstrates late 

Precambrian to early Paleozoic rifting in the midcontinent. 

Suggested temporal limits associated with the evolution of the 

graben span a considerable period of geologic time, ranging 

from 0.55 to 1.5 b.y.

Moreover, magnetic basements flanking the graben may 

have differing tectonic histories. The magnetic basement 

northwest of the graben is thought to contain < 1.5-b.y.-old 

mafic intrusions analogous to those emplaced in the St. Francois Mountains. Apparently, these intrusive rocks were 

emplaced on or near an old crustal surface and represent part 

of the Central Province intrusive suite. On the basis of 

magnetic trends and drill-hole information, magnetic basement southeast of the graben is interpreted as being different 

in lithology and age than that northwest of the graben. We 

suggest that l.l-b.y.-old basement, possibly Grenville-age 

rocks, flanks the graben to the southeast. The graben associated with inferred rifting may have, therefore, developed 

along a boundary separating different basement-rock types.

The proposed hypothesis, that the Mississippi Embayment is 

the site of a continental rift, is compatible with earlier interpretations made by Kumarapeli and Saul (1966), Heyl (in

White, 1967, p. 382), Burke and Dewey (1973), and Ervin and 

McGinnis (1975). In the model derived by Ervin and McGinnis, the rift evolved in late Precambrian time by the emplacement of high-density magma into the crust, associated uplift, 

and subsequent isostatic subsidence and trough formation. 

The present study, however, indicates that the graben, herein 

called the Mississippi Valley graben, is not as extensively intruded by mafic masses as those regions flanking the graben. 

In addition, if uplift occurred during the initial phases of rifting, the volcanic rocks of the St. Francois Mountains would 

have been extensively eroded, but this is not observed. A different model for the evolution of the Mississippi Valley graben 

is postulated herein on the basis of the new structural information derived from the interpretation of the magnetic data.

In the proposed model, the lithologic boundary, separating 

the Grenville Province and the Central Province, represented 

a zone of crustal weakness along which the rift evolved in late 

Precambrian or early Paleozoic time. Tensional stresses, acting along the boundary separating different basement-rock 

types, caused the crust to fail by brittle fracture and shearing. 

This model for rift basin evolution has been proposed by 

Salveson (1978). In this model, a rift zone forms from extension and not from initial uplift and subsequent subsidence due 

to isostatic adjustment. Fracturing by normal faulting or block 

faulting probably led to crustal thinning. Anomalous mass exess underlying the embayment is suggested in gravity interpretations by Cordell (1977) and Ervin and McGinnis (1975). The 

sparsity of mafic intrusions lying within the rift zone and the 

lack of any evidence for major volcanism in the embayment indicate that the raising of the mantle and associated partial 

melting of the crust did not provide a source of plutonism or 

volcanism within the shallow regions of the crust during the initial phases of rifting. The Tanganyika and Jordan depressions 

are examples of other rifts that have no associated volcanism.

The post-rifting phase included thick accumulation of 

Paleozoic sediments within the rift zone (Ervin and McGinnis, 

1975) and emplacement of alkalic intrusions both within the 

rift zone and along its margins. Paleozoic sediments exceeding 

4.5 km in thickness (Bond and others, 1971) may have been 

deposited in the Reelfoot basin (fig. 4). Age determinations of 

kimberlite and peridotite dikes and sills, encountered in wells 

in the vicinity of the Kentucky-Illinois fluorspar district and of 

the crest of the Pascola arch, suggest that Late Pennsylvanian 

and Permian igneous events occurred within the rift zone. A 

belt of interpreted mafic intrusions,lying within the rift zone 

and trending perpendicular to its axis in western Kentucky, 

may indicate the presence of a transverse shear zone that provided a channelway for magma. Inferred dikes, sills, and large 

plutons of post-middle Mesozoic age intruded the border 

faults of the rift zone. The extensive distribution of igneous 

bodies along the margins of the rift zone suggests that the 

anomalous mantle material in the lower crust solidified by 

post-middle Mesozoic time and, thus, represented an obstruc-
52 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

tion for raising magma. The rift-border faults would then 

have provided better channelways for the ascending magma 

than the rift-axial faults. The geographic locations of these 

magmatic intrusions strongly suggest that the associated igneous events are intimately related to the rift zone. The igneous intrusions may reflect reactivation of the rift or simply 

tectonic events unrelated to rifting but associated with magma 

emanating from pre-existing deep-seated faults within the rift 

zone.

The geographic extent of rifting has yet to be determined. 

The available magnetic data indicate that the northern terminus of the rift zone lies within western Kentucky and 

southern Illinois. In this region, characterized by intense igneous activity and faulting, the margins of the rift zone appear 

to widen substantially. The rift zone is clearly defined at the 

southern boundary of the surveyed area and must, therefore, 

extend somewhat farther southwest. Continuation of the rift 

zone may be supported by the existence of alkalic intrusions in 

southeast Arkansas that include the Magnet Cove ring-dike 

complex and syenite bodies near Little Rock. If projected to 

the southwest, the rift intersects the Ouachita Foldbelt, where 

it bends beneath the embayment. The late Precambrian to 

early Paleozoic Oklahoma aulacogen also extends into a 

craton at a high angle from a bend in the Ouachita Foldbelt. 

The strong correlation in structural configuration and age of 

these two rifts suggests that they had a common origin. Additional magnetic data north and south of the present area of 

coverage will be helpful in delineating the geographic extent 

of the rift zone and in better understanding continental rifting 

in the embayment.

Pertinent research areas for future consideration include (1) 

delineating the total extent of rifting, (2) determining the relationship between seismicity and rift-related features that include deep-seated faults and plutonic masses, (3) developing a 

detailed model describing the evolution of the rift zone, (4) understanding the relation between infra-sedimentary plutonism 

and tectonic structures, (5) delineating the lithologies and 

structures of Precambrian basement, and (6) investigating the 

possible dynamics of continental rifting in the embayment. 

The magnetic interpretations presented here will certainly be 

refined or modified when more detailed quantitative analyses 

are carried out and additional geophysical and geologic data 

are acquired. The results indicate that the Mississippi Embayment region experienced a complex tectonic history. Further 

studies will be very helpful in understanding intra-era tonic 

tectonics.

REFERENCES CITED

Arkansas Geological Commission, 1967, Aeromagnetic survey of northwestern Arkansas: scale 1:100,000.

Baer, A. J., 1976, The Grenville Province in Helikian times- A possible 

model of evolution: Philosophical Transactions of the Royal Society of 

London, A., v. 280, p. 449-515.

Bhattacharyya, B. K., Sweeney, R. E., and Godson, R. H., 1979, Integration of aeromagnetic data acquired at different times with varying elevations and line spacing: Geophysics, v. 44, no. 4, p. 742-752.

Bickford, M. E., and Mose, D. G., 1975, Geochronology of Precambrian 

rocks in the St. Francois Mountains, southeastern Missouri: Geological 

Society of America Special Paper 165, 48 p.

Bond, D. C., and others, 1971, Possible future petroleum potential of region 

9-Illinois Basin, Cincinnati arch, and northern Mississippi embayment: American Association of Petroleum Geologists Memoir 15, v. 2, 

p. 1165-1218.

Burke, K., and Dewey, J. F., 1973, Plume-generated triple junctions Key 

indicators in applying tectonics to old rocks: Journal of Geology, v. 81, 

p. 406-433.

Caplan, W. M., 1954, Subsurface geology and related oil and gas possibilities of northeastern Arkansas: Arkansas Resources and Development 

Commission, Division of Geology Bulletin 20, 124 p.

Clegg, K. E., and Bradbury, J. C., 1956, Igneous intrusive rocks in Illinois 

and their economic significance: Illinois State Geological Survey Report 

of Investigations 197, 19 p.

Cohee, G. V., and others, 1962, Tectonic map of the United States: U.S. 

Geological Survey and American Association of Petroleum Geologists, 

scale 1:2,500,000.

Cordell, Lindrith, 1977, Regional positive gravity anomaly over the 

Mississippi embayment: Geophysical Research Letters, v. 4, p. 

285-287.

Erickson, R. L., and Blade, L. V., 1963, Geochemistry and petrology of the 

alkalic igneous complex at Magnet Cove, Arkansas: U.S. Geological 

Survey Professional Paper 425, 95 p.

Ervin, C. P., and McGinnis, L. D., 1975, Reelfoot rift- Reactivated 

precursor to the Mississippi embayment: Geological Society of America 

Bulletin, v. 86, p. 1287-1295.

Freeman, L. B., 1950, Paleozoic structure and stratigraphy of the Jackson 

Purchase region, Kentucky: Kentucky Geological Survey Bulletin, ser. 

9, no. 4, p. 12-36.

Fuller, M. L., 1912, The New Madrid earthquake: U.S. Geological Survey 

Bulletin 494, 119 p.

Gordon, M., Jr., Tracey, J. I., Jr., and Ellis, M. W., 1958, Geology of the

Arkansas bauxite region: U.S. Geological Survey Professional Paper

229, 268 p. 

Grant, F. S., and West, G. F., 1965, Interpretation theory in applied

geophysics: New York, McGraw-Hill Book Company, 583 p. 

Grohskopf, J. G., 1955, Subsurface geology of the Mississippi embayment of

southeast Missouri: Missouri Division of Geological Survey and Water

Resources, v. 37, 2nd ser., 133 p. 

Hadley, J. B., and Devine, J. F., 1974, Seismotectonic map of the eastern

United States: U.S. Geological Survey Miscellaneous Field Studies Map

MF-620. 

Hendricks, J. D., and Hildenbrand, T. G., 1979, Total field aeromagnetic

map of northeast Arkansas: U.S. Geological Survey Open-File Report

79-1208, scale 1:250,000.

Herrmann, R. B., andCanas.J., 1978, Focal mechanism studies in the New 

Madrid seismic zone: Seismological Society of American Bulletin, v. 68, 

no. 4, p. 1095-1102.

Heyl, A. V., Jr., Brock, M. R., Jolly, J. L., and Wells, C. E., 1965, Regional 

structure of the southeast Missouri and Illinois-Kentucky mineral 

districts: U.S. Geological Survey Bulletin 1202-B, 20 p.

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map of 

central Mississippi valley and environs: U.S. Geological Survey Miscellaneous Field Studies Map MF-1011.

Hildenbrand, T. G., 1978, Approximate ages of igneous intrusions in the 

upper Mississippi embayment utilizing the total magnetic field 

anomalies: American Geophysical Union 1978 Midwest Meeting, 

Abstracts, p. 6.
MAGNETIC BASEMENT IN THE UPPER MISSISSIPPI EMBAYMENT REGION 53

Hildenbrand, T. G., and Johnson, R. W., 1977, Aeromagnetic map of the 

northern Mississippi embayment: U.S. Geological Survey Open-File 

Report 77-229.

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic 

and gravity anomalies in the northern Mississippi embayment and their 

spatial relation to seismicity: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-914.

Johnson, R. W., Jr., Haygood, C., and Kunselman, P. M., 1978, Residual 

total intensity aeromagnetic map of Kentucky, western sheet: University of Kentucky, Kentucky Geological Survey.

Kane, M. F., 1977, Correlation of major eastern earthquake centers with 

mafic/ultramafic basement masses, in Rankin, D. W., ed., Studies 

related to the Charleston, South Carolina, earthquake of 1886--A 

preliminary report: U.S. Geological Survey Professional Paper 1028-O, 

p. 199-204.

Kidwell, A.L., 1951, Mesozoic igneous activity in the northern Gulf Coast 

Plain: Gulf Coast Association of Geological Societies, 1st Annual 

Meeting, Transactions, p. 182-199.

Kisvarsanyi, E. B., 1974, Operation basement: Buried Precambrian rocks of 

Missouri- Their petrography and structure: American Association of 

Petroleum Geologists Bulletin, v. 58, no. 4, p. 674-684.

Kisvarsanyi, G., 1976, Precambrian metallogeneses in the St. Francois 

Mountains igneous province, southeast Missouri (Contribution to Precambrian Geology, no. 6): Missouri Department of Natural Resources, 

Geological Survey Report of Investigations 61, 190 p.

Koeing, J. B., 1956, The petrography of certain igneous dikes of Kentucky: 

Kentucky Geological Survey Bulletin, ser. 9, no. 21, 27 p.

Kumarapeli, P. S., and Saull, V. A., 1966, The St. Lawrence valley system 

 A North American equivalent of the East African rift system: Canadian Journal of Earth Science, v. 3, p. 639- 658.

Lidiak, E. G., Marvin, R. F., Thomas, H. H., and Bass, M. N., 1966, Geochronology of the midcontinent region, United States, Part 4, Eastern 

area: Journal of Geophysical Research, v. 71, p. 5427-5438.

Lidiak, E. G., and Zietz, Isidore, 1976, Interpretation of aeromagnetic 

anomalies between latitudes 37° N. and 38° N. in eastern and central 

United States: Geological Society of America Special Paper 167, 37 p.

Long, L. T., 1976, Speculations concerning southeastern earthquakes, 

mafic intrusions, gravity anomalies, and stress amplifications: Earthquake Notes, v. 47, p. 29-35.

McKeown, F. A., 1975, Hypothesis- Mafic intrusions and their contact 

zones are source zones of many earthquakes in central and southeastern 

United States: Earthquake Notes, v. 46, p. 53.

Missouri Division of Geological Survey and Water Resources, 1963, 

Aeromagnetic contour map of the Advance 15' quadrangle: scale 

1:250,000.

______1968a, Aeromagnetic contour map of the Greenville 15 ' 

quadrangle: scale 1:250,000.

_1968b, Aeromagnetic contour map of the Zalma 15 ' quadrangle:

scale 1:250,000.

Moody, C. L., 1949, Mesozoic igneous rocks of the northern Gulf Coastal 

Plain: American Association of Petroleum Geologists Bulletin, v. 33, p. 

1410-1428.

Phelan, M., 1969, Crustal structure in the central Mississippi valley earthquake zone: St. Louis, Missouri, Washington University Ph.D. dissertation, 166 p.

Riggs, K. A., 1976, Subsurface Precambrian granite, Lafayette County,

Mississippi: American Association of Petroleum Geologists Bulletin, v.

60, p. 455-457. 

Salveson, J. O., 1978, Variations in the geology of rift basins- A tectonic

model: International Symposium on the Rio Grande rift, Santa Fe,

New Mexico, 1978, p. 82. 

Scharon, L., and Hsu, I., 1969, Paleomagnetic investigations of some

Arkansas alkalic igneous rocks: Journal of Geophysical Research, v. 74,

no. 10, p. 2774-2779. 

Seyfert, C. K., and Sirkin, L. A., 1973, Earth history and plate tectonics:

New York, Harper and Row, 504 p.

Stauder, William, Kramer, M., Fischer, G., Schaeffer, S., and Morrissey, S. 

T., 1976, Seismic characteristics of southeast Missouri as indicated by a 

regional telemetered micro-earthquake array: Seismological Society of 

America Bulletin, v. 66, no. 6, p. 1953-1964.

Sykes, L. R., 1967, Mechanism of earthquakes and nature of faulting on the 

mid-oceanic ridges: Journal of Geophysical Research, v. 72, no. 8, p. 

2131-2153.

Thoenen, J. R., Malamphy, M. C., and Vallely, J. L., 1945, Geophysical 

survey of Arkansas bauxite region: U.S. Bureau of Mines Report of Investigations 3791, 49 p.

U.S. Geological Survey, 1978, Aeromagnetic form-line contour map of the 

east part of the Rolla, Missouri, I°x2° quadrangle: U.S. Geological 

Survey Open-File Report 78-330.

Vacquier, V., Steenland, N. C., Henderson, R. G., and Zietz, Isidore, 1951, 

Interpretation of aeromagnetic maps: Geological Society of America 

Memoir 47, 151 p.

Van Alstine, D. R., 1978, A new technique for contracting apparent polar 

wander paths and the revised Phanerozoic path for North America: 

Geology, v. 6, p. 137-139.

Wasserburg, G. J., Wetherill, G. W., Silver, L. T., andFlawn, P. T., 1962, 

A study of ages of the Precambrian of Texas: Journal of Geophysical 

Research, v. 67, p. 4021-4047.

Watson, K. D., 1967, Kimberlites of eastern North America, in Wyllie, P. 

J., ed., Ultramafic and related rocks: New York, John Wiley and Sons, 

Inc., p. 312-323.

White, P. E., 1967, Outline of thermal and mineral waters as related to 

origin of Mississippi Valley ore deposits, with discussion, in Brown, J. 

S., ed., Genesis of stratiform lead- zinc-barite-fluorite deposits 

(Mississippi Valley type deposits) A symposium, New York, 1966: 

Economic Geology Monograph 3, p. 382.

Wickham, John, 1978, The southern Oklahoma aulacogen, in Field guide to 

structure and stratigraphy at the Ouachita Mountains and the Arkoma 

Basin: American Association of Petroleum Geologists 1978 Annual 

Meeting, p. 2-33.

Zartman, R. E., 1977, Geochronology of some alkalic rock provinces in 

eastern and central United States: Earth and Planetary Science Letters 

Annual Review, v. 5, p. 257-286.

Zartman, R. E., Brock, M. R., Heyl, A. V., and Thomas, H. H., 1967, K-Ar 

and Rb-Sr ages of some alkalic intrusive rocks from the central and 

eastern United States: American Journal of Science, v. 265, p. 848-870.

Tectonic Features of the

New Madrid Seismic Zone From

Seismic-Reflection Profiles

By R. M. HAMILTON and M. D. ZOBACK

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-F
CONTENTS

Page

Abstract .................................................... 55

Introduction................................................. 55

Geologic setting .............................................. 55

Seismicity ................................................... 57

Data acquisition and processing ................................. 57

Correlation of reflections with stratigraphy ........................ 60

Description and interpretation of the profiles ...................... 60

Northern profiles ......................................... 61

Profiles S-12 and S-11 ................................. 61

Profile S-l .......................................... 61

Profile S-2 .......................................... 61

Profile S-3 .......................................... 63

Profile S-10 ......................................... 63

Summary ........................................... 63

Ridgely area profiles ...................................... 64

Profile S-5 .......................................... 64

Profile S-4 .......................................... 66

Profile S-6 .......................................... 66

Interpretation ....................................... 67

Description and interpretation of the profiles -Continued ........... .Page

Caruthersville area profiles ................................. 68

ProfileS-7 .......................................... 68

Profile S-13 ......................................... 69

Profile S-8 .......................................... 69

Profile S-9 .......................................... 70

Deep-resolution profiles. ................................... 71

Profiles D-l and D-3 .................................. 71

Profile D-2 .......................................... 72

Nature of geologic structure and faulting in the region of the embayment

Illinois-Kentucky mineral district ............................ 73

Cottage Grove-Shawneetown-Rough Creek fault zone ........... 73

Wabash Valley fault zone .................................. 74

Ste. Genevieve fault zone ................................... 75

Summary ............................................... 75

Tectonic implications ......................................... 75

Acknowledgments ............................................ 77

References cited .............................................. 77

ILLUSTRATIONS

	Page 

FIGURE 1. Relationship of earthquake epicenters, plutons, rift boundaries, faults, and major geologic features in the northern

Mississippi Embayment region ....................................................................................... 56

2. Locations of the seismic-reflection profiles, principal faults inferred from the data, rift boundaries, and research well. .................... 58

3. Velocity analysis for VP 450, line D-l...................................................................................... 59

4. Correlation of stratigraphic units with a synthetic seismogram based on logs from the research well and part of nearby reflection profile S-10 . . 60

5. Profile 12 ............................................................................................................. 62

6. ProfileS-11........................................................................................................... 62

7. ProfileS-l............................................................................................................ 63

8. ProfileS-2............................................................................................................ 64

9. ProfileS-3............................................................................................................ 64

10. ProfileS-10. .......................................................................................................... 66

11. ProfileS-5............................................................................................................ 67

12. ProfileS-4. ........................................................................................................... 70

13. ProfileS-6............................................................................................................ 72

14. Enlargement of profile S-6 showing the Cottonwood Grove fault ................................................................ 74

15. Spatial relationship of faults and an igneous intrusive mass to topographic ridges and Bouguer gravity anomalies in the

Reelfoot Lake, Tenn., area .......................................................................................... 75

16. ProfileS-7............................................................................................................ 76

17. Profile S-13........................................................................................................... 76

18. ProfileS-8. . .......................................................................................................... 78

19. Profile S-9............................................................................................................ 78

20. Profile D-l ........................................................................................................... 80

21. ProfileD-3 ........................................................................................................... 80

22. Profile D-2 ........................................................................................................... 81
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

TECTONIC FEATURES OF THE NEW MADRID SEISMIC 

ZONE FROM SEISMIC-REFLECTION PROFILES

By R. M. HAMILTON and M. D. ZOBACK

ABSTRACT

Over 250 km of multichannel, common-depth-point seismic-reflection 

profiles in the New Madrid, Mo., seismic region show that the modern 

seismicity of the New Madrid area is associated with reactivated geologic 

features of ancient origin. The main seismic zone, which extends over 100 

km in a northeasterly direction from near Marked Tree, Ark., to Caruthersville, Mo., is associated with a zone of arched and faulted Paleozoic rocks 

and minor faulting of overlying Upper Cretaceous and Tertiary rocks. Vertical offset of the Paleozoic rocks is about 1 km. These zones of faulting and 

seismicity lie along the axis of a northeast-striking basement rift of late Precambrian or early Paleozoic age. In the southern part of the diffuse zone of 

seismicity that extends northward from the Ridgely, Tenn., area to the 

vicinity of New Madrid, a northeast-striking, high-angle, reverse fault shows 

about 80 m of post-middle Eocene vertical movement. An inferred igneous 

intrusive body lies northwest of the fault. The intrusive activity apparently 

preceded the faulting, but probably was also Cenozoic in age. In the vicinity 

of New Madrid, the upper Paleozoic section is intensely disrupted, possibly 

from late Paleozoic igneous activity. A northeast-striking seismic zone west 

of New Madrid appears to be associated with a fault showing mostly prePaleocene vertical movement.

INTRODUCTION

Assessment of seismic risk in the mid-continent region requires that the intense seismicity of the New Madrid zone be 

understood in relation to geologic structures and tectonic 

history. The main difficulty in establishing such a relation is 

that most of the seismic zone is covered by thick and poorly 

consolidated sediments of the Mississippi River valley. The 

bedrock geology is buried and any faulting of the bedrock 

could be greatly diminished and may not be well preserved in 

the soft sediments at the surface.

A variety of geophysical methods have been employed in an 

effort to improve knowledge of seismicity and structure in the 

New Madrid seismic zone. Installation of a seismograph network of moderate density has led to a marked improvement in 

resolution of the seismicity pattern (Stauder and others, 1976; 

Stauder and others, 1979). Lineations in the seismicity pattern 

can now be clearly seen, suggesting the location of faults, 

whereas earlier epicenter maps (for example, Hadley and 

Devine, 1974) show a much more diffuse pattern. Magnetic 

and gravity surveys have revealed several strong anomalies interpreted to be plutons and have suggested the presence of a 

northeast-striking basement graben or rift (Hildenbrand and

others, 1977; Hildenbrand and others, 1979; Hildenbrand 

and others, this volume). These and other important tectonic 

features of the New Madrid region are shown on figure 1.

To further study the relationship between seismicity and 

structure, an extensive program of multichannel, commondepth- point, seismic-reflection profiling was undertaken. 

Preliminary work was done in the Tiptonville, Tenn., area in 

1977 when 32 km of profiles were obtained (Zoback, 1979). 

Because the results from that study proved encouraging, a 

much larger survey was carried out in 1978 and 1979, when 

about 250 km of profiles were run. The most significant results 

of the more recent profiling were summarized by Zoback and 

others (1980). The data from both surveys were released in a 

report by Hamilton and Zoback (1979). This report constitutes 

a full presentation and comprehensive discussion of the profiles from the 1978-79 survey. Features of particular interest 

are analyzed in detail.

GEOLOGIC SETTING

Considerable attention must be paid to the geologic history 

of the New Madrid seismic region because of an apparently 

close relationship between current seismicity and geologic 

features of ancient origin. The reader is referred to the detailed summary of geologic history that is presented in the introductory chapter of this volume and to Hinze and others (1977).

The New Madrid seismic zone lies at the'northern end of the 

Mississippi Embayment, in a region covered with poorly consolidated Cenozoic and Upper Cretaceous sediments. These 

sediments overlie Paleozoic sedimentary rocks, which in the 

study area are mostly limestones and dolomites of Ordovician 

and Cambrian age. The nearest exposure of Precambrian 

rocks is about 150 km north-northwest of the seismic zone in 

the St. Francois Mountains of Missouri.

The geologic structure of the embayment largely reflects the 

topography of the Paleozoic surface, which forms a broad, 

southwest-plunging trough. In the center of the embayment, 

in the area of this study, Late Cretaceous, Paleocene, and 

Eocene deposits over this surface are generally conformable. 

Knowledge of Paleozoic structure is based on subsurface information from wells. Centered in southeastern Missouri, Cam55
56

92°00'

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

91°00' 90W 89°00' 88°00'

38°00'

37°00'

36°00'-

35°00'

*

£'IN DIANA

^P&ISTR'fUfe i&frtyw ,^

KlNTUCKY

MISSOURI 

ARKANSAS

TENNESSEE

Northtrn limit of Mississippi Embaymtnt 

coastal-ploin mattriol

A Mafic or ultramafic intrusion within the 

Mississippi Embaymtnt-ldtntifitd in 

drill corts

EXPLANATION 

i Mafic or ultromafic intrusion within the 

Mississippi Embaymtnt inttrprtttd from 

tht magnttic fitld -- Approximatt boundary 

of intrusion dtttrmintd from ztro contour of associattd anomaly on tht stcondvtrtical-dtrivotivt magnttic map

,- Fault 

o Earthquakt tpictnttr

 Boundary of inftrrtd 

subsurfact rift

FIGURE 1. Relationship of earthquake epicenters, plutons, rift boundaries, faults, and major geologic features in the northern Mississippi Embayment 

region. Epicenters, open circles (Stauder and others, 1979); plutons, lined pattern; rift boundaries, heavy solid lines (Hildenbrand and others, this 

volume); and faults, fine solid lines (Heyl and McKeown, 1978).

brian rocks lie at the Paleozoic surface surrounded by successive belts of Ordovician and Devonian rocks. Thus, the subcrop data reveal the presence of a structural high, which is 

named the Pascola arch (Grohskopf, 1955) (fig. 1).

Many of the wells in the northern Mississippi Embayment 

that have reached Paleozoic rocks have encountered igneous 

rocks of alkalic composition (Miser and Ross, 1925; Kidwell, 

1947, 1950; Clegg and Bradbury, 1956), which are indicative
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 57

of a deep magma origin (Zartman, 1977). A Permian age was 

determined by the K-Ar method for cuttings from one well in 

the area (Zartman and others, 1967; Zartman, 1977).

Numerous faults have been mapped in the region north of 

the Mississippi Embayment (Heyl and McKeown, 1978) (fig. 

1). Although activity in the New Madrid seismic zone extends 

into the region of the mapped faults, its distribution is diffuse 

there, and no clear correlation between faults and earthquakes has yet been established. Much of the fault movement 

occurred between Pennsylvanian and Cretaceous time 

(Krausse and Treworgy, 1979). Continuation of mapped 

faults under the sediments of the embayment is largely conjectural.

The deep structure of the northern embayment is indicated 

by geophysical data. A seismic-refraction survey along the 

northwestern margin of the embayment, between Cape 

Girardeau, Mo., and Little Rock, Ark., showed that the crust 

is 45 km thick and that a layer of material intermediate between normal crustal and mantle rocks in velocity (7.4 km/s) 

exists in the lower crust (McCamy and Meyer, 1966). Regional 

gravity data, when used with the seismic-refraction data, suggest that this layer thickens under the middle of the embayment, and this thickening has been interpreted as evidence for 

a broad, northeast-striking rift of Precambrian origin (Ervin 

and McGinnis, 1975). Recent, detailed magnetic and gravity 

data show a similarly striking graben in the upper crust that 

has structural relief of 1.6-2.6 km; this graben may have formed as early as late Precambrian or early Paleozoic time (Hildenbrand and others, 1977; Hildenbrand and others, this 

volume). These data also show the locations of numerous 

plutons, particularly along the graben, or rift, margin and 

axis (fig. 1).

SEISMICITY

The improved seismograph network in the New Madrid 

region greatly increased the resolution of the seismicity pattern 

(fig. 1). The major feature is a linear zone extending for about 

100 km from near Marked Tree, Ark., in a northeasterly 

direction to beyond Caruthersville, Mo. This zone lies approximately along the axis of the northeast-striking rift (fig. 1) 

defined by magnetic and gravity data (Hildenbrand and 

others, 1977; Hildenbrand and others, this volume). A more 

diffuse zone lies between the Ridgely, Tenn., area and New 

Madrid, Mo., to the north. Another northeast-striking zone 

passing northwest of New Madrid toward Charleston, Mo., is 

evident, as are other clusters of activity. Fault-plane solutions 

(Herrmann, 1979) suggest predominantly right-lateral, strikeslip movement along the northeast-striking seismic zones and 

essentially east-west compression. The nature of movement 

between the two northeast- striking zones, that is, in the zone 

between Ridgely and New Madrid, is evidently more complex 

because the fault-plane solutions are varied (Herrmann and 

Canas, 1978; Herrmann, 1979). Composite-fault-plane solutions from a microearthquake study in this zone, however, also 

indicate east-west compression (O'Connell and others, this 

volume). Focal depths for New Madrid earthquakes are mostly 

less than 15 km (Stauder and others, 1979).

Although the epicenters for the three main earthquakes in 

1811-12 are not reliably known, a case can be made for their 

approximate location based on intensity data (Nuttli, 1973). 

The first shock, on December 16, 1811, probably occurred in 

northeastern Arkansas toward the southwestern end of the 

seismic zone. The second shock, on January 23, 1812, was 

closer to New Madrid than the first one. The third shock, on 

February 7, 1812, was nearest to New Madrid. Thus, the first 

large earthquake, and perhaps the second one, may have occurred along the main Marked Tree-Caruthersville seismic 

zone. The third earthquake could be associated with the zone 

of current seismicity near New Madrid. Of course, it is also 

possible that the faults responsible for the 1811-12 earthquakes are not revealed by current seismicity.

DATA ACQUISITION AND PROCESSING

The seismic-reflection profiling made use of the Vibroseis1 

method. Data acquisition and processing were carried out by 

Geophysical Services Inc. (GSI) under contract to the USGS. 

Most of the profiles were located so as to cross linear zones in 

the seismicity pattern (fig. 2). To facilitate correlation, most of 

the profiles were tied to one another. As the land is agricultural and the water table seasonably high, the logistics of truck 

operations necessitated that the profiles lie along roads. Vibration points were located more than 120 m from buildings or 

water wells to prevent the possibility of damage. For the same 

reason, highly populated roads had to be avoided.

Three or four vibrator trucks were used with a 48-channel 

recording system. All profiles were run with a 122-m offset to 

the nearest recording group. Thirteen of the profiles were run 

with 30.5-m-group intervals in split spreads. These profiles 

were designated by the letter "S" because the parameters of the 

spread configuration and sweep were chosen to enhance 

shallow reflections. Three profiles run with 61-m-group intervals to enhance deep reflections were designated by the letter 

"D. Profiles D-l and D-3 were run as split spreads, and D-2 

was run as an off-end spread. Multiplicity was 12- and 24-fold 

on the S and D profiles, respectively. Generally, 15 sweeps of 

7-s duration were made at each vibration point with 10 s of 

recording time. The sweep frequency was usually 10-80 Hz for 

the S profiles and 10-50 Hz for the D profiles. Additional 

parameters are given by Hamilton and Zoback (1979).

The fieldwork was done in two stages. Profiles S-l through 

12, D-2, and the west third of D-l were completed before the 

end of 1978, when work was interrupted, first by icy road con-

'Trademark of Continental Oil Co. Any use of trade names is for descriptive purposes only and does 

not imply endorsement by the U.S. Geological Survey.
58

90°30'

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

90°00' 89°30' 89°00'

36°30' -- -

36WFAU 

'Reelfoot Lake RESEARCH-

: S-^COTTONWOOD 

w . i I. __ GROVE

FAULT j$ 

$>

 BlythHe

FIGURE 2. - Locations of the seismic reflection profiles (heavy lines), principal faults inferred from the data (dashed lines), rift boundaries, 

buried pluton (hachured pattern), and the research well. The T lines and line R-l refer to profiles reported on by Zoback (1979).

ditions and then by floodwaters. The interruption proved to 

be beneficial because it allowed time for preliminary data processing on the completed profiles. Consequently, profiles D-3 

and S-13 were added to the field program the former line to 

determine true dip and strike of Paleozoic strata seen on line 

D-l, and the latter line to attempt to trace the southwesterly 

continuation of a fault seen on lines S-6 and S-4.

Data processing was done by standard procedures (see, for 

example, Dobrin, 1976; Sheriff, 1978; and Waters, 1978). 

The data were not migrated, except for a part of line S-6. In 

addition to the final-stack record sections, which are shown 

here in figures 5-14 and 16-22, the data available for analysis 

include correlated field records, brute-stack record sections 

(before residual static and residual normal-moveout corrections were made), single-fold-coverage sections (unstacked 

records from the individual vibrating points), velocity 

analyses, and computer listings of the residual static corrections.

Several features on the profiles that appear as deformation

of reflections are thought instead to be artifacts of the data 

processing. In places where a number of sequential vibration 

points were skipped, a wedge-shaped gap appears at the top of 

a section and the reflections generally drape toward the gap. 

Artificial reflection offsets apparently can result both from a 

bend in a profile and from residual-static processing; the 

residual static problems appear to occur only near the ends of 

a profile. Examples are cited later in the section "Description 

and Interpretation of the Profiles." In general, static effects 

appear to have been treated adequately in the data processing. 

Most zones in which significant corrections have been made 

are correlated with low-lying areas; the shot and receiver corrections are mostly less than 10 ms.

Seismic-wave velocities increase sharply at the Upper 

Cretaceous-Paleozoic contact. Stacking velocities for the 

reflections above this contact mostly fall in the range from 2.0 

to 2.1 km/s. Thus, two-way travel time in milliseconds for 

reflections in the Tertiary and Upper Cretaceous sections is 

approximately equal to distance in meters. Correlation be-
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 59

tween known depths to Paleozoic rocks in wells and the reflection times from nearby profiles yield estimates of the average 

velocity that are similar to each other in value. (See discussion 

in Crone and Brockman, this volume.) Velocity analysis of the 

profile data reported here did not generally yield estimates of 

stacking velocity below the Paleozoic surface because, except 

for profiles D-l and D-3, good reflections are not seen there. 

Data processing was done assuming a linear increase in stacking velocity from about 2 km/s at the Paleozoic surface to 

about 5.5 km/s at 3.0 s. The corresponding interval velocity

below the Paleozoic surface is about 6.2 km/s, in agreement 

with seismic- refraction evidence (McCamy and Meyer, 1966). 

An example of a velocity analysis from profile D-l is shown 

in figure 3. Reflections are clearly seen from the Paleozoic surface at 0.87 s, the Upper Cretaceous surface at 0.71 s, and 

shallower horizons. Weak reflections seen for the stackingvelocity functions VI and V2 between 1.57 and 1.70 s are probably multiples of the Paleozoic reflection. Most of the apparent multiple reflections arrive at about double the time for 

the primary reflections. A series of reflections below 1.7 s is

AB

o.o

RMS VELOCITY FT/SEC 5000 18000

6000 7000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000 0.0

 0.5

FIGURE 3. Velocity analysis for VP 450, line D-l, which is shown in figure 20. From left to right, the data shown are A, a collection of the traces that reflected 

below VP 450, uncorrected for moveout; B, the same traces corrected for moveout using stacking-velocity function V5; C, stacked sections for 23 traces 

centered on VP 450 using stacking-velocity functions VI through V9; and, D, a plot of the stacking-velocity functions against two-way travel time with the 

velocity picks derived from the reflections. The symbols under D are defined as follows: circle, highest amplitude in a 100-ms interval; cross, events from 0 

to -6 db; and triangle, events from -6 to 12 db.
60 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

seen to stack well at the higher velocities, which is part of the 

basis for their interpretation as primary reflections rather than 

multiples.

reflector that could be correlated over much of the study area, 

but not too well in the northern part. Reflections from within 

the Eocene section could be correlated only locally.

CORRELATION OF REFLECTIONS 

WITH STRATIGRAPHY

As part of the New Madrid studies, a research well was drilled in 1978 to a depth of 700 m in extreme southeastern 

Missouri (Crone and Russ, 1979) (fig. 2). Paleozoic rocks of 

possible Cambrian age were reached (John Repetski, written 

commun., 1979). Much of the hole was cored and numerous 

samples of cuttings were collected. A complete suite of logs was 

obtained in the well, including density and sonic logs from 

which a synthetic seismogram was derived. Profile S-10 was 

run within 10 m of the well. The actual record section, synthetic section, logs, and stratigraphic column from Crone and 

Russ (1979) are compared in figure 4. This information permits detailed correlation of the reflections on profile S-10 with 

stratigraphy above the Paleozoic surface. Because most of the 

profiles cross, or come close to doing so, reflections could 

usually be correlated between profiles. Two strong reflectors, 

the Paleocene-Upper Cretaceous contact and the Upper Cretaceous- Paleozoic contact, could be correlated over most of 

the study area. The Eocene-Paleocene contact is also a good

DESCRIPTION AND INTERPRETATION 

OF THE PROFILES

In order to provide coherent organization to the presentation of the large number of record sections, the profiles are 

discussed systematically from north to south (fig. 2). The profiles are treated in four groups: (1) a northern group from the 

area generally west of New Madrid S-l, 2, 3, 10, 11, and 12; 

(2) a group from the Ridgely area S-4,5, and 6; (3) a group 

from the area south of Caruthersville S-7, 8, 9, and 13; (4) 

and the three deep-resolution profiles (two lines being located 

southwest of Blytheville, Ark. D-l and D-3 and one line 

running east-northeast of Dyersburg, Tenn. D-2). The most 

significant tectonic features are seen on profiles S-4, 5, and 6 

of group 2, and on profiles D-l and 3 of group 4.

For brevity, the reflectors are referred to by the geologic age 

symbol of the underlying unit: the contact between Eocene 

and Paleocene rocks is referred to as the Pa reflector, K is used 

for the Paleocene-Upper Cretaceous contact, and Pz for the 

Late Cretaceous-Paleozoic contact. The K and Pz reflections 

are marked on all profiles; the Pa reflection is marked only on

DENSITY 

GR RES SONIC

REFL 

COEF

SYNTHETIC 

RECORD

REFLECTION

PROFILE

S-10

MEMPHIS 

SAND

TWO 

WAY 

TIME

-0.2

-0.3

«

1 FLOUR ISLAND 

WJLCOX ___ FM

GROUP L-________ 

< FT PILLOW SD

PORTERS CREEK 

CLAY

CLAYTON FM

CRETACEOUS 

SEDIMENTS

PALEOZOIC 

ROCKS

ii^PPSlMB

FIGURE 4. - Correlation of stratigraphic units with a synthetic seismogram based on logs from the research well and part of nearby reflection profile S-10. GR, 

gamma-ray log; RES, resistivity log; and REFL COEF, reflection coefficient. Two-way travel time is in seconds (from Crone and Russ, 1979). The 

Porters Creek Clay is of Paleocene age; shallower beds are of Eocene age.
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 61

the profiles where it is discussed. In the following discussion, 

interpretive comments closely follow descriptive comments in 

order to avoid redescribing features.

NORTHERN PROFILES

PROFILES S-12 AND S-ll

scarp, unless the sense of fault movement associated with the 

scarp has reversed. In any case, geologic evidence indicates 

that Sikeston Ridge is an erosional rather than structural 

feature (Russ, this volume). It may nevertheless be significant 

that the gradient near VP 162 is at about the place where profile S-ll crosses the seismic zone.

PROFILE S-l

Profiles S-12 (fig. 5) and S-ll (fig. 6), which are nearly colinear, are located north of New Madrid at the northern 

margin of the study area (fig. 2). They extend from near the 

flank of the Bloomfield pluton at the west end of line S-12 onto Sikeston Ridge at the east end of line S-ll, where a 

northeast-striking zone of seismicity is crossed. (See also fig. 1.)

The K, Pz, and other reflections show a slight easterly, 

regional apparent dip. The Pz reflection is disrupted and discontinuous in places, progressively more so toward the east 

end of profile S-ll. Below the Pz reflection, the level of incoherent energy is high, with many diffractions in evidence.

A possible explanation for the disruption of the upper 

Paleozoic section could be fracturing, rock alteration, and 

mineralization associated with intrusive igneous activity. The 

proximity of the lines to the Bloomfield pluton (fig. 2) and the 

finding of cuttings of igneous rocks in nearby wells support this 

interpretation. The lack of corresponding disruption of the 

Upper Cretaceous section indicates that the inferred intrusive 

activity occurred before Late Cretaceous deposition. The 

Paleozoic rocks in the area of lines S-ll and S-12 are probably 

Ordovician in age (Grohskopf, 1955), which fixes the earliest 

date of the igneous activity. A Permian age was determined for 

intrusive rocks from a well near Tiptonville, Tenn. (Zartman 

and others, 1967). Magnetic data suggest a post-early 

Mesozoic age for the Bloomfield pluton (Hildenbrand and 

others, this volume). Both of these ages fall within the range 

suggested by the seismic data.

The main feature of interest on profiles S-ll and S-12 is a 

low arch in the Pz, K, and shallower reflections centered under 

vibration point (VP) 172 on S-ll, reversal in the regional dip 

occurs under VP 188 at the west margin of the arch, and an increased gradient at VP 160 marks the east margin, although 

some reflections at about 0.25 s show unusual easterly dips to 

VP 148. The significance of the arch is not clear. Because all 

reflections have a similar arch, at first glance the feature seems 

to have the characteristics of a static effect; however, the position of the steepest gradient, near VP 160, is actually located 

progressively more easterly on the shallower reflections in the 

section. Thus the arch is not thought to be a static effect, but it 

is thought to represent a structural feature. At VP 162, in the 

zone where the steeper gradient occurs, the line crosses the 

western flank of Sikeston Ridge, a 6-m-high, north-trending 

topographic feature. The sense of displacement of the reflections at VP 162 is opposite to that of the topography, which 

argues against interpretation of the ridge flank as a fault

Profile S-1 (fig. 7) extends west from New Madrid across the 

same northeast-striking seismic zone that cuts the east end of 

profile S-ll (fig. 2).

The K reflection is fairly smooth and continuous along the 

whole profile. The Pz reflection is similar in character to the K 

reflection except in the eastern quarter of the line, east of 

about VP 437, where it is much more discontinuous. As in the 

case of profiles S-ll and S-12, diffractions and a high level of 

incoherent energy are seen below the Pz reflection on profile 

S-l. All reflections show regional apparent dip to the east. 

Superimposed on this trend are several features worth noting. 

Between VP 171 and 175 the trend is reversed, and between 

VP 278 and 290 the trend is steepened. These two places also 

mark changes in the general dip of the reflections. Reflections 

west of VP 171 dip more steeply to the east than they do east of 

VP 175, where they are nearly horizontal. East of VP 290 the 

reflections actually rise in a low arch, reaching their crest between VP 355 and 359. From there the regional dip to the east 

continues with only minor undulations in the reflections.

The reversal in trend between VP 171 and 175 is interpreted 

to be a fault that offsets the K and Pz reflections by about 20 

ms, west side down. Apparent diffractions immediately below 

the Pz reflection support this interpretation. Evidence for the 

minimum age of faulting is not entirely clear, as the reflections 

in the Tertiary section are not continuous; however, the reflections in the lower part of the Tertiary section appear to be offset at least 10 ms. This then may be a case of fault reactivation 

with initial movement between deposition of sediments during 

the Late Cretaceous and Paleocene and further movement 

after Paleocene deposition.

The change in dip between VP 278 and 290 could be a fault 

or a flexure. The Pa reflection (at about 0.3 s) is conformable 

with the K and Pz reflections, but shallower reflections (at 

about 0.2s) apparently are not. Thus, the deformation occurred before the Eocene.

The disruption of the Pz reflection in the eastern quarter of 

line S-l does not occur in shallower reflections. Following the 

reasoning used in interpreting profiles S-ll and S-12, this 

disruption may indicate intrusive activity between Ordovician 

and Late Cretaceous time.

PROFILE S-2

Profile S-2 (fig. 8) runs south from the west end of profile 

S-l to near the west end of profile S-3 (fig. 2). The profile
62

K 

Pz

W

O 

O

0 

W

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Profile S-12

West 220 140 East S-2J West 171 175 200 

\ / I

K 

Pz

W O

C 

O 

O 

0 

V)

0

E

FIGURE 5. Profile S-12. K, Tertiary-Upper Cretaceous contact; Pz, 

Upper Cretaceous-Paleozoic contact. Vibration point (VP) number is 

shown on the horizontal scale; the vibration point interval is 30 5 m. To 

estimate interval thickness in kilometers, multiply interval time in 

seconds by 1 above Pz reflector and by 3 below it.

FIGURE 7. Profile S-l. K, Tertiary-Upper Cretaceous contact; Pz, Upper 

Cretaceous-Paleozoic contact. Vibration point (VP) number is shown 

on the horizontal scale; the vibration point interval is 30.5 m. Arrow indicates where profile S-2 crosses.

Profile S-1 1

West East

FIGURE 6. Profile S-11. K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point (VP) 

number is shown on the horizontal scale; the vibration point interval is 30.5 m.

passes between a cluster of epicenters located to the west and 

the north-northwest-striking seismic zone to the east.

Again, the K reflection is generally smooth and continuous, 

whereas the Pz reflection is somewhat disrupted. All reflections show a southerly regional dip. The most important 

feature on the profile is an offset or flexure of about 25 ms in 

the K and Pz reflections between VP 370 and 374. A diffraction appears below the Pz reflection. Reflections above the K
TECTONIC FEATURES. NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 63

Profile S-1

278 290 300 

I I I

355 359 

\

500 East

FIGURE 7. - Continued

reflection do not show the offset. The brute-stack and singlefold-coverage record sections show the feature more clearly as 

an offset than does the final-stack section in figure 8. Apparently, processing the data for residual statics had the effect of 

smoothing out the offset somewhat in the final-stack section. 

The offset is interpreted as a normal fault, of about 25 m offset, south side down. Movement occurred after Late Cretaceous deposition but before deposition of the Cenozoic sediments. There is no evidence in the record section of fault reactivation. The fault, however, lies at the point where the northeast-striking seismicity zone crosses the profile.

PROFILE S-3

From near the south end of profile S-2, profile S-3 (fig. 9) 

runs easterly across the seismic zone to near the Mississippi 

River (fig. 2).

The K and Pz reflections are both fairly continuous and 

show easterly regional apparent dip, with a drop of about 100 

ms over the line length. Much of this drop occurs from VP 234 

to 329 and from VP 432 to the east end of the line. Several arches appear in the reflections, for example, between VP 329 

and 386, but their significance is unclear.

The only feature on profile S-3 that can be confidently interpreted as a fault is the offset near VP 488. The brute- stack 

section shows the fault more clearly than does the final- stack 

section. A normal offset of about 20 ms in the K and Pz reflections appears to carry into the Eocene section with similar 

amplitude. Thus, the age of movement would be post-middle 

Eocene.

PROFILE S-10

Tying with the east end of profile S-3, profile S-10 (fig. 10) 

runs in a south-southeasterly direction along the seismic zone 

(fig. 2). At VP 411, the line passes near the well used for stratigraphic correlation (Crone and Russ, 1979).

The K and Pz reflections are smooth and show regional apparent dip to the south with a total drop of about 90 ms. This 

drop occurs in an irregular manner. Between VP 340 and 452 

it is negligible. Near VP 452 the drop is about 40 ms. The 

features near VP 245 and near VP 228 that have offsets of 15 

ms in all reflections are interpreted to be processing effects 

associated with a line bend. The discontinuities and diffractions near the south end of profile S-10 may be the result of 

pre-Late Cretaceous igneous intrusive activity.

SUMMARY

The profiles in the northern part of the study area (S-1, 2, 

3, 10, 11, and 12) reveal only a few features that are clearly of 

structural significance.

The best example of faulting in the area is seen on profile 

S-2 at the point where the northeast-striking seismic zone 

crosses the profile (fig. 8, VP 370-378). To the northeast, 

where the seismic zone crosses profile S-1, there is a flexure 

that could be a fault (fig. 7, VP 278-290) with the same sense 

of offset as that observed on profile S-2. Farther northeast 

along the zone, there is a similar feature with the same sense of 

offset on profile S-ll (fig. 6, VP 164-160). Thus, using the 

seismic zone as a guide, one can identify similar features that 

suggest the presence of a through-going fault. A fault-plane
66 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Because line S-5 crosses the middle of the embayment, the 

reflections show little regional dip across the profile. The reflections show a broad arch, which may be quite significant, as 

will be discussed later in this section. The arch can best be seen 

in the K reflection, which rises from 0.68 s beneath VP 755 to 

less than 0.60 s at VP 265, only to fall again to 0.67 s beneath 

VP5.

Several features on profile S-5 that at first glance appear to 

be faults may in fact be data-processing effects. The offset in 

the K reflection between VP 755 and 763 at the west end and 

the offsets at four places east of VP 79 near the east end are 

associated with relatively large residual static corrections, 

which have a large reversal in sign between two depth points. 

It is thought, therefore, that the features resulted from the 

residual-statics processing routine "skipping a leg" in the correlation process.

At VP 219 a trough appears in the K and shallower reflections. The Pz reflection shows similar structure, but owing to 

its discontinuous nature, this is not entirely clear. Note the 

character of the Pz reflection between the trough and VP 187, 

because this feature may correlate with features on profiles S-4 

and S-6. An arch in the reflections between VP 133 and 105 

also may correlate with features on the other lines.

Below the Pz reflection near the eastern end of profile S-5 

(east of VP 105), there are numerous short, strong reflections 

and many lower amplitude reflections. A gentle westerly dip of 

strata is indicated by the reflections east of about VP 87, but 

flattening and a possible reversal appears farther west. This 

evidence for the orientation of Paleozoic sirata is seen more 

clearly on profiles D-l and D-3.

Interpretation of the features on profile S-5 will follow the 

descriptive sections on profiles S-4 and S-6.

PROFILE S-4

Running in a southerly direction from west of Reelfoot 

Lake, profile S-4 (fig. 12) crosses the middle of profile S-5, 

passes through Ridgely, and ties with the east end of profile 

S-6 (fig. 2). It passes through and along the eastern margin of 

the seismic zone.

Overall, the reflections show regional dip to the south 

amounting to about 90 ms between the ends of the profile. 

The Pa. K, and Pz reflections are clear and fairly continuous 

at the north end of the line. South of about VP 217, the Pz 

reflection becomes discontinuous and the Paleozoic section 

shows many diffractions. South of VP 237, the Pa, K, and 

possibly the Pz reflections dip into a trough at about VP 243. 

From VP 243 to 393, all reflections form an arch that crests at 

about VP 329. What appears to be the Pz reflection is relatively tightly arched and the Upper Cretaceous section is substantially thinned. The Paleocene section is only slightly thinned.

Centered at VP 395, an arched reflection, or diffraction, is 

seen slightly below and tangent to the K reflection. Between 

VP 393 and 413 the reflections rise about 80 ms. The data gap

North S-3 228 245

co

0

E

FIGURE 10. Profile S-10. K, Tertiary-Upper Cretaceous contact; Pz, Upper 

Cretaceous-Paleozoic contact. Vibration point (VP) number is shown 

on the horizontal scale; the vibration point interval is 30.5 m. Arrows 

indicate where profile S-3 crosses and where profile S-10 passes the 

research well.

near this feature is at the town of Ridgely. Between VP 545 

and 609, the Pz reflection is again strong, but between VP 609 

and 680 the reflection is disrupted.

Interpretation of profile S-4 will follow the description of 

profile S-6.

PROFILE S-6

Profile S-6 (fig. 13) bridges the interval between the Marked Tree-Caruthersville seismic zone and the zone centered on 

Ridgely. It extends from the Mississippi River near Caruthersville in an easterly direction to tie with profile S-4 (fig. 2).

The K reflection is clear across the whole line with one major disruption. The Pz reflection is weak and disrupted from 

the west end of the line to about VP 536. Diffractions are 

clearly seen below the Pz surface. The K and Pz reflections 

gently rise from the west end of the line to VP 416, where they 

reverse their dip and form a trough at VP 400. From there to 

VP 249 they form an arch that crests in the vicinity of VP 324. 

Over the arch the Upper Cretaceous section is substantially 

thinned. The Pa reflection is slightly arched and the 

Paleocene section is somewhat thinned. A reflection is seen 

about 50 ms below the Pz reflection between VP 364 and 300 

under the crest of the arch.

Bewteen VP 249 and 236 the Pz reflection and all reflections 

above it are offset; an enlarged view of this portion of profile 

S-6 is shown on figure 14. This is the most significant zone of 

post-Paleozoic offset seen on any of the profiles. Numerous apparent diffractions are seen in the zone. Also shown in figure
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 67

Profile S-10

300 340 400 jWell South

FIGURE 10. Continued

14 is an interpretation of the zone of offset. The primary 

feature is a high-angle reverse fault with about 80 ms of 

displacement, east side up. The nature of the faulting in the 

zone is not entirely clear and there is some uncertainty in this 

interpretation. Migration of the part of profile S-6 centered 

on the Cottonwood Grove fault was carried out without significant improvement in record clarity (J. J. Miller, written commun., 1979).

The thinning of the Paleocene and Upper Cretaceous sections seen across the arch west of the fault zone continues east 

of the zone. At about VP 221 (fig. 13) the Pz reflection curves 

downward in a possible diffraction, and overlying reflections 

indicate subtle offsets. Between VP 216 and 160 the Pz reflection is highly disrupted, with one slight disruption of the K 

reflection. Another zone of arched Pz and K reflections with 

associated thinning of the Upper Cretaceous section is 

centered at VP 156.

INTERPRETATION

Several features seen on profiles S-4 and S-6 are quite 

similar and are probably associated with the same structures 

(fig. 15). These correlations are also discussed by Zoback and 

others (1980).

The offset of 80 ms on profile S-4 (fig. 12) at VP 405 and the 

one of 80 ms on profile S-6 (figs. 13, 14) at VP 249 are unique 

in that no other offsets of similar size and character are seen on 

any of the sections. The feature delineated by connecting 

them is interpreted to be a reverse fault with about 80 m of offset, northwest side down, which strikes N. 40° E. The nature 

of the offsets shows that most of the movement occurred during post-middle Eocene time. The name Cottonwood Grove 

fault has been given to the structure because it passes under 

the community of that name on profile S-6, where the offset is 

best seen.

The Cottonwood Grove fault appears to cross profile S-5 at 

about VP 215, where a trough is seen in the reflections. The 80 

ms of offset seen on profiles S-4 and S-6 is certainly not in 

evidence on profile S-5, but similarities in reflection character 

occur between VP 215 and 183 on profile S-5 (fig. 11) and between VP 249 and 216 on profile S-6 (fig. 13), including 

similar westward thinning of the Upper Cretaceous section.

The arch centered at VP 324 west of the fault on profile S-6 

and the arch centered at VP 329 north of the fault on profile 

S-4 are also unique features that appear to be correlative. 

They too define a northeast-striking structure. The trend 

when projected to profile S-5 intersects the section near the 

arch centered at VP 265. This arch is not as distinctive as the 

ones on profiles S-4 and S-6, but it does show slight thinning 

of the Upper Cretaceous section. Correlation with the other 

arches, therefore, seems reasonable.

The arch on profile S-6 between VP 168 and 148 appears to 

have a counterpart on profile S-5 between VP 133 and 105. It 

could represent a horst block. Further correlation suggests 

that the troughs at VP 400 on profile S-6, VP 243 on profile 

S-4, and VP 357 on profile S-5 may be associated.

To recapitulate, correlation among profiles S-4, 5, and 6 

has delineated the northeast-striking Cottonwood Grove fault, 

which is associated with a similarly striking arch located northwest of it (fig. 15). Movement on this fault clearly occurred 

after middle Eocene time. That most of it occurred after the
68 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Profile S-5

FIGURE 11.- Profile S-5. Pa, Paleocene; K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point (VP) 

number is shown on the horizontal scale; the vibration point interval is 30.5 m. Arrow indicates where profile S-4 crosses.

arch was formed is indicated by northwestward thinning of the 

Upper Cretaceous section on the southeast side of the fault. It 

appears that the arch originally spanned the zone of the fault. 

The broad arch seen on profile S-5 apparently is one that did 

not collapse to the same extent as those seen on profiles S-4 

and S-6.

Determination of the time of arching is linked with assumptions about its origin. One possibility is that the arch was caused by topographic relief on the Paleozoic surface during postPaleozoic deposition. The argument would be that when Late 

Cretaceous sedimentation began, a ridge existed on the 

Paleozoic surface. The resulting thinner section of sediments 

over the arch would have been less compacted than the thicker 

section on the flanks, which resulted in the arching of the Tertiary strata. This idea is not favored because the Pz arch seen 

on S-4 and S-6 is unique on the profiles; many topographic 

features should be seen in the 250 km of profiles if significant 

relief on the Paleozoic surface existed at the time of renewed 

deposition.

The preferred explanation for the arching is that an igneous 

body intruded the Paleozoic section after middle Eocene 

deposition. Previous evidence of post-Late Cretaceous intrusive activity in the northern Mississippi Embayment has 

been found at the Newport pluton in central Arkansas along 

the western edge of the embayment (Click, this volume). A sill 

of possible Tertiary age has also been reported in a well located 

7 km northwest of New Madrid (E. B. Kisvarsanyi, Missouri 

Geological Survey, written commun., 1979). The presence of 

Tertiary intrusive masses in nearby areas of the northern embayment lends support to the hypothesis that the arch is the 

product of warping caused by igneous intrusions of Tertiary

age. Gravity data provide additional backing for the existence 

of the intrusions because the location of the inferred intrusive 

body coincides with a local gravity high (fig. 15). Also note in 

figure 15 that the upthrown block southeast of the Cottonwood Grove fault approximately coincides with Ridgely 

Ridge. The Ridgely fault (Zoback, 1979) bounds the southeast 

margin of the ridge.

The greatest disruption of the Pz reflection and the upper 

Paleozoic section occurs on the western part of profiles S-5 and 

S-6. Recall that the greatest disruption in the northern area is 

on the eastern part of profiles S-ll and S-l and on the southeastern part of profile S-10. In both sets of profiles the more 

intense disruption is nearer the river and in the vicinity of the 

seismic zone. As mentioned before, the disruption is interpreted to be evidence of igneous intrusive activity.

CARUTHERSVILLE AREA PROFILES

PROFILE S-7

Profile S-7 (fig. 16) extends west from near Caruthersville 

across the northern end of the Marked Tree-Caruthersville 

seismic zone (fig. 2).

The profile shows remarkably smooth and continuous reflections. In the western half of the profile, the reflections dip 

uniformly to the east. At VP 346 there is a V-like feature and 

possible diffractions. Farther east the reflections are virtually 

flat lying to about VP 148, where they dip east to the end of the 

line. The brute-stack sections show a sharp offset of 20 ms between VP 137 and 133.

The V-like feature at VP 346 is interpreted to be a fault at 

the hinge line between two tilted blocks of rock. The hinge line
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 69

389 Profile

FIGURE 11. Continued

coincides with the northeastern margin of the Marked TreeCaruthersville seismic zone. The feature at the east end of profile S-7 can be interpreted as a fault, but it may instead be a 

processing effect.

PROFILE S-13

Extending south from profile S-7 near Caruthersville, profile S-13 (fig. 17) lies at the northern end of the Marked TreeCaruthersville seismic zone (fig. 2). It was added to the 

original profiling program in an effort to locate the southwestward extension of the Cottonwood Grove fault.

The character of profile S-13 is different from that of the 

other S lines because the vibrator sweep ranged only up to 50 

Hz instead of to 80 Hz. The profile shows many gaps in the 

shallow data because many vibration points were skipped owing to the large number of buildings along the road. Nevertheless, the K and Pz reflections are strong. The K reflection is 

almost flat across the profile; the Pz reflection dips to tne south 

with a 50 ms net drop.

Structurally, profile S-13 is virtually featureless and certainly shows no evidence for the continuation of the Cottonwood 

Grove fault. It must be concluded that the fault either dies out 

northeast of the line or misses it (fig. 2).

PROFILE S-8

Profile S-8 (fig. 18) runs near the east shore of the Mississippi River from 4 km south of profile S-13 in a southerly direction to near the east end of profile S-9 (fig. 2). Profile S-8 

crosses the inferred location of an east-southeast-trending 

fault (Heyl and McKeown, 1978; Buschbach, 1978, fig. A-l).

133 105 100 79 East

FIGURE 11. Continued

Both the K and Pz reflections are strong and show a broad, 

shallow basin across the profile that reaches its low point at 

about VP 287. A few discontinuities and short undulations are 

seen in the Pz reflection. At VP 89 a tight undulation is seen 

with some possible diffracted energy below the Pz reflection. 

Above this point, between VP 81 and 85, the reflections in the 

Eocene section are disrupted. The sharpest discontinuity in 

profile S-8 lies at VP 137, where a 20-ms offset in the Pz 

reflection occurs. The overlying K reflection shows an undula-
70 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

tion. Farther south at VP 177, the Pz reflection is offset about 

10 ms; this offset appears to extend toward the surface as a 

change in shape of the K reflection. Disruption of shallower reflections is seen between VP 177 and 173. The thickness of the 

Upper Cretaceous section thins about 10 ms from VP 177 to 

137. Such thinning is similar to that seen southeast of the Cottonwood Grove fault. Association of this feature with that fault 

would require that the fault become more southerly in strike 

south of profile S-6.

North

Between about VP 367 and 443 on profile S-8, a set of reflections immediately below the Pz reflection reveals a basin 

structure. Other short reflections are seen in the lower part of 

the profile over much of its length. Near the south end, VP 523 

to 543, a set of reflections is seen below 1.5 sec that indicates a 

northerly apparent dip of strata. There do not, however, appear to be any discontinuities of sufficient clarity to indicate 

faults within the Paleozoic section.

If the feature at VP 137 on profile S-8 is not the Cottonwood Grove fault, then it is possible that the Cottonwood 

Grove fault and the associated features pass between profiles 

S-13 and S-8; however, this seems unlikely. The K reflection at 

the southern end of profile S-13 is at 0.70 s; at the northern 

end of profile S-8 it is at 0.73 s. Thus it would be difficult to 

accommodate the 80 ms of slip seen on profiles S-6 and S-4, 

and there is hardly room in the interval between profiles S-13 

and S-8 for the fault and associated arch structure.

PROFILE S-9

Profile S-9 (fig. 19) runs westerly from a point across the 

Mississippi River from the southern end of profile S-8 (fig. 2).

The K and Pz reflections are strong and continuous. They 

show a regional dip to the east. Gentle undulations are seen, 

the most pronounced of which is centered between VP 207 and 

230. No discontinuities are seen.

Profiles S-9 and S-8 were sited to look for the possible 

northeastward continuation of an interpreted fault on a proprietary seismic line located southwest of profile S-9. The proprietary data suggested that the fault might continue to the

217 Profile S-4

393 400 413 

\ I I

FIGURE 12.- Profile S-4. Pa, Paleocene; K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point 

(VP) nufnber is shown on the horizontal scale; the vibration point interval is 30.5 m. Arrows indicate where profiles S-5 and S-6 cross.
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 71

northeast but there is no evidence of the fault on either profiles 

S-8 or S-9.

DEEP-RESOLUTION PROFILES

PROFILES D-l AND D-3

Profile D-l (fig. 20) crosses the Marked Tree-Caruthersville 

seismic zone in an east-west direction southwest of Blytheville, 

Ark. (fig. 2). Profile D-3 (fig. 21) runs south from near the 

west end of profile D-l into and possibly across the seismic 

zone.

In contrast with the other profiles, profiles D-l and D-3 

show good reflections below the Pz reflection. The K and Pz 

reflections are strong and fairly continuous.

Structure in the reflections below the Paleozoic surface is 

clearly seen in profile D-1. A strong reflection is observed to 

dip from about 1.7 s at VP 118 to about 2.0 s at VP 8. A 

similar dip is shown by other reflections above and below it, 

particularly those between 1.2 and 2.7 s. East of VP 118, the 

dipping reflections are not clearly seen, although some reflections are seen shallower in the section between VP 199 and 

250. Weak sub horizontal reflections are present between VP 

136 and 250. In the eastern part of profile D-1, a set of nearly 

horizontal reflections is seen that dips slightly to the west. In 

the vicinity of VP 400, a shallow basin appears in the K and Pz 

reflections, and the deeper reflections flatten out. West of VP 

400 they reverse dip, and between about VP 400 and 363 a set 

of easterly dipping reflections is seen. The D-l profile thus 

shows a complex broad arch in Paleozoic rocks that appears to 

be disrupted and faulted.

The zone in the middle of profile D-1, between VP 310 and 

363, shows a high level of incoherent energy and little continuity in reflections. Some of this disruption could be due to 

problems in the static corrections because the right side of the 

disrupted zone occurs where a river is crossed; however, this 

explanation could account for only a small part of the disruption. The shallower reflections show several offsets and disruptions across this zone; the K and Pz reflections drop about 80 

ms between VP 281 on the west and VP 392 the east.

The amount of structural relief on reflections across the 

disrupted zone is uncertain because reflections cannot be correlated across it reliably. The best correlation appears to be 

between the strongest reflections on each side; that is, the ones 

at 1.75 s at VP 105 and 2.0 s at VP 375. Such a correlation implies about 1 km of vertical structural relief.

The west-dipping reflections near the west end of profile 

D-l correlate very well with a set of reflections on profile D-3 

(fig. 21) that show northerly dip. The true dip of these strata is 

about 12° NW., with a strike of N. 45° E. The termination 

points of the reflections, though not sharply defined, also 

define a strike of about N. 45 ° E. True dip cannot be determined for strata on the southeast side of the disrupted zone.

The structure in the disrupted zone is interpreted as faulting 

of arched Paleozoic rocks. The fault zone strikes about N. 45° 

E. (fig. 2), with about 1 km of vertical structural relief. 

Cenozoic and Tertiary strata across the zone show about 80 m 

of cumulative vertical displacement, southeast side down. The 

fault zone coincides with the Marked Tree-Caruthersville 

seismic zone and has a similar strike (fig. 2). Fault-plane solutions (Herrmann, 1979) indicate that movement on this fault

Profile S-4 545

680 South

FIGURE 12. - Continued
72 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

West

Profile S-6

600 536 500 416 400 

I I

364 324

FIGURE 13. Profile S-6. Pa, Paleocene; K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point (VP) 

number is shown on the horizontal scale; the vibration point interval is 30.5 m. Arrow inidcates where profile S-4 crosses.

is predominantly right-lateral, strike-slip, with a component 

of vertical displacement indicating northwest side up.

Correlation of the reflections below the Paleozoic surface 

with lithologic units is uncertain because there are no wells 

near the lines. On the basis of regional well data, Ordovician 

rocks are thought to lie at the Paleozoic subcrop (Caplan, 

1954; Grohskopf, 1955). As the thickness of the CambrianOrdovician rocks is believed to be approximately 4 km 

(Buschbach, 1978; Caplan, 1954), reflections in the lower part 

of the section could be from Precambrian strata. This depth is 

in approximate agreement with the depth to magnetic basement (Hildenbrand and others, this volume). However, Precambrian rifting could have resulted in the deposition of an 

anomalously thick section of Cambrian sediments. The northwest dip calculated for the strata northwest of the zone of 

disruption contrasts with the south to southwesterly dip that 

would be expected from the general structure of the Pascola 

arch situated to the north. Profile D-l crosses the north flank 

and profile D-3 crosses the west flank of a circular magnetic 

anomaly interpreted to be caused by intrusive igneous rocks 

(Hildenbrand and others, this volume); the northwest-dipping 

reflections in the profiles may indicate deformation caused by 

intrusion of the rocks.

The offset in the Pz, K, and other shallow reflections near 

the east end of line D-l at VP 580 may possibly indicate the extension of the Cottonwood Grove fault, although the nature of 

the zone of deformation is somewhat different than that seen 

on lines S-6 and S-4.

PROFILE D-2

Profile D-2 (fig. 22) strikes east-southeast in the area west of 

Dyersburg, Tenn. It passes near a well that reached rocks 

thought to be of Precambrian age at a depth of 2100 m 

(Buschbach, 1978; A. J. Crone, oral commun., 1979), and it 

crosses the southeast boundary of the rift defined by magnetic 

data (fig. 2).

On profile D-2, the Pz reflection is strong, but it is discontinuous. The K reflection on profile D-2 is weak compared 

with that on the other profiles; it cannot be traced continuously across the profile.

The data on profile D-2 are disappointing, perhaps due to 

static problems. There are no particularly interesting features 

in the shallow section and no good reflections below the Pz 

reflection.

NATURE OF GEOLOGIC STRUCTURE

AND FAULTING IN THE REGION

OF THE EMBAYMENT

The main findings from the seismic-reflection profiling are 

(1) a fault zone coincident with the main zone of seismicity and 

the nature of deformation of Paleozoic strata, (2) the Cottonwood Grove fault and evidence for post-middle Eocene movement that appears to be reverse faulting, (3) deformation inferred to be the result of igneous intrusive activity that occur-
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 73

249 236 221 216 200 160 156 East iS-4

FIGURE 13. Continued

red after middle Eocene time but before movement on the 

Cottonwood Grove fault, (4) disrupted areas of Paleozoic rocks 

that indicate igneous intrusive activity, and (5) a possible fault 

associated with the northeast-striking seismic zone northwest 

of New Madrid.

The seismic-reflection profiles provide new evidence of the 

nature of faulting in the northern Mississippi Embayment. A 

major question is the relationship between faults in the embayment and those in nearby regions. This relationship is of particular importance with regard to seismic-risk assessment, 

wherein the continuity and length of fault systems are critical 

elements. It has been suggested that the fault systems in the 

northern Mississippi Embayment, the Illinois-Kentucky 

mineral district, and the Wabash valley are continuous (fig. 1; 

Woollard, 1958; Heyl and Brock, 1961; Heyl and others, 

1965). Therefore, a brief discussion of those fault systems and 

others in the region is relevant here as a basis for comparison 

with the faults of the embayment.

ILLINOIS-KENTUCKY MINERAL DISTRICT

The larger structure of the mineral district is an arch that is 

about 45 km wide and has about 600 m of structural relief. 

Superimposed on it is Hicks Dome, a cryptovolcanic structure 

about 15 km in diameter that has 1200 m of structural relief 

(Hook, 1974).

The main fault system of the mineral district can be traced 

northeasterly from the edge of the Mississippi Embayment to 

the southern margin of the Shawneetown-Rough Creek fault

zone (fig. 1), but not across it. The mineral district faults are 

predominantly high-angle normal faults that form a complex 

horst-and-graben system. Vertical offsets are commonly 

160-460 m, with a maximum of 900 m. Evidence exsists for 

horizontal movement as well, but it is considered to be less 

than vertical movement (Hook, 1974; Trace, 1974). Overall, 

the mineral district is a regime of extensional tectonics.

The northeast-trending faults crosscut and postdate a 

system of northwest-trending faults that are filled in places by 

mafic dikes, which are Early Permian in age (Zartman and 

others, 1967). Thus the northeast-trending faulting is postEarly Permian. The lower limit on the age of the faulting can 

be placed as pre-Late Cretaceous, although some movement 

continued through the Cretaceous and Tertiary (Trace, 

1974).

COTTAGE GROVE-SHAWNEETOWN-ROUGH 

CREEK FAULT ZONE

The east-west-striking fault complex that lies north of the 

mineral district exhibits a variety of faulting styles including 

high-angle reverse, block, and scissors faulting. These styles of 

faulting and the presence of many sliver-shaped grabens are 

the basis for considering strike-slip or wrench faulting to be the 

predominant mechanism of displacement (Clark and Royds, 

1948). Fault movements occurred in Middle Cambrian and 

Devonian time with net vertical displacement down to the 

south, but the predominant vertical displacement occurred, in 

a reverse sense, after Pennsylvanian and probably after Early
74 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

West

.» »*ft PP'PTC . »< > *\ > X' <' " » ^ I I Vt 1 i

FIGURE 14. Enlargement of part of profile S-6 showing the Cottonwood Grove fault. Lines show interpreted reverse faults; arrows indicate direction of 

movement. The seismic-reflection line crosses the fault zone at about a 45° angle; thus reflections may be coming from both the upthrown and 

downthrown strata where the line crosses the fault zone.

Permian time (Krausse and Treworgy, 1979). The Cottage 

Grove-Shawneetown- Rough Creek fault zone may be part of 

an even larger feature called the 38th Parallel Lineament, 

which is thought to have a late Precambrian origin (Heyl, 

1972). On the basis of the pattern of subsidiary faults and 

folds, movement is right-lateral on the Cottage Grove fault 

and left-lateral on the Rough Creek fault; the Shawneetown 

fault shows a strong component of reverse or thrust movement 

(Heyl and others, 1965; Heyl, 1972).

WABASH VALLEY FAULT ZONE

The Cottage Grove-Shawneetown-Rough Creek fault zone 

apparently marks a discontinuity between the fault systems of 

the mineral district and the Wabash valley because faults cannot be traced across the zone and their strike changes significantly (Heyl and McKeown, 1978). The fault system of the 

Wabash valley is characterized by generally north-northeaststriking, high- angle normal faults that bound horsts and 

grabens. Individual fault displacements reach 146 m. Arcuate 

and overlapping fault segments are common. Pleistocene 

glacial deposits that rest on Pennsylvanian rocks in the region 

are not faulted. Faulting is believed to extend downward at 

least to basement at about 4 km in depth (Bristol and 

Treworgy, 1979).

Thus, although the style and age of faulting in the Wabash 

valley fault zone is similar to that of the faulting in the mineral 

district, the break in strike and continuity of faulting across 

the Shawneetown-Rough Creek fault zone suggest a zone of 

detachment between the systems.
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 75

FIGURE 15. Spatial relationship of faults and of an igneous intrusive mass 

(as inferred from seismic-reflection profiles) to topographic ridges (stippled areas), and Bouguer gravity anomalies (contour lines, contour interval 1 milligal, modified from Stearns, 1979) in the Reelfoot Lake, 

Tenn., area.

STE. GENEVIEVE FAULT ZONE

The Ste. Genevieve fault zone is a northwest-striking system 

of high-angle normal and reverse faults with the southwestern 

block upthrown. Maximum displacement is 900 m. Movement 

occurred in post-Devonian and in post-Mississippian or postPennsylvanian time (McCracken, 1971). Strong gravity and 

magnetic anomalies apparently associated with the Ste. 

Genevieve fault zone extend southeasterly into Kentucky 

(Lidiak and Zietz, 1976; Hildenbrand, this volume).

SUMMARY

An important episode of faulting occurred in the region 

north of the Mississippi Embayment after the Pennsylvanian. 

Estimates of the minimum age of this faulting are uncertain, 

but in some places a pre-Cretaceous age can be established. 

Some of the fault systems were active in the early and middle 

Paleozoic as well. In some areas, doming and intrusive activity 

are associated with the faulting. A Permian age has been 

assigned to some intrusive rocks in the mineral district (Zartman and others, 1976). Formation of the Pascola arch probably occurred in Late Pennsylvanian or Permian time

(Buschbach and Atherton, 1979; Sloss, 1979). These various 

happenings suggest that a major episode of tectonic activity 

occurred near the end of the Paleozoic, possibly in the Permian. This tectonic activity may have been associated with the 

Allegheny and Ouachita orogenies. Extensional deformation 

occurred on many of the fault systems, except along the Cottage Grove-Shawneetown-rough Creek fault system, where 

wrench and thrust faulting occurred.

TECTONIC IMPLICATIONS

Formation of the arched and faulted structure seen on profiles D-l and D-3 can be dated from the seismic-reflection 

data as post-Ordovician and pre-Late Cretaceous, although 

some minor movement occurred later. Considering the 

geologic history of the region, a likely time for formation of the 

structure is late Paleozoic, possibly Permian, perhaps 

associated with development of the Pascola Arch. Because the 

fault zone lies along the rift axis, which is thought to have 

formed in Early Cambrian or late Precambrian time (Hildenbrand, this volume), its origin is probably the result of reactivation of rifting. Modern movement on the fault, however, is 

predominantly right-lateral, strike-slip, with a vertical component of southeast side down, as indicated by fault-plane solutions (Herrmann, 1979). This indicates an east-west compressional stress regime. The fault zone may have been the source 

for two of the 1811 and 1812 earthquakes, as the estimated 

epicenters for two of the three largest shocks are near the fault 

(Nuttli, 1973).

The northwest dip determined for Paleozoic and possibly 

Precambrian strata northwest of the fault zone adds new information on regional structure. As the area lies on the south 

flank of the Pascola arch, a south or southwest dip would be 

expected. The structure seen on profiles D-l and D-3 may be 

the result of post-Ordovician igneous intrusive activity, as suggested by its coincidence with a magnetic anomaly.

The style of faulting associated with the Cottonwood Grove 

fault is particularly significant if the interpretation of reverse 

movement is correct. Such movement is characteristic of compressional tectonics. The age of the movement is post-middle 

Eocene. Composite-fault-plane solutions indicate that the 

area is currently under east-west compression (O'Connell and 

others, this volume), as is the entire region (Zoback and 

Zoback, 1980). Thus, movement on the Cottonwood Grove 

fault is consistent with the modern east-west compressional 

stress field causing the current seismicity rather than with the 

extensional stress regime that prevailed in the Paleozoic. The 

normal fault on profile S-2 indicates an extensional stress 

regime in the Late Cretaceous as well.

The evidence for disruption of Paleozoic rocks seen on the 

seismic profiles, which is interpreted as the result of igneous intrusive activity, occurs mostly in the vicinity of the northnorthwest-striking seismic zone in the Ridgely-New Madrid 

area. This area also is the location of the inferred intrusive
76

West

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Profile S-7

FIGURE 16. Profile S-7. K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point (VP) number is shown 

on the horizontal scale; the vibration point interval is 30.5 m. Arrow indicates where profile S-13 crosses.

North | S-7

Profile S-13

W

C SSSSCSS^^^

SSfefflBC&asSSSS^SSSgS^^ _ 

MgtSSKSSSE&SgSKSSi&S^^

FIGURE 17. Profile S-13. K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. 

Vibration point (VP) number is shown on the horizontal scale; the vibration point interval is 30.5 m. Arrow indicates where profile S-7 crosses.

body which was delineated on the basis of overlying arched 

strata near the Cottonwood Grove fault. In addition, 

topographic domes and ridges in the Ridgely-New Madrid 

area, which apparently experienced uplift, in part, during the 

1811-12 earthquakes, correlate with structure on the 

Paleozoic surface (Crone and Brockman, this volume).

On the basis of a fault-plane solution (Herrmann, 1979), 

the faulting that correlates with the northeast-striking seismic 

zone northwest of New Madrid is right-lateral, strike-slip.

The strikes of fault zones in the northern Mississippi Embayment region are similar to those of the fault system in the 

Illinois-Kentucky mineral district. Evidence of post-Late Cre-
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 77

S 1 3 i 200

137 the embayment in some respects, but differences in strike and 

lack of fault continuity argue against structural continuity 

with those fault systems.

These various results provide a partial picture of the sequence of tectonic events that are the underlying cause of New 

Madrid seismicity. In the late Precambrian or Early Cambrian, a large-scale rift formed along the axis of what later 

became the Mississippi Embayment. Through the Paleozoic 

some of the fault systems in the region were active, but the 

main episode of deformation and fault activity came in the 

late Paleozoic. This was an episode of mostly extensional 

faulting on fault systems north of the embayment. During the 

Late Cretaceous and early Tertiary, little deformation occurred. Faulting after the middle Tertiary and the nature of 

modern seismicity reflect a different stress regime than that of 

the Paleozoic. Thus, modern seismicity is seen as the result of 

east-west compression of fault zones and zones weakened by intrusive activity that originally formed as a result of rifting.

FIGURE 16. Continued

South

lf?^Silw«S!^^

K 

Rz

FIGURE 17. Continued

taceous faulting in western Kentucky exists, but its nature is 

unknown (Rhoades and Mistier, 1941). The idea of continuity 

between the fault systems of the two regions cannot be ruled 

out, although magnetic data suggest that the rift zone terminates in western Kentucky and southern Illinois (Hildenbrand 

and others, this volume). The fault systems of the Wabash 

valley and the Ste. Genevieve fault zone are similar to those of

ACKNOWLEDGMENTS

We greatly benefited from discussions with USGS colleagues 

A. J. Crone, E. E. Click, T. G. Hildenbrand, F. A. McKeown, 

and D. P. Russ. Migration of profile S-6 was carried out byj. 

J. Miller and A. H. Balch of the USGS. Partial support for the 

test well was provided by the U.S. Nuclear Regulatory Commission.

REFERENCES CITED

Bristol, H. M., and Treworgy, J. D., 1979, The Wabash Valley fault system 

in southeastern Illinois: Illinois State Geological Survey Circular 509, 

19 p.

Buschbach, T. C., 1978, New Madrid seismotectonic study Activities during fiscal year 1978: U.S. Nuclear Regulatory Commission 

NUREG/CR-0450, 129 p.

Buschbach, T. C., and Atherton, E., 1979, History of the structural uplift of 

the southern margin of the Illinois Basin, in Palmer, J. E., and Butcher, 

R. R., eds., Depositional and structural history of the Pennsylvanian 

system of the Illinois Basin, Part 2: International Congress of Carboniferous Stratigraphy and Geology, 9th, Guidebook for Field Trip No. 9, 

pt. 2, p. 112-115.

Caplan, W. M., 1954, Subsurface geology and related oil and gas 

possibilities of northeastern Arkansas: Arkansas Division of Geology 

Bulletin 20, 124 p.

Clark, S. K., and Royds, J. S., 1948, Structural trends and fault systems in 

Eastern Interior Basin: of the American Association of Petroleum Geologists Bulletin, v. 32, p. 1728-1749.

Clegg, K. E., and Bradbury, J. C. 1956, Igneous intrusive rocks in Illinois 

and their economic significance: Illinois Geological Survey Report of 

Investigations 197, p. 5-19.

Crone, A. J., and Russ, D. P., 1979, Preliminary report on an exploratory 

drill hole New Madrid test well-l-X in southeast Missouri: U.S. Geological Survey Open-File Report 79-1216, 12 p.

Dobrin, M. B., 1976, Introduction to geophysical prospecting: New York, 

McGraw-Hill, 630 p.

Ervin, C. P., and McGinnis, L. D., 1975, Reelfoot rift- Reactivated 

precursor to the Mississippi embayment: Geological Society of America 

Bulletin, v. 86, p. 1287-1295.
78 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

iNorin 81 85 89 100 

\ \ 1 1

137 

1

177 200

J,03 iia 211 Himnain m

FIGURE 18.- Profile S-8. K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. 

Vibration point (VP) number is shown on the horizontal scale; the vibration point interval is 30.5 m.

Grohskopf, J. G., 1955, Subsurface geology of the Mississippi embayment of 

southeast Missouri: Missouri Division of Geological Survey and Water 

Resources Report, ser. 2, v. 37, 133 p.

Hadley, J. B., and Devine, J. F., 1974, Seismotectonic map of the eastern 

United States: U.S. Geological Survey Miscellaneous Field Studies Map 

MF-620.

Hamilton, R. M., andZoback, M. D., 1979, Seismic reflection profiles in the 

northern Mississippi embayment: U.S. Geological Survey Open-File 

Report 79 1688.

Heinrich, R. R., 1941, A contribution to the seismic history of Missouri: 

Seismological Society of America Bulletin, v. 31, p. 187-224.

Herrmann, R. B., 1979, Surface wave focal mechanisms for eastern North 

American earthquakes with tectonic implications: Journal of Geophysical Research, v. 84, p. 3543-3552.

Herrmann, R. B., and Canas, J. A., 1978, Focal mechanism studies in the 

New Madrid seismic zone: Seismological Society of America Bulletin, v. 

68, p. 1095-1102.

Heyl, A. V., 1972, The 38th Parallel Lineament and its relationship to ore 

deposits: Economic Geology, v. 67, p. 879-894.

Heyl, A. V., and Brock, M. R., 1961, Structural framework of the IllinoisKentucky mining district and its relation to mining deposits, in Short 

papers in the geologic and hydrologic sciences: U.S. Geological Survey 

Professional Paper 424-D, p. D3 D6.

Heyl, A. V., Brock, M. R., Jolly, J. L., and Wells, C. E., 1965, Regional 

structure of the southeast Missouri and Illinois-Kentucky mineral 

districts: U.S. Geological Survey Bulletin 1202 B, 20 p.

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map of 

central Mississippi Valley and environs: U.S. Geological Survey Miscellaneous Field Studies Map MF-1011.

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic 

and gravity anomalies in the northern Mississippi embayment and their 

spatial relation to seismicity: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-914.

Hildenbrand, T. G., Kucks, R. P., Kane, M. F., andHendricks, J. D., 1979, 

Aeromagnetic map and associated depth map of the upper Mississippi 

embayment region: U.S. Geological Survey Miscellaneous Field Studies 

Map MF 1158.

West 200 207 230

K 

Pz

c 

o 

o

0) 

052

FIGURE 19. Profile S 9. K, Tertiary-Upper Cretaceous contact; 

Pz, Upper Cretaceous-Paleozoic contact Vibration point (VP) 

number is shown on the horizontal scale; the vibration point 

interval is 30.5 m.

Hinze, W. J., Braile, L. W., Keller, G. R., and Lidiak, E. G., 1977, A tectonic overview of the central Midcontinent: U.S. Nuclear Regulatory 

Commission NUREG-0382, 106 p.

Hook, J. W., 1974, Structure of the fault systems in the Illinois-Kentucky 

fluorspar district, in Hutchinson, D. W., ed., A symposium on the 

geology of fluorspar: Proceedings of the Ninth Forum on Geology of Industrial Minerals, Kentucky Geological Survey, Series X, Special Publication 22, p. 77-86.
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES 79

Profile S-8

287 300 

1 I

523 o *u 543 South

FIGURE 18. Continued

Profile S-9

230 300 400 500

1

East

FIGURE 19. Continued

Kidwell, A. L., 1947, Post-Devonian igneous activity in southeastern 

Missouri: Missouri Division of Geological Survey and Water Resources 

Report of Investigations, v. 4, 83 p.

_______1950, Mesozoic igneous activity in the northern Gulf Coastal 

plain: Gulf Coast Association of Geological Societies Transactions, 1st 

Annual Meeting, p. 182 199.

Krausse, H. F., and Treworgy, C. G., 1979, Major structures of the southern 

part of the Illinois Basin, in Palmer, J. E., and Dutcher, R. R., eds., 

Depositional and structural history of the Pennsylvanian system of the

Illinois Basin, Part 2: International Congress of Carboniferous Stratigraphy and Geology, 9th, Guidebook for Field Trip No. 9, pt. 2, p. 

115-120.

Lidiak, E. G., and Zietz, Isidore, 1976, Interpretation of aeromagnetic 

anomalies between latitudes 37 ° N and 38 ° N in the eastern and central 

United States: Geological Society of America Special Paper 167, 37 p.

McCamy, K., andMeyer, R. P., 1966, Crustal results of fixed multiple shots 

in the Mississippi embayment, in Steinhart, J. S., and Smith, T. J. , eds., 

The.e^i/th beneath the continents: American Geophysical Union 

Geophysics Monograph 10, p. 370-381.
80

\West

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Profile D-1

136 

I

FIGURE 20.- Profile D-1. K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point (VP) 

number is shown on the horizontal scale; the vibration point interval is 61 m. Arrow indicates where profile D-3 crosses.

South

Profile D-3

130 D-1 North

CO a

c 1

o

o

0 

CO

o,

K 

Pz

FIGURE 21.- Profile D-3. K, Tertiary-Upper Cretaceous contact; Pz, Upper CretaceousPaleozoic contact. Vibration point (VP) number is shown on the horizontal scale; the 

vibration point interval is 61 m. Arrow indicates where profile D-1 crosses.
TECTONIC FEATURES, NEW MADRID SEISMIC ZONE FROM SEISMIC-REFLECTION PROFILES

Profile D-1

300 310 363 375 392 400 

II II East

®SSS»SS^^

9f^%&»^^^

81

FIGURE 20. Continued

Profile D-2

West 200 300 East

&aeif*K&!ss^'!&!l&wJt*&!&5 !&St

FIGURE 22. - Profile S-2. K, Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact. Vibration point (VP) number is shown on

the horizontal scale; the vibration point interval is 61 m.
82 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

McCracken, M. H., 1971, Structural features of Missouri: Missouri Division 

of Geological Survey and Water Resources Report of Investigations 49, 

99 p.

Miser, H. D., andRoss, C. S., 1925, Volcanic rocks in the Upper Cretaceous 

of southwestern Arkansas and southeastern Oklahoma: American Journal of Science, 5th ser. 5, v. 9, p. 113 126.

Nuttli, O. W., 1973, The Mississippi Valley earthquakes of 1811 and 1812- 

Intensities, ground motion and magnitudes: Seismological Society of 

America Bulletin, v. 63, p. 227-248.

Rhoades, R., and Mistier, A.J., 1941, Post-Appalachian faulting in western 

Kentucky: American Association of Petroleum Geologists Bulletin, v. 

25, p. 2046-2056.

Sheriff, R. E., 1978, A first course in geophysical exploration and interpretation: Boston, MA 02116, International Human Resources Development Corporation, 313 p.

Sloss, L. L., 1979, Plate-tectonic implications of the Pennsylvanian system 

in the Illinois Basin, in Palmer, J. E., and Dutcher, R. R. , eds., Depositional and structural history of the Pennsylvanian system of the Illinois 

Basin, Part 2: International Congress of Carboniferous Stratigraphy 

and Geology, 9th, Guidebook for Field Trip No. 9, p. 107-112.

Stauder, William, Herrmann, R., Singh, S., Perry, R., Haug, E., andMorrissey, S. T., 1979, Central Mississippi Valley earthquake bulletin: 

Saint Louis University Quarterly Report no. 21, 28 p.

Stauder, William, Kramer, M., Fischer, G., Schaefer, S., and Morrissey, S. 

T., 1976, Seismic characteristics of southeast Missouri as indicated by a 

regional telemetered microearthquake array: Seismological Society of 

America Bulletin, v. 66, p. 1953-1964.

Stearns, R. G., 1979, Recent vertical movement of the land surface in the 

Lake County uplift and Reelfoot Lake basin areas, Tennessee, Missouri 

and Kentucky: U.S. Nuclear Regulatory Commission NUREG/CR0874, 37 p.

Trace, R. D., 1974, Illinois-Kentucky fluorspar district, in Hutchinson, D. 

W. , ed., A symposium on the geology of fluorspar: Proceedings of the 

Ninth Forum on Geology of Industrial Minerals, Kentucky Geological 

Survey, Series X, Special Publication 22, p. 58-76.

Waters, K. H., 1978, Reflection seismology: New York, John Wiley, 377 p.

Woollard, G. P., 1958, Areas of tectonic activity in the United States as indicated by earthquake epicenters: EOS (American Geophysical Union 

Transactions), v. 39, p. 1135-1150.

Zartman, R. E., 1977, Geochronology of some alkalic rock provinces in 

eastern and central United States: Annual Review of Earth and 

Planetary Sciences, v. 5, p. 257-286.

Zartman, R. E., Brock, M. R., Heyl, A. V., and Thomas, H. H., 1967, K-Ar 

and Rb-Sr ages of some alkalic intrusive rocks from central and eastern 

United States: American Journal of Science, v. 265, p. 848-870.

Zoback, M. D., 1979, Recurrent faulting in the vicinity of Reelfoot Lake, 

northwest Tennessee: Geological Society of America Bulletin, pt. 1, v. 

90, p. 1019-1024.

Zoback, M. D., Hamilton, R. M., Crone, A. J., Russ, D. P., McKeown, F. 

A., and Brockman, S. R., 1980, Recurrent intraplate tectonism in the 

New Madrid seismic zone: Science, v. 209, p. 971-976.

Zoback, M. L., and Zoback, M. D., 1980, State of stress in the conterminous 

United States: Journal of Geophysical Research, v. 85, p. 6113 6156.
A Crustal Structure Study of the 

Northern Mississippi Embayrnent

By C. B. AUSTIN and G. R. KELLER

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1 2 3 6-G
CONTENTS

Page

Abstract ............................................................................... 83

Introduction............................................................................ 83

Previous work ........................................................................... 84

Data and analysis ........................................................................ 84

Results and discussion .................................................................... 88

Acknowledgements ...................................................................... 92

References cited ......................................................................... 92

ILLUSTRATIONS

Page

FIGURE 1. Index map of study area showing surface- wave dispersion path studied and results of previous refraction studies. ......................... 85

2. Flow chart of digital data analysis used in this study ........................................................................... 86

3. Samples of moving-window analysis applied to sample of cross- correlograms of legs of tripartite between Oxford, Miss., St. Louis Mo., and

Ann Arbor, Mich. ................................................................................................. 88

4. Sample output of the cross-multiplication technique.......................................................................... 89

5. Group-velocity (A) and phase-velocity (B) data gathered in this study ............................................................ 90

6. Comparison of observed to theoretical group (A) and phase (B) velocities for a one-layer model ....................................... 91

7. Generalized crustal cross section along the gravity profile of Ervin and McGinnis ................................................... 92

TABLES

Page

TABLE 1. Epicentral data for earthquakes used as surface-wave sources ................................................................... 87

2. Mississippi Embayment Crustal models ..................................................................................... 89
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

A CRUSTAL STRUCTURE STUDY 

OF THE NORTHERN MISSISSIPPI EMBAYMENT

By C. B. AusTiN1 and G. R. KELLER*

ABSTRACT

Short- to intermediate-period (5-80 s) Rayleigh waves recorded at 

Worldwide Seismograph Station Network stations FLO (near St. Louis, 

Mo.) and OXF (Oxford, Miss.) were analyzed digitally to obtain group- and 

phase- velocity dispersion curves. These data were used to determine the 

crustal and upper mantle shear-velocity structure of the northern Mississippi 

Embayment. The models derived suggest that the crustal thickness near the 

axis of the northern Mississippi Embayment is probably in the range from 50 

to 55 km. An anomalously high velocity layer (VP «"*-" 7.0 km/s, Vs<x>4.0 

km/s) is found in the lowermost crust throughout the region. The crustal 

model derived is in good agreement with previous refraction studies.

The results of this study were integrated with previous work to produce a 

hypothetical crustal cross section for the northern Mississippi Embayment. 

This Earth model is similar to that which would be expected to be associated 

with an ancient rift zone (aulacogen?).

INTRODUCTION

The Mississippi Embayment is a broad structural trough or 

reentrant of the gulf coastal plain which extends into the 

North American craton from the south. This feature, whose 

axis generally parallels the course of the Mississippi River, 

plunges toward the Gulf of Mexico where Cretaceous and Tertiary sediments reach a maximum thickness of 5.5 km 

(Gushing and others, 1964). A popular theory for the origin of 

the embayment is that it represents a reactivated late Precambrian aulacogen (Ervin and McGinnis, 1975).

'Kidd Seismological Observatory, Department of Geological Sciences; University of Texas at El 

Paso, El Paso, TX 79968.

The presence of the New Madrid seismic zone (Stauder and 

others, 1976) indicates that the northern Mississippi Embayment is tectonically active. However, the forces responsible for 

this activity are poorly understood. Because crustal discontinuities may be related to contemporary tectonism (Hinze and 

others, 1977), a knowledge of crustal structure is important. 

Crustal structure has been of particular recent interest because 

of studies which suggest that the northern embayment may be 

underlain by anomalous crust (McCamy and Meyer, 1966; 

Kurita, 1973; Ervin and McGinnis, 1975; Mitchell and 

Hashim, 1977; and Mitchell and others, 1977).

This study was designed to determine the general crustal 

and upper mantle shear-velocity structure of the northern 

Mississippi Embayment by measuring Rayleigh-wave dispersion between WWSSN (Worldwide Seismograph Station Network) seismograph stations in the area and to integrate these 

results with those of previous studies in the area. Tectonic implications of variations in crustal structure are considered, as 

well as rift models which have been proposed for the development of the embayment (Burke and Dewey, 1973; Ervin and 

McGinnis, 1975; Hildenbrand and others, 1978).

Burke and Dewey (1973) originally suggested that the 

present-day Mississipppi Embayment was a Mesozoic 

aulacogen (failed rift) originating from a triple junction near 

Jackson, Miss. Ervin and McGinnis (1975), using gravity data 

tied to the refraction line of McCamy and Meyer (1966), examined this model and concluded that the current embayment is the result of readjustment of the Earth's crust along 

zones of weakness created by a Precambrian aulacogen 

(Reelfoot Rift). They suggested that the Reelfoot Rift is of the 

same age and origin as the southern Oklahoma aulacogen

83
84 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

(Burke and Dewey, 1973; Hoffman and others, 1974) and that 

both of these features formed as a Precambrian land mass 

broke up prior to the formation of the Appalachian- Ouachita 

mountain belt.

By the Late Cretaceous, the present Atlantic Ocean basin 

was forming and subsidence of the gulf coastal plain created 

tensional forces that initiated reactivation along the ancient 

rift zone (Ervin and McGinnis, 1975). Intrusion of syenite and 

lamprophyre along the axis of the Mississippi Embayment 

(Bond and others, 1971) marked the initial stage of reactivation and may have initiated isostatic adjustments that produced the present-day embayment (McGinnis, 1970). Although 

geologic evidence points to a stable embayment since Neogene 

time (Stearns and Wilson, 1972), both the extent of historical 

seismicity (Nuttli, 1974) and level of recent seismic activity 

(Stauder and others, 1976) indicate contemporary tectonism.

PREVIOUS WORK

The crustal structure of the Central United States seems to 

be relatively uncomplicated (McEvilly, 1964; Stewart, 1968; 

Warren, 1968; Long and Mather, 1972; and others). In 

general, the crust of the midcontinent region can be modeled 

as being composed of two layers overlain by a veneer of 

Paleozoic sediments. The sedimentary layer varies in thickness 

from ^ 5 km in various basins to 0 km on the Ozark uplift and 

has an average compressional velocity of ~ 5.0 km/s. The upper crustal layer (granitic) averages ~ 20 km in thickness and 

has a compressional velocity of 6.0-6.4 km/s. The lower 

crustal layer (gabbroic) is 10-20 km thick and has a compressional velocity of 6.5-6.9 km/s. Thus, the crustal thickness is 

~40 km. Upper mantle velocities (Pn) range from 8.0 to 8.2 

km/s.

However, seismic studies in the vicinity of the embayment 

(fig. 1) suggest the presence of a thick crust that has a basal 

high-velocity ( > 7.0 km/s) layer overlying the Moho. Interpretation of a refraction line from Hercules to Gladden (fig. 1) in 

southern Missouri (Stewart, 1968) indicated a 42-km-thick 

crust containing an 11-km-thick lower layer that has a compressional velocity of 7.3 km/s. However, Stewart (1968) noted 

that it may be inappropriate to regard the crust of southeastern Missouri as being simply layered due to pronounced 

lateral changes in the velocity structure. The refraction line of 

McCamy and Meyer (1966) from Cape Giradeau, Mo., to Little Rock, Ark., approached much nearer the axis of the embayment and the New Madrid seismic zone (fig. 1). This study 

reversed the earlier line of Steinhart and Meyer (1961) and, 

thus, is the best control for deep crustal structure in this area. 

McCamy and Meyer (1966) found a 45-km-thick crust containing a basal layer 15 km thick that has a compressional 

velocity of 7.4km/s. The upper mantle velocity they determined was 8.1 km/s.

Kurita (1973), modeling long-period P-wave spectra 

beneath the seismograph station FLO near St. Louis, Mo.,

found the crust to be about 57 km thick and to have a mafic 

basal crustal layer of compressional velocity 7.4 km/s. He also 

found an upper mantle velocity of approximately 8.1 km/s. 

Kurita (1973) suggested that a complex crustal tructure existed beneath station FLO.

More recent studies utilizing data from the Saint Louis 

University microearthquake array (Stauder and others, 1976) 

have also suggested the presence of a basal crustal layer having 

velocities greater than 7.0 km/s in the area of the New Madrid 

seismic zone. Mitchell and Hashim (1977) derived an Earth 

model from travel times for local earthquakes. Their model 

consisted of a 5.6-km-thick layer that had a compressional 

velocity of 6.19 km/s overlying an 18.1-km-thick layer that 

had a compressional velocity of 6.38 km/s. These layers were 

underlain by a basal crustal layer that had a compressional 

velocity of 7.17 km/s.

Mitchell and others (1977), using travel-time residuals of 

teleseismic P- waves, produced a three-dimensional crustupper mantle model of the New Madrid seismic zone. They 

proposed the existence of a region of low velocities extending 

to a depth of at least 150 km and dipping westward beneath 

the most active region of the seismic zone. They suggested that 

the New Madrid seismic zone may be underlain by a mantle 

hot spot such as that proposed for Yellowstone, Wyo.

Ervin and McGinnis (1975) analyzed and modeled the 

positive gravity anomaly associated with the embayment as being due to a thick lower crustal layer of high density. In a 

similar study, Cordell (1977) corrected for the low-density 

sediments of the embayment and found a continuous, longwavelength, positive anomaly with amplitude (15-45 mgals) 

increasing southward along the embayment's axis.

Hildenbrand and others (1978) interpreted the extensive recent aeromagnetic and gravity surveys in the northern 

Mississippi Embayment and delineated several prominent 

anomalies that appear to reflect the presence of major geological or tectonic features. A narrow, linear, northeast-trending 

zone of subdued magnetic anomalies was found which indicates a downfaulted block (graben?) in the basement.

DATA AND ANALYSIS

Analysis of Rayleigh-wave dispersion is a powerful tool for 

determining Earth structure through modeling in which theoretically derived values are compared with those observed. 

The recent articles by Dziewonski and Hales (1972) and 

Kovach (1978) provide good reviews of the use of Rayleigh 

waves in crustal studies. Rayleigh-wave dispersion is most sensitive to variations in shear-wave velocity (Der and others, 

1970). Because shear velocities are difficult to obtain from 

refraction profiling, Rayleigh-wave data provide complementary information to the compressional velocities derived from 

other studies and provide data needed for determination of 

Poisson's ratios. These ratios have considerable tectonic significance, especially considering recent studies of dilatancy and
A CRUSTAL STRUCTURE STUDY OF THE NORTHERN MISSISSIPPI EMBAYMENT 85

St Joseph

Sea Level

£ 10

Kansas City 

Kansas City ] Independence

KANSAS

OKLAHOMA

SCALE 

50

i' i I i '| i I -

50 100 KILOMETERS

_ Campbell______________ __ _

ALABAMA

FIGURE 1. Index map of study area showing surface-wave dispersion path studied (screened pattern) and results of previous refraction studies (fence 

diagrams). The fence diagrams show crustal thickness, P-wave velocities, and the M-discontinuity. St. Joseph to Hannibal and Hercules to Ste. 

Genevieve from Stewart (1968). Little Rock to Cape Girardeau from McCamy and Meyer (1966). Campbell to Chattanooga from Warren (1968). 

Results of Warren and others (1968) indicate a complex crustal structure in central and southern Mississippi (near and south of the Ouachita Front) and 

that is not shown.

earthquake prediction. Furthermore, a low-velocity zone such 

as that proposed for the New Madrid seismic zone area (Mitchell and others, 1977) may be more pronounced for shear 

waves than for compressional waves.

However, it is important to remember that Rayleigh-wavedispersion studies produce Earth models that represent the 

gross Earth structure averaged over the path of propagation. 

Therefore, to obtain geologically meaningful results, propagation paths confined to a single geological province must 

be studied. Modern digital techniques allow this to be done 

with relative ease, and the resolution obtainable with this 

method, combined with the low cost of such studies, makes it 

an ideal technique for determining shear-velocity crustal 

structure.

In this study, modern digital techniques were used to determine crustal and upper mantle structure along one leg of a tripartite array of stations (Press, 1956; Knopoff and others, 

1966). The method of analysis is illustrated in graphical form 

by the flow chart shown in figure 2.

The propagation path studied (fig. 1) traversed the area 

between the stations OXF (Oxford, Miss.) and FLO (near St. 

Louis, Mo.). This path was of interest because it crosses the

New Madrid seismic zone and the Mississippi Embayment. A 

third station (AAM, Ann Arbor, Mich.) was used to perform a 

tripartite solution to check for horizontal refractions which 

could be produced by major discontinuities in crustal structure.

The data for this study consist of long-period verticalcomponent seismograms from five earthquakes (table 1) 

selected on the basis of their epicenters lying very near the 

great-circle path joining the staions OXF and FLO. The 

earthquakes were selected from the earthquake data file of the 

Environmental Data Service, National Oceanic and Atmospheric Administration, at Boulder, Colo.; and records for 

each event were handpicked to insure quality data for each 

station. The paper seismograms were digitized to provide data 

at a rate of two samples per second.

In order to determine the interstation group velocities, a 

cross- correlation was performed for the paths OXF-FLO and 

OXF-AAM (Dziewonski and Hales, 1972). A time-variable, 

bandpass filter (Pilant and Knopoff, 1964; Dziewonski and 

Hales, 1972) was applied to the cross-correlograms to separate 

modes and smooth the data.

The moving-window technique of Landisman and others
86 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

DETERMINE GEOMETRY OF TRIPARTITE

DETERMINE DESIRED GREAT-CIRCLE AZIMUTH 

THROUGH THE TRIPARTITE

SELECT EARTHQUAKES THAT LIE 

ALONG GREAT-CIRCLE PATH

DETERMINE GREAT-CIRCLE 

AZIMUTH AND DISTANCE

DIGITIZE EARTHQUAKE

DETERMINE GROUP VELOCITY

TO STATION I 

FOR PHASE-VELOCITY INPUT

DETERMINE GROUP VELOCITY

TO STATION 2 

FOR PHASE-VELOCITY INPUT

DETERMINE GROUP VELOCITY

TO STATION 3 

FOR PHASE-VELOCITY INPUT

CROSS - CORRELATION

TO ISOLATE DISPERSION

ALONG TWO-STATION PATH

STATION I - STATION Z 

STATION I- STATION 3

TIME-VARIABLE BAND-PASS 

FILTER IF NEEDED

DETERMINE PHASE VELOCITY

ALONG LEGS OF TRIPARTITE

STATION I - STATION 2

STATION I- STATION 3

MOVING WINDOW ANALYSIS 

TO OBTAIN GROUP VELOCITY 

ALONG LEGS OF TRIPARTITE

TRIPARTITE SOLUTION 

FOR GROUP VELOCITIES

TRIPARTITE SOLUTION 

FOR PHASE VELOCITIES

LEAST-SQUARES FIT 

OF GROUP VELOCITIES

LEAST-SQUARES FIT 

OF PHASE VELOCITIES

SIMULTANEOUS INVERSION

OF GROUP AND PHASE VELOCITIES

TO OBTAIN EARTH MODEL

FIGURE 2. Flow chart of digital data analysis used in this study.
A CRUSTAL STRUCTURE STUDY OF THE NORTHERN MISSISSIPPI EMBAYMENT

TABLE 1. Epicentral data for earthquakes used as surface-wave sources

87

Number

Origin 

Time (GMT) Latitude Longitude Magnitude Station Distance to Azimuth from

Date h min s ("O code epicenter (km) epicenter (deg)

2 04/21/1969 02 19 07.1 14 06.0 91 00.0 5.5 OXF

3 05/01/1970 08 35 24.2 14 36.0 93 12.0 5.4 OXF

4 05/01/1970 20 03 27.9 14 36.0 93 36.0 5.0 OXF

5 05/02/1970 02 06 56.3 14 42.0 93.42 5.4 OXF

6 05/15/1970 09 44 45.2 14 30.0 92 48.0 5.4 OXF

2273.36

FLO

AAM

2244.58

FLO

AAM

2251.71

FLO

AAM

2242.67

FLO

AAM

2249.15

FLO

AAM

4.4147

2745.05 1.4556

3211.3514.8612

10.6754

2703.05 6.6589

3221.5820.1643

11.7793

2707.35 7.5930

3221.5820.1643

12.1050

2697.47 7.8525

3213.7220.4116

9.5246

2710.39 5.7014

3210.5318.5643

(1969) was applied to the filtered cross-correlograms to determine group velocities along the legs of the tripartite. This 

technique involves a cos2 window function that moves down 

the digitized time trace centering on each velocity of interest. 

The width of the window is a function of the period being examined, and for this study, a window factor of 3.0 was used. 

At each time, corresponding to a step in group velocity, the 

windowed data are Fourier-analyzed at the period of interest. 

This process is repeated for each period of interest. The individual Fourier-amplitude values obtained are displayed in 

matrix form as a function of group velocity and period. The 

plotted amplitudes are normalized so that the largest value is 

set to 100 db. The mapped values are then contoured and the 

resulting group-velocity dispersion curve is drawn through the 

contoured data. Figure 3 shows the results of a movingwindow analysis.

Corresponding phase velocities for each leg of the tripartite 

were determined using the cross-multiplication method of 

Bloch and Hales (1968). In this technique seismograms 

recorded at the two stations of interest are windowed by a cos2 

window function, centered at the time of interest determined 

by the group-velocity arrival time. The seismograms are then 

filtered and a windowed section of one seismogram, whose 

length is two thirds the period of interest, is point-by-point 

multiplied by the other trace. The results are summed and 

normalized to obtain an amplitude value for the corresponding period and velocity. When the two traces are in phase, 

the amplitude is a maximum of 99 db. At 180 degrees out of 

phase, a minimum of -99 db is plotted. Results are displayed 

in the same format as group-velocity (fig. 4) determinations.

Using the group and phase velocities calculated between 

station pairs OXF-FLO and OXF-AAM as input, a tripartite 

solution was computed for each period of interest. The angle 

of approach of the wave front and its velocity across the array 

of stations were determined using standard equations (Press, 

1956).

The geometry of the tripartite used in this study was designed so that the tripartite velocities derived represent velocities 

along the leg OXF-FLO. The accuracy of the angles of approach used to calculate these velocities depends on the validity of the assumption that no significant lateral refractions take 

place within the array. Our present knowledge of crustal variations found in the midcontinent outside the embayment suggests that these variations are too small to significantly affect 

this assumption. The tripartite velocity obtained for each 

period was plotted and a least- squares fit of the data provided 

the dispersion curve for inversion.

The scatter in these data (fig. 5) is somewhat greater than 

that usually found in the results of simple two-station analyses. 

However, the lateral refractions observed in this study (usually 

10-15) suggest that the tripartite calculation was worthwhile.

The generalized inversion program of Braile and Keller 

(1975) was employed to invert both group- and phase-velocity 

data simultaneously to obtain an Earth model. This inversion 

technique uses an iterative procedure based on the BackusGilbert inversion formulation (for example, Backus and 

Gilbert, 1970). In the inversion, theoretical data are calculated using the Haskell-Thompson matrix approach (Thompson, 1950; Haskell, 1953) as modified by Dunkin (1965). Fora 

given layer, variations in compressional velocity and density do
88 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

3.5-

2.5-

25 35 45 55

PERIOD (S)

65 75 85

3O2.5-

25 35 45 55 

PERIOD (S)

65 75

60 5 OXF-RRM UNFIITERED 60 S OXF-flflM FILTERED

B

FIGURE 3. Samples of moving-window analysis applied to sample of cross- correlograms of legs of tripartite between Oxford, Miss., St. Louis, Mo. 

and Ann Arbor, Mich. A, results for unfiltered correlogram; B, results for filtered correlogram.

not strongly influence theoretical dispersion curves (Der and 

others, 1970). Thus, the shear-wave velocity is the only independent variable considered in the inversion process. Compressional velocities are calculated from derived shear 

velocities using assumed Poisson's ratios, and densities are 

derived from empirical compressional velocity and density 

relations (summarized by Cook, 1962).

RESULTS AND DISCUSSION

The group- and phase-velocity data obtained in this study 

are shown in figure 5, along with the curves which represent 

the best "least-squares fit" to these data. Assuming no major 

lateral refractions occur within the tripartite of stations OXFFLO-AAM, these dispersion curves reflect the general crustal 

structure along the path OXF-FLO.

Two approaches were employed to derive estimates of the 

uniqueness of the Earth models derived in this study. First of 

all, the inversion program provides standard-deviation 

estimates for the parameters treated as unknowns. Secondly, a 

wide variety of inversion runs were made with differing 

numbers of layers and radically different starting models. 

With the resolution of the technique in mind, the models 

discussed are those which satisfy the observed data and have 

the fewest number of layers. Also, the main aspects of embay - 

ment crustal structure were common to all of the Earth models 

derived and had standard-deviation estimates suggesting that 

their geological implications are valid.

Two relatively thin layers representing the Cenozoic and
A CRUSTAL STRUCTURE STUDY OF THE NORTHERN MISSISSIPPI EMBAYMENT 89

Paleozoic- Mesozoic sedimentary rocks in the embayment were 

required in all models on the basis of geological evidence. The 

thicknesses and velocities derived for these layers (0.5 km, 4.3 

km/s; 2.3 km, 5.4 km/s) represent reasonable averages for the 

propagation path, but have little geologic significance because 

they represent averages over such a large area.

A simple one layer (exclusive of sedimentary cover) crustal 

model (table 2) approaches an acceptable fit to the observed 

data but fails to fall within the error bounds (95 percent confidence interval) for the observed data (fig. 6) and does not 

honor the trend of the data. However, this model and the 

results of previous studies suggest some general conclusions: 

(1) 47 km represents a good minimum value for the crustal 

thickness of the northern Mississippi Embayment, and (2) the 

average velocities of the crust are anomalously high (Vp ~ 6.8 

km/s; Vs ~ 3.9 km/s).

The model providing the best fit to the observed data is a 

more complex three-layer (exclusive of sedimentary cover) 

crustal model (table 2). This model is interpreted as the best 

representation of the average crustal structure along the propagation path (OXF-FLO) which could be derived from these 

data, because the addition of more layers did not improve the 

fit to the observed data. This model shows the average crustal 

thickness near the axis of the northern Mississippi Embayment 

to be 50-55 km. However, the inversion results showed that the

TABLE 2. Mississippi Embayment Crustal models

[Numbers in parentheses are total crustal thickness]

5.0 -I

5 B 25 35 45 96 65 75 85 

PERIOD (S)

FIGURE 4. Sample output of the cross-multiplication technique. 

This example is for the path from Oxford, Miss., to Ann Arbor, Mich.

Reference

HcCamy and Meyer (1966)- 

Stewart (1963) :

Hercules to Gladden --

Gladden to Ste. Genevleve -- 

Warren ( 1968) ----------------

Kurita (1973)--------------- 

Mitchell and Hashim (1977)--- -

Thickness

(km)

3.0

5.0

22.0

15.0

(45.0)

1.0

8.0

22.0

11.0

(42.0)

.5

3.5

21.0

17.0

(42.0)

15.0

25.0

(40.0)

1.0

9.0

38.0

9.0

(57.0)

1.0

1.0

5.6

18.1

7

P-velocity

(km/s)

4.7

6.2

6.5

7.4

8.1

5.0

6.0

6. 1

7.3

8.0

5.0

6.0

6.3

6.9

7.8

6.1

6.6

3.0

5.0

6.2

6.5

7.4

8.2

2.4

4.0

6.2

6.4

7.2

This study (one-layer crust; exclusive of surficial layers) 

(a = 0.25) ---- --- 

This study (three-layer crust; 

exclusive of surficial laylayers) (o = 0.25) -------

.6 

3.0 

43.3 

(46.9)

.5

2.3

12.6

12.1

25.8

(53.3)

4.0 

4.7 

6.8 

7.9

4.3 

5.4 

6.3 

6.8 

7.1 

7.9

resolution of the depth to the Moho provided by these data is 

relatively poor for two reasons: First of all the various inversion 

runs all converged to models with thick (~26 km), highvelocity (Vp ~ 7.1 km/s; Vs ~ 4.0 km/s) layers in the lower 

crust. Thus, the velocity contrast between the lower crust and 

upper mantle is considerably less than normal, and the Moho 

is more difficult to resolve. Secondly, the Moho may be a transitional boundary in this region.

The general aspects of crustal structure indicated by the 

results of this study compare favorably with previous work in 

the area (table 2). Inversions with complicated velocity structures in the mantle were run, but it should be noted that an 

upper mantle, low-velocity layer at a depth of ~ 100 km (Mitchell and others, 1977) is consistent with but not required by 

the data. The thick, high-velocity, lower crustal layer is of particular tectonic significance. Discrepancies in the thickness of 

this layer (compare this study to McCamy and Meyer, 1966)
90 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

o

_J

LU 

>

Q_

4. 5^ 

4, 0 

3. 5 

3. 0

_J_________L_

30 40 50 

PERIOD (S)

Explanation

X EQ.- 6

+ EQ.- 5

A EQ. - 4

O EQ.- 3

CD EQ.- 2

70

5. 0

o

LU

LU 

CO

en 

o_

2

10

1 1 1 1

n a

O

1 1

Explanation

X EQ.

+ EQ.

A EQ.

O EQ.

Q EQ.

20 30 40 50 60 70

B PERIOD (S)

FIGURE 5.- Group-velocity (A) and phase-velocity (B) data gathered in this study. Least-squares fits to observed data are solid lines.

80
A CRUSTAL STRUCTURE STUDY OF THE NORTHERN MISSISSIPPI EMBAYMENT 91

5.0

4.5

4.0

O 3.5

3

UJ

> 3.0

CT 2.5 

O

2.0

Explanation

---- ONE-LAYER MODEL

 THREE-LAYER MODEL

0 10 20 30 40

PERIOD (s)

50 60 70 80

CO

3

LU 

>

UJ 

CO

Q_

B

5.0

4.5

4.0

3.0

25 ^'J

2.0

Explanation

----ONE-LAYER MODEL

 THREE-LAYER MODEL

0 10 20 30 40

PERIOD (s)

50 60 70 80

FIGURE 6. Comparison of observed to theoretical (A) and phase (B) velocities for a one-layer model (dashed lines) and a three-layer model (solid lines) 

calculated from the one-and three-layer crusted models shown in table . The observed data are plotted as error bars at the 95 percent confidence 

interval.
92 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

may be accounted for by consideration of geographical locations of the paths transversed with respect to the large positive 

gravity anomaly associated with the embayment (Ervin and 

McGinnis, 1975; Cordell, 1977). The path considered in this 

study passes near the axis of the embayment where gravity 

modeling (Ervin and McGinnis, 1975) suggests that this layer 

thickens.

The good agreement between this study, refraction data 

(McCamy and Meyer, 1966; Stewart, 1968; Warren, 1968), 

and gravity data (Ervin and McGinnis, 1975; Cordell, 1977) 

prompted inversion runs in which Poisson's ratios were varied 

so as to bring the compressional velocities of this study in line 

with those determined by McCamy and Meyer (1966). The integration of these data to produce a hypothetical cross section 

of the northern Mississippi Embayment is shown in figure 7. 

This generalized cross section lies along the gravity profile 

modeled by Ervin and McGinnis (1975) and includes compressional velocities, shear velocities, densities, and Poisson's ratios 

for the various layers.

The tectonic implications of these results are important but 

can be briefly stated. The cross section is highly suggestive of a 

failed rift zone (aulacogen?). When the rift was active, the 

Moho could have been the boundary between the highvelocity lower crust and the upper crust. The high-velocity 

lower crust could have been a rift cushion, possibly representing a mixture of crust and upper mantle material (Cook, 

1962). Thus, an upwarp of the mantle could have existed 

beneath the embayment. However, by the Mesozoic, this rift 

cushion would have cooled to form a block of high-density 

material which could have played a role in the isostatic subsidence of the embayment (McGinnis, 1970).

ACKNOWLEDGMENTS

The authors would like to thank L. W. Braile for his helpful 

comments and suggestions. Computing facilities were provided by the computation center, University of Texas at El Paso.

Support for this study has been provided by National Aeronautics and Space Administration, contract number 

NA55-25030, and the Kidd Memorial Seismic Observatory.

REFERENCES CITED

Backus, G., and Gilbert, F., 1970, Uniqueness in the inversion of inaccurate 

gross earth data: Philosophical Transactions of the Royal Society (London), ser. A, v. 266, p. 123-192.

Bloch, S., and Hales, A. L., 1968, New techniques for the determination of 

surface wave phase velocities: Seismological Society of America 

Bulletin, v. 58, p. 1021-1034.

so ^

Vp-4.2-4.3 

V, 2.3

/>«2.30

er-.30

Vp -4.9-5.0

V,-2.9 

/>«2.60 

er-.25

Vp-6.0-6.2 

V,-3.6

0-2.88

er-.25

Vp = 6.3-6.8 

V.-3.8 

/>-2.97 

er-.25

Vp -7.3-7.4 

V, -4.0

/>-3.l7

cr-,29

Vp-8.0-8.1 

V,-4.6 

/>-3.30 

er- .27

FIGURE 7.- Generalized crustal cross section along the gravity profile of Ervin and McGinnis (Yellville to Scottsboro, fig. 1).

Seismic data used for control are labeled.
A CRUSTAL STRUCTURE STUDY OF THE NORTHERN MISSISSIPPI EMBAYMENT 93

Bond, D.C., and others, 1971, Possible future petroleum potential of region 

9, in Cram, I. H., ed., Future petroleum provinces of the United States 

 Their geology and potential: American Association of Petroleum 

Geologists Memoir 15, v. 2, p. 1165-1218.

Braile, L. W., and Keller, G. R., 1975, Fine structure of the crust inferred 

from linear inversion of Rayleigh-wave dispersion: Seismological Society of America Bulletin, v. 65, p. 71-83.

Burke, K., and Dewey, J. F., 1973, Plume-generated triple junctions- Key 

indicators in applying plate tectonics to old rocks: Journal of Geology, 

v. 81, p. 406-433.

Cook, K. L., 1962, The problem of the mantle-crust mix- Lateral inhomogeneity in the uppermost part of the earth's mantle: Advances in 

Geophysics, v. 9, p. 295-360.

Cordell, Lindrith, 1977, Regional positive gravity anomaly over the 

Mississippi Embayment: Geophysical Research Letters, v. 4, p. 

285-287.

Gushing, E. M., Boswell, E. H., and Hosman, R. L., 1964, General geology 

of the Mississippi Embayment: U.S. Geological Survey Professional 

Paper 448-B, 28 p.

Der, Z., Masse, R., and Landisman, M., 1970, Effects of observational errors on the resolution of surface waves at intermediate distances: Journal of Geophysical Research, v. 75, p. 3399-3409.

Dunkin, J. W., 1965, Computation of model solutions in layered, elastic 

media at high frequencies: Seismological Society of America Bulletin, 

v. 55, p. 335-358.

Dziewonski, A. M., and Hales, A. L., 1972, Numerical analysis of dispersed 

seismic waves: Methods in Computational Physics, v. 11, p. 39-85

Ervin, C. P., and McGinnis, L. D., 1975, Reelfoot rift- Reactivated precursor to the Mississippi Embayment: Geological Society of America 

Bulletin, v. 86, p. 1287-1295.

Haskell, N. S., 1953, The dispersion of surface waves on multilayered 

media: Seismological Society of America Bulletin, v. 43, p. 17-34.

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1978, Magnetic 

and gravity anomalies in the northern Mississippi Embayment and their 

spatial relation to seismicity: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-914.

Hinze, W. J., Braile, L. W., Keller, G. R., and Lidiak, E. G., 1977, A tectonic overview of the central midcontinent: U.S. Nuclear Regulatory 

Commission NUREG-0382, 106 p.

Hoffman, P., Dewey, J. F., and Burke, K. A. C., 1974, Aulacogens and their 

genetic relation to geosynclines, with a Proterozoic example from Great 

Slave Lake, Canada, in Dott, R. H., Jr., and Shaver, R. H., Modern 

and ancient geosynclinal sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 19, p. 38-55.

Knopoff, L., Mueller, S., and Pilant, W. L., 1966, Structure of the crust 

and upper mantle in the Alps from phase velocity of Rayleigh waves: 

Seismological Society of America Bulletin, v. 56, p. 1009-1044.

Kovach, R. L., 1978, Seismic surface waves and crustal and upper mantle 

structure: Geophysical and Space Physics Reviews, v. 16, p. 1-13.

Kurita, Tuneto, 1973, Regional variations in the structure of the crust in the 

United States from P-wave spectra: Seismological Society of America 

Bulletin, v. 63, p. 1663-1687.

Landisman, M., Dziewonski, A. M., Sato, Y. 1969, Recent improvements in 

the analysis of surface wave observations: Geophysical Journal of the 

Royal Astronomical Society, v. 17, p. 369-403.

Long, L. T., and Mather, U. P., 1972, Southern Appalachian crustal structure from the dispersion of Rayleigh waves and refraction data: Earthquake Notes, v. 43, p. 31-40.

McCamy, K., and Meyer, R. P., 1966, Crustal results of fixed multiple shots 

in the Mississippi Embayment: in, Steinhart, J. S., and Smith T. J., 

eds., The earth beneath the continents: American Geophysical Union, 

Geophysics Monograph 10, p. 370-381.

McEvilly, T. V., 1964, Central United States crust- upper mantle structure 

from Love and Rayleigh wave phase velocity inversion: Seismological 

Society of America Bulletin, v. 54, p. 1997-2015.

McGinnis, L. D., 1970, Tectonics and the gravity field in the continental interior: Journal of Geophysical Research, v. 75, p. 317-331.

Mitchell, B. J., Cheng, C. C., and Stauder, William, 1977, A three-dimensional velocity model of the lithosphere beneath the New Madrid 

seismic zone: Seismological Society of America Bulletin, v. 67, p. 

1061-1074.

Mitchell, B.J., andHashim, B. M., 1977, Seismic velocity determination in 

the New Madrid seismic zone A new method using local earthquakes: 

Seismological Society of America Bulletin, v. 67, p. 413-424.

Nuttli, O. W., 1974, Magnitude-recurrence relation for central Mississippi 

Valley earthquakes: Seismological Society of America Bulletin, v. 64, 

p. 1189-1207.

Pilant, W. L., and Knopoff, L., 1964, Observations of multiple seismic 

events: Seismological Society of America Bulletin, v. 54, p. 19-39.

Press, F., 1956, Determination of crustal structure from phase velocity of 

Rayleigh waves, pt. 1, Southern California: Geological Society of 

America Bulletin, v. 67, p. 1647-1658.

Stauder, William, Kramer, M., Fischer, G., Schaefer, S., and Morrissey, S., 

1976, Seismic characteristics of southeast Missouri as indicated by a 

regional telemetered micro-earthquake array: Seismological Society of 

America Bulletin, v. 66, p. 1953-1964.

Stearns, R. G., and Wilson, C. W., Jr., 1972, Relationship of earthquakes 

and geology in west Tennessee and adjacent areas: Nashville, Tenn., 

Vanderbilt University, Report for Tennessee Valley Authority, 344 p.

Steinhart, J. S., and Meyer, R. P., 1961, Explosion studies of continental 

structure Arkansas-Missouri: Carnegie Institution of Washington 

Publication 622, 409 p.

Stewart, S. W., 1968, Crustal structure in Missouri by seismic refraction 

methods: Seismological Society of America Bulletin, v. 58, p. 291-323.

Thompson, W. T., 1950, Transmission of elastic waves through a stratified 

solid medium: Journal of Applied Physics, v. 21, p. 89-93.

Warren, D. H,, 1968, Transcontinental geophysical survey (35-39 N.), 

seismic refraction profiles of the crust and upper mantle from 87 to 100 

W. longi tude: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-534-D, scale 1:1,000,000.

Warren, D. H., Healy, J. H., andjackson, W.J., 1966, Crustal seismic measurements in southern Mississippi: Journal of Geophysical Research, v. 

71, p. 3437-3458.

Style and Significance of 

Surface Deformation in the 

Vicinity of New Madrid, Missouri

By D. P. RUSS

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-H
CONTENTS

Page

Abstract ............................................................................... 95

Introduction............................................................................ 95

Lake County uplift....................................................................... 97

Tectonically induced subsidence- Reelfoot Lake Basin ......................................... 106

Evidence for regional deformation .......................................................... 106

Significance of deformation and association with historic seismicity ............................... 110

References cited .........................................................................114

ILLUSTRATIONS

Page

FIGURE 1. Map of New Madrid region showing location of geomorphologic features, towns, and limits of Lake County uplift........................ 96

2. Isobase map showing amount and pattern of deformation of Lake County uplift ................................................... 98

3. Map showing area of Lake County uplift modified by Mississippi River meander belt that has been reworked by river migration from 1820 to

1970; relict braided river channels on the Little River surface also shown ..................................................... 99

4. Longitudinal profiles along Mississippi River between channel mileposts 845 and 930 ............................................... 100

5. Index map showing locations of Cronanville, Lakelsom, and Tiptonville meanders shown in profile on figure 6 .......................... 101

6 Longitudinal profiles of natural levees of selected abandoned Mississippi River meanders ............................................ 102

7. Regional map of upper Mississippi Embayment showing location of braided-stream surfaces, Mississippi River meander belt,

and other selected physiographic features............................................................................... 103

8. Photograph of 0.5-m-wide fault zone exposed in USGS exploratory trench located at base of Reelfoot scarp ............................. 103

9. Regional topographic map of part of upper Mississippi Embayment ............................................................. 106

10. Cross section X-X ' showing topographic and sedimentologic relationships of Mississippi River meander belt, Little River braided-stream

surface, and Portageville bulge ....................................................................................... 108

11. Aerial photograph of Portageville, Mo., area showing part of Little River braided-stream surface, Mississippi River meander belt,

and Portageville bulge .............................................................................................. 109

12. Map showing spatial relationship between modern microearthquakes and area of Lake County uplift .................................. 112

13. Plan view and cross section of hypothetical tectonic model for origin of Lake County uplift ........................................... 113

TABLES

Page

TABLE 1. Comparison of geological and geophysical characteristics of topographic components of the Lake County uplift ..........................111
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION 

IN THE VICINITY OF NEW MADRID, MISSOURI

By DAVID P. Russ

ABSTRACT

A geomorphic analysis of tectonic surface deformation in the vicinity of 

New Madrid, Mo., shows that the Lake County uplift is an elongate, composite Quaternary structure that is associated with active faults and modern 

seismicity. The uplift is about 50 km long and 23 km wide; it upwarps the 

Mississippi River valley as much as 10 m in parts of northwestern Tennessee, 

southeastern Missouri, and southwestern Kentucky. The composite nature 

of the uplift is apparently the result of vertical movement along several ancient subsurface structures that are oriented with the modern stress field so as 

to optimize tectonic movement. The majority of the vertical movement has 

occurred during earthquakes. The Lake County uplift can be subdivided into several topographic bulges, the most prominent of which are Tiptonville 

dome, Ridgely Ridge, and the south end of Sikeston Ridge.

Tiptonville dome is an asymmetrical, monoclinal ridge that exhibits the 

greatest upwarping and the highest topographic relief on the uplift. Its 

relatively steep eastern flank is bounded by Reelfoot scarp, a complex 

monoclinal fold that has a zone of normal faults (displacement about 3 m) at 

its base. The zone of faults overlies, and is probably continuous with, 

Reelfoot fault, a high-angle dip-slip fault that offsets subsurface Paleozoic, 

Mesozoic, and Tertiary rocks. Seismic-reflection profiles reveal that the 

rocks beneath Tiptonville dome are upwarped in a manner that is spatially 

similar to the surface warping. The surface uplift accounts for about onefifth of the total deformation. Most of Tiptonville dome formed between 200 

and 2000 years ago. Additional uplifting deformed the northwest and 

southeast parts of the dome during the 1811-12 New Madrid earthquakes. 

Ridgely Ridge is a northeast-trending symmetrical bulge underlain by a 

similarly trending zone of faults. Although investigations are currently incomplete, none of the faults has been found to displace surface sediments. As 

with the Tiptonville dome, the surface expression of Ridgely Ridge mimics 

the structural pattern of the subsurface rocks. Much of Ridgely Ridge appears to be older than the Tiptonville dome, but geomorphic evidence suggests that it is younger than 6000 years B.P. The southern end of Sikeston 

Ridge and adjacent areas of the Mississippi River valley have undergone 

broad shallow upwarping. The deformation in this area probably occurred 

during late Wisconsin or early Holocene time.

The Portageville bulge, located 25 km southwest of New Madrid, is not a 

product of tectonism, as has been suggested, but was formed by geomorphic 

and sedimentologic processes. The bulge owes its topographic expression to 

the buildup of overbank deposits that have spilled from the adjacent aggrading Mississippi River meander belt onto an old braided-stream surface. 

The anomalously high topography in the uplands along the Mississippi River 

valley bluffs between Hickman, Ry., and Memphis, Tenn., is the result of an 

inversion of relief that was caused by differential erosion and is not the product of tectonism.

A strong correlation exists between modern seismicity and the Lake County uplift. Approximately 75 percent of the microearthquakes recorded from 

July 1974 through June 1978 in the general region between Ridgely, Tenn., 

and New Madrid, Mo., have occurred in the upwarped area. The patterns of 

epicenter locations reveal intense activity along the northwestern part of the 

Tiptonville dome and in the vicinity of Ridgely Ridge. The seismotectonics 

of the Ridgely area, however, are quite complex. Whereas Ridgely Ridge 

and underlying near-surface structures have a northeast trend, seismicity 

seems to have a northwest trend. Geomorphic investigations of surface 

sediments and structures, as well as historic reports, provide evidence that 

the New Madrid region has been shaken by at least three earthquakes of 

mfc >6.2 in the last 2000 years. A recurrence interval of about 600 years for 

these large earthquakes is therefore hypothesized.

Fault-plane solutions and structural relationships are used to develop a 

tectonic model for the formation of the Lake County uplift. The focal 

mechanisms reveal that the major compressive stress of the modern stress 

field in the New Madrid region is horizontal and trends approximately eastwest. The mechanisms also show that two of the major linear zones of 

seismicity in the area are best represented as northeast-trending, en echelon, 

right-lateral strike-slip faults. Movement on the two faults generates compression in the region between the faults. The compression is translated into 

vertical strain, resulting in uplift above the area of opposed fault movement. 

This area corresponds precisely with the location of abundant epicenters and 

with the area of the Lake County uplift.

INTRODUCTION

The identification and characterization of tectonic surface deformation are often important elements in the establishment 

of relationships between structural framework and seismicity. 

This paper describes the occurrence and style of Holocene surface deformation in the general region surrounding New

95
96 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Madrid, Mo., and examines the significance of the deformation in terms of the modern stress field, of a tectonic model, 

and of patterns of historic earthquakes.

New Madrid, Mo., is located on the Mississippi River along 

the eastern margin of the Mississippi River valley (fig. 1). The 

valley occupies the western one-half of the upper Mississippi 

Embayment which, physiographically, is the northern extension of the gulf coastal plain. Popular accounts, largely in

p^vate correspondence, citing evidence of surficial deformation in this area were reported as long ago as 1811 and 1812, 

during and shortly following the devastating New Madrid 

earthquake sequence. The first scientific article on the subject 

(Usher, 1837) investigated uplift along the banks of the 

Mississippi River in northwestern Tennessee. Of the several 

studies addressing the problem, almost all have considered 

Holocene tectonic deformation to be in genetic association

89°52'30 36°45'00" 89°45'00" 89°30'00" 89°15'00" 89°07'30'

36"30'00" 

EXPLANATION 

Island 

/ Topographic scarp

/ Limit of Lake County uplift 

.-" Physiographic boundary 

.: .;.'. Braided stream surfaces 

11111 Uplands

36°00'00"

FIGURE 1. Map of New Madrid region showing location of geomorphologic features, towns, and limit of the Lake County uplift.
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 97

with earthquakes; little is known about large-scale aseismic deformation.

The surficial tectonic structures identified in the present investigation and, in part, described in previous reports are low 

uplifts that have only subtle expression in the landscape and 

folds and faults exposed in a shallow exploratory trench. Consequently, sensitive geomorphological methods capable of 

detecting and resolving very minor anomalies and small-scale 

structural features, as well as standard geologic techniques, 

have been used in the investigation. With the exception of the 

Reelfoot Lake area, basins or "sunk lands" are not included in 

this report. Although many of these features may have been 

directly or indirectly generated by tectonic processes, they are 

not tectonic structures by strict definition. The structures that 

are identified near New Madrid have resulted in interpretations that afford a clearer picture of the temporal and spatial 

interrelations among surface and subsurface tectonic deformation, the geomorphic evolution of the Mississippi River 

valley, and seismicity.

LAKE COUNTY UPLIFT

Lake County uplift is a gently sloping, irregularly shaped 

topographic bulge whose surface has been upwarped as much 

as 10 m above the general level of the Mississippi River valley 

in parts of northwestern Tennessee, southwestern Kentucky, 

and southeastern Missouri (figs. 1 and 2). The deformed area 

has a maximum length (roughly north-south) of about 50 km 

and a maximum width of about 23 km. Relief across the surface of the uplift is uneven and asymmetric. The surface is 

largely dominated by two elongated bulges: one trending 

north-south and located northeast of Tiptonville, Tenn. (Tiptonville dome), and the other trending northeast in the vicinity 

of Ridgely, Tenn. (Ridgely Ridge). The former feature is 

strongly asymmetric, being bordered along its eastern flank by 

Reelfoot scarp. The Lake County uplift encompasses parts of 

four different geomorphic surfaces: the modern Mississippi 

River meander belt (the Mississippi River flows obliquely 

across the uplift from northeast to southwest) and three 

separate Mississippi River braided-stream terraces (Little 

River surface, Sikeston Ridge, and an unnamed surface east of 

Sikeston Ridge) (fig. 2). Because these surfaces have had different origins, are of different ages, and have experienced 

varying forms of geomorphic modification subsequent to their 

formation, determination of the magnitude, character, and 

timing of the uplift is complex and must be evaluated carefully. For example, lateral planation of the Mississippi River during and following the most active periods of deformation has 

eroded a considerable amount of the uplift's surface. Figure 3 

delineates that part of the uplift which the river modified between 1820 and about 1970.

Although the presence of an uplift in Lake County, Tenn., 

has been suggested for more than 150 years, few detailed 

studies that characterize the deformation have been made.

Usher's (1837) early work inferred an area of uplift east of the 

Mississippi River from just south of New Madrid to the mouth 

of the Obion River. He attributed the uplift to deformation 

produced during the New Madrid earthquakes of 1811-12. 

McGee (1892) recognized that the deformation extended from 

Tennessee into southeastern Missouri, and he named it the 

Lake County uplift. In his classic bulletin on the New Madrid 

earthquake, Fuller (1912) called the uplift the Tiptonville 

dome. More recently, in a lengthy study, Stearns (1979) revived the name "Lake County uplift" and subdivided the structure into physiographic bulges or "flood islands," of which the 

two most prominent are Tiptonville dome and Ridgely Ridge. 

This report will continue the use of the name "Lake County 

uplift" because of the precedents in the literature, and because 

such a broad term permits the individual parts of the uplift to 

be described with greater flexibility, clarity, and precision.

The investigations listed above, as well as those by Glenn 

(1933), Fisk(1944), and Krinitzsky( 1950), concur with Usher's 

conclusion that the topographically high area in Lake County, 

Tenn., is of tectonic origin. Proof, however, of the tectonic 

origin of the structure and of its time of formation has generally been inconclusive, largely because subsurface drilling, geophysical information, and absolute age data have not been 

available, and because appropriate studies were simply not 

undertaken. Evidence gathered during the current investigation, combined with previously published data confirms the 

tectonic origin of the topographic bulge. This evidence is as 

follows:

1. Comparison of longitudinal topographic profiles made 

along the modern natural levee of the Mississippi River 

(generally the highest elevations on the flood plain) with 

profiles made on adjacent parts of the Lake County uplift 

reveals that the surface of the uplift is significantly higher 

than any naturally occurring landform in the modern 

meander belt (fig. 4A). This observation agrees with previously stated qualitative assessments of an anomalously high 

surface in the area in question (Usher, 1837; McGee, 1892; 

Fisk, 1944).

2. Interpretation of longitudinal topographic profiles made 

of abandoned river channels and natural levees situated on 

the old meander-belt surface of the uplift shows, conclusively, that the original channel and levee gradients have 

been significantly warped, some to the extent that slopes, 

representing the original river-flow direction, have been 

reversed (figs. 5, 6A, 6B, 6C).

3. Transverse topographic profiles drawn across Reelfoot 

scarp, which forms the boundary of Tiptonville dome on 

the east side, reveal a predominantly convex slope, consistent with an uplift origin (Russ and others, 1978). If the 

scarp had formed solely by erosion, a straight or convexconcave slope would have most likely developed. Reelfoot 

scarp vertically offsets abandoned Mississippi River channels, which once crossed the area.
98

89°52'30 36°45'00"

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

89°45'00" 89°30'00" 89°15'00 89°07'30'

36°30'00"

36 C 15'00"| :

TENNESSEE

j^ Topographic scarp 

:' ' '.' '.' : Braided stream surface 

ttltt Uplands

36°07'30'

FIGURE 2. Isobase map showing amount and pattern of deformation of Lake County uplift. Bold lines indicate contours of equal uplift; solid where determined by direct measurement; dashed where calculated by reconstruction process (see text). Thin lines indicate idealized preuplift meander-belt contours; solid where determined by direct measurement; long and short dashed where determined by reconstruction process (see text). Values are in feet; 

to convert to meters divide values by 3.281.

4. Structures identified in an exploratory trench excavated 

across Reelfoot scarp indicate that the majority of the 

scarp is a monoclinal flexure formed as the adjacent Tiptonville dome was uplifted (Russ and others, 1978; Russ, 

1979).

5. Standard Vibroseis reflection profiles, part of an extensive 

series of lines run in the upper Mississippi Embayment during the past 4 years (Zoback, 1979; Hamilton and Zoback, 

1979; Crone and Brockman, this volume), show that the 

well-resolved Late Cretaceous-Paleozoic, Paleocene-Late 

Cretaceous, and Eocene-Paleocene reflectors are arched 

upwards over an area generally coincident with the Lake 

County uplift. The subsurface arching reaches a maximum precisely where the surface uplift is greatest: beneath Tiptonville dome and Ridgely Ridge. Beneath the
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 99

uplift, the post-Paleozoic section is generally characterized 

by intense deformation including local doming, fracturing, and faulting. Apparent igneous intrusive masses have 

also been identified.

The deformation that generated the Lake County uplift is 

reflected in the surface topography. Constructional and erosional fluvial processes operating before, during, and after the 

most intense periods of upwarping, together with the original 

relief, have resulted in a surface that is uneven and irregular. 

The uplifted surface is relatively broad and flat north of the

Tennessee State line (lat 36°30 ' N.). Most of the relief in this 

area is erosional in origin. It was produced during the cutting 

of the Sikeston Ridge scarps by braided streams, during lateral 

migration of the Mississippi River as it eroded parts of Sikeston 

Ridge and older uplifted areas within the meander belt, and 

by incision along local streams in response to man's activities. 

Positive relief of a nontectonic origin is especially evident 

within the large meander loop immediately south of New 

Madrid and on the Little River surface west of the meander 

belt. The former location has been aggraded by deposition of

89°52'30" 

36°45'00"

89°45'00" 89°15'00"

36°30'00

36°15'00

36°07'30

New Madrid 

~w MISSOURI

 ft

Reelfoot 

Lak

:: u P L A N D s

EXPLANATION

Limit of Lake County uplift

Outline of Figure 11

-^.^Topographic scarp

 ': : ; :> Braided stream surface

 %M Relict braided stream channels

--T-ri Area of Mississippi River meander 

belt modified by river migration 

from 1820 to 1970

; tt I Uplands

89°07'30"

FIGURE 3. ~ Map showing area of Lake County uplift modified by Mississippi River meander belt that has been reworked by river migration 

from 1820 to 1970; relict braided river channels on the Little River surface also shown. Data from Fisk, 1944.
100 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

SOUTH NORTH

County

B

t

£ 310- 

z 300- 

z 290- 

9 280-

S 270- 

2 260-

________ 

1 1 1

c:

1 I Lowland Flood Plain

1 1 1

r-310 

-300 

-290 

-280

-2/0 

-260

K 320 n 

? 310-

2 290-

< 280- 

5 270-

8

. . ^ _______ ^ 1

.... . . ... ^-^ ^~ Post - 1R90 Flood Plnm

K 850 

III 

860 870 880 890 

III

900 910 920 93

-320 -310

-290

-280 -270

0

Milepost

FIGURE 4. Longitudinal profiles along Mississippi River between channel mileposts 845 and 930. Mileposts and elevation data from U.S. Geological Survey 

7-minute topographic quadrangles; locations plotted on figure 7. Locations of islands shown on figure 1. A , natural-levee profile and projected elevations of adjacent Lake County uplift; low water reference plane from data collected in 1962 and adjusted using 20-yr average minimum discharge data 

from 1954 to 1975 (U.S. Army Corps of Engineers, 1976). B, lowland flood-plain profile. C, profile of Mississippi River meander belt that was reworked 

from 1820 to 1970.

natural-levee deposits and by the accumulation of more than 3 

m of dune sands. The latter surface has received a sizable 

amount of natural-levee deposits.

Between the Kentucky-Tennessee State line and the city of 

Tiptonville, the topography is dominated by Tiptonville 

dome. This north-south-trending elliptical bulge has the 

greatest uplift and consequently the highest elevation of any 

point within the Lake County uplift. Relief between the dome 

and the adjoining area east of the uplifted area reaches a maximum value of slightly more than 10 m. The east side of Tiptonville dome is bounded by Reelfoot scarp, a folded and 

faulted structure whose convex slope has a maximum dip of 

about 3 ° (which in an area of very low relief like the Mississippi 

valley is a prominent slope!). The northwestern physiographic 

edge of Tiptonville dome (about 0.5 km west of Cronanville, 

Tenn.) is marked by a low but conspicuous 1.5-m scarp. The 

scarp, and the area immediately west of it, was formed by 

lateral migration of an ancient Mississippi River course and is 

not directly the product of tectonic deformation. The 

meandering channel incised the older meander-belt deposits 

and left behind a well-developed set of point-bar ridges and 

swales. It is difficult to establish whether the incision occurred 

prior to, during, or between pulses of uplift on the Tiptonville

dome. The incised surface is itself warped upwards about 4.5 

m. The southwestern edge of Tiptonville dome is sharply truncated by an abandoned Mississippi River channel, which cut 

into the edge of the dome as recently as the 1890's.

Between Reelfoot Lake and Ridgely, Tenn., the Lake 

County uplift narrows. The surface in this area is dominated 

by northeast-trending Ridgely Ridge. The flanks of the ridge 

have dips of 1 ° or less. Maximum relief on the ridge is about 

6.0m.

In general, the physiographic boundary of the Lake County 

uplift is only approximately known. This is because as the 

uplift diminishes outward toward areas of no deformation, the 

erosional and depositional relief on the various fluviatile surfaces begin to exceed the amount of uplift and thus obscure 

the zero-uplift boundary. Along most of this boundary the 

slope is less than 1 °. Only along Reelfoot scarp between the 

Mississippi River and the southern edge of Reelfoot Lake and 

in an area southwest of Ridgely, Tenn., is the border of the 

uplift sharp and easily distinguishable.

Determination of the pattern and amounts of absolute 

uplift for the Lake County uplift is a complex process involving 

an interrelated study of soils, subsurface boring data, landform morphologies, and the topographic framework of the
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 101

three abandoned braided-stream surfaces and of the modern 

Mississippi River meander belt. Figure 2 is an isobase map 

showing the cumulative gross upwarping of Lake County 

uplift. Because of substantial differences in original elevation 

and slope, mode and time of formation, and type and amount 

of post-formation geomorphic modification, each of the four 

surfaces must be analyzed individually before being combined 

into a single map displaying total uplift such as is shown in 

figure 2. The thin contours shown on figure 2 (275, 280, 285, 

290, 295,) represent an idealized configuration of the 

downstream slope of the modern Mississippi River meander 

belt as it may have existed prior to modification. (The possibility of uplift in the vicinity of the meander belt prior to the 

meander belt's formation will be discussed shortly.) The position of these contours was ascertained by dividing the area between the nearest undisturbed upstream and downstream contours (270 and 300 ft, on fig. 2) into generally equally spaced 

segments. The idealized flood plain has, therefore, a relatively 

smooth slope, characteristic of the meander belt as a whole. 

On the other hand, examination of a detailed topographic 

map of the area reveals that the present slope of the meander 

belt is highly uneven and has been significantly disturbed. The 

bold isobase contours drawn on figure 2 consist of solid lines 

representing lines of equal uplift determined by direct comparison of the modern deformed topographic surface with the

89°30'

36°15 /::

A

8 :-ic

EXPLANATION

;§ Cronanville meander / Topographic scarp 

H Lake Isom meander ^/ Limit of Lake County uplift

Tiptonville meander .. ' Physiographic boundary

FIGURE 5. Index map showing locations of Cronanville, Lake Isom, and 

Tiptonville meanders shown in profile on figure 6.

reconstructed "preuplift" meander-belt surface and of short 

dashed lines representing lines of equal uplift, determined by 

comparing the reconstructed "preuplift" meander-belt surface 

with a deformed surface substantially modified by geomorphic 

processes. Thus, the positions of the bold dashed contours take 

into account degradation of the uplifted area by river erosion 

and aggradation produced by natural levee, backswamp, 

crevasse, and eolian deposition. Geomorphic modification occurred both concurrently and subsequent to the uplift. When 

comparing the uplift measured on the modern Mississippi 

meander belt with that on the braided terraces, it is important 

to consider that the meander belt is younger than, slopes at a 

different angle and direction than, and was formed by modification quite different from that on the braided terraces. 

Careful comparison of the reconstructed meander-belt contours of figure 2 with reconstructed contours on the adjacent 

braided surfaces (not shown) reveals that throughout most of 

the region the meander belt has aggraded itself above the level 

of the older Little River surface and the unnamed braided surface east of Sikeston Ridge by amounts ranging from 0 to 4.5 

m and averaging 3.7 m. In places, the aggradation has dammed off the mouths of small stream valleys tributary to the 

Mississippi River from the the east. Hatchie River valley, for 

example, lies 1.2-2.7 m below the level of the Mississippi 

meander belt (fig. 7). In the far northeastern part of the area, 

the meander belt has entrenched the braided terrace by about 

1.5m. Failure to recognize basic sedimentologically produced 

differences in the elevations of the surfaces under study would 

result in substantial errors in the amount of calculated uplift.

In the deformed area where the braided-stream terraces 

flank the northwestern edge of the Mississippi River meander 

belt, the upwarping has been less than 2 m. Consequently, 

precise quantitative delineation of the deformation here is difficult to determine. This is especially true on lower terraces 

that have received natural-levee and backswamp sediments 

from the Mississippi River since it has occupied its meander 

belt. Examination of topographic profiles on Sikeston Ridge 

and the surface immediately east of the ridge constructed 

parallel to the slope of these braided surfaces shows that the 

gradients in their southernmost parts flatten significantly. The 

flattening exceeds that which might reasonably be expected to 

have been produced by the combination of the normal 

downstream decrease in flood-plain gradient and the deposition of the natural-levee sediments. On the Lewis Prairie segment of Sikeston Ridge (fig. 1), soil data supplement the topographic evidence that the surface there has received only a 

minor amount of natural-levee deposits.

Structural deformation of the near-surface sediments of the 

Lake County uplift (other than the general upwarping shown 

in fig. 2) has been mapped and studied in a 260-m-long exploratory trench excavated across the northern part of 

Reelfoot scarp (location shown on fig. 1; discussion in Russ 

and others, 1978). Sediments in the trench are faulted and 

folded into a broad, shallow monoclinal arch which mimics
102 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

O 

O

o: 

<

O

o

o

Q_

O u_ 

O O z 

o

32

30

280

h320 

 300 

280

Original channel slope direction"

0.5 1.0 KILOMETERS

zo 

oz

o 

o

> o 

zoo. O "- CK:

LU

o 

o

z

B =

o

E 280

320, sooi 9ftnu

320 ^300

Original channel slope direction0 1.0 KILOMETERS

Original channel slope direction0 1.0 KILOMETERS

Figure 6. Longitudinal profiles of natural levees of selected abandoned Mississippi River meanders. Location of channel meanders shown on figure 5. 

Elevation data from U.S. Geological Survey 7 -minute topographic quadrangles. A, profile of the Cronanville meander. Uplift has deformed channel 

profile, reversing the slope direction between Cronanville and the site of the USGS trench and offsetting the channel levee at Reelfoot scarp. B, profile of 

the Lake Isom meander. Channel profile has been obviously deformed by uplift of Tiptonville dome and Ridgely Ridge. C, profile has been significantly 

warped by formation of Tiptonville dome, Reelfoot scarp, and Reelfoot Lake. The lake bottom has been downdropped relative to the dome.

the geometry of the subaerial scarp. Small-scale monoclines, 

anticlines, and synclines were mapped in the trench. The 

monoclines are located along the upper one-third of the scarp 

and occur in inward-dipping pairs that form incipient 

grabens. The monoclines and numerous small faults identified 

in the upper part of the trench were apparently formed as a 

result of tensional strain when the sediments of Tiptonville 

dome were arched upwards. A 0.5-m-wide zone of eastdipping normal faults with an aggregate vertical displacement 

greater than 3 m was mapped near the scarp base (fig. 8). The 

faults overlie, in a parallel fashion, a subsurface fault (named 

Reelfoot fault) detected by Vibroseis-reflection profiling

(Zoback, 1979), thus suggesting that the surficial faults are 

tectonic, deep-seated, and related to the uplift that created 

Tiptonville dome. These faults are the only known examples 

of surficial Holocene tectonic faulting in the upper Mississippi 

Embayment. Examination of the reflection profiles also reveals that the Tertiary-Cretaceous and Cretaceous-Paleozoic 

reflectors located about 450-600 m beneath Reelfoot scarp are 

upwarped into a broad, east-dipping monocline (Crone and 

Brockman, this volume). Cumulative structural relief on the 

monocline and Reelfoot fault amounts to 36-46 m as compared to about 9 m on Reelfoot scarp. The spatial coincidence 

and similar geometries of the surface and subsurface faults
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 103

and folds suggest that they are genetically related. If this is so, 

then the Holocene deformation on Tiptonville dome may account for about 20 percent of the total vertical deformation to 

occur in this area since the Late Cretaceous. If, however, because of compaction or other mechanical processes, the deformation associated with any single event is unevenly propagated to the surface, then few conclusions can be made 

concerning the rate of subsurface deformation in comparison 

with surface deformation. Geologic investigation of sand-blow 

dikes and of all the faults exposed in the exploratory trench 

suggests that the dikes and faults formed simultaneously during strong earthquakes. Evidence indicates, however, that little, if any, movement occurred on the faults during the 

1811-12 New Madrid earthquake (Russ, 1979). Nevertheless, 

popular historic accounts indicate that, during these earthquakes, uplift occurred along the south end of Reelfoot scarp 

at the southern end of Reelfoot Lake. The uplift caused the 

damming of Reelfoot River (not shown) and the enlargement 

of the lake.

Although no evidence is currently available that documents 

possible faulting of near-surface sediments along Ridgely

MAIDEN Coruthersvilie

EXPLANATION

[:'. .'-\ Braided stream surfaces 

A River mile markers 

opographic scarp 

Uplands

FIGURE 7. Regional map of upper Mississippi Embayment showing location 

of braided-stream surfaces, Mississippi River meander belt, and other 

selected physiographic features. River mile markers (fig. 4) and cross 

section X-X' (fig. 10) are shown.

FIGURE 8. Photograph of 0.5 m-wide fault zone (between arrows) exposed 

in TJSGS exploratory trench located at base of Reelfoot scarp. Faults 

displace alluvium in a normal sense and may be continuous with the 

subsurface Reelfoot fault. Location of trench shown on figure 1.

Ridge, Vibroseis-reflection profiling recently completed in the 

area (Hamilton and Zoback, 1979) shows subsurface warping 

related to the surface uplift. The subsurface warping is observed on Eocene, Late Cretaceous, and Paleozoic reflectors, the 

shallowest of which comes to within about 150 m of the surface. The reflections show a linear zone of faults that trends 

northeast, from southeast of Ridgely to the vicinity of Reelfoot 

Lake (Zoback and others, 1978; Zoback and others, 1980). 

Close examination of the seismic profiles suggests that all of 

the major reflectors are upwarped into a broad, shallow, 

northeast-trending arch. The faults and arch are largely 

parallel to, and coincident with, the overlying Ridgely Ridge. 

A part of the arched area extends beyond the limits of the 

overlying surface uplift into the area southwest of Ridgely, 

Tenn. It is possible that uplifting has not occurred in this area 

during the lifetime of the Mississippi River meander belt. The 

maximum upwarping measured on any structure beneath 

Ridgely Ridge amounts to about 80 m. This structural relief 

occurs on the Cottonwood Grove fault, the largest dip-slip 

fault observed on the seismic profiles in this area (Zoback and 

others, 1978; Zoback and others, 1980). The maximum surface uplift on Ridgely Ridge is about 6 m.

Although the earliest workers (Usher, 1837; McGee, 1892) 

believed that the Lake County uplift was formed during the 

1811-12 New Madrid earthquakes, a significant amount of 

evidence suggests that the deformation has occurred in spatially irregular pulses over a period of at least several thousand 

years. The oldest landform to be affected by the uplift is 

Sikeston Ridge, an early- Wisconsin braided-stream surface 

that probably formed more than 30,000 years ago (Saucier, 

1974). The Little River surface and the unnamed braided surface to the east of Sikeston Ridge formed during the late 

Wisconsin and earliest Holocene and may have elements as 

young as 6000 years old (Saucier, 1974). The key to possible 

early deformation of these landforms is the elevation of the 

surface of the abandoned meander, which carved the reentrant into Sikeston Ridge just northeast of New Madrid
104 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

(figs. 1 and 2). Careful examination of the meander's surface 

reveals that it has not undergone uplift or recently been 

reworked. Saucier's (1974) studies suggest that the maximum 

age of the Mississippi River meander-belt deposits is about 

6000 years. Geomorphic analysis, using crosscutting relationships, continuity of form, and degree of preservation, indicates that the meander northeast of New Madrid is one of 

the oldest in the meander belt and that it is quite a bit older 

than the ancient meanders deformed by Tiptonville dome. 

The absence of uplift on the old meander surface suggests that 

Sikeston Ridge, the area now occupied by the old meander, 

and probably the other braided surfaces were upwarped 

before the old meander was created. The surfaces, therefore, 

must have been deformed more than 4000 years ago and 

possibly several tens of thousands of years ago. Because the 

New Madrid area has experienced only a minor amount of 

total uplift, no obvious discordancy exists among the gradients 

of the three braided-stream surfaces, and thus it is very difficult to determine exactly when the uplift took place. Nevertheless, it is known that the meander northeast of New Madrid 

was carved considerably before those on the Tiptonville dome, 

and thus much of the uplift on the braided-stream surfaces 

antedates that to the south.

It is apparent that because most of the Lake County uplift is 

situated within the Mississippi River meander belt, the majority of the uplift as expressed at the surface can be no older than 

the oldest meander-belt sediments or about 6000 years B.P. 

If Holocene uplift in this area occurred prior to the formation 

of the meander belt, when the surface still had a braided pattern, then it is possible that as much as 3 m of the total uplift 

now measured across the area may represent deformation inherited from the braided-stream surface. It is probable, 

however, that when the Mississippi River meander belt formed, the eroding activity of the migrating belt would have 

altered the gradients of surfaces that had been upwarped more 

than 3 m. Radiometric C-14 ages obtained on freshwater 

gastropod and pelecypod shells collected from the alluvium in 

the USGS exploratory trench on Reelfoot scarp indicate that 

the Tiptonville dome segment of the meander belt has a maximum age of about 2000 years (Russ, 1979). Thus, most of the 

deformation on the dome has occurred within the last 2,000 

years. Structural evidence observed in the trench indicates 

that most of the uplift that produced Reelfoot scarp and the 

adjacent Tiptonville dome occurred during two pre-1800, 

high-intensity earthquakes (Russ, 1979; Russ and others, 

1978). Uplift also took place on parts of the dome during the 

1811-12 New Madrid earthquakes (evidence for post-1800-- 

modern uplift will be discussed in more detail later in this 

section). It is therefore concluded that Tiptonville dome is the 

product of at least three distinct pulses of uplift associated with 

seismicity, as well as an unknown component of aseismic 

deformation.

The timing of the deformation on Ridgely Ridge is less clear

than is that for Tiptonville dome. It appears, however, that 

the majority of the uplift is out of phase with, and occurred 

earlier than, the uplift of the dome. Evidence for this conclusion is as follows:

1. Ridgely Ridge has a subdued physiography with no sharp 

topographic breaks. Reelfoot scarp on the eastern edge of 

Tiptonville dome, on the other hand, is relatively high and 

steep. It is possible that subaerial erosion has diminished 

the relief on a hypothetically higher Ridgely Ridge, whereas the youthful dome topography is quite apparent.

2. As seen on topographic maps and bathymetric charts of 

Reelfoot Lake, Reelfoot scarp swings to the southeast 

around the southern shore of Reelfoot Lake and cuts across 

the projection of the buried faults associated with Ridgely 

Ridge. Reelfoot scarp, it is to be remembered, overlies 

Reelfoot fault. Thus one of the major structures of the 

Tiptonville dome truncates structures related to Ridgely 

Ridge. No youthful-appearing surficial features have yet 

been identified that overlie the subsurface structures of 

Ridgely Ridge as Reelfoot scarp does the Reelfoot fault.

3. Somewhat contradictory soils data support the hypothesis 

that Ridgely Ridge is older than the Tiptonville dome. As 

pointed out by Stearns (1979), soils with "B" horizons (particularly "B" horizons with illuviated clay) are much more 

common on Ridgely Ridge than on Tiptonville dome. The 

presence of a textural "B" horizon is supposedly indicative 

of greater age: if the surface upon which the soil has developed has been uplifted high enough, then the soil will 

not have been affected by recurrent overbank deposits. 

Soils thus positioned will undergo a more rapid pedogenic 

development than soils that are frequently inundated and 

consequently will appear older than flooded soils. Unfortunately, Lake County, Tenn., also has a large area of soils 

with "B" horizons that are situated on the lowest floodplain levels outside of the area of uplift. Thus, the relationship of soil-horizon development to relative flood-plain 

evaluation is not altogether clear.

Post-1800 deformation on the Lake County uplift near Portageville and Linda, Mo., is supported by several lines of 

evidence (fig. 1). Professor J. W. Foster recounts that keelboats 

used to regularly make passage from the Mississippi River 

through Portage Bayou (not shown), past the present town of 

Portageville (fig. 2), and on to St. Francis River (not 

shown)(Broadhead,1902). Following the 1811-12 earthquakes, however, such passage was no longer possible because 

the land had apparently been uplifted, thus causing Portage 

Bayou to shallow. The upwarping most probably occurred in 

the area southeast of Point Pleasant, Mo., along the northwestern edge of the Tiptonville dome (fig. 1). The uplift may have 

been related to deformation associated with a waterfall which 

reportedly formed across the Mississippi River during the 

earthquake of February 7, 1812 (Penick, 1976). In order to in-
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 105

vestigate the possibility of post-1800 deformation, I have constructed longitudinal profiles along the modern channel of the 

river using elevations taken from the most recent USGS 7 

1/2-minute topographic maps. (Bayouville, Mo.-Ky., Bondurant, Ky.-Mo.-Tenn., Caruthersville, Mo.-Tenn., 

Hickman, Ky.-Mo.-Tenn., Hubbard Lake, Mo.-Ky.-Tenn., 

Mooring, Mo.-Tenn., New Madrid, Mo.-Ky., Point Pleasant, 

Mo.-Tenn.-Ky., Ridgely, Tenn., Stanley, Mo.-Tenn., Tennemo, Tenn.-Mo., Tiptonville, Tenn.-Mo.-Ky., and Wolf 

Island, Mo.-Ky., figs. 4/4, B, C). They cover the area between 

Hickman, Ky., and Caruthersville, Mo. (fig. 7). Three 

separate sections were constructed in order to clarify whether 

plotted anomalies are the result of geomorphic or tectonic processes and to distinguish deformation that may have occurred 

during the 1811-12 earthquakes from that which may be even 

younger. A cursory examination of figure 4 reveals that all of 

the sections have similar profiles, suggesting that they may be 

the result of identical processes. Over their total length the 

profiles are convex upward, a configuration that is commonly 

associated with areas that have been uplifted. Fisk (1944, pi. 

19) noted that this same stretch of valley had a convex-upward 

profile in 1874. A more comprehensive interpretation of figure 

4 follows. The positions of the islands plotted on the cross sections are shown on figure 1. Figure 7 shows Mississippi River 

mileage markers.

As delineated on figure 4/4, the levee gradient north of 

island No. 8 is 0.33 m/km, a value about average for most of 

the levee in the upper Mississippi Embayment. South of island 

No. 8 the levee gradient flattens significantly, to a value of 

0.19 m/km. This degree of flattening is greater than that normally produced as a result of excessive meandering by the 

Mississippi River in a direction oblique to the regional valley 

slope. In fact the change in gradient commences on a long 

reach that parallels the valley slope, a stretch in which flattening would not normally be expected. The flattening is most 

probably the result of upwarping along the northern edge of 

Tiptonville dome. In the vicinity of island No. 11, the levee 

gradient steepens sharply over a distance of 11 km. This 

stretch is in the approximate location where one of the two 

waterfalls was reported to have formed during the February 7, 

1812, earthquake. The faulting(?) that produced the waterfall 

probably also caused uplift and tilting of the adjacent flood 

plain. The direction of slope change and the restricted length 

of the anomaly are consistent with this interpretation. A possible determination of the amount of uplift that occurred in the 

area can be obtained by drawing a line between "normal" or 

undeformed segments of the profile in such a way as to approximate a preuplift gradient. Measuring vertically from this 

line to the high point on the profile near island No. 11 suggests 

that a maximum of 2 m of uplift is represented. Between Linda and island No. 13, the natural-levee gradient flattens once 

again. Here the flattening occurs in a zone where the river 

flows directly down the regional slope. Such a gradient change

would not normally be expected to occur as a result of fluvial 

processes. Minor uplift may have deformed this section of the 

river course also. Near island No. 13, the river leaves the 

vicinity of the Lake County uplift, and the natural levee has a 

slope typical of that of most of the meander belt in the upper 

embayment.

The lowland flood-plain profile of figure 4B is quite similar 

to the natural-levee profile. As in figure 4A a steepening of the 

profile gradient occurs between island No. 11 and Linda. 

Because the change in slope does not appear to be the product 

of any recognizable geomorphic or hydrologic process, the 

lowland flood plain and natural levee are inferred to have 

been simultaneously upwarped. Response by the two landforms to the deformation has been similar.

It is intriguing that the part of the Mississippi River flood 

plain which has formed since 1820 (fig. 4C) has a profile that 

closely matches those of the lowland flood plain and natural 

levee. It may be that the profiles are similar because the 

modern Mississippi River has not yet been able to erode away 

the effects of previous deformation to arrive at a profile of 

equilibrium. Alternatively, the post-1820 profile may be convex upward, with a relatively steeply sloping stretch between 

Tiptonville, Tenn., and island No. 11, because recent, and 

perhaps even current, deformation has warped the flood-plain 

surface. Supporting evidence for the latter interpretation is 

found on the LWRP (low water reference plane) shown in 

figure 4A. The LWRP has an anomalous convex-upward profile. In terms of possible recent movement, this profile is even 

more significant than the others, because the river's water surface would be expected to adjust itself relatively quickly to any 

imposed perturbation in its gradient, either by eroding its bed 

(and thus changing the gradient), its banks (and thus modifying the width-depth ratio), or both in an effort to achieve an 

equilibrium state. Such hydraulic equilibriums are generally 

believed to be attained relatively rapidly and certainly before 

significant modification of flood-plain landforms can be

achieved.

Several aspects of the meander pattern of the Mississippi

River suggest control by tectonic processes. Between Cairo, 

111., and Hickman, Ky., the river is relatively straight; from 

Hickman southward to Blytheville, Ark., however, it is highly 

sinuous (fig. 7). It is quite possible that the river has straightened its course in order to increase the slope in an area (Lake 

County uplift) where tectonism is reducing it. Since the uplift 

of the Tiptonville dome, the migration of the large New 

Madrid meander loop has undoubtedly been inhibited. The 

river has, in a sense, been wedged between the topographically 

high Sikeston Ridge to the north and the dome to the south. 

The position of the Mississippi River course within its meander 

belt also suggests the possibility of tectonic influence. Between 

Cairo, 111., and Hickman, Ky., and, in general, between Blytheville, Ark., and Memphis, Tenn., the river flows along the 

eastern edge of its meander belt (fig. 7). However, between
106 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Hickman and Blytheville the river "deflects" out into the 

western half of its meander belt. The excursion approximately 

covers the area of the Lake County uplift and is most pronounced at New Madrid. It is conceivable that the river has 

been deflected to the west as a result of tectonic uplift. Old 

channel maps indicate that the position of the river in 1765 

was similar to the river's modern location. Thus, any significant tectonic deflection must have occurred before 1765.

TECTONICALLY INDUCED SUBSIDENCE 

REELFOOT LAKE BASIN

Historic accounts by Lyell (1849), Usher (1837), McGee 

(1892), and Fuller (1912) reported that Reelfoot Lake, Tenn., 

formed as a result of subsidence during the 1811-12 New 

Madrid earthquakes. Located just east of Tiptonville dome, 

the lake is actually a series of irregularly shaped, interconnected water bodies, most of which occupy parts of abandoned 

Mississippi River channels (fig. 1). An analysis of bathymetric 

maps of the lake bottom discloses that much of the lake is less 

than 2 m deep, though some spots are as deep as 5 m. The lake 

bottom (contoured at a 1-ft interval) contains no discontinuities or scarps that cannot be attributed to relict fluviatile morphologies. It is therefore inferred that the lake bottom has not 

been broken by dip-slip faulting. The bathymetric survey also 

shows that relict bayous flanked by small levees (which are now 

beneath the lake's surface) once flowed between deeper 

pockets of water but were not present within them. The association of the lake with old river channels, the occurrence of 

water depths in excess of 3 m, particularly in the southern part 

of the lake, and the presence of discontinuous bayou levees 

suggest that remnants of old oxbow lakes were present in the 

area before the time of the New Madrid earthquakes. Drowned cypress trees lying just beneath the modern water surface, 

partially eroded Indian mounds located on the lake's northwest shore, and submerged sycamore, hackberry, and box 

elder trees that formerly marked the boundaries of several of 

the old George Doherty land grants (McGill and Craig, 1933; 

Tennessee State Library and Archives, 1979) are evidence that 

the lake was enlarged during the earthquakes. The available 

evidence is insufficient to prove whether the enlargement was 

caused by compaction of Holocene channel clays as suggested 

by Glenn (1933), by tectonic subsidence, or by faulting (?) and 

uplift which would have dammed off the lake's natural 

drainage across the abandoned Tiptonville meander (figs. 1 

and 6C). Figure 6C clearly shows the tectonic warping of the 

Tiptonville meander along Reelfoot scarp at the lake's 

southern edge. It may be that compaction, subsidence, and 

upwarping occurred simultaneously during the New Madrid 

earthquakes to effect the enlargement of the lake. Thus the 

genesis of Reelfoot Lake basin seems to have been associated 

with Reelfoot scarp and Tiptonville dome and not with the 

zone of faults that underlies Ridgely Ridge and that appears to 

continue into the subsurface beneath the lake.

EVIDENCE FOR REGIONAL 

DEFORMATION

Several areas of considerable areal extent adjacent to the 

Lake County uplift have been suggested as having been upwarped (Glenn, 1906; Fuller, 1912; Wells, 1933; Stearns, 

1979). Two such areas on the Little River surface and one 

within the uplands immediately east of Reelfoot Lake, Tenn., 

have been investigated. Using the technique of contour 

averaging, a procedure similar to the topographic-envelope 

method employed by Stearns (1979), I have generated a contour map (5-ft interval for the Mississippi River valley and 

100-ft for most of the Mississippi Embayment uplands north 

of Memphis, Tenn.) (fig. 9). Examination of the map reveals 

that the Little River surface west of the area between New 

Madrid, Mo., and Caruthersville contains anomalous deflections of topographic contours. Normally, the contours on this 

surface would be expected to trend in an approximate eastwest direction, perpendicular to the original river gradient. 

However, the map shows that the contours are deflected 

strongly to the southeast. Stearns (1979) has suggested that the 

contour excursions may have been caused by possible gentle 

warping of the flood plain, and he referred to the anomalous 

area as the Portageville bulge. More recently, Steven 

Obermeier (written commun., 1979) has collected more than 

600 logs of drill holes on the Little River surface. Analysis of 

the types, distributions, and thicknesses of sediments recorded 

in the logs conclusively shows that the anomalous bulging is 

solely the product of recent overbank sedimentation produced 

by floodwaters that spilled onto the old braided surface from 

the modern meander belt. Figure 10 shows, pictorially, the relationships of the Little River surface, the Mississippi River 

meander belt, and Portageville bulge. As the meander belt aggraded its surface above the level of the adjacent Little River 

plain, backswamp, natural-levee, and crevasse deposits spread 

over the surface and began to bury it. In particular, Right 

Hand Chute of Little River, a major crevasse of the Mississippi 

River, aggraded its channel and formed prominent natural 

levees of its own near Portageville (fig. 7). The bulging effect 

was enhanced at the critical location where aggrading crevasse 

deposits and levee deposits of an abandoned Mississippi River 

channel coalesce. The burial of the old braided channels of 

the Little River surface by the younger overbank sediments is 

clearly seen in the photograph of figure 11. The location of the 

photograph and a diagrammatic representation of the morphologies present are shown as an inset in figure 3. Significantly, the places on the photo where the braided pattern is truncated are also the exact locations where the topographic contours begin their deflection. The sum of the evidence, therefore, supports a nontectonic origin for the Portageville bulge.

Fuller (1912) described a topographic bulge on the Little 

River surface in northeastern Arkansas, which he named the 

Blytheville dome. He believed that the Blytheville dome was, 

along with the Tiptonville dome, one of the major tectonic 

features formed during the New Madrid earthquakes.
90°52'30" 

37°00 I r

STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI

90"00' _____ 89W

107

88W

36°00

35W

0 5

Scale

10 15 20 Miles

0 5 1015 20 Kilometers

FIGURE 9. Regional topographic map of part of the upper Mississippi Embayment. The Mississippi River meander belt, uplands, and individual braided - 

stream surfaces are contoured separately using contour intervals of 5 ft in the Mississippi River valley and 100 ft in the uplands north of Memphis, 

Tenn. Contours were determined by averaging method. Hachured symbols denote topographic scarps. Bold dashed lines define boundaries of 

meander belt.

However, detailed analysis of topographic maps and profiles 

and of subsurface data indicates that, like the Portageville 

bulge, the Blytheville dome is the product of depositional processes. Blytheville is situated at the junction of the Mississippi 

River meander belt and Left Hand Chute of Little River, a 

major Mississippi River crevasse. Natural-levee deposits from 

both the aggraded crevasse channel and adjacent meander 

belt combine to form a topographic bulge that has no association with tectonic upwarping.

In studies of ground water and stratigraphic sequences in 

the uplands east of the Mississippi River valley, Glenn (1906) 

and Wells (1933) recognized an inconsistency in the regional

slope of the land surface. The slope, which has a northwesterly 

dip, reverses its direction and rises anomalously in the vicinity 

of the Mississippi River valley bluffs. Projection of the regional 

slope across the uplands reveals that, near the bluffs, upland 

elevations should be about 350 ft above mean sea level. In actuality, however, elevations within a zone extending from the 

line of bluffs southeastwardly about 10-25 km range from 400 

ft to more than 500 ft above mean sea level (fig. 9). Glenn and 

Wells attributed the anomalous topographic rise to uplift 

associated with Mississippi River valley earthquakes. Because 

an uplift of about 150 ft far exceeded the inferred upwarping 

that they believed to have been produced during the New
108 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

X'

Wo

MILES

FIGURE 10. Cross section X-X ' showing topographic and sedimentologic relationships of the Mississippi River meander belt, the Little River braidedstream surface, and the Portageville bulge. Location of section shown on figure 7. Datum A represents the original level of the once-continuous composite Little River surface. Datum B represents the level of the aggraded Mississippi River meander belt prior to deformation by the Lake County uplift. 

Patterned area represents extent and thickness of the Portageville bulge, composed of overbank sediments deposited during and following aggradation 

of the adjacent meander belt. Vertical exaggeration, 211 times.

Madrid earthquakes in the Mississippi River valley, Glenn and 

Wells postulated that the rise was the cumulative result of 

uplift associated with numerous previous earthquakes. 

Although Glenn's and Well's explanation for the topographic 

rise sounded plausible, no substantial evidence was advanced 

to support it. They relied solely on spatial coincidence.

Geomorphic and tectonic investigations of the uplands and 

nearby Mississippi River strongly indicate that the anomalously high topography in the uplands east of the Mississippi River 

valley bluffs is the product of differential erosion and eolian 

deposition unrelated to tectonism. As seen in figure 9, topographically high areas occur all along the bluffs from Memphis, Tenn., northward to Hickman, Ky. The pattern produced by the distribution of these "highs" is neither closely related 

to any known tectonic feature or mechanism, nor is it spatially 

coincident with the pattern of historic or modern earthquake 

epicenters (Nuttli, 1973; Stauder and others, 1979). Field 

studies and geologic maps (for example, Schreurs and Marcher, 1959) indicate that the topographically high areas are, 

however, coincident with a belt of Pliocene (?) gravel 

(Lafayette Gravel of former usage), a widespread fluviatile 

deposit of variable thickness, and thick sections of Pleistocene 

loess that overlie the gravel. The gravel deposits also occur 

along a high stream divide adjacent to the Tennessee River but 

are not present between this divide and the general area of the

Mississippi River valley bluffs (Hardeman and Miller, 1959). 

Outcrops in this intervening zone are dominated by unconsolidated Pleistocene, Eocene, and Cretaceous sands (Miller 

and others, 1966). Reconstruction of the geomorphic history 

of western Tennessee indicates that immediately prior to 

deposition of the Pliocene (?) gravel, the relatively low area occupied by the Cenozoic and Mesozoic sands was a broad, 

regional topographic high. The center of the high ran in a 

northeasterly direction through Martin, Tenn., and formed 

the divide between the Mississippi River and Tennessee River 

drainages. (See, for example, Olive, 1981.) Primary deposition of the gravel probably occurred during Pliocene time 

(Olive, 1981) and filled the ancestral Mississippi and Tennessee River valleys, which were somewhat wider than their 

modern counterparts. No significant gravel deposition, 

however, took place on the northeast-trending divide region.

FIGURE 11. - Aerial photograph of the Portageville, Mo., area showing part 

of Little River braided-stream surface, Mississippi River meander belt, 

and Portageville bulge. Note extremely sharp contact between darktoned braided- stream pattern and light-toned Portageville bulge. Contact occurs where topographic contours are deflected from an east-west 

to a northwest-southeast trend and represents boundary of significant 

overbank deposition. Outline of area of photograph shown on figure 3.
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 109
110 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Since the Pliocene, differential erosion has removed the soft 

unconsolidated sands of the ancient divide area much more 

easily and rapidly than the gravel deposits. The gravel has 

acted as an armoring veneer and has undergone insignificant 

denudation relative to the sandy area. Thus, since the 

Pliocene, much of western Tennessee has suffered a striking 

inversion of topographic relief. Formation of the modern 

Mississippi and Tennessee River valleys has accentuated the 

inversion. Beds of Pliocene (?) gravel originally deposited in 

and along topographically low valleys today survive as remnant, narrow bands along high topographic divides. The once 

broad and high sandy divide has been reduced to a relatively 

low and rolling plain. Deposition of eolian loess during the IIlinoian and Wisconsin Glaciations has resulted in deposits 

6-18 m thick on and immediately east of the Mississippi River 

valley bluffs (Leighton and Willman, 1950). Though these 

sediments continue to the east for a considerable distance, 

their thickness rapidly diminishes away from the bluffs.

All of the available evidence indicates that the topographically high area adjacent to the Mississippi River valley bluffs 

formed as a compound result of a profound inversion of 

topographic relief due to differential erosion, followed by the 

deposition of significant sections of loess. There is no need to 

invoke a tectonic explanation for the high area, nor is there 

sufficient evidence to support such an explanation.

SIGNIFICANCE OF DEFORMATION

AND ASSOCIATION 

WITH HISTORIC SEISMICITY

The evidence gathered in this investigation indicates that 

the surface- geologic structures in the New Madrid region are 

complex, composite features that formed during a series of 

tectonic events. Most of the deformation occurred in association with large-magnitude earthquakes. Correlations show 

that the geometry and location of the surficial structures are to 

a large measure controlled by underlying, ancient and deepseated geologic structures. Most of these structures lie along 

the axial zone of an inferred Precambrian crustal rift interpreted from aeromagnetic and gravity data (Hildenbrand and 

others, 1977; Hildenbrand and others, this volume).

The complexity of the Lake County uplift is revealed by the 

presence of several distinct topographic bulges within the 

greater zone of upwarping. The bulges exhibit differences in 

relief, geometry, orientation, time of formation and relation 

to subsurface structures, gravity, and locations of modern 

seismicity. Yet, as will be discussed later in this section, the 

component elements of the uplift are probably genetically interrelated by the common modern stress field responsible for 

their formation. The composite nature of the uplift is clearly 

seen by comparing the characteristics of Tiptonville dome, 

Ridgely Ridge, and the warped area located at the southern 

end of Sikeston Ridge (table 1).

The uplifted area at the southern end of Sikeston Ridge is a 

broad, gentle flexure, which apparently formed more than 

4000 years ago. Recordings by the Saint Louis University 

seismic network reveal, however, that this region and an adjacent part of the Little River surface are the sites of numerous 

recent microearthquakes (fig. 12). Thus the stresses that produced the uplift are probably still active today. Farther to the 

south, Tiptonville dome deforms sediments that were 

deposited about 2000 years ago (Russ, 1979). The shape and 

origin of Tiptonville dome are closely associated with Reelfoot 

fault and the monoclinal fold that underlies Reelfoot scarp. 

The exploratory trench that was excavated across Reelfoot 

scarp in the summer of 1977 revealed a close genetic relationship between structures exposed in the trench walls and the 

morphology of the scarp and adjacent dome (Russ, 1979; Russ 

and others, 1978). In addition, evidence in the trench indicates that much of Tiptonville dome was formed during at 

least two earthquakes of intensities MM > VIII that occurred 

before 1800. Other evidence not associated with the trench 

suggests that some uplift took place along the western edge of 

the dome during and possibly subsequent to the 1811-12 

earthquakes. The New Madrid region therefore has experienced at least three large earthquakes (mfc > 6.2)in about 

the last 2000 years. The seismogenic significance of Reelfoot 

fault is somewhat problematic. It is possible that the fault may 

shallow out and die above depths of about 5-15 km, which are 

the depths generally calculated for earthquake hypocenters in 

the New Madrid region (Stauder and others, 1979). Plots of 

recent microearthquake epicenters determined by the Saint 

Louis University network show that most of Tiptonville dome, 

including the Reelfoot fault area, has been relatively quiet 

seismically at least for the past 5 years (Stauder and others, 

1979). Only along the westernmost part of the dome, near the 

Mississippi River, are modern earthquakes abundant. And as 

was previously pointed out, the Mississippi River profile at this 

position is convex, possibly as a result of minor active upwarping.

In contrast to the general seismic quiescence of much of 

Tiptonville dome, Ridgely Ridge is currently the locus of the 

most abundant seismic activity in the entire New Madrid 

region. The great amount of seismicity is somewhat perplexing 

because most of the deformation of the ridge appears to 

predate the creation of Tiptonville dome. And whereas parts 

of Tiptonville dome were uplifted during the 1811-12 earthquakes, there is no evidence to suggest that deformation occurred along Ridgely Ridge at that time. The seismotectonic 

framework of the Ridgely Ridge area has not yet been resolved 

satisfactorily. The ridge coincides spatially with an underlying 

zone of faults and a well-defined gravity anomaly. All of these 

features trend northeast-southwest and may possibly be genetically related to the geologic structures associated with a 

well-defined zone of earthquake epicenters trending southwest 

from Caruthersville for about 100 km. Conversely, some recent epicentral plots in the Ridgely area (Stauder and others,
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 111 

TABLE 1. Comparison of geological and geophysical characteristics of topographic components of the Lake County uplift

Topographic Relative

component topographic

relief

Southern end Low 

of Sikeston

Ridge.

Tiptonville dome High 

Ridgely Ridge Moderate

Crosssectional Orientation

geometry of axis

Shallow, broad Generally

arch. northnorthwest.

Strongly asymmetric North 

arch; relatively

steeply dipping

limb on the east;

moderate width.

Generally symmetric Northeast 

arch; relatively

narrow width.

Time of

formation

Late Pleistocene

to about 6000

yr B.P.

Generally less than

2000 yr B.P.

Generally less than

6000 yr B.P.

Surface faults

None known 

3+ m

of normal

offset along

eastern edge

of dome ;

believed continuous with

subsurface offset of Reelfoot

fault.

None known -

Subsurface

faults

Single identified

dip-slip

fault.

Several

high-angle faults

that have general

strike parallel to

orientation of

axis.

Parallel and coincident with a

northeast- trending

zone of faults.

Correlations of 

gravity with topographic component

(from Hildenbrand

and others, 1979;

and Stearns, 1979)

No correlation.

Moderate correlation with

gravity high.

Strong correlation

with northeasttrending linear

gravity high.

1979; Nicholson and Singh, 1979; O'Connell and others, this 

volume; R. M. Hamilton, written commun., 1979) reveal the 

presence of a northwest-trending zone of seismic events. The 

relationship of this zone to the northeast-trending faults and to 

the overall occurrence of seismicity in the Ridgely area is currently unknown.

In summary, it is worthwhile emphasizing two points:

1. A very close spatial relationship exists between modern 

seismicity and the geographic extent of tectonic surface deformation (fig. 12). Approximately 75 percent of the microearthquakes recorded in the New Madrid region between 1974 and 1978 have occurred within the area of the 

Lake County uplift. The northwestern part of Tiptonville 

dome that was uplifted during the 1811-12 earthquakes is 

still active today, whereas the eastern part of the dome 

along Reelfoot fault (including the southern part that is believed to have been uplifted in 1811-12) is not an area of 

abundant modern seismicity. Though there is no evidence 

of deformation on Ridgely Ridge in 1811-12, that area is 

currently seismically active.

2. At least three earthquakes of intensity strong enough to 

cause tectonic faulting, folding, and liquefaction of surface 

sediments have occurred in the New Madrid region in the 

last 2000 years. A recurrence rate of about 600 years for 

earthquakes of mD ^6.2 (the approximate threshold of 

liquefaction in the New Madrid region) is therefore indicated. Inasmuch as this rate is similar to the recurrence 

interval for earthquakes of mfc ^7 as determined from 

earthquake intensity data (Nuttli, 1974), it is suggested that 

the faulting, folding, and liquefaction are related to earthquakes of mb ^7 that recur about every 600 years.

Until recently, few data have been available to help explain 

the cause of the Lake County uplift. Even with the recently obtained geomorphic and seismic evidence, the origin of the 

uplift remains somewhat problematic. After considering 

several causes, Fuller (1912) concluded that the most probable 

explanation for the upwarping involved subsidence of the deep 

center of the Mississippi Embayment. He thought that such 

subsidence might produce a depression that would cause the 

overlying unconsolidated sediments to slide or slump toward 

the embayment center and that would produce, in the process, 

a corrugated surface. The high parts of the surface would have 

the configuration of domes. Recent subsurface drilling and 

seismic evidence, however, do not support this hypothesis. Offsets of the base of the Mesozoic section of sufficient magnitude 

to produce the required depression have not been identified. 

In addition, the deep center of the embayment is located 

beneath the Lake County uplift and not farther to the west, in 

southeastern Missouri, as Fuller suspected.

Detailed examination of the seismicity and the structures 

beneath the uplift strongly suggests that the uplift is related to 

tectonic processes localized on the ancient subsurface structures. Such processes appear to be strongly dependent upon 

the orientation and magnitude of the modern stress field. 

Herrmann and Canas (1978) and Herrmann (1979) have determined composite and single-event fault-plane solutions, respectively, for earthquakes in the New Madrid region. Their 

results indicate that the maximum regional compressive stress 

is horizontal and trends in an approximate east- west direction. The mechanisms they calculated also show that major 

zones of seismicity extending in a linear trend southwest from 

Caruthersville and northeast from Lilbourn, Mo., are best 

modeled as continuous faults having right-lateral strike-slip 

displacement. Focal-plane mechanisms for the numerous 

earthquakes that have occurred between Ridgely, Tenn., and 

Lilbourn, Mo. (fig. 12), have been determined by Herrmann 

and Canas (1978), Herrmann (1979), O'Connell and others
116 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

This paper describes the significant characteristics and 

areal extent of the uplift as they are expressed on the Paleozoic 

bedrock. In the study area (fig. 1), two-way travel times to the 

reflection that represents the top of the Paleozoic rocks were 

read from the profiles and gridded to produce an evenly spaced network of data points. Hand- and computer-contoured 

plots of the gridded data illustrate the complex nature of the 

Paleozoic bedrock surface. Trend-surface analyses of the gridded data define the regional extent of the warping and emphasize anomalously high and low areas. The amount of relief 

on the uplift has been estimated from reconstructed contours.

The purpose of this study was to identify the regional structural trends in the area and to evaluate them in terms of the 

overall pattern of seismicity. Detailed analyses of the reflection 

profiles show a variety of complex faulted features and bodies 

of intrusive igneous rock (Zoback and others, 1978; Zoback 

and others, 1980). Because of the regional scope of this study, 

the effects of individual faults and other local features were 

not incorporated into this analysis.

GENERAL GEOLOGIC SETTING

The upper Mississippi Embayment is a broad, slightly asymmetrical, southwest-plunging syncline of unconsolidated Cretaceous and Tertiary sediments (Murray, 1961; Stearns and 

Marcher, 1962; Gushing and others, 1964) which rest on 

deformed bedrock of Cambrian to Pennsylvanian age 

(Freeman, 1953; Caplan, 1954; Grohskopf, 1955). A major 

unconformity, representing erosion which may have begun as 

early as the Pennsylvanian and continued into the Late Cretaceous (Marcher and Stearns, 1962), developed on the Paleozoic rocks. Downwarping of the Paleozoic rocks began in the 

Late Cretaceous, and by the end of Cretaceous time, the first 

major marine invasion deposited sands, silts, and clays 

throughout the area (Stearns, 1957). The embayment continued to subside as repeated transgressive-regressive cycles 

persisted into the middle Eocene and deposited thick sequences of marine and nonmarine sediments. By the end of 

the Eocene, subsidence generally ceased and the upper embayment filled with sediments (Gushing and others, 1964; 

Russell and Parks, 1975). In the Pliocene or early Pleistocene, 

a veneer of coarse alluvial and fluvial gravels blanketed the 

area (Potter, 1955; Russell and Parks, 1975). In response to 

the Pleistocene glaciations, the ancestral Mississippi and Ohio 

River systems filled lowland areas with braided-stream 

deposits while prevailing winds mantled the adjacent uplands 

with loess (Saucier, 1974). Starting about 6000 years ago, the 

braided river systems evolved into a meandering regime and 

began reworking the older unconsolidated deposits.

Because of the thick veneer of Cretaceous and Tertiary 

sediments and sparse exploratory drilling, the configuration of 

the Paleozoic bedrock surface in the upper embayment is only 

generally known. Only a handful of widely separated wells

have penetrated Paleozoic rocks in the New Madrid region. 

Most of these wells are old and have poor records, which usually consist of crude drillers' logs. A few of the wells have geophysical logs, which are necessary for accurate correlations, 

but those wells are so scattered that detailed structural interpretations are impossible or meaningless. From the limited 

subsurface information, numerous regional studies have confirmed the basic synclinal character of the Paleozoic bedrock 

surface (fig. 2) (Roberts and Gildersleeve, 1950; Caplan, 1954; 

Grohskopf, 1955; Gushing and others, 1964; Schwalb, 1969; 

Stearns and Wilson, 1972). However, subtle structures remain 

undetectable because of insufficient and scattered subsurface 

data. On the basis of limited well control and topographic 

lineations, Stearns and Zurawski (1976) constructed a series of 

interpretative structural contour maps which attempted to 

portray the configuration of the Paleozoic bedrock surface 

more accurately. Nevertheless, to date, the reflection profiles 

are the most complete and most reliable source of subsurface 

structural information available in the upper Mississippi Embayment.

The assumption that the bedrock surface in the study area 

was of low relief prior to Upper Cretaceous sedimentation is 

critical to all of the structural interpretations presented here. 

The top of the Paleozoic bedrock gently slopes toward the axis 

of the embayment at about 35 ft/mi (6.6 m/km) (Boswell and 

others, 1965) with only minor interruptions (Grohskopf, 

1955). Any significant deformation recognized on the bedrock 

surface other than the broad downwarping of the embayment 

is attributed to and interpreted in terms of tectonic processes.

General knowledge of the geologic history of the northern 

Mississippi Embayment suggests that there was very little 

topographic relief on the bedrock by Upper Cretaceous time. 

The Paleozoic bedrock in the study area is predominantly 

Cambrian and Ordovician carbonates that were uplifted as 

part of the Pascola arch (Grohskopf, 1955). For perhaps as 

long as 180 m.y. (Davis and others, 1973) during the Paleozoic 

and Mesozoic, the arch was extensively eroded and several 

kilometers of sediments were removed (Marcher and Stearns, 

1962). By Late Cretaceous time, the Paleozoic bedrock was 

beveled to a mature, low-relief surface with a well-developed 

drainage system (Pryor, 1960).

The character of the Paleozoic reflector on profile line S-10 

and data from the USGS New Madrid test well adjacent to the 

profile line (drill hole C, fig. 1) further support the belief that 

only minor relief remained on the bedrock surface. The USGS 

well shows that the bedrock beneath line S-10 is altered, 

highly fractured dolomite (Crone and Russ, 1979). Because 

the test well is located near the crest of the Pascola arch, which 

is an uplift that may have been structurally elevated more than 

8000 ft (2625 m) (Marcher and Stearns, 1962) beginning in the 

late Paleozoic (Bond and others, 1971), it is reasonable to expect that the brittle dolomites beneath line S-10 would have 

been fractured prior to the erosion that produced the Paleozoic-Late Cretaceous unconformity. When exposed, soluble,
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 

TABLE 1. Comparison of geological and geophysical characteristics of topographic components of the Lake County uplift

111

Topographic Relative

component topographic

relief

Southern end Low 

of Sikeston

Ridge.

Tiptonvllle dome High 

Ridgely Ridge Moderate

Crosssectional Orientation

geometry of axis

Shallow, broad Generally

arch. northnorthwest.

Strongly asymmetric North 

ari_h; relatively

steeply dipping

limb on the east;

moderate width.

Generally symmetric Northeast 

arch; relatively

narrow width.

Time of

formation Surface faults

Late Pleistocene None known 

to about 6000

yr B.P.

Generally less than 1+ m

2000 yr B.P. of normal

offset along

eastern edge

of dome ;

believed continuous with

subsurface offset of Reelfoot

fault.

Generally less than None known 

6000 yr B.P.

Subsurface

faults

Single identified

dip-slip

fault.

Several

high-angle faults

that have general

strike parallel to

orientation of

axis.

Parallel and coincident with a

northeast- trend ing

zone of faults.

Correlations of 

gravity with topographic component

(from Hildenbrand

and others, 1979;

and Stearns, 1979)

No correlation.

Moderate correlation with

gravity high.

Strong correlation

with northeasttrending linear

gravity high.

1979; Nicholson and Singh, 1979; O'Connell and others, this 

volume; R. M. Hamilton, written commun., 1979) reveal the 

presence of a northwest-trending zone of seismic events. The 

relationship of this zone to the northeast-trending faults and to 

the overall occurrence of seismicity in the Ridgely area is currently unknown.

In summary, it is worthwhile emphasizing two points:

1. A very close spatial relationship exists between modern 

seismicity and the geographic extent of tectonic surface deformation (fig. 12). Approximately 75 percent of the microearthquakes recorded in the New Madrid region between 1974 and 1978 have occurred within the area of the 

Lake County uplift. The northwestern part of Tiptonville 

dome that was uplifted during the 1811-12 earthquakes is 

still active today, whereas the eastern part of the dome 

along Reelfoot fault (including the southern part that is believed to have been uplifted in 1811-12) is not an area of 

abundant modern seismicity. Though there is no evidence 

of deformation on Ridgely Ridge in 1811-12, that area is 

currently seismically active.

2. At least three earthquakes of intensity strong enough to 

cause tectonic faulting, folding, and liquefaction of surface 

sediments have occurred in the New Madrid region in the 

last 2000 years. A recurrence rate of about 600 years for 

earthquakes of mb>6.2 (the approximate threshold of 

liquefaction in the New Madrid region) is therefore indicated. Inasmuch as this rate is similar to the recurrence 

interval for earthquakes of mfc ^7 as determined from 

earthquake intensity data (Nuttli, 1974), it is suggested that 

the faulting, folding, and liquefaction are related to earthquakes of mfc >7 that recur about every 600 years.

Until recently, few data have been available to help explain 

the cause of the Lake County uplift. Even with the recently obtained geomorphic and seismic evidence, the origin of the 

uplift remains somewhat problematic. After considering 

several causes, Fuller (1912) concluded that the most probable 

explanation for the upwarping involved subsidence of the deep 

center of the Mississippi Embayment. He thought that such 

subsidence might produce a depression that would cause the 

overlying unconsolidated sediments to slide or slump toward 

the embayment center and that would produce, in the process, 

a corrugated surface. The high parts of the surface would have 

the configuration of domes. Recent subsurface drilling and 

seismic evidence, however, do not support this hypothesis. Offsets of the base of the Mesozoic section of sufficient magnitude 

to produce the required depression have not been identified. 

In addition, the deep center of the embayment is located 

beneath the Lake County uplift and not farther to the west, in 

southeastern Missouri, as Fuller suspected.

Detailed examination of the seismicity and the structures 

beneath the uplift strongly suggests that the uplift is related to 

tectonic processes localized on the ancient subsurface structures. Such processes appear to be strongly dependent upon 

the orientation and magnitude of the modern stress field. 

Herrmann and Canas (1978) and Herrmann (1979) have determined composite and single-event fault-plane solutions, respectively, for earthquakes in the New Madrid region. Their 

results indicate that the maximum regional compressive stress 

is horizontal and trends in an approximate east- west direction. The mechanisms they calculated also show that major 

zones of seismicity extending in a linear trend southwest from 

Caruthersville and northeast from Lilbourn, Mo., are best 

modeled as continuous faults having right-lateral strike-slip 

displacement. Focal-plane mechanisms for the numerous 

earthquakes that have occurred between Ridgely, Tenn., and 

Lilbourn, Mo. (fig. 12), have been determined by Herrmann 

and Canas (1978), Herrmann (1979), O'Connell and others
112

36°45'00"

36°30'00" 

36°15'00" 

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION 

89°45'00" 89°30'00" 89°15'00'

FIGURE 12. Map showing spatial relationship between modern microearthquakes (small dots) and the area of the Lake County uplift (dashed line). 

Microearthquakes shown here were recorded during the time interval from 1 July 1974 to 30 June 1978 by the Saint Louis University network. 

Modified from Stauder, this volume.

(this volume), and Nicholson and Singh (1978). Results of the tions and the calculated direction of the maximum compresanalyses show a variety of solutions, indicating a complex tec- sive stress, a theoretical model can be constructed that offers a

tonic regime. The majority of the analyses, however, have pro- plausible explanation for the origin of the Lake County uplift,

duced reverse-fault solutions. Based upon the fault-plane solu- As displayed in figure 13, the maximum compressive stress of
STYLE AND SIGNIFICANCE OF SURFACE DEFORMATION, NEW MADRID, MISSOURI 113

the regional modern stress field is oriented in a horizontal eastwest direction. This direction is consistent with right-lateral 

displacement on the faults of the two major northeasttrending seismic zones. Movement on the two faults generates 

compression in the region between the faults. The compression would be highest in the area where the opposite-moving 

blocks abut one another. The area coincides precisely with the 

locations of abundant epicenters, some with reverse-fault focal 

mechanisms, and with the area of the Lake County uplift. As 

displayed in the cross-sectional diagram of figure 13 (X-X '), 

the compression is translated into vertical strain (Poisson 

bulge), resulting in uplift above the area of opposed block 

movement. The theoretical model just presented is quite 

similar to the concept of en echelon surface folds that are produced by wrench tectonics (Wilcox and others, 1973). In the 

New Madrid region the transverse offset in the strike-slip 

system between Caruthersville and New Madrid, Mo., would 

serve to increase the horizontal compression and localize the 

folding in the area of offset.

It is doubtful that any of the surface uplift could be the product of warping associated with the emplacement of intrusive 

rocks. Evidence interpreted from seismic-reflection profiles 

suggests that intrusions occurred beneath Ridgely Ridge some 

time following the middle Eocene (Zoback and others, 1980). 

The absence of significant subsurface heat-flow anomalies in 

this area (Swanberg and others, this volume), however, suggests that intrusions have not occurred during the last few 

thousand years. Consequently the surface sediments, which 

are about 2000-6000 years old, were not deformed by stresses 

caused by intrusive activity.

The Lake County uplift can be compared to similar surface 

warping associated with large earthquakes in other parts of the 

country. The Ocotillo Badlands in southern California is an 

anticlinal dome that lies astride left- stepping en echelon 

segments of Coyote Creek fault (Clark, 1972; Sharp and Clark, 

1972; Segall and Pollard, 1980). Through geologic time the 

predominant movement on segments of the fault has been 

right-lateral strike slip. During the 1968 Borrego Mountain 

earthquake, three segments of Coyote Creek fault broke, causing compression in the Ocotillo Badlands (Clark, 1972; Sharp 

and Clark, 1972). Recurring offset on these faults during the 

Quaternary has caused sedimentary strata to be uplifted, producing a complexly-folded dome with a topographic relief of 

and a width of 2 km. Fold axes in the ~ 200 m Ocotillo 

Badlands are oriented to strikes of segments of the Coyote 

Creek fault in a manner similar to the orientation of fold axes 

to faults in the Lake County uplift area of northwest Tennessee. In studies of the great Alaska earthquake of 1964 

(Plafker, 1972), it was determined that a large part of Montague Island adjacent to Prince William Sound had been upwarped 10 m. The deformed area of the Island has an 

elongated elliptical shape and is of a considerably larger 

magnitude than the Lake County uplift. As with the Lake 

County uplift, upwarping on Montague Island appears to

Limit of Lake County uplift

0 10 20 KILOMETERS

FIGURE 13.- Plan view (A) and cross section (B) of hypothetical tectonic 

model for the origin of the Lake County uplift. Model interrelates the 

modern stress field with fault movement and the location of uplift. 

Barbed arrows indicate cf\, the direction of maximum compressive 

stress; single arrows indicate right-lateral displacement on faults of major northeast-trending seismic zones; large open-headed arrows indicate compression generated by movement along faults; and saw teeth 

indicate area of highest compression. See text for explanation.

have been produced by strong horizontal compression. The 

Alaskan surface deformation, however, was caused by elastic 

rebound along the upward-moving plate of a megathrust and 

was locally intensified by subsidiary reverse faults that broke 

through the upper plate to the surface (Plafker, 1972). In contrast, surface deformation in the Lake County area is probably 

the result of compression along a deflection or offset in a major 

strike-slip fault system. Seismic data and focal mechanisms 

reveal the presence of reverse faults and antithetic normal 

faults in the region of offset between Caruthersville and New 

Madrid. There appears to be little similarity in origin between 

the Lake County uplift and the recently discovered Palmdale 

bulge in southern California. In contrast to the seismically 

generated Lake County uplift, the Palmdale bulge appears to 

be the product of aseismic warping caused by the accumulation of elastic strain (Castle and others, 1976). But like the 

Palmdale bulge, the significance of the Lake County uplift as 

it pertains to potential future earthquakes is unknown.
114 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

REFERENCES CITED

Broadhead, G. C., 1902, The New Madrid earthquake: The American

Geologist, v. 30, p. 76-87. 

Castle, R. O., Church, J. P., and Elliott, M. R., 1976, Aseismic uplift in

southern California: Science, v. 192, p. 251-253. 

Clark, M. M., 1972, Surface rupture along the Coyote Creek fault, in The

Borrego Mountain earthquake of April 9, 1968: U.S. Geological Survey

Professional Paper 787, p. 55-86. 

Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower

Mississippi River: Vicksburg, U.S. Corps of Engineers Mississippi River

Commission, 78 p. 

Fuller, M. L., 1912, The New Madrid earthquake: U.S. Geological Survey

Bulletin 494, 118 p. 

Glenn, L. C., 1906, Underground waters of Tennessee and Kentucky west of

Tennessee River: U.S. Geological Survey Water Supply and Irrigation

Paper 164, 173 p. 

_______1933, The geography and geology of Reelfoot Lake: Tennessee

Academy of Science Journal, v. 8, no. 1, p. 2-12. 

Hamilton, R. M., andZoback, M. D., 1979, Seismic reflection profiles in the

northern Mississippi embayment: U.S. Geological Survey Open-File

Report 79- 1688. 

Hardeman W. D., and Miller,R. A., 1959, Map of mineral resources and

mineral industries of Tennessee: Tennessee Department of Conservation and Commerce, Division of Geology, scale 1:500,000. 

Herrmann, R. B., 1979, Surface wave focal mechanisms for eastern North

American earthquakes with tectonic implications: Journal of Geophysical Research, v. 84, p. 3543-3552. 

Herrmann, R. B., and Canas, J. A., 1978, Focal mechanism studies in the

New Madrid seismic zone: Seismological Society of America Bulletin, v.

68, no. 4, p. 1095-1102. 

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic

and gravity anomalies in the northern Mississippi embayment and their

spacial relation to seismicity: U.S. Geological Survey Miscellaneous

Field Studies Map MF-914, scale 1:1,000,000. 

Hildenbrand, T. G., Rucks, R. P., and Kane, M. F., 1979, Simple Bouguer

gravity map of the Dyersburg 1° by 2° quadrangle, Kentucky, Missouri,

and Tennessee: U.S. Geological Survey Open-File Report 79-1183,

scale 1:1,000,000. 

Krinitzsky, E. L., 1950, Geological investigation of faulting in the lower

Mississippi Valley: U.S. Army Corps of Engineers Waterways Experiment Station Technical Memorandum 3-311, 50 p. 

Leighton, M. M., and Willman, H. B., 1950, Loess formations of the

Mississippi Valley: Journal of Geology, v. 58, no. 6, p. 599-623. 

Lyell, Charles, 1849, A "second visit to the United States, v. 2: New York,

Harper and Brothers, 287 p. 

McGee, W. J., 1892, A fossil earthquake [abs.]: Geological Society of

America Bulletin, v. 4, p. 411-415. 

McGill, J. T., and Craig, W. W., 1933, The ownership of Reelfoot Lake:

Tennessee Academy of Science Journal, v. 8, no. 1, p. 13-21. 

Miller, R. A., Hardeman, W. D., and Fullerton, D. S., 1966, Geologic map

of Tennessee: Tennessee Department of Conservation and Commerce,

Division of Geology, scale 1:250,000. 

Nicholson, C., andSingh, S., 1978, A detailed microearthquake study of the

New Madrid fault system's western Tennessee segment [abs.]:

Seismological Society of America Earthquake Notes, Eastern Section, v.

49, no. 4, p. 19. 

Nuttli, O. W., 1973, The Mississippi Valley earthquakes of 1811 and 1812

intensities, ground motion and magnitudes: Seismological Society of

America Bulletin, v. 63, no. 1, p. 227-248.

Nuttli, O. W., 1974, Magnitude-recurrence relation for central Mississippi 

Valley earthquakes: Seismological Society of America Bulletin, v. 64, 

no. 4, p. 1189-1207.

Olive, W. W., 1981, Geologic map of the Jackson Purchase region, Kentucky: U.S. Geological Survey Miscellaneous Investigations Map 

I 1217.

Penick, James, Jr., 1976, The New Madrid earthquakes of 1811-1812: 

Columbia, Mo., The University of Missouri Press, 181 p.

Plafker, George, 1972, Tectonics, in The great Alaska earthquake of 1964, 

seismology and geodesy: Washington, National Academy of Sciences, 

p. 112-188.

Russ, D. P., 1979, Late Holocene faulting and earthquake recurrence in the 

Reelfoot Lake area, northwestern Tennessee: Geological Society of 

America Bulletin, pt. 1, v. 90, p. 1013 1018.

Russ, D. P., Stearns, R. G., and Herd, D. G., 1978, Map of exploratory 

trench across Reelfoot scarp, northwestern Tennessee: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-985, scale 1:39.4.

Saucier, R. T., 1974, Quaternary geology of the lower Mississippi Valley: 

Arkansas Archaeological Survey Research Series 6, 26 p.

Schreurs, R. L., and Marcher, M. V., 1959, Geology and groundwater 

resources of the Dyersburg quadrangle, Tennessee: Tennessee Department of Conservation, Division of Geology, Report of Investigations 7, 

61 p.

Segall, P., and Pollard, D. D., 1980, Mechanics of discontinuous faults: 

Journal of Geophysical Research, v. 85, p. 4337-4350.

Sharp, R. V., and Clark, M. M., 1972, Geologic evidence of previous 

faulting near the 1968 rupture on the Coyote Creek fault, in The Borrego Mountain earthquake of April 9, 1968: U.S. Geological Survey 

Professional Paper 787, p. 131-140.

Stauder, William, Herrmann, R. B., Singh, Sudarshan, and others, 1979, 

Central Mississippi Valley earthquake bulletin: Saint Louis University, 

Department of Earth and Atmospheric Sciences, Quarterly Report 19, 

51 p.

,Stearns, R. G., 1979, Recent vertical movement of the land surface in the 

Lake County uplift and Reelfoot Lake basin areas, Tennessee, 

Missouri, and Kentucky: U.S. Nuclear Regulatory Commission 

NUREG/CR - 0874, 37 p.

Tennessee State Library and Archives, 1979, George Doherty land grant 

records: Roll no. 5, p. 62, 63, 81.

U.S. Army Corps of Engineers, 1976, Mississippi River hydrographic 

survey Cairo, Illinois, to White River, Arkansas: Memphis, Tenn., 

U.S. Army Engineer District, 98 p.

Usher, F. C., 1837, On the elevation of the banks of the Mississippi in 1811: 

Silliman's Journal (American Journal of Science, First Series), v. 31, p. 

291-294.

Wells, F. G., 1933, Ground-water resources of western Tennessee: U.S. Geological Survey Water-Supply Paper 656, 319 p.

Wilcox, R. E., Harding, T. P., and Seely, D. R., 1973, Basic wrench tectonics: American Association of Petroleum Geologists Bulletin, v. 57, 

no. 1, p. 74 96.

Zoback, M. D., 1979, Recurrent faulting in the vicinity of Reelfoot Lake, 

northwestern Tennessee: Geological Society of America Bulletin, pt. 1, 

v. 90, p. 1019-1024.

Zoback, M. D., Hamilton, R. M., Crone, A.J., Russ, D. P., andBrockman, 

S. R., 1978, Seismic reflection profiling to delineate tectonic features of 

the New Madrid seismic region [abs.]: Seismological Society of America 

Earthquake Notes, Eastern Section, v. 49, no. 4, p. 27-28.

Zoback, M. D., Hamilton, R. M., Crone, A. J., Russ, D. P., McKeown, F. 

A., and Brockman, S. R., 1980, Recurrent intraplate tectonism in the 

New Madrid seismic zone: Science, v. 209, no. 4460, p. 971-976.
Configuration and Deformation of the 

Paleozoic Bedrock Surface in the 

New Madrid Seismic Zone

By A. J. CRONE and S. R. BROCKMAN

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1236-1
CONTENTS

Page

Abstract ...............................................................................115

Introduction ............................................................................115

General geologic setting...................................................................116

Previous work...........................................................................118

Collection and processing of data ........................................................... 118

Analysis ofthe gridded data ............................................................... 120

Tiptonvilledome ....................................................................121

Ridgely ridge ....................................................................... 123

Reelfoot Lake basin .................................................................. 123

Other positive features ................................................................ 123

Trend-surface analyses ................................................................... 123

Third-degree trend surface and residuals ................................................. 125

Sixth-degree trend surface and residuals ................................................. 125

Discussion ..............................................................................126

Acknowledgments ....................................................................... 131

References cited ......................................................................... 134

ILLUSTRATIONS

Page

FIGURE 1. Location map of seismic-reflection profiles and subsurface control in the New Madrid region . ........................................ 117

2. General synclinal configuration of the Paleozoic bedrock surface in the Mississippi Embayment ....................................... 118

3. Configuration of the Paleozoic bedrock surface interpreted from the calculated two-way travel times .................................. 119

4. Plot of calculated two-way travel time vs depth to Paleozoic bedrock using well control .............................................. 120

5. Areas of deformation on the bedrock surface ................................................................................ 122

6. Third-degree trend-surface map .......................................................................................... 124

7. Contour map of third-degree trend-surface residuals ......................................................................... 126

8. Sixth-degree trend-surface map .......................................................................................... 128

9. Contour map of sixth-degree trend-surface residuals .......................................................................... 130

10. Comparison of ground-surface deformation and bedrock deformation of the Lake County uplift. ..................................... 132

11. Comparison of seismicity with ground-surface and bedrock-surface deformation in the area of the Lake County uplift .................... 133

TABLE

Page

TABLE 1. Well control for depth to bedrock in the New Madrid area and travel-time data .................................................... 121
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

CONFIGURATION AND DEFORMATION OF THE

PALEOZOIC BEDROCK SURFACE IN THE

NEW MADRID SEISMIC ZONE

By ANTHONY}. CRONE and STANLEY R. BROCKMAN

ABSTRACT

More than 280 km of multi-channel seismic-reflection profiles provide the 

first detailed information about the configuration of the Paleozoic bedrock 

surface beneath the unconsolidated sediments in the upper Mississippi Embayment. In the area of intense seismicity, roughly between Ridgely, Tenn. 

and New Madrid, Mo., data representing two-way travel times to the top of 

the Paleozoic bedrock reflector were orthogonally gridded by computer. 

Uplifted and downwarped areas were recognized and the amount of deformation was estimated by using reconstructed contours. Trend-surface 

analyses of the travel times produced mathematical models of the bedrock 

surface and allowed regional trends to be recognized.

Analysis of the gridded data, the trend-surface maps, and the trend- surface residuals confirmed that the Lake County uplift, an area of anomalously high ground elevations in western Tennessee, is underlain by uplifted 

bedrock. The subsurface uplift encompasses about 367 km2 and, like the 

topographic feature, is divided into two main parts: Tiptonville dome and 

Ridgely ridge. Tiptonville dome, a roughly equidimensional uplift, is about 

95 km2 in area and locally has been uplifted as much as 42 m. Ridgely ridge is 

a large, complex uplift that trends northeast-southwest from the southern 

end of Reelfoot Lake to beyond Cottonwood Grove, Tenn. Parts of the ridge 

are highly faulted, and in some places, the bedrock has been upwarped more 

than 50 m. Several other small areas of uplift, apparently unrelated to the 

much larger Lake County uplift, are present in the study area.

Reelfoot Lake basin, a topographically low area directly east of Tiptonville dome that is partially occupied by Reelfoot Lake, coincides with an area 

of downwarped bedrock. This suggests that the basin may be, in part, a 

structural depression which may have influenced the location of the lake. 

Abundant evidence indicates that Reelfoot Lake was enlarged during the 

1811-12 earthquakes, but it remains unclear whether the enlargement was 

due to tectonic or nontectonic processes.

The orientation and maximum upwarping on the Lake County uplift, as 

expressed on the ground surface and on the bedrock surface, were determined by different techniques, but the results are remarkably similar. This 

strong similarity suggests that the surface and subsurface features are genetically related.

Different parts of the Lake County uplift have been tectonically active at 

different times. Tiptonville dome has been active most recently, the eastern 

portion of Ridgely ridge has been uplifted in the past 6000 years, and the 

western part of Ridgely ridge was deformed more than 6000 years ago. The

patterns of modern seismicity in the vicinity of the Lake County uplift do not 

clearly show which segments of the uplift are most active today.

The close spatial relationship between the modern seismicity and the areal 

extent of the uplift suggests that the forces responsible for the earthquakes 

are also producing the uplift, and that those forces are active today. The formation of the Lake County uplift is best explained by a tectonic model proposed by Russ (this volume) that is consistent with local and regional geologic 

and geophysical data, and with fault-plane solutions.

INTRODUCTION

As part of a series of continuing investigations to characterize the tectonic processes and to evaluate the seismic hazards in 

the upper Mississippi Embayment, the USGS (U.S. Geological 

Survey) has obtained about 280 km of multi-channel seismicreflection profiles in the area of intense seismic activity from 

northeastern Arkansas to New Madrid, Mo. The reflection 

profiles provide the first detailed information about the configuration of the bedrock surface beneath the unconsolidated 

embayment sediments and confirm the existence of a positive 

structural feature in Lake County, Tenn., which has deformed competent Paleozoic rocks.

An area of anomalously high ground has long been 

recognized in Lake County, Tenn. (Usher, 1837). The 

elevated ground has been presumed to be of tectonic origin 

(McGee, 1892; Fuller, 1912), but the relationship between this 

surface feature in unconsolidated alluvium and any subsurface structure in lithified bedrock has not been firmly established. The reflection profiles show for the first time that the 

surface deformation is a direct expression of deeper warping 

on the Paleozoic bedrock. Thus, the surface doming is a tectonic feature. Furthermore, the areas of surface and subsurface warping coincide with some of the most intense seismicity, 

suggesting that the tectonic stresses responsible for the formation of the uplift are still active today.

115
116 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

This paper describes the significant characteristics and 

areal extent of the uplift as they are expressed on the Paleozoic 

bedrock. In the study area (fig. 1), two-way travel times to the 

reflection that represents the top of the Paleozoic rocks were 

read from the profiles and gridded to produce an evenly spaced network of data points. Hand- and computer-con toured 

plots of the gridded data illustrate the complex nature of the 

Paleozoic bedrock surface. Trend-surface analyses of the gridded data define the regional extent of the warping and emphasize anomalously high and low areas. The amount of relief 

on the uplift has been estimated from reconstructed contours.

The purpose of this study was to identify the regional structural trends in the area and to evaluate them in terms of the 

overall pattern of seismicity. Detailed analyses of the reflection 

profiles show a variety of complex faulted features and bodies 

of intrusive igneous rock (Zoback and others, 1978; Zoback 

and others, 1980). Because of the regional scope of this study, 

the effects of individual faults and other local features were 

not incorporated into this analysis.

GENERAL GEOLOGIC SETTING

The upper Mississippi Embayment is a broad, slightly asymmetrical, southwest-plunging syncline of unconsolidated Cretaceous and Tertiary sediments (Murray, 1961; Stearns and 

Marcher, 1962; Gushing and others, 1964) which rest on 

deformed bedrock of Cambrian to Pennsylvanian age 

(Freeman, 1953; Caplan, 1954; Grohskopf, 1955). A major 

unconformity, representing erosion which may have begun as 

early as the Pennsylvanian and continued into the Late Cretaceous (Marcher and Stearns, 1962), developed on the Paleozoic rocks. Downwarping of the Paleozoic rocks began in the 

Late Cretaceous, and by the end of Cretaceous time, the first 

major marine invasion deposited sands, silts, and clays 

throughout the area (Stearns, 1957). The embayment continued to subside as repeated transgressive-regressive cycles 

persisted into the middle Eocene and deposited thick sequences of marine and nonmarine sediments. By the end of 

the Eocene, subsidence generally ceased and the upper embayment filled with sediments (Gushing and others, 1964; 

Russell and Parks, 1975). In the Pliocene or early Pleistocene, 

a veneer of coarse alluvial and fluvial gravels blanketed the 

area (Potter, 1955; Russell and Parks, 1975). In response to 

the Pleistocene glaciations, the ancestral Mississippi and Ohio 

River systems filled lowland areas with braided-stream 

deposits while prevailing winds mantled the adjacent uplands 

with loess (Saucier, 1974). Starting about 6000 years ago, the 

braided river systems evolved into a meandering regime and 

began reworking the older unconsolidated deposits.

Because of the thick veneer of Cretaceous and Tertiary 

sediments and sparse exploratory drilling, the configuration of 

the Paleozoic bedrock surface in the upper embayment is only 

generally known. Only a handful of widely separated wells

have penetrated Paleozoic rocks in the New Madrid region. 

Most of these wells are old and have poor records, which usually consist of crude drillers' logs. A few of the wells have geophysical logs, which are necessary for accurate correlations, 

but those wells are so scattered that detailed structural interpretations are impossible or meaningless. From the limited 

subsurface information, numerous regional studies have confirmed the basic synclinal character of the Paleozoic bedrock 

surface (fig. 2) (Roberts and Gildersleeve, 1950; Caplan, 1954; 

Grohskopf, 1955; Gushing and others, 1964; Schwalb, 1969; 

Stearns and Wilson, 1972). However, subtle structures remain 

undetectable because of insufficient and scattered subsurface 

data. On the basis of limited well control and topographic 

lineations, Stearns and Zurawski (1976) constructed a series of 

interpretative structural contour maps which attempted to 

portray the configuration of the Paleozoic bedrock surface 

more accurately. Nevertheless, to date, the reflection profiles 

are the most complete and most reliable source of subsurface 

structural information available in the upper Mississippi Embayment.

The assumption that the bedrock surface in the study area 

was of low relief prior to Upper Cretaceous sedimentation is 

critical to all of the structural interpretations presented here. 

The top of the Paleozoic bedrock gently slopes toward the axis 

of the embayment at about 35 ft/mi (6.6 m/km) (Boswell and 

others, 1965) with only minor interruptions (Grohskopf, 

1955). Any significant deformation recognized on the bedrock 

surface other than the broad downwarping of the embayment 

is attributed to and interpreted in terms of tectonic processes.

General knowledge of the geologic history of the northern 

Mississippi Embayment suggests that there was very little 

topographic relief on the bedrock by Upper Cretaceous time. 

The Paleozoic bedrock in the study area is predominantly 

Cambrian and Ordovician carbonates that were uplifted as 

part of the Pascola arch (Grohskopf, 1955). For perhaps as 

long as 180 m.y. (Davis and others, 1973) during the Paleozoic 

and Mesozoic, the arch was extensively eroded and several 

kilometers of sediments were removed (Marcher and Stearns, 

1962). By Late Cretaceous time, the Paleozoic bedrock was 

beveled to a mature, low-relief surface with a well-developed 

drainage system (Pryor, 1960).

The character of the Paleozoic reflector on profile line S-10 

and data from the USGS New Madrid test well adjacent to the 

profile line (drill hole C, fig. 1) further support the belief that 

only minor relief remained on the bedrock surface. The USGS 

well shows that the bedrock beneath line S-10 is altered, 

highly fractured dolomite (Crone and Russ, 1979). Because 

the test well is located near the crest of the Pascola arch, which 

is an uplift that may have been structurally elevated more than 

8000 ft (2625 m) (Marcher and Stearns, 1962) beginning in the 

late Paleozoic (Bond and others, 1971), it is reasonable to expect that the brittle dolomites beneath line S-10 would have 

been fractured prior to the erosion that produced the Paleozoic-Late Cretaceous unconformity. When exposed, soluble,
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE

89°30'

117

36°30' -

36°15'-

MISSOURI W

024 Kilometers

FIGURE 1. Location map of seismic-reflection profiles and subsurface control in the New Madrid region. Reflection profile lines 

shown by heavy solid lines; wells listed in table 1 shown by triangles and letters. Drill hole C is the New Madrid test well. 

R-, S-, and T-lines are seismic-reflection profile lines.
118 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

92°

35°-

34°

100 MILES

0 50 100 150 KILOMETERS

FIGURE 2. General synclinal configuration of the Paleozoic bedrock surface 

in the Mississippi embayment (modified from Gushing and others, 

1964). Contour interval, 1000 ft; datum is mean sea level. Study area is 

shaded.

fractured dolomites would rapidly weather into a high-relief 

karst landscape. Thus, if any significant relief remained on the 

bedrock surface prior to Upper Cretaceous sedimentation, it 

should be preserved beneath profile line S-10. However, the 

Paleozoic reflector on line S-10 near the well is smooth, is continuous, and shows no evidence of substantial topographic 

relief.

PREVIOUS WORK

Numerous accounts describing the effects of the 1811-12 

earthquakes in the upper Mississippi Embayment reported 

startling changes in the land surface. Usher (1837) first 

described an area of elevated land east of the Mississippi River, 

between New Madrid, Mo., and the mouth of the Obion River 

in western Tennessee, and attributed it to movements during 

the 1811-12 earthquakes. Likewise, McGee (1892), Purdue 

(1916), and Glenn (1933) all attributed the topographically 

high area in Lake County, Tenn., and the adjacent low areas 

to deformation caused by the 1811-12 or earlier earthquakes. 

In his classic study, Fuller (1912) described the size, shape, and 

amount of relief of this topographic feature and formally 

named it the Tiptonville dome.

Born (1935) presented the first evidence that the area of 

elevated ground in western Tennessee could be related to subsurface structure. In a north- south stratigraphic cross section, 

he showed a structurally high well at Proctor City, on top of 

the uplift. More recently, from drill data, Fisk (1944) and 

Krinitzsky (1950) interpreted faulting along the east edge of 

the Tiptonville dome and regarded the dome as a tectonic 

feature. Stearns (1979) and Russ (this volume) mapped the size

and shape of the dome, divided it into component parts, and 

likewise concluded that it has a tectonic origin.

COLLECTION AND PROCESSING OF DATA

The USGS program of seismic-reflection profiling in the upper embayment was conducted in two stages: first, 32 km of 

profiles (T-lines and line R-l) were collected in the vicinity of 

Reelfoot Lake (Zoback, 1979), and later about 250 km of profiles (S-lines) were run throughout the seismically active area 

(Hamilton and Zoback, 1979). Figure 1 shows the locations of 

the profiles. A synthetic seismogram from the sonic-velocity 

and density logs of the New Madrid test well (drill hole C, fig. 

1), drilled into Paleozoic rocks near line S-10, was used to correlate the reflectors on the profiles with geologic units. The 

synthetic data agree well with the real data (Crone and Russ, 

1979). Additional details of the reflection-profiling program 

are provided by Hamilton and Zoback (1979).

Two-way travel times to the Paleozoic reflector measured at 

4000-ft (1312 m) intervals (20 vibration points) along the profiles, are accurate to within 5 ms. The original 240 data points 

were processed using a minimum curvature program (M. W. 

Webring, unpub. computer program, 1978) designed to produce a smooth contour map that confidently represents the 

data (Briggs, 1974). The program generated a north-south 

east-west orthogonal grid of calculated two-way travel times 

spaced about 1.6 km apart. Hand- and computer-contoured 

maps of the calculated travel times approximate the configuration of the Paleozoic bedrock surface. Both versions of the 

contour maps are essentially identical, suggesting that the 

hand-contoured version was not noticeably biased. This study 

uses the hand-contoured maps because the map lines are 

generally smoother.

Two sources of potential error exist in the gridded data. 

First, a comparison of the calculated two-way travel times with 

the original data shows that the gridding process tends to 

reduce sharp, local variations; however, significant regional 

trends remain basically unaffected. Secondly, there are places 

near the edge of the study area, such as the region directly east 

of New Madrid and the region south of Portageville, where the 

gridded data were extrapolated beyond the data from the 

reflection profiles (fig. 3). East of New Madrid, limited well 

data act as a check on these gridded data and suggest that contours of the gridded data in the area are reasonably accurate. 

The gridding program calculated a two-way travel time of 636 

ms for the Mt. Carmel Drilling-Florence Smith well (N, table 

1). Using the depth-travel time relationship of 0.94 m/ms 

developed later in this paper, the calculated depth to 

Paleozoic bedrock is 598 m compared to the actual depth of 

600 m (Finch, 1971) (both depths referenced to 91 m). Thus, 

in areas remote from the profile lines, the gridded data seem 

reliable.

No well control is available to check the gridded data in the 

area south of Portageville and thus to confirm the accuracy of 

the contour lines. However, the position of the contours is con-
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE

89°30'

119

36°30' -

36°15' -

750

 -~. -770

EXPLANATION 

Structural low calculated from seismic data with interpreted orientation

Structural high calculated from seismic data with interpreted orientation 

Structural high calculated from seismic data with no interpreted orientation

FIGURE 3. Configuration of the Paleozoic bedrock surface interpreted from the calculated two-way travel times. Contour 

interval, 25 ms; dashed contours are intermediate contours selected to show detail. Hachured contours show closed depressions. Longer two-way travel times indicate deeper bedrock. Reflection profiles shown by heavy screened lines and 

identified on figure 1. Areas of poor data control have not been contoured. Triangle is drill hole N (table 1). The 

outline of Lake County uplift, Tiptonville dome, Ridgely ridge, and Reelfoot Lake basin are shown on figure 5.
120 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

trolled by data from reflection profiles that lie to the north, 

east, and south. Also, the regular, even spacing of the con tours 

suggests that the gridding program has generated reasonable 

travel-time values as it did in the region east of New Madrid 

(fig. 3).

In areas of absent or sparse data, the gridding program 

calculates values that tend toward a planar surface (Briggs, 

1974) rather than create artificial structures. Despite the 

evidence that the gridding program works well in areas of poor 

data, we regard the contours in these areas skeptically because 

of the distance from the profile lines.

ANALYSIS OF THE GRIDDED DATA

On the basis of regional data, the Paleozoic bedrock in the 

study area dips gently and regularly to the south and southeast 

(fig. 2) (Gushing and others, 1964; Stearns and Wilson, 1972). 

In this study, significant deflections and clustering of contours 

contrary to the anticipated regular gradient of the embayment 

are interpreted as structures such as folds, faults, and flexures.

A contour map of the gridded two-way travel times (fig. 3) 

reveals a broad positive feature on the bedrock surface in 

western Tennessee which corresponds in size, location, and 

general shape to the Lake County uplift identified on the 

ground surface and described by Stearns (1979) and Russ (this 

volume). Furthermore, a distinct change in the trends of the 

contours suggests that the positive area may be subdivided into 

two component parts. North of Tiptonville, the Lake County 

uplift has an axis that trends northwest-southeast; in contrast, 

the southern part trends more northeast- southwest. In accordance with previously established terminology (McGee, 1892; 

Fisk, 1944; Stearns, 1979; Russ, this volume), this report refers 

to the entire broad, positive feature as the Lake County uplift; 

the northern portion of the uplift is identified as Tiptonville 

dome and the southern part is named Ridgely ridge.

Several lines of evidence suggest that the contoured two-way 

travel times (fig. 3) can be converted to depth estimates by using the approximate relationship that 1 ms equals 1 m of 

depth. The depths to Paleozoic bedrock from 13 drill holes in 

the area (fig. 1, table 1) are plotted against the two- way travel 

times predicted by the gridded data at the wells (fig. 4). Drill 

hole E is not included in figure 4 because of uncertainty in the 

depth to Paleozoic bedrock. The bedrock depths are referenced to the 91-m datum used to process the reflection profiles for 

statics corrections. For some of the wells, the gridded travel 

times were adjusted to compensate for predictable variations 

in local structure and for some smoothing effects of the gridding routine (table 1). The local variations in structure were 

predicted on the basis of detailed analysis of the reflection profiles (Zoback and others, 1980). A linear-regression line fitted 

to the adjusted data and designed to intercept the origin yields 

a slope of 0.95 m/ms. Over large depth intervals, the two-way

900

800

700

.600

500

Slope, l/V=0.95m/ms

Slope, l/V = lm/ms

500 600 700 

Depth below 91m datum, in meters

800

FIGURE 4. Plot of calculated two-way travel time vs depth to Paleozoic 

bedrock using well control. Letters identify wells listed in table 1; open 

circles are depth vs two-way travel time and triangles are depth vs adjusted two-way travel time. Depth values are referenced to the 91-m 

datum used to process the reflection profiles. Slope of 0.95 m/ms is 

linear regression line of the adjusted data required to intercept the 

origin (dashed line); slope of 1 m/ms is shown for comparison (solid 

line). Drill hole E is not included because of uncertainty in depth to 

Paleozoic bedrock.

travel time versus depth relationship is characteristically 

nonlinear; however, for the small depth interval considered 

here, a linear function is a reasonable assumption.

The mean value of the ratios of travel time versus depth for 

all wells listed in table 1 is 0.94 m/ms. Likewise, the ratio for 

the New Madrid test well (drill hole C, table 1) is 0.94 m/ms. 

Because the test well has excellent subsurface control and 

because profile line S-10 runs within a few meters of the well, 

no corrections are necessary and, thus, this ratio is very 

reliable. Finally, the best fit of the stacking velocities used to 

process the reflection profiles yields a relationship of 1 m/ms 

(Zoback and others, 1980). All of these various lines of 

evidence indicate that 1 m of depth per 1 ms in two-way travel 

time is a useful approximation.

The amount of error introduced by using this rule of thumb 

to convert travel time to depth is variable because of local variations in the average velocity of the Upper Cretaceous to Tertiary sediments. The local variability and other contributing 

errors in the conversion mean that travel times converted to 

meters are only approximate.

Unrecognized lateral velocity changes in the Upper Cretace-
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 121

TABLE 1. Well control for depth to bedrock in the New Madrid area and travel-time data

[Leader ( ) indicate data not calculated. Well depths were converted from feet to meters using a conversion factor of 0.3048 m/ft]

Hell"

A

B

c:

D

_E

_F

G

H

I

£

K

_L

M

N

Depth (m) to top 

I/ of Paleozoic rocks 

Well below 91-m datun

Ulbourn Oil - #1 Hunter

Union-Cordova 011-

#1 Phillips.

U.S.G.S. -WDW-l-X

Heelfoot Done Oil01 Kate Weight.

Henderson 011-

#1 Linda fbrris (A.C&D)

Frazier & Benz-

#1-B Sam Hays.

Oorley, Geiselman & Benz-

#1 J. E. Vaughn.

Benz 011-

#1 territt Estate.

Johnson Oil -

#1 S. J. Bradshsw.

rfenderson 011-

#1 S. J. Bradshaw.

Henderson Oil -

#1 F. B. Carroll.

Henderson Oil -

#1 A. E. ferkham.

Bsnderson Oil -

#1 J. E. Rice.

Mt. Camel Drlg.-

Florence Smith.

436

475

621

600

773, 752, 734

692

754

722?

795

807

772

680

785

600

TVjD-way 

travel time from 

gridded data

500

551

659

667

762

763

765

790

804

804?

784?

734

796

636

Katio of depth 

Adjusted to adjusted 

two-way travel travel time 

time (m/ms)

532

551

659

650

733

780

790?

804

804

784

727

796

636

0.82

.86

.94

.92

.94

.97

.91

.99

1.00

.98

.94

.99

.94

mean = .94

Comments

Adjusted +32 ms because of local structure.

No major correction

Do.

Adjusted -17 ms because of local structure.

Disregarded due to uncertainty in depth

to Paleozoic bedrock.

Adjusted -30 ns to compensate for

smoothing by gridding program.

Adjusted +15 ms to compensate for

smoothing by gridding program.

Adjustment cannot be estimated because

of inaccurate top of Paleozoic

bedrock and structural complications.

ND major corrections recognized.

Do.

Do.

Adjusted -7 ms because of local structure.

ND major corrections recognized.

Do.

Locations shown on figure 1, and data plotted on figure 4.

ous to Tertiary sediments may cause spurious structures. In 

areas of abnormally high velocities, two-way travel times are 

shorter, resulting in apparent bedrock highs. Likewise, in abnormally low velocity areas, apparent bedrock lows result. The 

average velocities from the well control data in table 1 (ratios 

of depth to adjusted travel time) provide evidence that the 

structures described in this report are not an artifact of velocity 

changes. The wells with the highest average velocities (/, /, K, 

and M in table 1) are situated in structurally low areas (figs. 1, 

3, 7, and 9). This indicates that lateral velocity variations 

alone cannot account for the structures developed on the 

bedrock surface.

TIPTONVILLE DOME

Tiptonville dome is a large positive feature that extends 

from the west edge of Reelfoot Lake north and northwest to 

the neck of the New Madrid meander (fig. 3). The axial trace 

of the dome trends north-northwest from the town of Tiptonville. The dome is bounded on the east by a large structural 

low that lies directly below Reelfoot Lake, and merges with 

Ridgely ridge on the south. To the west, the dome grades into 

a shallow structural low. Northward, the deformation

diminishes, and the uplift apparently merges with the normal 

slope of the embayment.

Reconstructed contours, which project across the warped 

areas, were drawn in an attempt to model the regional structure prior to any deformation (fig. 5). When compared with 

the gridded travel times, the reconstructed contours allow the 

amount of deformation to be estimated and permit uplifted 

and downwarped areas to be mapped (fig. 5). Gridded data 

points that clearly lie above or below the surface defined by the 

reconstructed contours are considered as evidence of tectonic 

deformation. The two segments of the Lake County uplift, 

Tiptonville dome and Ridgely ridge, are clearly shown on 

figure 5.

As expressed on the Paleozoic bedrock (fig. 5), Tiptonville 

dome has a maximum east-west dimension of 14 km, has a 

maximum north-south dimension of slightly more than 11 km, 

and covers an area of about 95 km2 . At the north end of the 

dome, the 650-ms contour (fig. 3) is deflected to the southeast 

to a point where the reconstructed contours (fig. 5) suggest 

that the bedrock should be at about 685 ms. This indicates 

about 35 m of uplift. Along profile line T-2, at least 42 m of 

uplift can be measured where a 658-ms data point on the grid-
122 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

89°30'

36°30' -

36"! 5' -

LAKE COUNTY 

UPLIFT

4 Miles

024 Kilometers

FIGURE 5. -Areas of deformation on the bedrock surface defined by reconstructed contours. Contour interval, 25 ms; solid 

where gridded data; dashed where reconstructed contours were projected across deformed areas. Downwarped areas 

(line pattern) and uplifted areas (stipple pattern) shown; queried where indefinite boundaries. Letters refer to unnamed deformed areas discussed in the text. Reflection profiles shown by heavy screened lines and identified on figure 1.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 123

ded surface (fig. 3) lies on the 700-ms reconstructed contour. 

This is the maximum amount of uplift detected on Tiptonville 

dome.

RIDGELY RIDGE

Ridgely ridge is a large northeast-southwest-trending uplift 

that extends from the southern shore of Reelfoot Lake to 

beyond Cottonwood Grove, Tenn. The ridge is a complex, 

faulted feature with numerous small intrusives (Zoback and 

others, 1980). Faulting of Ridgely ridge has produced local 

areas of high relief on the bedrock surface that are generally 

oriented northeast-southwest (Zoback and others, 1980). 

However, several of these areas, indicated by the closed contours in the vicinity of Ridgely and Cottonwood Grove in 

figure 3, are strongly oriented north-south and east-west. This 

is probably an inaccurate representation and is most likely due 

to the strong influence of the north-south gridding pattern 

upon small structures. Larger features whose orientations are 

determined by numerous data points are not unduly influenced by the orthogonal patterning of the gridded data.

Ridgely ridge covers more than 260 km2 , exceeds 20 km in 

length, and is about 11 km wide. The ridge is bounded on the 

northeast by Reelfoot Lake basin, on the northwest by a structural low (fig. 5, feature X), and on the north by the Tiptonville dome. On the basis of present data, the southern limit of 

the uplift is undefined because the ridge extends beyond the 

data.

Without the reconstructed contours, it is difficult to 

estimate the amount of uplift that has occurred on Ridgely 

ridge. However, in some areas, the uplift has been substantial. 

For example, just southeast of Cottonwood Grove, a closed 

750-ms contour (fig. 3) lies in an area where even a crude projection of the reconstructed contours (fig. 5) would indicate 

that the bedrock should be at least 800 ms. This means that 

parts of Ridgely ridge have been elevated at least 50 m.

REELFOOT LAKE BASIN

Reelfoot Lake basin1 (Fisk, 1944; Stearns, 1979) is a topographically low area that is partially occupied by Reelfoot 

Lake. The origin of the basin is unclear. Various workers 

believe that it is a tectonic depression produced by faulting 

(Fuller, 1912; Fisk, 1944; Krinitzsky, 1950), whereas others attribute at least part of the surface depression to compaction 

and settling of clays (Purdue, 1916; Glenn, 1933; Russ, 1979). 

The two areas of low bedrock along the east side of Tiptonville 

dome (fig. 5) appear to be the subsurface equivalents of the 

surficial Reelfoot Lake basin. The larger depression, covering

'Reelfoot Lake ba 

Schwalb (1969).

;in as discussed here is not the same feature as the Reelfoot basin defined by

about 28 km2 , lies directly below Reelfoot Lake. This relationship suggests that the location of the lake may be, in part, 

structurally controlled.

OTHER POSITIVE FEATURES

Four small areas of elevated bedrock in the northwest portion of the study area are indicated by perturbations in the 

contour lines. The four areas (fig. 5, features A, B, C, and D) 

are local features apparently unrelated to the Lake County 

uplift.

Feature A, directly southeast of Marston, covers about 17 

km2 and has been elevated about 7-9 m above the projected 

undeformed surface. It is bounded on the south by an area of 

low bedrock that separates it from the much larger Lake 

County uplift.

Features B, C, and D are small warped areas probably produced by local faulting and flexing. The limits of these small 

deformed areas are difficult to accurately define because the 

areas are based on only a few data points. The amount of warping on these features generally appears to be small, but the 

exact amount is difficult to estimate because reconstructed 

contours cannot be confidently projected across the features.

TREND-SURFACE ANALYSES

Trend-surface analyses are valuable mathematical techniques that identify trends in data sets and isolate the trends 

from local fluctuations (Merriam and Harbaugh, 1964; Doornkamp, 1972; Davis, 1973). In trend-surface analyses, linear 

regression of the data produces a series of polynomial equations which describe surfaces that model trends in the data. 

The higher the order of the polynomial equation, the more 

complex the surface, and the more accurately the trend surface fits the input data. The trend-surf ace program used in 

this study has the maximum capability of fitting a sixth-degree 

polynomial surface. Residuals, the difference between the input values and the values calculated for the trend surface, can 

be used for structural interpretations (Merriam and Harbaugh, 1964); they emphasize areas of particularly intense deformation superimposed on the regional trends.

Using the gridded data as input, trend-surface analyses in 

this study show the regional extent and characteristics of the 

Lake County uplift, whereas the residuals reveal the local 

complexities of the Paleozoic bedrock surface. The trend surfaces and residuals are plotted as contour maps (figs. 6-9).

The percentage reduction in the total sum of the squares is a 

common way of describing how much of the variation of the 

data can be explained by the regression surface (Merriam and 

Harbaugh, 1964; Doornkamp, 1972; Davis, 1973). It is similar 

to the coefficient of determination. Higher percentages indicate better fits; thus, if a trend surface perfectly matches all 

of the data, the reduction in the total sum of the squares is 100
124 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

89°30'

36°30' -

3615' -

024 Miles i 'i .' i ; '

024 Kilometers

FIGURE 6.--Third-degree trend-surface map. Contour interval, 25 ms. Areas of poor data control have not been contoured. 

Reflection profiles shown by heavy screened lines and identified on figure 1. Axial trace of uplift on third- degree surface indicated by dashed line.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 125

percent. The percentage reduction in the total sum of the 

squares for the surfaces generated in this study ranges from a 

low of 93 percent for the first-degree surface to more than 99 

percent for the fifth- and sixth-degree surfaces. These high 

values show that the surfaces are faithful representations of 

trends in the data, although, as indicated by the residuals, 

locally large deviations do exist.

Several mathematical tests are available to determine the 

statistical significance of a trend surface. Most of the tests require that the input data be normally distributed about the 

mean (Doornkamp, 1972). Chi-square tests on the original 

data and on the gridded data indicate that neither set is normally distributed; thus, the statistical significance of the trend 

surfaces cannot be evaluated using conventional hypothesistesting techniques. Despite this shortcoming, the surfaces are 

geologically valuable and are useful for detecting patterns and 

recognizing structural trends.

Severe distortion often occurs at the edges of trend-surf ace 

maps because, typically, the periphery of the surface is poorly 

constrained by data. This distortion is commonly described as 

"edge effects" (Davis, 1973). The "edge effects" on the trendsurface maps presented here appear to be minimal, with the 

exception of part of the sixth-degree surface, which will be 

discussed later.

THIRD-DEGREE TREND SURFACE 

AND RESIDUALS

The Mississippi Embayment, when mathematically modeled as a gently plunging syncline, can be most simply described 

by a third-degree polynomial equation. In the ideal model, 

contours of the Paleozoic bedrock surface in the study area 

would be slightly convex to the north or northwest (fig. 2). 

Areas where the configuration of the bedrock surface deviate 

significantly from this model are assumed to have been structurally disturbed. An analysis of the third-degree trend surface 

can make these deviations obvious.

A prominent positive feature that corresponds to the Lake 

County uplift is defined by deflected contours that are convex 

to the south-southeast; it is apparent on the third-degree trend 

surface (fig. 6). On the basis of these deflected contours, the 

uplift can be seen to extend from south of Ridgely, where the 

deformation is most pronounced, to north of Tiptonville. The 

675-ms contour north of Tiptonville has been deflected, indicating deformation, but the 600-ms contour and those farther north are slightly convex to the northwest, indicating little 

or no detectable deformation. The available data fail to define 

the southern limit of the uplift, although the break in the 

800-ms contour suggests that it extends at least as far as the 

southernmost data points.

The crest of the uplift can be roughly located by connecting 

the point of maximum southward deflection for each contour

line (fig. 6). The overall trend of the crest is generally northsouth.

A contour map of the third-degree surface residuals (fig. 7) 

shows that the Lake County uplift can be divided into Tiptonville dome to the north and Ridgely ridge to the south. These 

two features are separated by an east-west saddle that lies 

under the town of Tiptonville.

On the basis of the residuals, the highest point of Tiptonville 

dome lies about 4 km north of Tiptonville and rises about 31m 

above the computed trend surface. Three arms radiate out 

from this high point: one arm extends to the northeast, a second arm trends north-north west toward the neck of the New 

Madrid meander, and a third arm strikes directly west toward 

drill hole C.

The eastern boundary of Tiptonville dome is marked by 

north-south- striking contours that separate the uplifted area 

from a large low area, the Reelfoot Lake basin. The basin extends from near the Mississippi River to beneath the western 

portion of Reelfoot Lake. As previously discussed, the'location 

of the lake within an area of low bedrock suggests possible 

structural control. The boundary between Tiptonville dome 

and Reelfoot Lake basin that is suggested by the contours 

closely coincides with the location, orientation, and sense of 

movement of the Reelfoot fault defined by Zoback (1979). Apparently some parts of the Lake County uplift are fault bounded.

Ridgely ridge, the segment of the Lake County uplift that 

extends southwesterly from the southern end of Reelfoot Lake, 

is a complex feature with a highly irregular bedrock surface 

(fig. 7). The ridge is actually a system of northeast-southwesttrending structures (Zoback and others, 1980) that locally 

have as much as 40 m of relief. The irregularity of the bedrock 

surface is obvious from the numerous small, closed contours of 

positive and negative residuals in figure 7, especially near the 

towns of Ridgely and Cottonwood Grove. As already noted, 

the north-south and east-west orientations of the closed contours are probably artifacts of the gridding process; they do 

not represent the true orientation of these structures.

SIXTH-DEGREE TREND SURFACE 

AND RESIDUALS

The sixth-degree trend surface represents the most accurate 

model of the Paleozoic bedrock surface produced by the study 

(fig. 8). The Lake County uplift, indicated by deflected contours, is shown as a prominent north- northeast-trending 

feature. North of Tiptonville it is a broad, low-amplitude 

structure. To the south near Ridgely, it increases in 

amplitude, becomes very clearly defined, and is bounded on 

both the east and west by areas of low bedrock. The southern 

end of the uplift cannot be firmly established.
126 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

89°30'

36°30'

36°15'-

4 Miles

0 4 Kilometers

FIGURE 7. -Contour map of third-degree trend-surface residuals. Contour interval, 10 ms. Negative residuals lie above and positive 

residuals lie below the trend surface. Areas with residuals less than-10ms shown by stipple pattern; areas with residuals greater than 

+10 ms shown by line pattern. Reflection profiles shown by heavy screened lines and identified on figure 1. Triangle is New Madrid test well (drill hole C, table 1). The outline of Lake County uplift, Tiptonville dome, Ridgely ridge, and Reelfoot Lake basin 

are shown on figure 5.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 127

When compared with the third-degree surface (fig. 6), the 

axial trace of the uplift, interpreted from the sixth-degree surface, is rotated slightly to the northeast (fig. 8). Even though 

deflections in the contours suggest that the axial trace could be 

extended (fig. 8), it has not been projected north of the river 

because of the lack of control from reflection profiles in the entire northeast quadrant of the study area.

The western edge and the extreme southeastern corner of 

the sixth-degree trend surface are distorted by "edge effects." 

The apparent north-south- trending high area just west of 

Portageville is, most likely, artificially introduced by the trend 

program, because the trend surface in this area is not controlled by any data points. Likewise, the gradient indicated by the 

775-ms contour in the extreme southeast corner of the map is 

not constrained by data points and is probably inaccurate.

The contour map of the sixth-degree residuals (fig. 9) is very 

similar to that for the third-degree surface. Tiptonville dome 

is separated from Ridgely ridge by a conspicuous saddle 

located beneath the town of Tiptonville. The saddle breaches 

the uplift and connects the Reelfoot Lake basin on the east 

with another low area under the Mississippi River, southwest 

of Tiptonville.

The characteristics of Ridgely ridge shown by the sixthdegree residuals are virtually the same as those shown by the 

third-degree residuals. Despite the increased capability of the 

sixth-degree surface to model complicated surfaces, numerous 

negative and positive residuals (areas of high and low bedrock) 

still exist along the ridge. Once again, this demonstrates the 

complex character of this portion of the uplift. Most of these 

anomalies cover such small areas that they cannot be 

reasonably accommodated by any trend surface.

DISCUSSION

The Lake County uplift is a clearly defined positive feature 

covering about 367 km2 on the Paleozoic bedrock surface. Portions of the uplift have been elevated more than 50 m above 

the reconstructed, undeformed surface. In some areas the 

uplift is probably bounded by faults, but, as seen on the 

seismic profiles (see the western part of S-6 in Hamilton and 

Zoback, this volume, fig. 13), in many places the vertical 

movements have simply flexed the bedrock surface without 

any evidence of substantial breaks.

The general shape, orientation, and areas of maximum 

uplift on the bedrock surface are similar to the ground-surface 

uplift documented by Russ (this volume), suggesting that the 

two features are genetically related (fig. 10). Furthermore, 

both Tiptonville dome and Ridgely ridge are distinct, recognizable features on the ground surface and in the subsurface. 

The areas of maximum ground deformation are nearly coincident with the crest of the Lake County uplift as defined by 

both the third- and sixth-degree trend-surface analyses. This 

relationship is especially striking when the axial trace of the 

third-degree surface is compared with the 30-ft contour of 

Russ on Tiptonville dome and with his 20-ft contour on

Ridgely ridge immediately northeast of Ridgely. The comparison for the sixth-degree surface is not quite as strong; 

nevertheless, the similarity is still obvious.

In several areas the surface and subsurface characteristics of 

the Lake County uplift are noticeably different. One or more 

of the following factors contribute to these differences: (1) 

resolution of the surface data is considerably better than that 

of the seismic-reflection data, (2) all of the subsurface deformation may not propagate through the unconsolidated 

sediments to the ground surface, (3) the morphology of the 

surface feature is subject to modification by erosional and depositional processes, and (4) subsurface deformation that 

predates the development of the modern meander belt will not 

be mirrored on the ground surface. Careful analysis of the effects of these factors provides insight into the evolution of the 

uplift.

The surface uplift can be accurately defined using modern 

topographic maps (Russ, this volume); by comparison, the 

subsurface feature is defined by much less precise data. 

Reading errors of several milliseconds and gridding the twoway travel times reduce the resolution of the reflection data to 

perhaps ± 3 m. As a result, slightly deformed areas in the subsurface may escape unnoticed while equally deformed areas 

on the ground surface can be recognized.

Before it warps the ground surface, deformation of the 

Paleozoic bedrock in the upper embayment must propagate 

through approximately 600 m of unconsolidated sediment. 

Soft-sediment flowage, slumping, and compaction may compensate for much of the bedrock deformation; perhaps only 20 

percent of the subsurface warping is actually expressed on the 

ground surface (Russ, this volume). This would produce a significantly smaller surface uplift with much less relief than the 

corresponding subsurface feature.

The surface expression of the Lake County uplift is subject 

to rapid modification by various geomorphic processes, the 

most important of which is probably erosion and deposition by 

the Mississippi River. Older portions of the surface feature 

may have been partially eroded or otherwise obscured by the 

river, whereas younger portions, essentially unmodified, can 

be easily recognized by their prominent scarps or other 

geomorphic characteristics (Russ, this volume). Thus, areas of 

old uplift may be detectable in the subsurface but have been 

destroyed or concealed on the ground surface.

The warped Paleozoic bedrock examined in this study 

records the cumulative deformation that has occurred on the 

Lake County uplift since the weathered bedrock surface was 

buried by sediments of Late Cretaceous age more than 65 m.y. 

ago. By comparison, the deformation recorded on the ground 

surface has occurred only within the past 6000 years, since the 

development of the Mississippi River meander belt (Saucier, 

1974). Thus, considerably more deformation than is actually 

recorded on the modern ground surface, perhaps encompassing an even larger area, may have occurred on the bedrock 

surface.

The northern portion of the Lake County uplift defined by 

Russ (this volume) covers a considerably larger area than the
128

36°30' -

36°15' -

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

89°30'

4 Miles

024 Kilometers

FIGURE 8. Sixth-degree trend-surface map. Contour interval, 25 ms. Areas of poor data control have not been contoured. 

Reflection profiles shown by heavy screened lines and identified on figure 1. Axial trace of uplift on sixth- degree surface indicated by dashed line.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 129

corresponding uplift on the bedrock surface (fig. 10). Most of 

the discrepancy in area occurs where the ground surface has 

been only slightly elevated; poor resolution of the subsurface 

data appears to be primarily responsible for this discrepancy. 

In the area directly north of Tiptonville where the ground has 

been elevated at least 15 ft, the surface and subsurface uplifts 

agree fairly well. The match between the surface and subsurface features would probably improve if the resolution of the 

subsurface data could be increased by reducing the inherent 

uncertainties and if the northern limits of the subsurface uplift 

could be more accurately defined by additional profile lines.

The topographic expression of Ridgely ridge is generally 

restricted to the area east of the town of Ridgely; a large area 

of warped bedrock west of the town of Ridgely has essentially 

no surface expression (fig. 10). The absence of surface deformation above warped bedrock may have three possible explanations: (1) soft-sediment deformation and compaction in 

the Cretaceous and Tertiary sediments may have effectively 

masked part of the subsurface uplift, (2) the surface feature 

may have been partially eroded by the Mississippi River, or (3) 

only the eastern portion of the subsurface uplift may have been 

tectonically active in the last 6000 years.

It is unlikely that compaction and soft-sediment deformation are entirely responsible for the limited extent of Ridgely 

ridge on the ground surface. If soft-sediment processes were 

solely responsible, then the surface feature, even though subdued, should generally mimic the subsurface feature and the 

topographically highest areas should correspond to the areas 

of maximum uplift on the bedrock. In fact, the residual maps 

from this study (figs. 7 and 9) show that this is not true. For example, the ground surface near Cottonwood Grove has been 

elevated less than 5 ft (fig. 10), while the residuals suggest that 

the underlying bedrock has been elevated at least 40 m (fig. 7). 

It might be argued that the amount of soft-sediment compaction and deformation is not uniform everywhere and that 

perhaps nearly all of the subsurface uplift near Cottonwood 

Grove has been absorbed by the sediments. The nonuniformity of soft-sediment processes is certainly true on a small scale, 

but for a feature as large as the one considered here and for a 

column of sediments exceeding 600 m in thickness, the local 

variations should equalize and the amount of compaction and 

deformation should be nearly uniform. Because soft- sediment 

processes cannot completely account for the limited size of the 

surface feature, other factors must be involved.

Partial erosion of Ridgely ridge by modern and ancient 

Mississippi River courses also fails to entirely explain the 

limited size of the surface uplift. In his classic study of the 

Mississippi River meander belt, Fisk (1944) showed that the 

former channels which traverse Ridgely ridge belong to several 

of the older stages of the meander belt. There is no evidence 

from his work, however, that a more recent course of the river

has eroded the western portion of the surface uplift and left the 

eastern portion as the remnant of a larger topographic 

feature.

The best explanation for the size difference between the surface and subsurface uplifts is that different portions of the 

uplift have been tectonically active at different times. Careful 

interpretation of the reflection profiles by Zoback and others 

(1980) shows that, on the bedrock surface, Ridgely ridge is a 

broad, complex, northeast-trending arch broken by numerous 

faults that run subparallel to the trend of the arch. This northeast orientation of Ridgely ridge is consistent with the orientation suggested by the trend-surf ace residuals (figs. 7 and 9). 

The surface warping (fig. 10) is essentially coincident with 

several of the faults discussed by Zoback and others (1980). 

Recurrent movement has been demonstrated on several faults 

in the New Madrid area (Zoback, 1979; Zoback and others, 

1979) and on other faults in the Eastern United States 

(Ratcliffe, 1971; Mixon and Newell, 1977). Thus it is plausible 

that parts of the uplift may have been tectonically active for a 

period of time and then have become temporarily quiescent. If 

this is true, the eastern part of Ridgely ridge may have been 

uplifted and tectonically active in the past 6000 years, thus 

warping the modern ground surface, while deformation of the 

western part may have occurred much earlier. The patterns of 

modern seismicity around Ridgely ridge show a broad, irregular distribution of epicenters (fig. 11) that neither confirms nor refutes this hypothesis.

The exact reason why the topographic expression of Ridgely 

ridge is small compared to the subsurface feature is still not 

completely clear, but localized tectonic movements offer the 

simplest explanation. Geomorphic and sedimentologic processes cannot solely account for the size difference between the 

surface and subsurface feature, suggesting that the area of 

elevated ground east of Ridgely must have been produced by 

tectonic processes.

Reelfoot Lake and the Reelfoot Lake basin (Stearns, 1979) 

are two other physiographic features that may also reflect 

bedrock warping. Reelfoot Lake basin is a topographic 

depression whose origin has been explained by both tectonic 

and nontectonic processes. The bedrock contour map (fig. 3) 

and the trend-surface-residual maps (figs. 7 and 9) all show a 

depression on the bedrock surface that lies directly beneath the 

topographic depression. Reelfoot Lake basin lies on the 

downthrown block of the Reelfoot fault, which has experienced recurrent movement since the the Late Cretaceous 

(Zoback, 1979) and may have moved several times in the 

Holocene (Russ, 1979). After movement along the fault, 

Reelfoot Lake basin would be tectonically depressed, producing an area of relatively low ground where drainages could be 

disrupted and ponding could occur. Within this relatively low 

area, fine-grained sediments may have preferentially ac-
130 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

89°30'

36°30' -

36°15'-

4 Kilometers

FIGURE 9.- Contour map of sixth-degree trend-surface residuals. Contour interval, 10 ms. Negative residuals lie above and positive 

residuals lie below the trend surface. Areas with residuals less than -10 ms shown by stipple pattern; areas with residuals greater than 

+ 10 ms shown by line pattern. Reflection profiles shown by heavy screened lines and identified on figure 1. Triangle is New Madrid test well (drill hole C, table 1). The outline of Lake County uplift, Tiptonville dome, Ridgely ridge, and Reelfoot Lake basin 

are shown on figure 5.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 131

cumulated owing to one of two mechanisms. In the Holocene, 

when the Mississippi River adopted a meandering regime, the 

presence of a relatively low area could have influenced the 

river to occupy the Reelfoot Lake basin more frequently and 

deposit thick plugs of channel-filling clays (Saucier, 1964). Alternatively, the low area may have simply provided a favorable 

site where large quantities of overbank silts and clays could 

collect. Any thick accumulations of fine-grained sediment 

would be susceptible to settling and compaction, particularly 

during the periods of intense ground motion produced by major seismic events. Thus, under these circumstances, the 

ground surface in the basin could have subsided without any 

associated tectonic movements.

Historical accounts and field evidence indicate that 

Reelfoot Lake, the major physiographic feature in the basin, 

was enlarged during the 1811-12 earthquakes (Russ, this 

volume). However, there is conflicting evidence about movement on the Reelfoot fault having occurred at that time 

(Fuller, 1912, p. 58; Russ, 1979). If movement on the Reelfoot 

fault did occur during the 1811- 12 events, then the enlargement of Reelfoot Lake was tectonically induced. From the 

evidence available, it is unclear whether the lake enlargement 

was due entirely to compaction or to faulting; both processes 

most likely contributed.

In recent years increased instrumentation has firmly established the patterns of seismicity in the upper Mississippi Embayment and defined the major trends (Stauder and others, 

1976; Stauder, this volume). Most of the earthquakes occur in 

three major zones: (1) a northeast-southwest linear trend that 

extends from northeastern Arkansas to Ridgely, Tenn.; (2) a 

broad zone of intense seismicity that trends north-northwest 

from near Ridgely toward Lilbourn, Mo.; and (3) a second 

northeast-southwest linear trend, subparallel to the first, 

which extends from near Lilbourn to the northeast. Focalmechanism solutions for the two linear northeast-southwest 

zones of seismicity indicate predominantly right-lateral strikeslip movement (Herrmann and Canas, 1978; Herrmann, 

1979; O'Connell and others, this volume); solutions for the 

broad zone between Ridgely and Lilbourn are poorly constrained and variable, but mainly indicate reverse-fault 

movements.

The major tectonic element in the upper Mississippi Embayment is believed to be a northeast-trending rift, possibly of 

Precambrian age (Ervin and McGinnis, 1975), whose existence has been supported by Hildenbrand and others (1977, 

this volume) on the basis of gravity and magnetic data. Hildenbrand and others (this volume) believe that much of the 

modern seismicity is associated with ancient zones of crustal 

weakness produced by extensive faulting along the axis and 

margins of the rift. Some of these weak zones may have been 

reactivated by the contemporary east-west compressional 

stress field (Sbar and Sykes, 1973), producing strike-slip movement along formerly extensional features (Hildenbrand and 

others, this volume). Also, these weak zones may have acted as 

conduits for Paleozoic and Mesozoic intrusive igneous rocks 

(Hildenbrand and others, this volume), which have been recognized throughout the embayment (Zoback and others, 

1980).

The precise causes of the Lake County uplift are, at present, 

unknown, although geologic, geophysical, and seismic studies 

are continuing to clarify the seismotectonic framework of the 

New Madrid region. The close spatial relationship between 

the current seismicity and the areal extent of the Lake County 

uplift (fig. 11) suggests that the forces responsible for the 

earthquakes are also producing the uplift. Therefore, an improved knowledge of the contemporary stress field is essential 

to understanding the origin of the uplift.

Russ (this volume) offers a model for the origin of the Lake 

County uplift that is consistent with both the local and 

regional geologic and geophysical data and with the faultplane solutions. The model essentially proposes that the Lake 

County uplift has developed within an area of compression 

caused by a major bend in a strike-slip fault system. Within 

the area of compression, strain has caused vertical bulging 

that has produced the observed uplift on the bedrock and the 

ground surface.

Evidence from surface and subsurface studies shows that 

different portions of the Lake County uplift have been tectonically active at different times. Sharp scarps and high 

topographic relief indicate that Tiptonville dome has been active most recently (Russ, this volume); historical accounts suggest that parts of the dome may have been elevated during the 

1811-12 New Madrid earthquakes. The eastern portion of 

Ridgely ridge between the town of Ridgely and the southern 

end of Reelfoot Lake was uplifted within the past 6000 years 

but, based on geomorphic evidence, is apparently older than 

Tiptonville dome. This study suggests that the oldest part of 

the Lake County uplift is the western portion of Ridgely ridge, 

where the Paleozoic bedrock was deformed more than 6000 

years ago, prior to the development of the modern Mississippi 

River meander belt.

The extensive modern seismicity in the area of the uplift 

and the similar characteristics of the deformation on the 

ground surface and on the bedrock surface indicate that the 

stresses which produced the Lake County uplift still exist today. Furthermore, if different portions of the uplift have been 

tectonically active at different times in the past, then apparently the stresses responsible for the seismicity shift with 

time. Improved knowledge of the geologic and seismotectonic 

framework of the upper Mississippi Embayment will help to 

determine where and how the stresses accumulate and how 

they are released, thus providing a more accurate and realistic 

assessment of seismic hazards in the area.

ACKNOWLEDGMENTS

We would like to acknowledge the many beneficial discussions with D. P. Russ, R. M. Hamilton, D. M. Perkins, and 

other colleagues at the U.S. Geological Survey. Reviews by L. 

E. Cordell and R. L. Wheeler provided valuable suggestions to 

improve the clarity of the manuscript.
132 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

89°30'

36°30' -

36°15' -

4 Miles

4 Kilometers

FIGURE 10. Comparison of ground-surface deformation (dashed contours, in feet) and bedrock deformation of the Lake 

County uplift (solid line; queried where uncertain). Ground-surface deformation modified from Russ (this volume). 

Axial trace of uplift from third-degree trend surface (heavy dashed line) and from sixth-degree trend surface (dotted 

line). Reflection profiles shown by heavy screened lines and identified on figure 1. The outline of the Lake County 

uplift, Tiptonville dome, Ridgely ridge, and Reelfoot Lake basin are shown on figure 5.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 133

36°45'00"

89°45'00" 89°30'00" 89°15'00"

36°30'00" 

36°15'00" -

. * / ^Lilbourn jffiNew Madrid

ur.

FIGURE 11. Comparison of seismicity with ground-surface and bedrock-surface deformation in the area of the Lake County uplift. Dots show locations of 

microseismic events recorded by the Saint Louis University network between July 1, 1974, and June 30, 1978 (modified from Stauder, this volume). 

Dashed line shows the limit of ground-surface deformation (modified from Russ, this volume); heavy line shows limit of bedrock-surface deformation, 

queried where indefinite.
134 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

REFERENCES CITED

Bond, D. C., Atherton, Elwood, Bristol, H. M., Buschbach, T. C., Stevenson, D. L., Becker, L. E., Dawson, T. A., Fernalld, E. C., Schwalb, 

Howard, Wilson, E. N., Statler, A. T., Stearns, R. G., and Buehner, J. 

H., 1971, Possible future petroleum potential of region 9--Illinois 

basin, Cincinnati arch, and northern Mississippi embayment, in Crum, 

I. H., ed., Future petroleum provinces of the United States Their 

geology and potential: American Association of Petroleum Geologists 

Memoir 15, v. 2, p. 1165-1218.

Born, K. E., 1935, Notes on the Upper Cretaceous and Tertiary sub-surface 

stratigraphy of western Tennessee: Tennessee Academy of Science Journal, v. 10, p. 248-264.

Boswell, E. H., Moore, G. K., MacCary, L. M., and others, 1965, Cretaceous aquifers in the Mississippi embayment: U.S. Geological Survey Professional Paper 448-C, 37 p.

Briggs, I. C., 1974, Machine contouring using minimum curvature: 

Geophysics, v. 39, no. 1, p. 39-48.

Caplan, W. M., 1954, Subsurface geology and related oil and gas possibilities of northeastern Arkansas: Arkansas Resources and Development 

Commission, Division of Geology Bulletin 20, 124 p.

Crone, A. J., and Russ, D. P., 1979, Preliminary report on an exploratory 

drill hole- New Madrid test well-l-X in southeast Missouri: U.S. Geological Survey Open-File Report 79-1216, 12 p.

Gushing, E. M., Boswell, E. H., and Hosman, R. L., 1964, General geology 

of the Mississippi embayment: U.S. Geological Survey Professional 

Paper 448-B, 28 p.

Davis, J. C., 1973 [1974], Statistics and data analysis in geology: New York, 

John Wiley and Sons, Inc., 550 p.

Davis, R. W., Lambert, T. W., and Hansen, A. J., Jr., 1973, Subsurface 

geology and ground-water resources of the Jackson Purchase region, 

Kentucky: U.S. Geological Survey Water-Supply Paper 1987, 66 p.

Doornkamp, J. C., 1972, Trend-surface analysis of planation surfaces, with 

an East African case study, in Chorley, R. J., ed., Spatial analysis in 

geomorphology: New York, Harper and Row, p. 247-281.

Ervin, C. P., andMcGinnis, L. D., 1975, Reelfoot rift - Reactivated precursor to the Mississippi embayment: Geological Society of America 

Bulletin, v. 86, p. 1287-1295.

Finch, W. I., 1971, Geologic map of part of the Bondurant quadrangle, 

Fulton County, Kentucky, and New Madrid and Mississippi Counties, 

Missouri: U.S. Geological Survey Geologic Quadrangle Map GQ-944, 

scale 1:24,000.

Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower 

Mississippi River: Vicksburg, Mississippi River Commission, 78 p.

Freeman, L. B., 1953, Regional subsurface stratigraphy of the Cambrian 

and Ordovician in Kentucky and vicinity: Kentucky Geological Survey 

Bulletin 12, ser. 9, 352 p.

Fuller, M. L., 1912, The New Madrid earthquake: U.S. Geological Survey 

Bulletin 494, 119 p.

Glenn, L. C., 1933, The geography and geology of Reelfoot Lake: Tennessee Academy of Science Journal, v. 8, no. 1, p. 3-12.

Grohskopf, J. G., 1955, Subsurface geology of the Mississippi embayment of 

southeast Missouri: Missouri Geological Survey and Water Resources, 

ser. 2, v. 37, 133 p.

Hamilton, R. M., andZoback, M. D., 1979, Seismic reflection profiles in the 

northern Mississippi embayment: U.S. Geological Survey Open-File 

Report 79-1688, 8 p., 25 pis.

Herrmann, R. B., 1979, Surface wave focal mechanism for eastern North 

American earthquakes with tectonic implications: Journal of Geophysical Research, v. 84, B-7, p. 3542 3552.

Herrmann, R. B., and Canas, J. A., 1978, Focal mechanism studies in the 

New Madrid seismic zone: Bulletin of the Seismological Society of 

America, v. 68, no. 4, p. 1095 1102.

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977. Magnetic 

and gravity anomalies in the northern Mississippi embayment and 

their spatial relation to seismicity; U.S. Geological Survey Miscellaneous Field Studies Map MF-914, 2 sheets.

Krinitzsky, E. L., 1950, Geological investigation of faulting in the lower 

Mississippi Valley: U.S. Army Corps of Engineers Waterways Experiment Station, Technical Memorandum 3 311, 91 p.

Marcher, M. V., and Stearns, R. G., 1962, Tuscaloosa formation in Tennessee: Geological Society of America Bulletin, v. 73, p. 1365-1386.

McGee, W. J., 1892, A fossil earthquake [abs.]: Geological Society of 

America Bulletin, v. 4, p. 411-414.

Merriam, D. F., and Harbaugh, J. W., 1964, Trend-surface analysis of 

regional and residual components of geologic structure in Kansas: Kansas State Geological Survey Special Distribution Publication 11, 27 p.

Mixon, R. B., and Newell, W. L., 1977, Stafford fault system- Structures 

documenting Cretaceous and Tertiary deformation along the Fall Line 

in northeastern Virginia: Geology, v. 5, p. 437-440.

Murray, G. E., 1961 [1962], Geology of the Atlantic and Gulf coastal province of North America: New York, Harper & Brothers, 692 p.

Potter, P. E., 1955, The petrology and origin of the Lafayette gravel, pt. 2, 

Geomorphic history: Journal of Geology, v. 63, no. 2, p. 115-132.

Pryor, W. A., 1960, Cretaceous sedimentation in upper Mississippi embayment: American Association of Petroleum Geologists Bulletin, v. 44, 

no. 9, p. 1473-1504.

Purdue, A. H., 1916, Oil and gas conditions in the Reelfoot Lake district of 

Tennessee: Tennessee Geological Survey, Resources of Tennessee, v. 6, 

p. 17-36.

Ratcliffe, N. M., 1971, The Ramapo fault system in New York and adjacent 

northern New Jersey- A case of tectonic heredity: Geological Society of 

America Bulletin, v. 82, p. 125-142.

Roberts, J. K., and Gildersleeve, Benjamin, 1950, Geology and mineral 

resources of the Jackson Purchase region, Kentucky: Kentucky Geological Survey Bulletin 4, ser. 9, 114 p.

Russ, D. P., 1979, Late Holocene faulting and earthquake recurrence in the 

Reelfoot Lake area, northwestern Tennessee: Geological Society of 

America Bulletin, v. 90, p. 1013-1018.

Russell, E. E., and Parks, W. S., 1975, Stratigraphy of the outcropping Uuper Cretaceous, Paleocene, and lower Eocene in western Tennessee (including descriptions of younger fluvial deposits): Tennessee Division of 

Geology Bulletin 75, 118 p.

Saucier, R. T., 1964, Geological investigation of the St. Francis basin: U.S. 

Army Corps of Engineers Waterways Experiment Station, Technical 

Report 3-659.

_______1974, Quaternary geology of the lower Mississippi valley: Arkansas Archeological Survey Research Series 6, 26 p.

Sbar, M. L., and Sykes, L. R., 1973, Contemporary compressive stress and 

seismicity in eastern North America And an example of intra-plate 

tectonics: Geological Society of America Bulletin, v. 84, p. 1861-1882.

Schwalb, H. R., 1969, Paleozoic geology of the Jackson Purchase region, 

Kentucky, with reference to petroleum possibilities: Kentucky Geological Survey, ser. 10, Report of Investigations 10, 40 p.

Stauder, William, Kramer, Mark, Fischer, Gerard, Schaeffer, Stephen, and 

Morrissey, S. T., 1976, Seismic characteristics of southeast Missouri as 

indicated by a regional telemetered microearthquake array: Seismological Society of America Bulletin, v. 66, no. 6, p. 1953-1964.

Stearns, R. G., 1957, Cretaceous, Paleocene and lower Eocene geologic 

history of the northern Mississippi embayment: Geological Society of 

America Bulletin, v. 68, p. 1077-1100.

_______1979, Recent vertical movement of the land surface in the Lake 

County uplift and Reelfoot Lake basin areas, Tennessee, Missouri and 

Kentucky: U.S. Nuclear Regulatory Commission, NUREG/CR-0874, 

37 p.
CONFIGURATION AND DEFORMATION OF THE PALEOZOIC BEDROCK SURFACE 135

Stearns, R. G., and Marcher, M. V., 1962, Late Cretaceous and subsequent

structural development of the northern Mississippi embayment area:

Geological Society of America Bulletin, v. 73, p. 1387-1394. 

Stearns, R. G., and Wilson, C. W., Jr., 1972, Relationship of earthquakes

and geology in west Tennessee and adjacent areas: Knoxville, Tenn.,

Tennessee Valley Authority, 302 p. 

Stearns, R. G., and Zurawski, Ann, 1976, Post-Cretaceous faulting in the

head of the Mississippi embayment: Southeastern Geology, v. 17, no. 4,

p. 207-229. 

Usher, F. C., 1837, On the elevation of the banks of the Mississippi, in 1811:

American Journal of Science, v. 31, p. 294-296. 

Zoback, M. D., 1979, Recurrent faulting in the vicinity of Reelfoot Lake,

northwestern Tennessee: Geological Society of America Bulletin, v. 90, 

p. 1019-1024.

Zoback, M. D., Hamilton, R. M., Crone, A.J., Russ, D. P., and Brockman, 

S. R., 1978, Seismic reflection profiling to delineate tectonic features of 

the New Madrid seismic region [abs.]: Seismological Society of America 

Earthquake Notes, Eastern Section, v. 49, no. 4, p. 27-28.

_______1979, Preliminary interpretation of faulting in the New Madrid 

seismic zone [abs.]: American Geophysical Union, EOS, Transactions, v. 60, no. 18, p. 310.

Zoback, M. D., Hamilton, R. M., Crone, A. J., Russ, D. P., McKeown, F. 

A., and Brockman, S. R., 1980, Recurrent intraplate tectonism in the 

New Madrid seismic zone: Science, v. 209, no. 4460, p. 971-976.

High-Resolution Seismic-Reflection 

Surveying on Reelfoot Scarp, 

Northwestern Tennessee

By JOHN L. SEXTON, E. P. FREY, and DAVE MALICKI

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-J
CONTENTS

Page

Abstract ............................................................................... 137

Introduction............................................................................ 137

Study area..............................................................................137

Field method ........................................................................... 140

Results ................................................................................ 140

Comparison to Vibroseis records ............................................................ 149

Conclusions ............................................................................ 149

References cited ......................................................................... 149

ILLUSTRATIONS

Page

FIGURE 1. Index map showing location of study area .................................................................................. 138

2. Stratigraphic column based on log from the Kate Wright no. 1 well located near Reelfoot scarp 1 mi (1.6 km) north of Proctor City, Tenn. . . 139

3. Map showing seismic lines JL-1, AM-1, and LDC-1 on Reelfoot scarp, Tennessee. ................................................ 141

4. Diagram illustrating field method. A, source, receiver array, and schematic raypath diagram. B, plan view of source and receiver

array for successive shots ................................................................................................ 142

5. North-south seismic-reflection line AM-1 .................................................................................. 143

6. East-west seismic reflection lineJL-1 ...................................................................................... 144

7. East-west seismic reflection line LDC-1 .................................................................................... 145

8. Shotpoints on seismic lines LDC-1, JL-1, and AM 1 with similar features ........................................................ 146

9. East-west USGS Vibroseis reflection record T-5 .............................................................................. 147

10. North-south USGS Vibroseis reflection record T-7 ........................................................................... 147

11. Locations of faults suggested by Zoback (1979) based on Vibroseis reflection survey records .......................................... 148
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

HIGH-RESOLUTION SEISMIC-REFLECTION SURVEYING 

ON REELFOOT SCARP, NORTHWESTERN TENNESSEE

By JOHN L. SEXTON1 , E. P. FREY*, and DAVE MALiCKi3

ABSTRACT

A high-resolution seismic-reflection survey consisting of three short lines, 

two that are 2,000 ft (610 m) long and one that is 1,000 ft (305 m) long, has 

been conducted in the region of Reelfoot scarp in northwestern Tennessee. 

The purpose of the survey was to examine the feasibility of using high-resolution seismic methods to delineate shallow geologic structures within a limited 

area of the scarp. Results show that shallow reflectors (60 400 ms) can be 

mapped well when they are reasonably continuous. Reflections from the 

base of the Tertiary rocks and reflections from the Paleozoic rocks are also 

well resolved. No faults exhibiting significant vertical offset have been 

found. Thus, if faulting is associated with Reelfoot scarp, it must be located 

farther west from this location than was previously believed. Monoclinal 

flexuring is one possible explanation for the scarp structure that appears to 

be consistent with the seismic-reflection data.

INTRODUCTION

Studies designed to define the structural setting and tectonic 

history of the Mississippi Embayment area have recently increased in number and intensity. Understanding the tectonism 

of the area is important because of the potential earthquake 

hazard posed by the New Madrid fault zone. Although the 

seismicity of the area is being monitored (Stauder, 1977), the 

relation between seismicity and regional tectonics is poorly understood. Understanding the relation requires knowledge of 

the location, extent, geometry, and activity of faults in the 

area. Faults of the New Madrid seismic zone are poorly defined because they are concealed by a thick cover of unconsolidated sediments. Most of the active faulting is at depth and 

usually does not produce detectable surface displacements.

'Department of Geosciences, Purdue University, West Lafayette, IN 47907.

department of Geology. Southern Illinois University, Carbondale, IL 62901.

3 Amoco Production Company, Houston Division, 500 Jefferson Building, Houston, TX 77001.

The geologic field investigations of Russ (1978) of the USGS 

(U.S. Geological Survey) have been of great importance in 

locating fault zones and interpreting the regional tectonics. To 

supplement these geologic studies, the USGS has initiated a 

seismic-reflection program to help delineate subsurface faults. 

As part of the USGS program, 32 km of 24-fold conventional 

Vibroseis (registered trademark of Continental Oil Company) 

reflection lines were run near the western edge of Reelfoot 

Lake and on Reelfoot scarp, north of Tiptonville, Tenn. 

Results have been summarized by Zoback (1979).

The seismic-reflection program was designed to search for 

subsurface structure associated with Reelfoot scarp and the 

Ridgely lineament. The best quality reflections on the 

Vibroseis records are associated with (1) the Porters Creek 

Clay, (2) a limestone of the Clayton Formation at the base of 

the Tertiary, and (3) the top of the Paleozoic rocks. Above the 

Porters Creek reflector, no continuous reflections were observed on the Vibroseis records.

The present study examines the feasibility of using highresolution seismic-reflection techniques with explosive sources 

to map not only the deeper Paleozoic, Cretaceous, and 

Paleocene units, but also shallow reflectors that were poorly 

resolved in the Vibroseis survey. The study area was selected to 

coincide with the area where an exploratory trench was excavated by the USGS and where several Vibroseis lines were 

run.

STUDY AREA

Reelfoot scarp is located west and north of Reelfoot Lake in 

the Tiptonville 7^-minute quadrangle in Lake County, 

Tenn. (fig. 1). This area rests on the alluvial plain of the 

Mississippi River. The post-Paleozoic sediments consist of 

about 2,000 ft (610 m) of poorly consolidated sand, silt, and 

clay. A stratigraphic column (fig. 2) based on a driller's log

137
138 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

FIGURE 1. Index map showing location of study area (circled).

(Wells and Foster, 1933) from the Kate Wright No. 1 well, 

located on the scarp near Proctor City, indicates that the 

alluvium is approximately 145 ft (44 m) thick. Gravels of 

Pleistocene age are included in the alluvium. The remaining 

sediments are poorly consolidated down to the irregular 

Paleozoic surface, although several exceptions, such as the 

hard sandstone at the base of the Cretaceous, are noted. It 

should be stressed that because the stratigraphic column (fig. 

2) is based on a driller's log, it may contain errors.

The Jackson Formation underlies the alluvium; it is 

reported to be 45 ft (14 m) thick and to consist of silt, clay, and 

thin beds of sand. The formations of the Claiborne Group, unconformably underlying the Jackson Formation, cannot be 

differentiated in well logs because they all consist of sand intercalated with beds and lenses of clay. The Claiborne Group is 

underlain by the Wilcox Group. Layers of coarse sand are

found in the Wilcox Group, and the well log (Wells and 

Foster, 1933) includes a hard "sandrock" at a depth of 900 ft 

(274 m), still within the Wilcox Group. However, there is some 

question concerning the exact location of the boundary between the Claiborne and Wilcox Groups. The total combined 

thickness of the Claiborne and Wilcox Groups is 1,075 ft (328 

m).

The Wilcox Group is underlain by the Paleocene Midway 

Group, represented in northwestern Tennessee by the Porters 

Creek Clay. The Porters Creek Clay is between 150 and 250 ft 

(46-76 m) thick in this area; it consists of a carbonaceous dark 

clay with glauconitic silt and sand. The Paleocene Clayton 

Formation is a thin limestone layer at the base of the 

Paleocene that unconformably overlies the Upper Cretaceous 

Ripley Formation. The Ripley consists of sand and clay, is 

about 250 ft (76 m) thick, and contains clay lenses. These Up-
HIGH-RESOLUTION SEISMIC-REFLECTION SURVEYING ON REELFOOT SCARP, TENNESSEE 139

Age

Q 

U 

A 

T 

E 

R 

N 

A 

R 

Y

E 

0

C

E 

N

E

ALLUVIUM

JACKSON 

FORMATION

C

L

A 

1 

B 

0 

R

E

6

R

0 

U 

P

w

1

L 

r

0

X

6 

R 

0

U 

P

m_-_-_-TJ ""*-''

~ ->

\---^:}] ~* - -

rrr

 '. : ' ' '. '.'.' 

 '/ .; ;-X ;-

**"- --

i *i $a

;: ^ ;S:'

V X$

: ':r : ' . 

\KOCK [

-I-^r-IH

v.-.'-v ' '.' ' 

- - -.

*"'*" .'-".'-'i -'.**

(Fee 

. 145

.190

 900

1265

P

A

L

E 

0

C

E

N

E

U

P

P

E

R

C 

R

E 

T

A

C

E

0

U

s

P

A

L

E

0

z

0

1

C

M

1

5, PORTERS

A CREEK

Y CLAY

6

R

0

U

P

CLAYTON 

FORMATION r \j T\ m M i iwi^

R

1

P

L

E

Y

F 

0 

R

M

A

T

1

0

N

UNDIFF

FORMATIONS

nr~_n.~

"~ " r~

r£HHr

X^r

_^~ * V"i_V

=^1-3-"

~ _ " _ _n

r^in_~

7^j~^n_m ~~_^~

 ^~^-^~L

~i_n_^^i

~ -^~~ " '' " ~"

u^ru^J"

i_r^^J^-T"

^ - T^-~

Lr^nrrj"

ri>I-I£

iS8^!L '. 'I'fT^i

^^_ i * i i * i

^g.'i!! .'! ! '

^W

1265

1620

^

1974

Meters 0 T 0 Feet 

50

EXPLANATION

| | No Sample 

H Unconformity

E3 Sand

ES3 Alluvium

El Sand and Gravel

O Gumbo

SD Sandstone

ES Shale

S Limestone

IIlGumbo, Sand and Shale

FIGURE 2. Stratigraphic column based on log from the Kate Wright no. 1 well located near Reelfoot scarp 1 mi (1.6 km) north of

Proctor City, Tenn.
140 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

per Cretaceous sediments lie unconformably on the eroded 

surface of the Paleozoic rocks. The formations of the Paleozoic 

are consolidated and consist of limestone, shale, and chert.

An exploratory trench across Reelfoot scarp has revealed 

numerous faults, folds, and sandblow dikes on Holocene 

sediments (Russ, 1978). A 1.5-ft (0.5-m)-wide zone of eastdipping normal faults having a vertical displacement greater 

than about 10 ft (3 m) was mapped at the base of the scarp and 

is believed to be of deep-seated origin. According to Russ, 

faults within this zone are the only ones of probable tectonic 

origin known to cut Holocene sediments in the upper 

Mississippi Embayment. Russ also found that little or no nearsurface movement occurred across the zone, and he inferred 

that the structures exposed in the trench were not related to 

the 1811-12 New Madrid earthquakes. Further, Russ's 

analysis of the observations in the trench and of historical 

earthquake data indicate that three earthquakes strong 

enough to liquefy sediments and generate surface faulting 

have occurred in the vicinity of the trench sites, two of which 

predate 1800.

Seismic-refraction profiles across Reelfoot scarp (Zoback 

and others, 1978) have been interpreted as showing vertical 

offsets of 180 + 33 ft (55 + 10 m)(down to the east) on the top of 

the Paleozoic bedrock. This interpretation is in agreement 

with interpretations made from the USGS seismic-reflection 

program (Zoback, 1979).

Results of the USGS seismic-reflection program indicate 

numerous faults, the most significant of which are two eastdipping, high-angle normal faults that have about 165-200 ft 

(50-60 m) of vertical offset at the contact of the Upper Cretaceous sediments and Paleozoic bedrock (Zoback, 1979). One 

of the faults is associated with a major northeast-trending 

lineament (Ridgely lineament) located near the southeastern 

edge of Reelfoot Lake. This lineament extends from 15 km 

southwest of Ridgely, Tenn., for about 60 km to near 

Hickman, Ky. (Heyl and McKeown, 1978). The second 

reported fault is essentially coincident with Reelfoot scarp. 

The nature of the offsets on these faults is interpreted by 

Zoback as indicating reactivation of motion on faults of Late 

Cretaceous or older age.

FIELD METHOD

In May 1978, a seismic-reflection survey on Reelfoot scarp 

using high- resolution digital recording equipment and explosive sources was initiated. A digital sample rate of Vi ms 

was used to obtain adequate frequency resolution. Shot and 

geophone separations were kept small (20 ft or 6.1 m) to gain 

spatial resolution. A 4-ms sample rate, vibrator-point spacings of 200 ft (61 m), and group intervals of 200 ft (61 m) were 

used in the Vibroseis survey. The seismic system used for the

b; ~h-resolution survey consisted of an 8-channel digital 

seismic-recording system with a digital magnetic tape 

recorder; one detector per channel was used. Initially, hydrophones placed in shallow holes were used in the survey; 

however, use of these was abandoned because the holes caved 

in and because of sporadic failure of the hydrophones. The hydrophones could have provided higher frequency records than 

geophones had field conditions allowed their use. This apparently would not have been significantly advantageous, 

however, as several good records obtained with the hydrophones were judged only slightly, if at all, better than 

geophone records. The increased time required to use hydrophones is considerable in this area owing to inflow of water 

and sidewall material into the drill holes.

Three seismic lines were placed such that one 2,000-ft 

(610-m)-long, east-west line (JL-1) crossed Reelfoot scarp at 

the location (fig. 3) of the exploratory trench and also overlapped Vibroseis line T-5. The second seismic line (AM-1), also 

2,000 ft (610 m) long and oriented in a north-south direction, 

was located so that it intersected line JL-1 and was positioned 

in the same location as Vibroseis line T-7 (fig. 3). The third 

line (LDC-1) was 1,000 ft (305 m) long and was oriented eastwest so that it crossed the scarp at a location about 2,000 ft 

(610 m) south of line JL-1 (fig. 3). Along each 2,000-ft 

(610-m) line 101 shots were fired, and along the 1,000-ft 

(305- m) line 51 shots were fired. The three lines were shot in a 

manner that produced four-fold common-depth-point 

coverage (fig. 4). The shot-point spacing, detector spacing, 

and offset to the first detector were all 20 ft (6.1 m). Each successive shot was moved 20 ft forward, replacing the nearest 

geophone, which, in turn, was moved forward to become the 

last or number 8 geophone. The source used for each shot was 

one charge of Atlas kinestik1 (K-l/3). All shot depths and 

geophone elevations were surveyed to an accuracy of 0.1 ft 

(0.03 m). Shot holes were drilled below the water table when 

possible and, due to hole cave-in, ranged from 8 to 22 ft (2.4 to 

6.7 m) deep. The maximum shot-hole depth of 22 ft was 

limited by drilling costs and equipment capabilities. Forward 

and reverse refraction profiles that were 840 ft (256 m) long 

and had a detector spacing of 105 ft (32 m) were shot for each 

line. The information was used as an aid in determining static 

corrections for the reflection survey.

RESULTS

Results of the high-resolution seismic-reflection feasibility 

study (figs. 5-7) show that very good reflections from shallow

'Any use of a brand nan 

USGS.

i for descriptive purposes only and does not imply endorsement by the
HIGH-RESOLUTION SEISMIC-REFLECTION SURVEYING ON REELFOOT SCARP, TENNESSEE 141

300 FEET

0 80 METERS

I

I

4v P no 

i

N

 T-7

V.P. 115

BM 

29076

+ - 

I 

I 

i

I

V.P. 136

V P 120

EXPLANATION

USGS VIBROSEIS LINE 

SIU SEISMIC LINE 

SIU SEISMIC LINE 

V.P .120 USGS VIBROSEIS POINT 

SIU SHOT POINT

x TEMPORARY BENCH MARK

FARM ROAD 

PAVED ROAD

FIGURE 3. Map showing seismic lines JL-1, AM-1, and LDC-1 on Reelfoot scarp, Tennessee. 

Shotpoints are 20 ft (6 m) apart.
142 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

RECEIVER ARRAY

SOURCE 1

0 20 FEET 

024 METERS

RECEIVER ARRAY

0,00000

o 20 FEET

024 METERS

FIGURE 4. Diagram illustrating field method. A. source, receiver array, 

and schematic raypath diagram. B. plan view of source and receiver array for successive shots. All shot and receivers are colinear but shown in 

separate rows for clarity. Dashed lines are schematic raypaths to illustrate how four-fold common-depth-point average is achieved. Vertical line represents common depth point for each successive shot.

Tertiary horizons, in addition to reflections from Cretaceous 

and Paleozoic rocks, can be obtained. Reflections were obtained at two-way travel times of about 60, 80, 130, 180, 350, 420, 

580, and 660 ms. The reflections at 60 and 80 milliseconds 

were strong but generally discontinuous and cannot be correlated across the entire records. Reflections at 130, 180, 350, 

580, and 660 milliseconds were strong and can be correlated 

across most parts of the records. Several zones on the reflection 

records show disruptions of the reflectors, but in most cases 

these are due to bad data caused by either near- surface or shot 

conditions. Preliminary depth estimates based on two different (but similar) velocity functions, one derived from this 

survey and the other from the USGS Vibroseis survey, indicate 

that reflections are obtained from the top of the Jackson Formation (60 ms), the top of the Claiborne (80 ms), several units 

within the Claiborne and Wilcox Groups (130, 180, and 350 

ms), the top of the Midway Group (Porters Creek Clay) (420 

ms), and the Paleocene Clayton Formation of the Midway 

Group which overlies the Cretaceous rocks (580 ms) and the 

Cretaceous-Paleozoic unconformity (660 ms). The stratigraphic column (fig. 2), based on the de Armand driller's log 

(Wells and Foster, 1933), has been used as a guide for interpreting reflections.

Exact depths cannot be calculated because of the uncertainties in the calculated interval-velocity function. Use of a 

stacking-velocity function derived from the reflection data 

gives an estimate of the depths. Each of the lines shows good

reflections from the various reflectors noted above. Lines 

AM-1 and JL-1 intersect at shotpoint 24 on line AM-1 and 

shotpoint 26 on line JL-1. Overlaying the seismograms at this 

tie point reveals that all reflectors on both lines match almost 

perfectly, as they should. Line LDC-1 does not intersect the 

other two lines; however, it is roughly parallel to JL-1 and is 

nearly perpendicular to the strike of Reelfoot scarp, as is JL-1. 

Therefore, if no dip parallel to the strike of the scarp exists, it 

can be assumed that subsurface reflections should correlate 

well on both lines at shotpoints connected by straight lines 

roughly parallel to the scarp. Accordingly, all reflections at 

shotpoint 1 on line LDC-1 correlate very well with reflections 

at shotpoint 46 on line JL-1. Reflections at subsequent shotpoints on LDC-1 and JL-1 also correlate well, except beyond 

about shotpoint 25 on line LDC-1 where very few good reflections are recorded.

On line AM-1, shotpoints 1-20 were recorded using hydrophones; the general character of the record (fig. 5) for those 

shotpoints is of slightly higher frequency content than for those 

shotpoints for which geophones were used. Disruptions in the 

continuity of reflections on line AM-1 occur for all reflectors 

between shotpoints 12 and 20 and between 90 and 98. The 

poor reflection quality between 12 and 20 is associated with 

the previously mentioned hydrophone problem. Between shotpoints 90 and 98, reflections are not traceable for the Clayton 

unit; however, the only vertical offset is due to dip, and no 

faulting of the Cretaceous or Paleozoic rocks is inferred for this 

region of the scarp. The shallow reflectors for shotpoints 80-98 

are generally broken, discontinuous, and difficult to follow 

across the record. This reflection character may be the result 

of shallow faulting or of inadequate static corrections. The 

time intervals between the shallow reflectors at 130 and 180ms 

remain essentially constant between shotpoints 1 and 90 but 

appear to increase slightly between shotpoints 90 and 100.

A similar shallow feature exists on line JL-1 between shotpoints 45 and 55; however, no time-interval change is observed 

(fig. 6) between the same shallow reflections. This feature can 

also be correlated with a disrupted shallow reflection (130 ms) 

on line LDC 1 beginning at shotpoint 25 (fig. 7). A straight 

line connecting shotpoints 25 on LDC-1, 53 on JL-1, and 80 

on AM-1 has a strike roughly parallel to the strike of the scarp 

and could possibly be a shallow fault zone (fig. 8) associated 

with the scarp. These features may or may not be related to 

each other and to a common, small- displacement (a few 

meters), shallow fault zone such as the one located at the base 

of the scarp by Russ (1978). The correlation is possible, but 

certainly open to question.

Between shotpoints 40 and 55 on line JL-1, the slope of the 

reflections decreases. Beginning at shotpoint 55 on line JL-1, 

the shallow reflections gradually bend upwards by a total of 

about 7 ms (one-way travel time) when compared to the same 

reflection between shotpoints 1 and 40. This gradual rise is 

estimated to be about 39 ft (12 m) using a one-way travel time 

of 7 ms and a velocity (based on refraction and reflection data)
SHOTPOINT NUMBERS 

60 50

aaigaij3igig«gi«geis§^

FIGURE 5. North-south seismic reflection line AM-1; location of seismic line AM-1 is shown on figure 4. Traces are from reflection points spaced 1
146 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

0 300 FEET j ,01. 

I ! ' I 'l I 1

0 80 METERS /

/ r- '

 - - ._-, ___.__._______._ ----, y

AM-I /

/ 51

V /

101 ^^^" ^>» /

) """- 1 

 ....y.... ......................

1

1

' N

iv p no ' 

I/

/i

 T-7

1 

1 V P 1(5

1 

1 

1 

1 

1 

TBM 

^9076

Q / / *_ __ _: 4-'- __I^li.

S7 / V P 140^ T-5 V P 136 

o / /

CO £ /

£ r / 

/( 1

/'"" / ' V P 120

\ / EXPLANA1

r3 / --- USGSVIBRO 

) / ........... SIU SEISMIC

r I ........... SIU SEISMIC

/ *V.P .120 USGS VIBRC 

^ / -si SIU SHOT P 

) / - POSSIBLE F> 

^7 ^; / ^oTx TEMPORYB 

__ J ""::^ / Z = = ZI FARM ROA

~^^ '' / LDC-I DA\/Cn DHA

'ION

SEIS LINE 

: LINE 

: LINE

)SEIS POINT

OINT 

\ULT 

ENCH MARK

C) 

^D

3I806J / -.V;: ::::::::::;::;:::::::::::::-::::-:::::---:.:::- -

FIGURE 8. Shotpoints on seismic lines LDC-1, JL-1, and AM-1 at which similar features are noted are connected with heavy

dashed line.
HIGH-RESOLUTION SEISMIC-REFLECTION SURVEYING ON REELFOOT SCARP, TENNESSEE 147

USGS 

REFLECTION LINE T-5

WEST TIE LINE T-7

V.P. 117 EAST

w ELEVATION 

140T 135 130 125 120 115 110 105 (FEET)

300

200

o£ 1000i

1200

1400

1600

1800

2000

FIGURE 9. East-west USGS Vibroseis reflection record T-5. Seismic line 

T-5 overlaps lineJL-1, and the location is shown in figure 3. Lines T-7 

and T-5 cross at vibration point (V.P.) 117.

face and water table, producing severe ground roll. This is 

observed as low-frequency noise on the reflection record, in 

contrast to the higher frequency content on that part of the 

record section where good reflections are recorded. Because 

most of the energy went into the generation of surface waves, 

little energy reached the subsurface reflectors. Therefore, the 

reflections were weak and difficult, if not impossible, to correlate across the record. Most of these reflections can, with difficulty, be followed across the record sections, except for those 

on the LDC-1 line.

SOUTH

USGS 

REFLECTION LINE T-7

TIE LINE T-5 

V.P. 137.5

V

120 115 110 105

NORTH

0.0

300

0.9

1-Ofctato

1.7

1.8

1.9

ELEVATION 

(FEET)

FIGURE 10. - North-south USGS Vibroseis reflection record T-7. Seismic 

line T- 7 overlaps line AM-1, and the location is shown in figure 3. 

Lines T-7 and T-5 cross at vibration point (V.P.) 137.5 on line T-7.
148 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

FIGURE 11. Locations of faults suggested by Zoback (1979) based on Vibroseis reflection survey records.
HIGH-RESOLUTION SEISMIC-REFLECTION SURVEYING ON REELFOOT SCARP, TENNESSEE 149

COMPARISON TO VIBROSEIS RECORDS

Vibroseis line T-5 is an east-west line which overlaps seismic 

line JL-1 (fig. 9). T-5 extends much farther to the east than 

does JL-1. Line T-7 is a north-south line coincident with line 

AM-1, but T-7 extends farther to the north by about 2,000 ft 

(610 m) (fig. 10). Seismic sections for T-5 and T-7 are 

presented in figures 9 and 10. The time scale for the Vibroseis 

record is twice that for the JL-1, AM-1, and LDC-1 records, 

which will be collectively called the SIU (Southern Illinois University) records.

Resolution, both in time and space, of shallow and deep 

reflectors is greater on the SIU records. Examination of the 

SIU field records shows a frequency of about 125 Hz for the 

reflection from the Paleocene Clayton Formation, whereas the 

Vibroseis reflections are about 30 Hz or less. Harmonic 

analysis of the SIU records for the shallower reflections reveals 

significant energy at 250-300 Hz.

Several faults have been suggested (Zoback, 1979) for the 

scarp area on the basis of line T-4, located several thousand 

feet north of the SIU lines, and also from line T-3, located 

several thousand feet west and extending south. Figure 11 

gives the estimated location of possible faults, as discussed by 

Zoback. These suggested faults all cross one or more of the SIU 

lines. However, no fault with any significant displacement is 

observed on the SIU seismic records. Thus, if faulting is 

associated with Reelfoot scarp in the area of the SIU survey, it 

must be located west of the JL-1 line. Zoback (1979) has also 

commented on the questionable nature of the offsets observed 

on line T-4 because of apparent errors in static corrections. 

Other Vibroseis lines not within the very localized area of investigation of this study have shown larger offsets in the 

Tertiary-Cretaceous and Cretaceous-Paleozoic contacts. On

those lines, evidence for faulting is more positive than on line 

T-4 (Zoback, 1979).

CONCLUSIONS

Results of the high-resolution seismic-reflection feasibility 

study show that mapping shallow reflectors (60 to 400 ms) in 

this region is possible if reflectors are continuous. The highresolution record sections also have excellent reflections from 

the Paleocene Clayton Formation (580 ms), which overlies the 

Cretaceous strata, and from the Paleozoic rocks (660 ms). Profiling across Reelfoot scarp has shown no significant vertical 

offset in Paleozoic and younger rocks directly beneath the 

scarp. If Reelfoot scarp is associated with faulting in the 

localized area of this investigation, then the fault must be 

located farther west. The scarp appears to be associated with a 

monoclinal flexure.

REFERENCES CITED

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map of 

central Mississippi Valley and environs: U.S. Geological Survey Miscellaneous Field Studies Map MF-1011.

Russ, D. P., 1978, Recent surface faulting and earthquake recurrence in the 

Reelfoot Lake area, northwestern Tennessee [abs.]: American Geophysical Union, Midwestern Meeting, St. Louis, Mo., 1978, Program of 

Abstracts, St. Louis University, p. 8.

Stauder, William, 1977, Microearthquake-array studies of the seismicity in 

southeast Missouri: U.S. Geological Survey Earthquake Information 

Bulletin, v. 9, no. 1, p. 8-13.

Wells, F. G., and Foster, M. D., 1933, Ground-water resources of western 

Tennessee: U.S. Geological Survey Water-Supply Paper 656, 319 p.

Zoback, M. D., 1979, Recurrent faulting in the vicinity of Reelfoot Lake, 

northwestern Tennessee: Geological Society of America Bulletin, pt. 1, v. 

90, p. 1019-1024.

Zoback, M. D., Healy, J., Roller,]., McKeown, F., and Russ, D., 1978, A 

geophysical assessment of suspected faults in the New Madrid seismic zone: 

EOS (American Geophysical Union Transactions), v. 59, no. 4, p. 229.

Stratigraphy and Structure of 

Post-Paleozoic Sediments Above the 

Newport Pluton of Northeastern Arkansas

By E. E. CLICK

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-K
CONTENTS

Page

Abstract ............................................................................... 151

Introduction ............................................................................151

Regional setting .........................................................................151

Previous investigations.................................................................... 153

This investigation ........................................................................153

Geologic history .........................................................................156

Structural computations .................................................................. 156

The pre-Late Cretaceous erosion surf ace ................................................. 156

Regional tilting and regional warping ................................................... 158

Pre-Late Cretaceous topography ....................................................... 158

Local uplifts ........................................................................ 160

Combined features................................................................... 160

Upper Cretaceous and younger depositional units above and geographically near the Newport Pluton .. . 160 

General Statement ................................................................... 160

Unit 1: The weathered Paleozoic substratum .............................................. 164

Unit 2: The basal conglomerate of the Upper Cretaceous sequence ............................ 164

Unit 3: The main body of the Upper Cretaceous sequence ................................... 165

Unit 4: Paleocene limestone ........................................................... 166

Unit 5: Upland gravel ................................................................ 167

Unit 6: The Eocene sequence........................................................... 167

Unit 7: Post-Eocene deposits ........................................................... 172

Summary .............................................................................. 173

References cited .........................................................................173

ILLUSTRATIONS

Page

FIGURE 1. Index map of Arkansas ................................................................................................. 152

2. Map of the Newport area showing outcrop localities or generalized limits of post-Paleozoic sedimentary units. .......................... 154

3. Diagrammatic cross section along lincA-A '................................................................................ 155

4. Map of three distinct parts of the Newport region with contours drawn on top of the reconstructed pre-Late Cretaceous erosion surface ..... 157

5. Map of the Newport region showing the cumulative effect of regional tilting and regional warping of the pre-Late Cretaceous erosion surf ace 159

6. Map of the Newport region showing an interpretation of the topography at the time of the advance of the Late Cretaceous sea ............ 161

7. Map of the Newport area showing the location and size of three local uplifts ...................................................... 162

8. Map of the Newport region showing the cumulative deformation of the pre-Late Cretaceous erosion surface ........................... 163

9. Map of the Newport area showing the middle Cretaceous topography as altered by the subsequent effect of local uplift................... 164

10. Diagrammatic cross sections along line B-B' in the Newport region north of the local uplifts ........................................ 169

11. Diagrammatic cross sections along line A -A ' in the Newport region through the area underlain by local uplifts .........................171
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

STRATIGRAPHY AND STRUCTURE OF POST-PALEOZOIC

SEDIMENTS ABOVE THE NEWPORT 

PLUTON OF NORTHEASTERN ARKANSAS

By ERNEST E. CLICK

ABSTRACT

The Newport area, at the border of the Mississippi Embayment and the 

Ozark region of northeastern Arkansas, in part is underlain by the deeply 

buried Newport pluton of probable Late Cretaceous age. The area also contains three local uplifts that are closely associated geographically with the 

pluton. The uplifts developed during the time of deposition of Upper Cretaceous and lower Tertiary sediments in the embayment.

At about the end of Cretaceous time, the sea floor above the newly 

emplaced Newport pluton was arched slightly- just enough to allow marine 

erosion of a few meters of soft sediment and the formation of rounded pebbles from the underlying beds of Pennsylvanian sandstone. Some of those 

pebbles accumulated in slight depressions and became a foundation for 

Paleocene limestone; some were incorporated into the limestone.

At about the end of Paleocene time, three local areas within or near the 

geographic limit of the Newport pluton were uplifted well above their 

former level in the embayment. Apparently the uplifts resulted from the injection of magma into the lower part of the sedimentary sequence. If so, 

more than 50 km3 of material was emplaced at a depth of more than 1.76 km 

below sea level. The two uplifts near the edge of the embayment have been 

stripped of most of their post-Paleozoic cover; the southeastern uplift, about 

25 km basinward from the Paleocene shoreline, still is covered by Quaternary alluvium and older sediments.

INTRODUCTION

Three local uplifts have been outlined by surface and subsurface geologic data in the vicinity of the Newport pluton 

along the edge of the Mississippi Embayment of northeastern 

Arkansas. This report discusses the general stratigraphy, 

structure, and depositional history of post-Paleozoic sediments 

in a broad area that includes the pluton and the uplifts, and

indicates which stratigraphic and structural anomalies of the 

area are related to the local uplifts.

Geologic and geophysical data from the general area of the 

deeply buried pluton show that there is only a partial 

geographic correlation between the uplifts and the mass of 

dense and highly magnetic rock that makes up the main body 

of the pluton. No attempt is made herein to force the data to 

fit, or to blend the two sets of data, or even to present the raw 

geophysical data. However, because of the general agreement 

among various observations and concepts, one of the prime 

working hypotheses in this report is that the emplacement of 

the pluton or the emplacement of local lenticular masses of 

igneous material stemming from the pluton caused the 

dome-shaped uplifts.

REGIONAL SETTING

The rectangular Newport area, Arkansas (fig. 1), the principal geographic part of the region of interest in this report, 

extends from lat 35 °15 ' to 36 °00' N. and from long 91 °15' to 

91 °45' W., a total of a little more than 3,600 km2 . The three 

uplifts described in this report are within its borders, as are all 

but the easternmost and southernmost parts of the deeply 

buried Newport pluton. For completeness and in order to add 

a more regional perspective, areas to the east and south of the 

Newport study area are shown in many illustrations. The term 

Newport region, rather than Newport area, is used to designate this larger segment of northeastern Arkansas.

The northeast-trending boundary of the Mississippi Embayment of the Gulf Coastal Plain province extends along an ir151
152 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

93° 92° 91"

SALEM PLATEAU

ARAGOULD PLUTON 

.(NOT EXPOSED) SPRINGFIELD? PLATEAU

JONESBORO

PLUTON 

(NOT EXPOSED)

BOSTON MOUNTAINS

~r- 

<

NEWPORT 

PLUTON

(NOT 

EXPOSED)

CROWLEYS 

RIDGE

LITTLE ROCK

PLUTON 

(EXPOSED 

LOCALLY)

I I ...

20 40 60 60 100 120 KILOMETERS

FIGURE 1. Index map of Arkansas showing the locations of the Newport area, the Batesville manganese district, Crowley Ridge, physiographic provinces 

and geologic phenomena discussed in text, the Mississippi valley graben or rift zone, and cross sections A-A ' and B-B ' shown on figures 3, 10, and 11.

regular line from the southwest corner to the northeast corner 

of the Newport area. In general, this is the boundary at which 

rocks of Paleozoic age that crop out to the northwest dip 

beneath the Cretaceous and younger deposits of the embayment. This generally accepted position of the edge of the embayment, however, actually is the northwestern limit of a 

30-m-thick layer of Quaternary clay, sand, and gravel. Cretaceous and Paleocene units crop out in a discontinuous band 

just northwest of the boundary and, farther northwest, in 

outliers throughout a zone several kilometers wide. No 

Paleozoic outcrops are found in the Arkansas part of the upper 

Mississippi Embayment, but many post-Paleozoic outliers occur in the eastern Ozark region.

The northwestern part of the Newport area, underlain by 

Paleozoic rocks, lies within the eastern ends of two principal 

physiographic provinces: the low-lying Arkansas Valley section of the Ouachita porvince on the south, and the relatively 

high Ozark Plateaus province on the north (fig. 1). The Ozark 

Plateaus province is in turn divisible into three distinct 

geomorphic units: (1) the Salem Plateau to the north 

(underlain by a variety of Ordovician units that are subject to 

truncation by the solution of their carbonate), (2) the Springfield Plateau (underlain by the highly resistant Boone Formation of Mississippian age), and (3) the Boston Mountains to the 

south (underlain by various siliceous sandstone beds of Early 

Pennsylvanian age). West of the Newport area, where these
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS 153

three geomorphic units are well delineated, the Boston Mountains stand nearly 400 m above the Springfield Plateau; and 

the Springfield Plateau, in turn, stands nearly 100 m above the 

Salem Plateau.

In the eastern Ozarks of the Newport area, facies changes 

all but eliminate any distinctive Boone-supported plateau 

along the edge of the embayment where Paleozoic units dip 

beneath younger deposits. The term Boston Mountains loses 

some of its usual significance when applied to topographic 

highs at the edge of the embayment, because the "mountains" 

there are uplifted areas that once were part of the embayment. 

Even though the standard concepts of Ozark geomorphic subdivisions are barely applicable at the eastern edge of the Ozark 

region, the terminology is retained to identify the distal ends of 

regionally meaningful geomorphic features. Furthermore, 

although the three geomorphic provinces of Arkansas (Ozark 

Plateaus, Ouachita, and Coastal Plain) theoretically join at a 

point within the Newport area (fig. 1), the effects of the 

emplacement of magma related to the Newport pluton have 

left the location of that junction quite vague.

PREVIOUS INVESTIGATIONS

In the Newport area, the geology of both the Paleozoic rocks 

to the northwest and the Cretaceous and younger sediments to 

the southeast have been investigated intermittently during the 

past century. The northwestern part of the Newport area includes the eastern half of the Batesville manganese district 

(fig. 1) which, along with the area to the south and east, has 

been extensively studied because it contains deposits of 

manganese ore, high-calcium limestone, agricultural 

limestone, building stone, glass sand, gravel and other roadmetal resources, important faunal zones, abundant ground 

water, and a trace of natural gas (Miser, 1922, 1941; Croneis, 

1930; Easton, 1942; Gordon, 1944,1964; Gordon and Kinney, 

1944; Kinney and Straczek, 1950; Sheldon, 1954; Frezon and 

Click, 1959; and Stroud and others, 1969).

The geology of the Arkansas portion of the Mississippi Embayment has been investigated in reference to ground water, 

gravel deposits, oil and gas possibilities, lignite deposits, and 

faunal zones (Stephenson and Crider, 1916; Spooner, 1935; 

Renfroe, 1949; Caplan, 1954; Sheldon, 1954; and Clardy, 

1979).

Geophysical investigations of the Newport area have had a 

special impact on geologic concepts related to deeply buried 

sedimentary units and to underlying or possibly intruding igneous masses that have not yet been reached by the drill bit 

(Jespersen, 1964; Ouellette, 1970; Ervin and McGinnis, 1975; 

Hildenbrand and others, 1977; Cordell, 1977; Hendricks, 

1979; Hendricks and Hildenbrand, 1979; and Hildenbrand 

and others, 1979). A recent map (Heyl and McKeown, 1978) 

summarizes the regional seismotectonic setting of the area.

THIS INVESTIGATION

The objective of this investigation of the Newport area was 

to reexamine on a regional basis many of the unresolved problems left by the local and topical geologic investigations of 

previous workers, and to organize and present both old and 

new data in various ways to depict the post-Paleozoic stratigraphic and structural development of the area. Previous 

workers left many geologic uncertainties in this particular part 

of the embayment area, because (1) some of the structural and 

depositional effects of the emplacement of the Newport pluton 

were recorded but not understood, and (2) remnants of postPaleozoic deposits west of the embayment lowlands are so 

widely scattered that they were largely ignored.

After this investigation was started, the discovery was made 

that the previously mapped rocks of Paleocene age which occur 70 m above sea level at the edge of the Mississippi Embayment have a counterpart of silicified limestone near the crest 

of the adjacent "mountain" 240 m above sea level (figs. 2 and 

3). As the carbonate unit of the Paleocene sequence is less than 

10m thick in this immediate area and rests on or within 3 m of 

the Paleozoic surface, its distortion is unrelated to compaction 

of underlying material. Post-Paleocene uplift is involved and 

the mountain of Paleozoic rock is younger structurally than 

the Paleocene rock resting on it. With some local uplift accepted as a certainty, available data were plotted on a series of 

maps (figs. 4-9) to quantitatively show the structural displacement that has occurred in the Newport area since middle Cretaceous time.

Much of the material deposited in the Newport area since 

that time has remained relatively unconsolidated; a small percentage of it is limestone, silicified limestone, and sandstone 

cemented by iron oxide. As the term rock does not correctly 

apply to most of the material, the term sediment is used in this 

report to designate a general class of material that includes 

largely unconsolidated detritus and some slightly consolidated 

marl. When applicable, the terms clay, silt, sand, and gravel 

are used to designate specific classes of unconsolidated 

detritus. Specific types of sedimentary rock are noted if important to a concept.

The formal terms Late Cretaceous and pre-Late Cretaceous 

are in part too general for the purposes of this report. Technically, Late Cretaceous is a span of about 35 m.y., whereas the 

Late Cretaceous sea probably did not reach the Newport area 

until about Maastrichtian time probably not more than 5 

m.y. before the end of the Cretaceous Period. Therefore, the 

Upper Cretaceous sequence of the Newport area probably was 

deposited during only the last 5 m.y. of Cretaceous time. The 

informal term middle Cretaceous is used to indicate much of 

the first 30 m.y. of Late Cretaceous time.

This study was supported by the National Earthquake 

Hazards Reduction Program and the Reactor Hazards Reduction Program of the U.S. Geological Survey. Cuttings and
154 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

91°45' 

36

Valley of ancestral 

Strawberry River

Valley of ancestral 

White River

Valley of ancestral 

Little Red River

EXPLANATION

Main body of alluvium of Quaternary age in the 

Mississippi Embayment-Valleys of streams 

entering the embayment area contain alluvium 

not shown here

Western limit of scattered deposits of upland 

gravel; probably late Paleocene in age

Areas that contain thicker, locally commercial, 

deposits of upland gravel

Outcrops of silicified Paleocene limestone

Paleocene limestone of the Grand Glaise terrace 

area, in part covered by loess and gravel

Generalized limit of sandstone-pebble gravel of 

Late Cretaceous and early Paleocene age. 

Widely scattered localities at which pebbles 

are abundant, locally in commercial quantities, 

indicated by cross

Western limit of widely scattered remnants of 

Upper Cretaceous sand and clay

Offshore bar of Late Cretaceous age

Geographic limit of main body of deeply buried 

dense and highly magnetic rock of the 

Newport pluton, as determined from 

geophysical data

Manmade exposure in SW%SE'/4SE% sec. 32, 

T. 11 N., R. 4 W., in which a layer of 

sandstone pebbles underlies a massive bed 

of silicified Paleocene limestone

Outcrop in SWV4NE'/4 sec. 32, T. 11 N., 

R. 4 W. in which pebbles of Ordovician and 

Mississippian chert and angular fragments of 

silicified Paleocene limestone occur together 

in an iron-rich sandy matrix

FIGURE 2. - Map of the Newport area showing outcrop localities or generalized limits of post-Paleozoic sedimentary units, and the geographic limit of the

main body of the deeply buried Newport pluton.

cores from test holes drilled in and near the area of study were 

obtained with the cooperation of the Arkansas Geological 

Commission and the Mobil Oil Corporation. Norman F. 

Williams, Director of the Arkansas Geological Commission,

and Elizabeth A. Elliot of the Mobil Oil Corporation were 

especially helpful. Donald R. Albin of the U.S. Geological 

Survey logged cuttings from the Newport Air Base test hole 

(% 10).
Total depth 1,829 4 m 

(1,763 3 m below sea level)

Lower Ordovician

and 

Upper Cambrian

Theoretical laccoliths drawn 

equal in size to local uplift 

components directly above

FIGURE 3. - Diagrammatic cross section along line A-A ' showing the relationship of the Boston Mountains, local uplifts, the Newport pluton, and the sedim

Mississippi Embayment. Dry hole MP- Sturgis drilled to the depth of 1,829.4 m shown in relation to the Newport pluton. Location of cross section A
156 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

GEOLOGIC HISTORY STRUCTURAL COMPUTATIONS

Based on current assumptions, the central part of the upper 

Mississippi Embayment area has been modeled as part of an 

ancient trough that may have been a rift valley or graben 

receiving deposits (volcanic, nonmarine, marine, or a mixture) during late Precambrian through Middle Cambrian 

time (Hildenbrand and others, 1977; and fig. 1). If so, deposition was effectively confined to the graben while drainage 

from the exposed igneous terrane of much of the upper midcontinent area was funneled into that trough. During Late 

Cambrian time the sea flooded the graben and spilled out 

across the adjacent lowlands. From then until Middle Pennsylvanian time, regional deposition and subsidence continued intermittently throughout the Ozark region and was nearly continuous in the trough. During early and middle Carboniferous 

time, a great quantity of detritus poured into at least the 

western part of the subsiding trough; much of the coarser 

detritus was reworked and swept westward by longshore currents while the central part of the trough received finer 

material.

An early phase of the Ouachita orogeny of Late Pennsylvanian time arched the trough or its outlet enough to block continental drainage and force the drainage to run around the 

north side of the Ozark region. No permanent deposits accumulated in the trough during the nearly 200 m.y. from Middle Pennsylvanian to Late Cretaceous time, but through a process of erosion, or erosion and subsidence, midcontinent 

drainage was re-established there. Marine deposition was 

renewed during the latter part of Cretaceous time as mafic or 

ultramafic magma was emplaced to form plutons along the 

margins of the trough (Hildenbrand and others, 1977). A final 

retreat of the sea at the end of Eocene time, and a resultant extension of the major midcontinent river across the embayment 

area, established the modern aspect of the Mississippi Embayment. Pleistocene glaciation blocked the northern outlet of the 

Missouri River, which then began to flow into the Mississippi 

Valley and down the trough.

The pre-Late Cretaceous development of the broad embayment plain is a geologic enigma. Ample time a substantial 

part of 200 m.y. was available for the widening of the valley 

once a channel was established, but a special sequence of 

events or circumstances seems to have been required to get a 

stream to cross the area. Apparently the post-Ouachitaorogeny route down the embayment and across the Ouachita 

Mountains was not as high topographically as geologic computations and projections seem to suggest, and this path was the 

best of several possible outlets for the ancestral Ohio River. 

Whatever the events may have been, a southward-flowing 

stream breached or circumvented the now-buried eastern end 

of the Ouachita Mountains during late Paleozoic or Mesozoic 

time. The Late Cretaceous sea advanced northward up a 

broad valley that had been cut directly across the structural 

grain of the Ouachita Mountains and across the ancient 

Mississippi Valley graben at an angle of about 20.

THE PRE-LATE CRETACEOUS EROSION SURFACE

The surface that was shaped by 200 m.y. of erosion prior to 

the advance of the Late Cretaceous sea is identifiable within 

broad limits over much of the Newport area. It is of critical 

importance in this study of subsidence and uplift during the 

past 70 m.y. Two significant types of features the Mississippi 

Embayment and the local uplifts in the vicinity of the Newport 

pluton developed in this area after the Late Cretaceous sea 

began to flood the erosion surface; those features can be 

measured quantitatively by measuring the distortion of that 

surface. As no other applicable horizon is available, any disadvantages of using the erosion surface as a datum are recognized as unavoidable factors in the overall problem.

Contours showing the reconstructed pre-Late Cretaceous 

erosion surface (fig. 4) are based on data and assumptions that 

differ according to the state of preservation of the surface in 

each of three distinct parts of the Newport region: (1) the embayment area in which the erosion surface is preserved extensively beneath Cretaceous deposits, (2) adjacent parts of the 

Ozark region and Arkansas Valley in which a few Cretaceous 

and Tertiary remnants can be found in areas of mostly 

Paleozoic outcrop, and (3) parts of the Ozark region and 

Arkansas Valley in which no Cretaceous or Tertiary deposits 

are known. As the general altitude of the top of the 

embayment-lowland surface of this area is only slightly more 

than 60 m above sea level, the + 60-m contour on figure 4 is 

used as the generalized boundary of the embayment on this 

and on figures 2 and 7.

The erosion surface in the embayment segment of the 

Newport area now extends from the + 60- to the -360-m contours and has a relatively uniform southeast dip of less than 1 °. 

Regionally, the southeast dip of the top of the undifferentiated 

Paleozoic beds remains relatively constant at less than 1° 

through the outer 75 km of the northwest limb of the Mississippi Embayment and then decreases gradually through the next 

45 km to the embayment axis near the present position of the 

Mississippi River (Capian, 1954, pi. 2; and Gushing and 

others, 1964, fig. 4). As no significant pre-Late Cretaceous 

hills, ridges, or valleys have been identified in the embayment 

area of Arkansas, the erosion surface is assumed to have had 

little relief at the beginning of Late Cretaceous time just 

enough southeast slope for drainage. The obvious distortion of 

the surface in the southeastern part of the Newport area is 

evidence of a post-Cretaceous uplift, which has been outlined 

by test drilling.

Where remnants of Late Cretaceous and Paleocene deposits 

are present on Paleozoic beds northwest of the embayment 

edge (northwest of the + 60-m contour, fig. 4), the pre-Late 

Cretaceous erosion surface is drawn to show as little undulation as possible while depicting a surface above all Paleozoic 

beds and below all Cretaceous and Paleocene deposits. The
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS

91°45' 

36°OO

35°15

157

90°45'

35°05'

EXPLANATION

i " i Embayment area in which the pre-Late Cretaceous erosion ~ 

t .. /'.' :i? i surface is preserved extensively beneath Cretaceous deposits

Eastern Ozark region in which a few Cretaceous and Tertiary 

remnants can be found in an area of mostly Paleozoic outcrops

Parts of the Ozark region and Arkansas Valley in which 

no Cretaceous of Tertiary deposits are known

Drill hole

FIGURE 4- Map of three distinct parts of the Newport region with contours drawn on top of the reconstructed pre-Late Cretaceous erosion surface. 

Contour interval, 30 m; datum, mean sea level. The + 60-m contour is the approximate boundary of the Mississippi Embayment.
158 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

surface dips into valleys that contain Upper Cretaceous 

deposits but is drawn above valleys that appear to have been 

cut since Cretaceous time. The younger valleys tend to have 

high, steep walls, whereas the old valleys that contain deposits 

of Late Cretaceous age tend to be more open.

The dip of the erosion surface changes little in crossing the 

Mississippi Embayment boundary, remaining at a little less 

than 1° to the southeast. Farther northwest in the Salem 

Plateau, the dip decreases. The largest local uplift of postPaleocene age in the Newport area (mentioned earlier in this 

report) distorts the surface in the south-central part of the 

area. A much smaller uplift is indicated by a closed + 90-m 

contour just south of the southern lobe of the large uplift.

The most controversial of the three parts of the erosion surface is in the western part of the Newport area, where the 

Boston Mountains, the Springfield Plateau, and part of the 

Arkansas Valley stood above the Late Cretaceous sea floor; 

therefore, no post-Paleozoic marine remnants exist to mark 

the present position of the former upland surface (figs. 2,4). 

In this area the erosion surface is drawn as a relatively smooth 

tentlike horizon slightly above the Paleozoic beds in such a way 

that only a small amount of erosion since middle Cretaceous 

time is accounted for. Any error caused by this procedure is a 

result of being too conservative in estimating erosion and is 

thought to be small. Long exposure prior to Late Cretaceous 

time must have resulted in a greatly decreased rate of erosion 

of these sandstone-capped and chert-residuum-capped 

uplands.

The reconstruction of the position of the pre-Late Cretaceous erosion surface, shown on figure 4, involves using data 

from an actual surface whose position can be verified by drilling in the embayment or by mapping post-Paleozoic outcrops 

in all but the western part of the upland, where no Upper Cretaceous units were deposited. The two requirements of the surface, that it be above the present position of all Paleozoic beds 

and below the present position of all Upper Cretaceous and 

Tertiary deposits of the area, give it a great deal of constraint. 

The three derivative maps described below are interpretive, 

but only to the extent permitted by the objective control built 

into figure 4. By definition, these three maps (figs. 5-7) each 

show a separate component, and the fourth (fig. 4) is a combination of all components; any one of the maps can be 

reproduced from a proper combination of the other three.

REGIONAL TILTING AND REGIONAL WARPING

The three, largely unrelated components that define the 

surface shown on figure 4 are (1) regional tilting and regional 

warping, (2) pre-Late Cretaceous topography, and (3) local 

uplifts. Regional tilting and warping, as implied by its name, 

significantly involves the entire area (fig. 5), and for the most 

part, relates to the normal development of the Mississippi Embayment. It is shown by smooth contours that tend to parallel 

the trend of the embayment. The pre-Late Cretaceous land 

surface rose significantly above sea level only in the western 

part of the Newport area (fig. 6); local uplifts are anomalies 

that shaped the embayment only in limited areas (fig. 7).

The amount of regional tilting and warping (fig. 5) was determined by using control points from areas of outcrop and 

from the subsurface, but all the control points were outside of 

the limits of topographic and structural anomalies. Contours 

were projected through the anomalous areas and tied to control points on either side. Separating Cretaceous topography 

and local uplifts from regional tilting is not difficult. Tilt is relatively uniform and is the only factor involved in much of the 

area. Abrupt changes in the dip of the erosion surface at an 

uplift or at the edge of the middle Cretaceous upland clearly 

indicate the addition of another factor. The maximum 

amount of regional tilting and warping of the erosion surface 

shown on figure 5 is less than 1 °; the maximum dip of the erosion surface in the vicinity of uplifts is as much as 4°.

As can be seen from figure 5, the southeast corner of the 

Newport area (lat 35°15'; long 91°15') has subsided more 

than 360 m and the northwest corner has risen more than 180 

m relative to sea level since the Late Cretaceous sea reached 

the area. Tilting toward the axis of the embayment is at a 

relatively uniform rate in the eastern part of the area but is 

reversed on the west side of the crest of a north-trending anticline that dominates the west-central and southwestern parts 

of the Newport area. This anticline is well outlined by field 

evidence along the south side of the White River valley and is 

extended southward from there on the basis of less positive 

evidence. The anticline may have been formed by the 

emplacement of an igneous body related to the Newport 

pluton, but probably it resulted from an entirely different 

stress. The structure is not similar to the more elliptical uplifts 

elsewhere in the area and is not a feature of middle Cretaceous 

topography. The anticline is present only in the area adjacent 

to and slightly east of the above-sea-level mass of the Boston 

Mountains. Apparently the Boston Mountains rose during the 

development of the Mississippi Embayment (as did the area 

directly to the north of them), but have since subsided. If so, 

the anticline was formed as a result of subsidence along the 

high side of a once uniformly tilted surface not as a result of 

local uplift along an axis.

PRE-LATE CRETACEOUS TOPOGRAPHY

The second-largest component of the surface that represents the reconstructed position of the pre-Late Cretaceous 

erosion level is the topography of the area at the time of the advance of the Late Cretaceous sea (fig. 6). Even though that 

long-since-altered topographic surface is unknowable in 

detail, some of its general features can be outlined. In the 

west-central part of the Newport area, the eastern end of the 

Boston Mountains stood at least 240 m above the level of the
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS

91° 15'

159 

90°45'

35°15

35°05'

EXPLANATION

Anticline

20 KILOMETERS 

I

FIGURE 5. Map of the Newport region showing the cumulative effect of regional tilting and regional warping of the pre-Late Cretaceous erosion surface.

Contour interval, 30 m; datum, mean sea level.

advancing Late Cretaceous sea; today that part of the area is a sloped gently eastward from a level more than 90 m above the

little more than 300 m above present sea level. Just north of sea; today the highest hills there are more than 240 m above

those mountains the eastern end of the Springfield Plateau present sea level. These measurements of Late Cretaceous
160 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

topography are made from the top of present-day topographic 

highs to the base of nearby post-Paleozoic deposits that accumulated at or near sea level. As such, they are conservative. 

Valleys of ancestral rivers also are marked by remnants of postPaleozoic deposits. Valleys of the ancestral Strawberry and 

Little Red Rivers (fig. 6) were respectively south and north of 

today's river valleys. The White River was then much as it is 

now, just north of the Boston Mountain escarpment. The proemial Mississippi Embayment lowlands sloped gently (perhaps 

even more gently than is shown on fig. 6) toward the advancing shoreline.

LOCAL UPLIFTS

The location and size of each of three local uplifts in the 

Newport area are shown on figure 7, along with their 

geographic relationships to the Newport pluton and to the 

western edge of the Mississippi Embayment. These uplifts are 

shown quantitatively as the residuum in this series of computations; they are what is left after middle Cretaceous topography 

(fig. 6) and regional tilting and warping (fig. 5) are subtracted 

from the reconstructed pre-Late Cretaceous erosion surface 

(fig. 4). Subsurface and outcrop data also indicate the general 

geographic positions of the uplifts, which are anomalously 

high masses of Paleozoic rock overlain by structurally high 

post-Paleozoic deposits. Only the indirect computations used 

here, however, yield a clear indication of their true shape and 

size.

Collectively the three local uplifts cover an area of nearly 

950 km2 , which is only a little less than the area underlain by 

the Newport pluton. They and the pluton, however, are not 

totally within the same boundaries (fig. 7). Apparently the 

uplifts developed when magma associated with the pluton was 

injected into the lower part of the sedimentary sequence. If so, 

more than 50 km3 of magma was emplaced in order to effect 

the arching of the overlying rocks. No post-Paleozoic faults are 

known to be related to the uplifts, and no dip greater than 4° 

has been measured on the flanks of the structures. Upper Cretaceous and younger depositional units above and geographically near the Newport pluton are discussed under that 

heading later in this report. The distribution of those units 

helps to identify the uplifts.

The locations of the two deepest holes that have been drilled 

on or near the uplifts are shown on figure 7. Both test holes 

reached total depth in rocks of Early Ordovician age, the 

northern one at 1,457 m below sea level and the southern one 

at 1,763 m below sea level (fig. 3). Neither test hole encountered igneous rock or unusually high temperatures.

The area occupied by the two northwestern uplifts is now 

generally included within the boundary of the Boston Mountains of the Ozark region, even though the uplifts are in areas 

once covered by embayment sediments. The southeasternmost 

uplift still is covered by embayment sediments because 

regional tilt exceeded uplift there. The "offset or reentrant" of

the embayment boundary (Jespersen, 1964) just south of the 

White River valley (fig. 7) is a result of post-Eocene erosion 

that stripped most of the younger sediment from the large 

uplift but left nearly all of the associated Paleozoic rock standing well above the embayment level. Close examination of the 

regional trend of the embayment boundary, as shown on the 

geologic map of Arkansas (Haley and others, 1976), reveals 

that the anomaly in the trend is a protuberance or bulge into 

the embayment area and that the reentrant into the Ozark 

region is a complementary feature on the north side of the 

bulge which is accented by the broad valley of the White 

River.

COMBINED FEATURES

Even though the three features middle Cretaceous 

topography, regional tilting and warping, and local uplifts 

appear to be unrelated genetically, selective combinations of 

them are informative. The combination of the effect of 

regional tilting and warping with the effect of local uplift (fig. 

8) shows the total deformation of the Newport region since 

middle Cretaceous time. This view of the data reinforces the 

conclusion that the uplifts are local anomalies rather than a 

part of the regional tilting toward the axis of the embayment. 

No faults are known to have been involved in the formation of 

the uplifts or in the tilting.

A somewhat different aspect of the uplifts is shown by their 

combination with middle Cretaceous topography (fig. 9). 

Without the effect of regional tilting and warping, the crest of 

the highest uplift is on a level with the crest of the Boston 

Mountains. This was never their relative standing, however, 

because at least one-third of the tilting had been accomplished 

prior to the final uplifting. Again, the uplifts are seen as 

anomalies of the embayment area, slightly detached from the 

Ozark region. Even a cursory comparison of the distribution of 

post-Paleozoic deposits (fig. 2) and the combination of middle 

Cretaceous topography and local uplifts (fig. 9) indicate how 

readily the topography and the uplifts can be differentiated. 

None of the younger marine deposits overlies the remnants of 

middle Cretaceous hills, but three post-Paleozoic marine units 

extend onto or across the highest uplift.

UPPER CRETACEOUS AND

YOUNGER DEPOSITIONAL UNITS

ABOVE AND GEOGRAPHICALLY

NEAR THE NEWPORT PLUTON

GENERAL STATEMENT

Beds of post-Paleozoic sand and clay in the area of outcrop 

of the Newport region have yielded no identifiable spore, 

pollen, or microfossils and only a few megafossils. The deposits 

that, in this report, are considered to be of Late Cretaceous
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS 161

90°45 r

FIGURE 6. - Map of the Newport region showing an interpretation of the topography at the time of the advance of the Late Cretaceous sea. 

Contour interval, 30 m; datum, mean sea level as of the time when the shoreline was as indicated on this map.
162 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

EXPLANATION 

-f- Deep drill hole

20 KILOMETERS

FIGURE 7. - Map of the Newport area showing the location and size of three 

local uplifts, all of probable pre-middle Eocene age, and their 

geographic relationship to the Newport pluton and to the western edge 

of the Mississippi Embayment. Contour interval, 30 m; datum (outer 

limit of each uplift) arbitrarily assigned to a horizontal surface.

age are so classified on the basis of the work of Stephenson and 

Crider (1916), on stratigraphic position, on lithologic correlation, and on the lack of convincing evidence that the deposits 

are younger than Cretaceous. Outcrops of Paleocene 

limestone and silicified limestone are fossiliferous and easily 

recognizable but are restricted to the area of the largest of the

three local uplifts, entirely south of the White River (fig. 2). 

No outcrops of Eocene deposits have been recognized in the 

Newport area, but Eocene deposits are present in the subsurface in the southeastern part of the area.

Rocks of Paleocene age that cap the Grand Glaise terrace 

along the edge of the embayment a few kilometers south of the 

White River (figs. 2, 3) yield an abundant fauna of Midway 

age. Stephenson and Crider (1916, p. 49-54) furnished details 

of the lithology, fauna, and stratigraphic relations of the 

Paleocene units in this area. (The Midway formation of 

Stephenson and Crider, 1916, and of earlier workers, originally was included in the Eocene Series.) Those authors also included two general comments in their report that, if accepted 

as stated, would counter some of the concepts of the present 

report:

1. p. 51 - "West of Bradford the Midway formation rests 

against the Paleozoic rocks along a steep eastward-sloping 

contact which probably represents an ancient sea cliff." 

Recent work (Click, unpub. data, 1978) has shown that 

silicified limestone of Midway age caps the hills northwest 

of Bradford and that the "ancient sea cliff is the unfaulted limb of a major post-Paleocene uplift.

2. p. 53 - "Independence County. The Midway formation 

extends from Jackson County northward into the southern part of Independence County. Harris (1894, p. 24) 

says: In Arkansas the Midway stage forms a low, terracelike elevation on the northeast flank of the Paleozoic 

upland just to the south and southwest of Bayou Departe, 

Independence County."

Apparently Stephenson and Crider did not quite understand what Harris intended. The Midway does not extend 

along the edge of the embayment from Jackson County northward into Independence County, and Harris did not actually 

say that it does. At any rate, on their map (1916, pi. 2), 

Stephenson and Crider showed a narrow band of Midway and 

younger deposits extending along the edge of the embayment 

northward from Grand Glaise in Jackson County to Departee 

Creek (formerly Bayou Departe) in Independence County. 

Authors of the 1929 Geologic Map of Arkansas (Arkansas 

Geological Survey, 1929), Fisk (1944, pi. 10, sheet 1), 

Jespersen (1964), and numerous others have perpetuated the 

error. Only at the northernmost end of the band of outcrop, 

shown on those maps do Cretaceous and possibly younger 

deposits form "a low, terrace-like elevation."

Seven lithologic units or stratigraphic sequences above the 

Paleozoic bedrock have been recognized in the outcrops and 

subsurface of the Newport area. Even though the units are not 

of equal stratigraphic rank, each is the product of a local or 

regional event that is important to the understanding of the 

structural and depositional history of the area. These units, 

listed below, are numbered from oldest to youngest and are 

discussed in that order in the following text:
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS

9 1 P4 5' 

36°00'

35° 15'-

35°05

FIGURE 8. - Map of the Newport region showing the cumulative deformation of the pre-Late Cretaceous erosion surface (that is, regional tilting and

warping plus local uplift). Contour interval, 30 m; datum, mean sea level.
164 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

(7) post-Eocene deposits of gravel, alluvium, and

windblown material 

(6) clay, silt, sand, and lignite of Eocene age (not

known to crop out in this area) 

(5) upland gravel, mostly chert pebbles 

(4) the Paleocene limestone; found to be silicified in

some locations; containing sandstone-pebble

clasts in many locations 

(3) the main body of the Upper Cretaceous sequence;

mostly sand but partly clay, with local deposits

of sandstone pebbles 

(2) the basal conglomerate of the Upper Cretaceous

sequence; scattered chert pebbles in locally derived residuum 

(1) the weathered and leached upper layer of the

Paleozoic substratum

UNIT 1: THE WEATHERED 

PALEOZOIC SUBSTRATUM

Outliers of post-Paleozoic deposits in the Newport area rest 

on a leached and weathered layer of Paleozoic material that, 

in many outcrops, may appear to be a depositional unit. The 

nature and significance of the weathered layer varies with the 

original character of the underlying rock, with the length of 

time of weathering, and with other more subtle factors. A correct interpretation of this leached zone in its various settings is 

helpful in appraising remnants of post-Paleozoic deposits.

Paleozoic limestone overlain by permeable Cretaceous 

deposits is especially susceptible to leaching; dolomite is a little 

less so. Shale is susceptible to the oxidation of its dark carbonaceous fraction and to other changes that make it plastic; 

sandstone and chert are less affected. The depth of weathering 

of the underlying Paleozoic rocks increases from east to west 35 

across the Newport area. West of Cave City in the northern 

part of the Newport area, manganese ore is mined from a 

residual zone beneath gravel (Miser, 1922, p. 105-107); that 

zone is the product of the sub-gravel leaching of as much as 30 

m of Ordovician limestone. To the east in the Newport area, 

the cover was thicker and more clayey, resulting in generally 

less sub-gravel solution and letdown.

All the weathering described here is of a type that is assumed to have taken place under a cover of permeable material. 

That cover absorbed rainwater, kept the substratum damp 

with slightly acid ground water, and prevented the mechanical 

erosion of the residue that resulted from leaching. The residue 

from Paleozoic carbonate is a layer of light-gray, plastic, 

clayey quartz silt or sand that, when wet, behaves like plastic 

clay. The clay fraction is illite, in contrast to the montmorillonitic clay of the overlying Cretaceous. The quartz content of 

the material generally is quite high in spite of the clayey appearance.

91 15'

FIGURE 9. Map of the Newport area showing the cumulative effect of the 

reconstructed middle Cretaceous topography (Paleozoic surface) and 

the post- middle-Cretaceous distortion of the Paleozoic surface caused 

by local uplift. Contour interval, 30 m; contour hachured where uplifting has closed the outlet of a middle Cretaceous valley; datum, arbitrary.

UNIT 2: THE BASAL CONGLOMERATE OF THE 

UPPER CRETACEOUS SEQUENCE

The surface of the truncated Paleozoic rocks in northeastern Arkansas was flooded in a gentle manner by the advancing low-energy sea of Late Cretaceous time (fig. 6). In the 

central part of the Newport area, the basal conglomerate of 

the resultant sequence is a thin layer of locally derived, poorly 

sorted residuum that may be largely what was left after a soil
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS 165

was washed away. There are, however, a few well-rounded 

pebbles of chert in most deposits, even where the substratum is 

not cherty.

In the area of outcrop along the edge of the embayment, 

and especially in the eastern outcrops north of the Newport 

area, the basal conglomerate of the Upper Cretaceous sequence is a little thicker and contains many well-rounded and 

polished chert pebbles, mostly less than 20 mm in longest 

dimension. These pebbles are found not only at the base of the 

unit but also as much as 2-3 m above the base, indicating that 

they were moved by a moderately energetic sea. In contrast, in 

the north-central part of the Newport area where the Cretaceous rests on St. Peter Sandstone (Middle Ordovician), the 

distinctive rounded and frosted sand grains of the St. Peter are 

reworked into only the lower several centimeters of the Cretaceous. In the subsurface of the embayment to the east, the 

base of the Cretaceous generally is marked by clay or sand, 

rather than by pebbles, suggesting that the valleys of pre-Late 

Cretaceous rivers of this area were paved with sand and clay.

UNIT 3: THE MAIN BODY OF THE 

UPPER CRETACEOUS SEQUENCE

For the purposes of this report, all sand and clay in the outcrop area that lies between the Paleozoic rocks below and the 

distinctive and extensive upland gravel unit above is included 

in the Upper Cretaceous sequence unless the material is a lens 

in the Paleocene limestone. The maximum thickness of 

material so included is about 30 m; a common thickness in a 

representative outcrop is 5 m. Part of the upper layer of this 

sand may be from decalcified sandy Paleocene limestone.

Small outliers of Upper Cretaceous sand commonly show little evidence of bedding or lamination because such features 

have been destroyed by extensive slump and creep. The total 

sequence basal conglomerate, sand body, and overlying bed 

of Tertiary gravel may drape over a hill and down a slope, 

partly because of dissolution of underlying rocks but primarily 

as a result of creep toward erosional lows. Where the sequence 

has been less disturbed, especially where the iron oxide cement 

is well developed, detailed bedding can be observed.

Outcrops of Upper Cretaceous sand are well exposed along 

the edge of the embayment north of the White River. Apparently these outcrops are remnants of an offshore bar that 

developed several kilometers seaward from the swampy 

shoreline (fig. 2). A typical outcrop on the northwestern side of 

the bar shows well-developed cross-laminations that dip 

westward or northwestward (shoreward) as much as 30°. Reinforced walls of vertical boreholes made by shallow-water 

marine animals stand out in sharp contrast to the laminae. 

The sand of the bar is medium to coarse, whereas that in the 

area shoreward from the bar is consistently fine. In the subsurface to the southeast of the bar, the Upper Cretaceous sequence consists of about 100 m of alternating, thick units of

both limy shale and fine to medium sand, and a few local 

lenses of limestone. Most of the subsurface sequence is fossiliferous. Nearly all of the sand, silt, and clay deposited in the 

area during Late Cretaceous time was derived from a northern 

source.

During this investigation, locations of remnants of the Upper Cretaceous sand and clay were recorded on maps at a scale 

of 1:24,000 and were compiled on a small-scale regional map 

to show their general western limit (fig. 2). The difference between the present western limit of the remnants and the 

original depositional limit of the layer of Upper Cretaceous 

sand is unknowable but could be sizable. In the western part of 

the Newport area, the sand is preserved only where protected 

by a layer of gravel; it may have been washed away even in 

areas where some chert pebbles remain. Remnants of the 

original layer of Upper Cretaceous sand are found farther up 

the valleys of the ancestral Strawberry and Little Red Rivers 

than up the valley of the ancestral White River, but that probably is a function of preservation rather than of original extent.

Locally in the southern half of the Newport area, pebbles of 

sandstone are incorporated into the Upper Cretaceous 

deposits, primarily at the western shoreline and offshore on the 

largest of the three uplifts. The pebbles were derived from 

beds of Lower Pennsylvanian sandstone that were exposed to 

wave action along low sea cliffs and valley walls, as well as from 

beds exposed across the sea floor in the area of incipient uplift. 

Few if any of the pebbles were transported very far from their 

respective sources, but all were rounded. Those pebbles that 

were derived from valley walls and cliffs are in remnant 

deposits that now are deeply weathered and that have sustained at least a moderate amount of distortion through creep. 

The distribution of these deposits helps outline the middle 

Cretaceous topographic highs that limited the advance of the 

Late Cretaceous sea.

Widely scattered sandstone pebbles and several commercial 

deposits of sandstone-pebble gravel are found on the largest of 

the three uplifts (fig, 2). Those pebbles were formed under 

special conditions that existed on the budding uplift during 

Late Cretaceous or early Paleocene time. A relatively vigorous 

sea rolled across the slightly arched sea floor, removing any 

Cretaceous cover, rounding fragments of the underlying Pennsylvanian sandstone, and polishing the exposed faces of sandstone beds and sides of boulders too large to be moved. The 

best example of that is on the seaward (southeastern) side of 

the uplift where the sea spent most of its energy. There today, 

rock layers along the flank of the uplift and large boulders 

bypassed in gravel-pit operations display polished faces unlike 

those developed under normal subaerial weathering. No 

similar conditions are known to have been involved in the 

development of the two smaller uplifts.

The bulk of the sandstone pebbles in the area of outcrop 

across this uplift rests on the Pennsylvanian and is covered by 

soil or loess. At least one deposit of sandstone pebbles, a unit
166 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

more than 2 m thick, is in a more definitive stratigraphic position near the crest of the uplift in the SWV4SEHSEV4 sec. 32, 

T. 11 N., R. 4 W., Independence County, Ark. (fig. 2, locality 

Z). That layer of sandstone pebbles and sandy matrix rests on 

Pennsylvanian beds and is overlain by more than 1 m of friable 

fine sand that contains sandstone pebbles and a few lenses of 

silicified sandy limestone of Paleocene(P) age. The sequence is 

capped by a massive bed of silicified Paleocene limestone that 

contains widely scattered small clasts of sandstone. On the 

flank of the uplift, the Paleocene (Midway Group) limestone 

in the vicinity of the Grand Glaise terrace contains sandstone 

clasts "small greenish pebbles of sandstone" (Stephenson and 

Crider, 1916, p. 52). The clasts there are much larger than 

those in the silicified limestone, some being as much as 10 cm 

long; they are present in varying abundance through at least 

the lower 3 m of the limestone unit at some localities. 

Presumably they are reworked from an older deposit.

The sandstone pebbles on the largest uplift were rounded 

prior to the beginning of the deposition of the closely 

associated Paleocene limestone;- how long prior is unknown. 

Many of the pebbles underlie the Paleocene limestone; some 

are reworked into it. In this report, the pebbles are assumed to 

have been rounded during latest Cretaceous time and, in part, 

to have been transported a short distance during early 

Paleocene time.

UNIT 4: PALEOCENE LIMESTONE

Unconsolidated sediments and rocks of Paleocene age that 

crop out around the northern part of the Mississippi Embayment are assigned to the Midway Group, which is made up of 

the calcareous and sandy Clayton Formation below and the 

much thicker Porters Creek Clay above. The only outcrops of 

this group in the Newport area are the sandy limestone of the 

Grand Glaise terrace area on the flank of an uplift and the 

silicified limestone west of there on the crest of the uplift (fig. 

2). Presumably all of those outcrops belong to the Clayton Formation, but herein they are assigned to the Midway Group, a 

broader classification. In the subsurface of the embayment in 

the southeastern part of the Newport area, the Paleocene sequence grades southeastward into clay, suggesting that the 

limestone of the outcrop area may be part of an upliftcontrolled facies. A slightly raised platform in an otherwise 

muddy sea may have favored a lime-producing fauna. No 

Paleocene limestone is known to have been encountered in the 

test holes drilled on the southeasternmost of the three uplifts. 

Apparently regional tilting kept that area well submerged 

throughout Paleocene time, in spite of any incipient uplifting.

The Paleocene outcrops of the Grand Glaise terrace extend 

about 10 km along the edge of the embayment, perched there 

on a platform of Paleozoic rock and partly protected by an intermittent cover of gravel and loess (fig. 2; and Stephenson 

and Crider, 1916, p. 44-45). The age of the overlying gravel is 

unknown. It is in the proper stratigraphic position to be either

a facies of the upland gravel or one of the younger gravels of 

the embayment. Fossils, mainly mollusks, shark teeth, crustaceans, and echinoids, are abundant locally in the Paleocene 

limestone (Stephenson and Crider, 1916, p. 48). Fine sand is 

present in nearly all beds of the limestone, and layers of friable 

sand are interbedded locally with the well-indurated 

limestone. About 8 m of the unit is exposed in the thickest outcrops, which may represent most of the total sequence of the 

Grand Glaise area.

The Paleocene unit near the crest of the uplift northwest of 

the Grand Glaise terrace (fig. 2) is the product of nearly total 

silicification of massive-bedded, fossiliferous, slightly sandy 

limestone. A few fossils that were especially resistant to silicification were eventually dissolved, leaving open molds or irregular soft areas that, in turn, weathered and created voids. 

Of the two major remnants of the silicified limestone, the 

southwestern one is higher on the uplift; it is a massive elliptical outcrop 300 m long and 100 m wide that caps a nearly 

flat-topped hill. The northeastern and more extensive remnant of the unit caps a narrow ridge and dips about 90 m down 

the flank of the uplift in a distance of 1.3 km (4°). The base of 

the unit there is exposed in a few places; but the top, being at 

the crest of the wooded ridge, is too distant from the base to 

allow a direct measurement of the thickness. The maximum 

thickness appears to be about 7 m.

Periodically, both major outcrops of the silicified limestone 

yield a few durable pebbles that creep down the slope and into 

the channels of intermittent streams. The distinctive 

subrounded pebbles of silicified limestone, commonly 

associated with well-rounded sandstone pebbles from the basal 

unit of the sequence, are discernibly different from the remainder of the bedload of the short streams that drain the 

uplift. Some boulders have become separated from the 

massive beds of the outcrop but tend to remain close to their 

source. The silicified limestone is extremely durable, being 

subject to little small-scale shattering, exfoliation, or conchoidal fracturing. For the most part, it is weathered only by 

the chemical solution of silica, an extremely slow process. For 

that reason, the assumption is made that essentially all of the 

original areas of silicification are still represented by outcrops. 

The remainder of the uplifted area in which no trace of 

silicified limestone is found probably once was covered by 

Paleocene limestone that has long since been removed by 

chemical weathering, leaving only scattered sandstone clasts.

The time and cause of silicification of the Paleocene 

limestone in the Newport area are unknown. Only beds well 

up toward the crest of the uplift are known to have been 

altered. The other known outcrop of silicified Paleocene 

limestone in Arkansas is about 175 km to the southwest in the 

SEV4 sec. 17, T. 4S., R. 17 W., Hot Spring County (fig. 1), in 

the vicinity of the Cretaceous Magnet Cove Complex of the 

Little Rock pluton but only about 20 m above the embayment 

lowland (J. D. McFarland III, written commun., 1978).

In one small area near the crest of the uplift in the
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS 167

SWV4NEV4 sec. 32, T. 11 N., R. 4 W. (fig. 2, locality Y), 

rounded pebbles of Ordovician and Mississippian chert and 

angular fragments of the silicified Paleocene limestone are 

joined in an iron-rich sandy matrix. The conglomerate, 

resting on the massive beds of silicified Paleocene limestone, 

appears to be a typical iron-cemented lens of upland gravel 

(unit 5), except that here it also contains fragments of the 

underlying rock, which appears to have been silicified prior to 

brecciation. That suggests early silicification of the limestone 

because the chert gravel was deposited prior to the probable 

late Paleocene uplifting. Fractures in the massive bed of 

silicified limestone also suggest that the bed may have been 

broken into large blocks by post-silicification uplift.

UNIT 5: UPLAND GRAVEL

Many of the commercially important gravel pits in the 

Paleozoic outcrop area of the Newport region are in the layer 

of upland gravel that in most areas rests on the Upper Cretaceous sand and clay sequence. West of the limit of the layers of 

sand and clay, the gravel rests on the pre-Late Cretaceous erosion surface. In the northern part of the area, the upland 

gravel is made up primarily of pebbles and cobbles of Ordovician chert (Jefferson City and Cotter Formations); in the 

southern part of the area it is made up primarily of Mississippian chert (Boone Formation), but a mixture of the two types 

of chert is common. Isolated remnants of a once-regional layer 

of gravel now cap the highest hills in the northwestern part of 

the Newport area, extend up ancestral river valleys in the 

southwestern part, and dip regionally southeastward across 

the Newport area to the edge of the embayment. Some of the 

thicker deposits (as much as 10 m thick) are found just northwest of the present embayment edge.

The exact age of the upland gravel generally has been left to 

speculation, because the gravel contains few if any indigenous 

fossils and has no known inorganic fraction from which its age 

can be determined. It rests on rock as young as Paleocene 

limestone and is overlain by Pleistocene loess. In the Ozark 

region just north of the Newport area, the gravel rests on the 

crests of hills of Ordovician dolomite as much as 120 m above 

the stream level of the adjacent valley. It is found on the crest 

and flanks of the large uplift northwest of Grand Glaise positions that it would have been unable to reach after the uplift 

had developed. All of the chert fraction, which is nearly 100 

percent of the gravel, was derived from Ordovician and 

Mississippian outcrops north of the White River; some of it was 

carried southwestward 50 km into the estuary of the ancestral 

Little Red River along a path that did not drop in altitude (fig. 

2). Such movement required strong marine currents that probably were characteristic of the late Paleocene sea but not of 

the Eocene sea. These bits of evidence suggest a late Paleocene 

age for the upland gravel. It is younger than the Paleocene 

limestone but probably not much younger.

UNIT 6: THE EOCENE SEQUENCE

No outcrops of sediment of Eocene age have been identified 

in the Newport area, but a thick sequence of Eocene deposits is 

present in the subsurface of the general embayment area. 

Regionally the Eocene is made up of thinner, predominantly 

continental units in the northern part of the Mississippi Embayment and of thicker, increasingly marine units to the 

south. An erosional wedge of the Wilcox Group (Eocene), consisting of clay, silt, sand, and lignite, extends into the southeastern part of the Newport area. There it is overlain by the 

Quaternary sand and gravel that blanket the embayment. A 

short distance farther southeast, outside of the Newport area, 

the Wilcox is overlain by the Claiborne Group, which in turn is 

overlain by the Jackson Group, both of Eocene age. The 

Jackson was deposited by the last sea that is known to have 

flooded the embayment northward beyond the vicinity of 

Memphis, Tenn.

The subcrop edge of the Eocene sequence is an erosional 

edge the present limit of a once more extensive deposit. As 

the fine-grained and unconsolidated Eocene of this area is 

very easily eroded where not protected, no outliers have survived the 40 m.y. of post-Eocene weathering to indicate the 

original extent of the unit. Indirect evidence, however, suggests that upper Paleocene and Eocene deposits did extend 

across a substantial part of the Newport area. Probably the 

best indication of that comes from Crowleys Ridge, about 60 

km to the east (fig. 1). Sediments of Eocene age there, protected by a layer of gravel, stand at least 70 m above the embayment lowlands. The gravel rests on a truncated surface, indicating that the Eocene deposits once stood even higher than 

they do today. The assumption that Crowleys Ridge has not 

been uplifted relative to the Newport area forces the conclusion that the northern part of the embayment region, including a significant part of the Newport area, once was filled 

by Tertiary sediments to a level higher than that of any of the 

Eocene on Crowleys Ridge.

Superimposed valleys near the edge of the Ozark region indicate that Tertiary units younger than Paleocene limestone 

once extended northwestward into the Newport area. In much 

of the central and eastern part of the Arkansas Valley physiographic province, resistant beds of Pennsylvanian sandstone 

form dip slopes that dominate the drainage pattern. However, 

in the southwestern part of the Newport area (at the easternmost end of the Arkansas Valley), those resistant beds of sandstone have little effect on the courses of southeastward-flowing 

streams. The valleys of major and minor streams there are 

superimposed upon the folded Paleozoic rocks. After the 

beginning of the retreat of the Jackson sea, streams in the 

southern part of the Newport area flowed out of well-established valleys in the southern Ozarks and the Arkansas Valley 

onto the soft sediment of the embayment. These streams 

swung south around the end of the uplifted area but, otherwise, were free to choose a direct route toward the edge of the
168 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

0\\

uoipas ui 

puaq O of

0) 0>

Qu

uoipas ui

Puaq 06 L

£ 3 5 Q CD

CO C .fc

l> CO CO

N-
Extensive subaerial erosion 

I

Gravel (chert) and 

in valley of early 

Mississippi River

Vertical exaggeration X 31 ill 

20 km II 

Scale (all sections)

Scattered chert pebbles on highest hills;

commercial deposits I I Commercial deposits 

of chert gravel where I ' of chert gravel 

let down during solution I No aravel found Qn Cretaceous 

of underlying carbonates' J,, ~.t , ' . «^kr - - / I 1 No 9ravel found

^:W^^^^^

i;^ssectedSpringfield^i||*SlSSStli)^^ 

i||||i|| Plateau Il|||ll|lll|lllllllliNtck^&^*tj7/7f^

Newport 

Air Base 

test hole 

I/

Quaterna

loess, sa

FIGURE 10. Diagrammatic cross sections along line B-B ' in the Newport region north of the local uiplifts, showing progressive structural develo

Cretaceous to present time. Cross section B-B ' shown on figure 1.
170 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

uoipas ui

uoipas ui 

puaq O g
EOCENE DEPOSITS 

ON UPLAND GRAVEL EXTENSIVE SUBAREIAL EROSION 

OF EOCENE AND OLDER DEPOSITS

SAN

Eocene and upper Paleo

UPLIFT PROBABLY COMPLETED

DURING EARLY EOCENE TIME

Oligocene time

Boston Mountains

I Western Lowlands

Pleistocene and Holocene alluvium

Eocene and upper Paleo

(mostly barren of marine 

La e Cretaceous erosion

as lec whe e restored

Vertical exaggeration X 31

1,829.4m (1,763.3m below sea level) in Lower Ordovician Scale (all sections)

FIGURE 11.- Diagrammatic cross sections along line A~A' in the Newport region through the area underlain by local uplifts, showing progressi

from Late Cretaceous to present time. Cross section A-A ' shown on figure 1.
172 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

retreating sea. As base level lowered and valley bottoms reached the underlying resistant Paleozoic rocks, the streams did not 

deviate around obstacles; instead, they cut trenches through 

them.

The pre-Late Cretaceous channel of the Little Red River 

just north of the present site of Bald Knob was cut by a stream 

that followed the least resistant path around Paleozoic structures. The present channel of the Little Red River, probably 

established after Jackson time, cuts across Paleozoic structures 

and enters the present embayment just south of Judsonia. The 

nearly 10 km shift of the river may have been influenced in 

part by the uplift to the north. Overflow Creek flows 

southward from the uplift and directly across the valley of the 

ancestral Little Red River. Two km west of Bald Knob the 

creek has cut a channel 30 m deep through a resistant sandstone that caps a topographically high area herein considered 

to be a small local uplift that was buried by Eocene sediments. 

Both the Little Red River and Overflow Creek have classic 

superimposed valleys, cut in their present positions only 

because the path was established in overlying Tertiary 

sediments.

The White River also has cut a channel across resistant units 

in spite of the availability of a much easier path just to the 

south. The valley of that river was superimposed upon an anticline, which might have been a growing structure. Previously 

discussed local post-Cretaceous warping there must be considered in determining the Tertiary development of the White 

River valley. Some evidence suggests that the anticline crossed 

by the White River is continuous with the major anticline that 

is crossed by the Little Red River (figs. 5, 8). If so, post-Cretaceous warping was an additional factor in the development of 

nearly all the present drainage patterns in the Newport area.

UNIT 7: POST-EOCENE DEPOSITS

Two post-Eocene sequences that differ appreciably in age 

are present in the upper Mississippi Embayment: (1) highterrace gravel, such as that on Crowleys Ridge, locally at least 

70 m above the adjacent lowlands; and (2) younger alluvium 

that underlies the Western and Eastern Lowlands on either 

side of Crowleys Ridge (fig. 3). For the most part, the alluvium 

of the lowlands accumulated during Pleistocene time; it is being reworked locally by streams of the area today. The highterrace gravel is much older and may have begun to accumulate not long after the Eocene sea retreated.

The gravel on Crowleys Ridge east of the Newport area is 

made up mostly of very resistant pebbles of Mississippian and 

Ordovician chert from the Ozark region and from other source 

areas to the north, northeast, and east. Most of the material 

may have been reworked from the formerly extensive deposits 

of upland gravel (unit 5 of this report). A few distinctive pebbles and cobbles that apparently were derived from as far 

north as the Lake Superior area are present in the gravel, but 

no suite of pebbles that might be indicative of glacial outwash

has been found in these deposits (Salisbury, 1891, p. 

240-243).

A blanket of Pleistocene and Holocene alluvium covers the 

1,200-km2 southeastern portion of the Newport area, leaving 

no exposures of the Paleozoic, Cretaceous, or Tertiary beds 

that underlie it (fig. 2). As the base of the unit is not exposed, 

thickness data are available only from test holes. The maximum thickness of the unit in the Newport area may be as 

much as 50 m, but the more common thickness is about 30 m. 

At no place in the Newport area is the base of the alluvium 

known to be as low as sea level. Apparently the unit thins 

slightly toward the edge of the embayment before it terminates 

abruptly against the erosional scarp that marks the northwestern edge of the embayment (figs. 3, 10, 11). Subsurface data 

suggest a slight thinning of the alluvium over the local southeastern uplift in the Newport area. Even if the thinning is as 

indicated in figures 3 and 11, various interpretations of its 

cause and significance are possible.

Several investigations of the alluvium have been made, including determinations of age, thickness, lithology, source 

areas, nature of contact with underlying sediments, environment of deposition, topographic features, ground-water 

resources, and tectonic features (Stephenson and Crider, 

1916; Fisk, 1944; Boswell and others, 1968; Krinitzski and 

Wire, 1964; Smith and Saucier, 1971; Saucier, 1974, 1978).

Even though the alluvium of the Western Lowlands may be 

largely glacial outwash, a substantial fraction of it is made up 

of well-rounded grains, granules, and small pebbles of quartz. 

This fraction may be recycled nonglacial detritus from the 

Missouri River basin. Another significant constituent of the 

alluvium in the Newport area is chert gravel that the White 

River and other streams carried out of the Ozark region and 

deposited along the Ozark side of the embayment long before 

Pleistocene time. Because that gravel was readily available, its 

redeposition could have begun as soon as erosion established 

the present level of the embayment. Gravel is relatively near 

the surface in the Grand Glaise-Olyphant area, where it is 

dredged and pumped from extensive pits just inside the embayment. Most of the chert pebbles of this gravel are similar to 

those of the upland gravel to the northwest and appear to be 

reworked from it; some igneous pebbles from distant sources 

are included in at least part of the sequence. To the east, farther out in the embayment, the alluvium grades upward from 

gravel and sand to silt and clay; the gravel there has a high 

percentage of quartz pebbles. Some of the gravel on Crowleys 

Ridge may have been brought in by the ancestral Ohio River; 

but the alluvium of the Western Lowlands, except for a small 

fraction of gravel reworked from Crowleys Ridge, is thought to 

have been derived from areas west of the Ohio River basin.

The alluvial sequence is capped locally by sand dunes and 

deposits of loess, especially just east of the relict channel of a 

braided stream that once flowed along the northwestern edge 

of the embayment (Saucier, 1978, p. 30). The most recent and 

best preserved eolian deposits accumulated there just prior to
SEDIMENTS ABOVE THE NEWPORT PLUTON, NORTHEASTERN ARKANSAS 173

the diversion of the ancestral Mississippi River to the eastern 

side of Crowleys Ridge.

During much of the past 20,000 years, major streams of the 

Western Lowlands have tended to flow at the extreme western 

edge of the embayment and rasp the exposed face of Paleozoic 

rocks. The braided ancestral Mississippi River conformed to 

that trend just prior to being diverted to the east about 18,000 

years ago (Saucier, 1978, p. 30 and 34); the meandering rivers 

of today (Current, Black, and White) persist in following 

much the same pattern. In the process, some of those rivers 

have nicked the Paleozoic rocks and thereby formed the Ozark 

Escarpment (figs. 3, 10, 11). The fact that the surface of the 

Western Lowlands slopes slightly to the west could be either a 

cause or a result of the stream patterns. Braided-stream 

deposits of early Wisconsin age underlie most of the eastern 

and central part of the Western Lowlands; Holocene alluvium 

underlies much of the western part (Saucier, 1974, fig. 1). The 

nick is well developed where bordered by deposits of either 

age, and therefore may be entirely the result of abrasion by 

Pleistocene braided streams. The modern streams may be 

locked in an old channel.

The straightness of the Ozark Escarpment (the face of the 

nick) in some areas has been cited incorrectly as evidence of 

faulting at the edge of the embayment (Fisk, 1944, fig. 6). Actually, that lineation has developed at the line of intersection 

of the tilted pre-Late Cretaceous erosion surface and the 

horizontal surface at the working depth of the streams. Further cutting has modified the position of the face but has left a 

remarkably straight feature in many areas. Where the preLate Cretaceous erosion surface is distorted over local uplifts 

at the edge of the Ozark region, the nick is curved. Where the 

erosion surface is low, as in the area of the Late Cretaceous 

estuary of the ancestral Strawberry River, the nick was never 

developed. At no place did the rivers cut very far back into the 

Ozark region, because the Paleozoic rocks are resistant to erosion and the combined thickness of the geologic sequences that 

would have had to be cut increases rapidly northwestward 

from the embayment. In areas away from the uplifts where the 

exposed face of the nick is about 5 m high and the adjacent 

alluvium is 20-25 m thick, one can expect to find no Cretaceous deposits under the alluvium within 2-3 km of the edge of 

the embayment because the dip of the pre-Late Cretaceous 

erosion surface is less than 1° (fig. 10).

SUMMARY

A series of structural, sedimentary, and erosional anomalies 

in the post-Paleozoic sequence above the deeply buried 

Newport pluton appears to have been caused by the emplacement of the pluton or by the emplacement of sills and laccoliths associated with the pluton. On the basis of evidence 

from the post-Paleozoic sedimentary sequence, uplifting over 

or near the pluton began during Late Cretaceous time and

continued at least into middle Paleocene time. That tectonic 

activity was closely associated in time with the early development of the Mississippi Embayment.

A seven-step diagrammatic interpretation of the Late Cretaceous to Holocene evolution of a segment of the western 

margin area of the Mississippi Embayment is shown for each of 

two localities: (1) along line A-A ' southeastward across uplifts associated with the Newport pluton (fig. 11), and (2) 

along line B-B ' southeastward across the Newport area just 

north of the pluton (fig. 10). Those two figures illustrate the 

concepts developed in this report and show the effects of the 

local uplifts.

REFERENCES CITED

Arkansas Geological Survey, 1929, Geologic map of Arkansas: scale 

1:500,000.

Boswell, E. H., Gushing, E. M., and Hosman, R. L., 1968, Quaternary 

aquifers in the Mississippi Embayment: U.S. Geological Survey Professional Paper448-E,15 p.

Caplan, W. M., 1954, Subsurface geology and related oil and gas possibilities of northeastern Arkansas: Arkansas Resources and Development Commission, Division of Geology Bulletin 20, 124 p.

Clardy, B. F., 1979, Arkansas lignite investigations: Arkansas Geological 

Commission Preliminary Report, 133 p.

Cordell, Lindrith, 1977, Regional positive gravity anomaly over the 

Mississippi Embayment: Geophysical Research Letters, v. 4, no. 7, p. 

285-287.

Croneis, C. G., 1930, Geology of the Arkansas Paleozoic area, with especial 

reference to oil and gas possibilities: Arkansas Geological Survey 

Bulletin 3, 457 p.

Gushing, E. M., Boswell, E. H., and Hosman, R. L., 1964, General geology 

of the Mississippi embayment: U.S. Geological Survey Professional 

Paper 448-B,28 p.

Easton, W.H., 1942, Pitkin limestone of northern Arkansas: Arkansas Geological Survey Bulletin 8, 115 p.

Ervin, C. P., and McGinnis, L. D., 1975, The Reelfoot Rift- Reactivated 

precursor to the Mississippi Embayment: Geological Society of America 

Bulletin, v. 86, no. 9, p. 1287-1295.

Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower 

Mississippi River: U.S. Army Corps of Engineers, Mississippi River 

Commission, 78 p.

Frezon, S. E., and Click, E. E., 1959, Pre-Atoka rocks of northern Arkansas: 

U.S. Geological Survey Professional Paper 314-H, p. 171-189.

Gordon, Mackenzie, Jr., 1944, Moorefield formation and Ruddell shale, 

Batesville district, Arkansas: American Association of Petroleum Geologists Bulletin, v. 28, no. 11, p. 1626-1634.

______1964, Carboniferous cephalopods of Arkansas: U.S. Geological 

Survey Professional Paper 460, 322 p.

Gordon, Mackenzie, Jr., andKinney, D. M., 1944, Geologic map and structure sections of the Batesville district, Independence County, Arkansas: 

U.S. Geological Survey Oil and Gas Investigations Preliminary Map 12, 

scale 1:20,000.

Haley, B. R., Click, E. E., Bush, W. V., Clardy, B. F., Stone, C. G., Woodward, M. B., andZachry, D. L., 1976, Geologic map of Arkansas: scale 

1:500,000.

Harris, G. D., 1894, The Tertiary geology of southern Arkansas: Arkansas 

Geological Survey Annual Report for 1892, v. 2, 207 p.

Hendricks, J. D., 1979, Bouger gravity and geologic map of northeast 

Arkansas: U.S. Geological Survey Geophysical Investigations Map 

GP-925, scale 1:250,000.
174 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

Hendricks, J. D., and Hildenbrand, T. G., 1979, Total field aeromagnetic 

map of northeast Arkansas: U.S. Geological Survey Open-File Report 

79-1208, scale 1:250,000.

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map of 

central Mississippi Valley and environs: U.S. Geological Survey Miscellaneous Field Studies Map MF-1011, scale 1:500,000.

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic 

and gravity anomalies in the northern Mississippi Embayment and their 

spatial relation to seismicity: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-914, scale 1:100,000.

Hildenbrand, T. G., Rucks, R. P., Kane, M. F., and Hendricks, J. D., 1979, 

Aeromagnetic map and associated depth map of the upper Mississippi 

embayment region: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-1158, scale 1:1,000,000.

Jespersen, Anna, 1964, Aeromagnetic prospecting for bauxite deposits in 

the Mississippi Embayment, Arkansas and Missouri: U.S. Geological 

Survey Geophysical Investigations Map GP-370, scale 1:125,000.

Kinney, D. M., and Straczek, J. A., 1950, Geologic map of the central part 

of the Batesville manganese district, Independence and Izard Counties, 

Arkansas: U.S. Geological Survey Mineral Investigations Field Studies 

Map MF-1, scale 1:31,680.

Krinitzsky, E. L., and Wire, J. C., 1964, Groundwater in alluvium of the 

lower Mississippi valley (upper and central areas): U.S Army Corps of 

Engineers, Waterways Experiment Station Technical Report 3-658, v. 

2, 361 p.

Miser, H. D., 1922, Deposits of manganese ore in the Batesville district, 

Arkansas: U.S. Geological Survey Bulletin 734, 273 p.

______1941, Manganese carbonate in the Batesville district, Arkansas: 

U.S. Geological Survey Bulletin 921-A, 97 p.

Ouellette, R. G., 1970, Upper crustal structure in the Newport, Arkansas

area: St. Louis, Missouri, Washington University M.A. thesis, 58 p. 

Renfroe, C. A., 1949, Petroleum exploration in eastern Arkansas with

selected well logs: Arkansas Resources and Development Commission,

Division of Geology, Bulletin 14, 159 p. 

Salisbury, R. D., 1891, On the relationship of the Pleistocene to the prePleistocene formations of Crowleys Ridge, and adjacent areas south of

the limit of glaciation: Arkansas Geological Survey Annual Report for

1889, v. 2, p. 224-248. 

Saucier, R. T., 1974, Quaternary geology of the lower Mississippi Valley:

Arkansas Archaeological Survey Research Series 6, 26 p. 

______1978, Sand dunes and related eolian features of the lower

Mississippi River alluvial valley: Geoscience and Man, v. 19, p. 23-40. 

Sheldon, M. G., 1954, Sample descriptions and correlations for selected

wells in northern Arkansas: Arkansas Resources and Development

Commission, Division of Geology Information Circular 17, 222 p. 

Smith F.L., and Saucier, R. T., 1971, Geological investigations of the

Western Lowlands area, lower Mississippi valley: U.S. Army Corps of

Engineers, Waterways Experiment Station Technical Report S-71-5,

47 p.

Spooner, W. C., 1935, Oil and gas geology of the Gulf Coastal Plain in 

Arkansas: Arkansas Geological Survey Bulletin 2, 474 p.

Stephenson, L. W., and Crider, A. F., 1916, Geology and ground waters of 

northeastern Arkansas: U.S. Geological Survey Water-Supply Paper 

399, 315 p.

Stroud, R. B., Arndt, R. H., Fulkerson, F. B., and Diamond, W. G., 1969, 

Mineral resources and industries of Arkansas: U.S. Bureau of Mines 

Bulletin 645, 418 p.
The Northeastern Extension of the 

New Madrid Seismic Zone

By L. W. BRAILE, W. J. HINZE, G. R. KELLER, and E. G. LIDIAK

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-L
CONTENTS

Page

Abstract ............................................................................... 175

Introduction............................................................................ 175

Data and analysis ........................................................................176

Gravitymap ............................................................................ 176

Magnetic map ..........................................................................177

Discussion.............................................................................. 179

Conclusions ............................................................................ 183

Acknowledgments ....................................................................... 184

References cited ......................................................................... 184

ILLUSTRATIONS

Page

FIGURE 1. Tectonic features in the New Madrid seismic zone and surrounding area.......................................................... 176

2. Preliminary Bouguer gravity map of the New Madrid seismic zone and surrounding area ............................................ 177

3. Preliminary aeromagnetic map of the New Madrid seismic zone and surrounding area .............................................. 178

4. Shaded gravity-anomaly contour map showing inferred boundaries of the New Madrid linear tectonic feature ........................... 180

5. Shaded magnetic-anomaly contour map showing inferred boundaries of the New Madrid linear tectonic feature ......................... 181

6. Selected profiles of gravity and magnetic anomaly data across the New Madrid linear tectonic feature . ................................. 182

7. Contour map of number of earthquakes per 104 -km2 area with intensity greater or equal to threefromlSOO to 1972 ...................... 183
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

THE NORTHEASTERN EXTENSION OF THE 

NEW MADRID SEISMIC ZONE

By LAWRENCE W. BRAiLE1 , WILLIAM J. HiNZE1 , 

G. R. K.ELLER2 , and EDWARD G. LIDIAKS

ABSTRACT

Linear trends of nearly circular, correlative, positive gravity and magnetic 

anomalies bound a central zone of low gravity and magnetic relief and are 

associated with the New Madrid seismic zone. These trends can be traced 

northeastward across the 38th parallel lineament into southern Indiana at 

least to lat 39° N. on regional gravity and magnetic anomaly maps. These 

anomalies, which are interpreted as having a source within the basement, 

are associated with the trend of earthquake epicenters in the New Madrid 

area and to the northeast. This association suggests that a linear basement 

structural feature is a primary control on the location of earthquake 

epicenters in the New Madrid region.

INTRODUCTION

The Mississippi Embayment is a major structural feature 

extending into the North American craton from the Gulf coast 

province. Geophysical data, subsurface geologic data, and the 

associated contemporary tectonism manifested in the New 

Madrid seismic zone suggest that both its paleotectonics and 

contemporary tectonics are complex. This complexity is especially evident in the region of the northern Mississippi Embayment, where the extension of the New Madrid seismic zone intersects the 38th parallel lineament. An understanding of this 

zone of intersection is of fundamental importance if we are to 

unravel the contemporary geodynamics and define the seismic 

regionalization of this region.

The major tectonic features of the upper Mississipp Embaymnet (fig. 1) have been described by Heyl (1972). Tectonic 

models that have been proposed to explain both the ancient 

and contemporary tectonic activity in the midcontinent have 

been reviewed by Hinze and others (1979). In an integrated

'Department of Geosciences, Purdue University, West Lafayette. IN 47907. 

department of Geological Sciences, University of Texas. El Paso, TX 79968. 

'Department of Earth and Planetary Sciences, University of Pittsburgh, PA 15260.

analysis of existing geophysical and geologic data, Ervin and 

McGinnis (1975) postulated that the Mississippi Embayment is 

a reactivated Precambrian aulacogen. This tectonic model 

has received considerable acceptance as a working hypothesis 

for further study, and more recent data seem consistent with 

this interpretation.

A significant amount of geological and geophysical data has 

recently become available in the area of the northern 

Mississippi Embayment. The studies of Nuttli (1974), Varma 

(1975), and Kovacs and Murphy (1977) provided a thorough 

review of the historical record of seismic activity. Although 

limited in geographical extent, the seismograph array 

operated by Saint Louis University has for the first time provided detailed data about the extent and nature of seismicity 

in the New Madrid seismic zone (Stauder and others, 1976; 

Herrmann and Canas, 1978). The pattern of seismicity in the 

midcontinent of North America has long been the subject of 

controversy and speculation. Largely on the basis of straightline projection of regions of more intense seismicity, many investigators have suggested a structural connection between the 

New Madrid area; the Anna, Ohio, area; and the St. 

Lawrence River valley (Woollard, 1958; Kumarapeli and 

Saull, 1966). Regional analyses of gravity and aeromagnetic 

data have recently been completed by Lidiak and Zietz (1976) 

and Hildenbrand and others (1977).

In this paper we examine available regional geophysical 

data in the area of the New Madrid seismic zone and its possible northeastern extension. A correlation is seen to exist between regional basement structure, as evidenced by gravity 

and magnetic anomaly patterns, and contemporary tectonism, as evidenced by historical seismicity data. This correlation implies a structural control on the locations of earthquake activity and indicates that areas of potential seismic activity may be identified by delineating the extent of the controlling structural feature. This structure is interpreted as extending northeastward from the New Madrid area into 

southwestern Indiana at least to lat 39° N.

175
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

90° 88° 86°

ILLINOIS INDIANA

OHIO

4- X/^5

j

+ + +

\

£

1 / 

COTTAGE GROVE " /M* ///<J

4~ ~\- 'f / Hh 1

'.>>^ ^ ,FAULT ZONE \( ffA

 <?n S ^\ 1 '/Rjf

 °*>>, N^ > IJBft

/>

(

S +

)

fe 

$

4* _i_

'r_/' (^ - '^~. ,'''

mi IKH rame J

+

/" j

/ ^

^\ r -N^' +

%

FAULT ZONE'

\ ~^! '$*'

KENTUCKY

NEW

MADRID c)^. ?# '' 

^~

^

v

, -*^;

PENNYRILE FAULT ZONE

+

ciP ~)s<

/& s

/ \

.<>

+

'ARKANSAS

S'^

~J

f

.^v TENNESSEE

0 TOO KILOMETERS

FIGURE 1. - Tectonic features in the New Madrid seismic zone and surrounding area (generalized from Heyl and McKeown, 1978; and Heyl, 1972). 

Faults are shown by thick solid lines. Thin solid line is outline of Mississippi Embayment.

DATA AND ANALYSIS

The data presented in this paper have been compiled from 

many sources and have been merged into a regional data set 

that is as consistent as current data and reduction procedures 

permit. The regional maps were prepared by compositing the 

data of individual surveys and adjusting levels to provide an 

adequate join at the boundaries. These data sets are currently 

undergoing further modifications and additional data are being added. Thus the maps presented here must be considered 

preliminary.

GRAVITY MAP

A regional gravity map of the northern Mississippi Embayment (fig. 2) was prepared from data reported by Hildenbrand and others (1977), McGinnis and others (1976), Keller 

and others (1978), and the National Geophysical and SolarTerrestrial Data Center, and from additional new gravity observations in southern Indiana. These data have been merged 

into a consistent data file and reduced using a sea-level datum, 

a Bouguer density of 2.67 g/cm3 , the 1967 international gravity formula and datum, and a curvature correction. The main
THE NORTHEASTERN EXTENSION OF THE NEW MADRID SEISMIC ZONE

MAGNETIC MAP

177

body (six 1 ° X 2 ° quadrangles) of the Bouguer gravity values 

was gridded on a 2-km spacing using the technique of Braile 

(1978). Because new data are to be added to the file in

southern Indiana, additional data from surrounding areas are Preparation of a consistent magnetic map of the area into be gridded and included in the data file, and the data volves several difficult problems. Various aeromagnetic 

shown in figure 2 south of lat 36° N. have not been gridded, surveys have been observed at different times and elevations, 

this map must be considered as a preliminary gravity map of and have been reduced using different geomagnetic field 

the area. models. We are currently adjusting datum levels, survey elevaMISSOURI /S)

0 100 KILOMETERS i i i i i i

FIGURE 2. - Preliminary Bouguer gravity map of the New Madrid seismic zone and surrounding area. Hachured contours indicate closed lows. 

Contour interval is 5 mgal. The data are corrected using a Bouguer density of 2.67 g/cm3 and a sea-level datum.
178

40

92C

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

90° 88° 86°

84C

0 100 KILOMETERS

FIGURE 3. Preliminary aeromagnetic map of the New Madrid seismic zone and surrounding area. Hachured contours indicate closed lows. Contour interval is

100 gammas; survey elevation, 2 km.

tions, and parameters for removal of geomagnetic-field effects 

to combine the existing and new aeromagnetic surveys of the 

area. The aeromagnetic map (fig. 3) was prepared by contouring the existing total-in tensity anomaly maps of southern Illinois, Indiana, western Kentucky, and the northern Mississippi Embayment in a consistent manner. Until additional data 

are added and adjustment for variable survey parameters is 

completed, the magnetic map must be considered preliminary. The primary difference between this map (fig. 3) 

and the final magnetic map will be in regional base levels of individual surveys. The character of the magnetic field and the 

pattern of anomalies are correct as shown on the preliminary 

map. In preparation of the magnetic map, southwestern Illinois aeromagnetic data (Heigold, 1976) were digitized and 

joined with new data observed in southeastern Illinois. These 

maps were joined to the geomagnetic-field-adjusted Indiana 

aeromagnetic map (Richardson, 1978). Western Kentucky 

data are from Johnson and others (1978), and the remainder 

of the data shown in figure 3 are from Hildenbrand and others 

(1977). Although the data for the various survey areas have not 

been completely adjusted, the map (fig. 3) is most consistent 

with a survey elevation of 2 km. An arbitrary datum has been 

used. A magnetic anomaly map (Johnson and others, 1980) 

and a gravity anomaly map (Keller andothers, 1980) covering 

the area of the maps in figures 2 and 3 at a scale of 1:1,000,000 

will soon be available.
THE NORTHEASTERN EXTENSION OF THE NEW MADRID SEISMIC ZONE 179

DISCUSSION

Although many anomalous features are evident on the 

gravity and magnetic anomaly maps (figs. 2, 3), our focus is 

the anomaly patterns associated with the New Madrid seismic 

zone and its possible extension to the northeast into the area of 

the Wabash Valley fault zone. The most prominent feature on 

the gravity and magnetic maps is a west-northwest-trending 

positive anomaly in southern Illinois and western Kentucky. 

This anomaly is particularly prominent on the magnetic map 

where the positive anomaly is flanked by a strong gradient and 

a parallel magnetic low to the southwest. Analysis of the gravity and magnetic data for this anomalous feature indicates a 

complex pattern of anomalous sources characterized by both 

correlative (positive- positive or negative-negative) and anticorrelative (positive-negative or negative-positive) gravity and 

magnetic anomalies. Preliminary modeling of the northwesttrending anomaly also indicates a considerable longwavelength character to the anomaly, suggesting that the 

depth of the causative bodies is significant. Although the 

northwest-trending anomaly closely parallels the Ste. 

Genevieve fault zone in southern Illinois and Missouri, the 

anomaly can be traced northwesterly across Missouri and 

southeasterly at least into central Tennessee. Furthermore, 

basement relief on the Ste. Genevieve fault zone is less than 1 

km, and so this fault zone is unlikely to be the source of the 

observed gravity and magnetic anomalies. Therefore, this 

northwest- trending anomaly is interpreted as being due to a 

major basement lithologic boundary. It appears likely, then, 

that faults associated with the 38th parallel lineament in 

southern Illinois and western Kentucky, which were active in 

late Precambrian and early Paleozoic time, may have been 

localized by the basement lithologic boundary. Although the 

northwest-trending gravity and magnetic anomaly is of considerable geologic significance, it appears to be unrelated to the 

possible northeastward extension of the New Madrid seismic 

zone. In addition as will be illustrated below, this basement 

lithologic boundary does not appear to affect the earthquake 

activity in the midcontinent area.

Another prominent feature on the gravity and magnetic 

maps is several nearly circular, positive gravity and magnetic 

anomalies. Hildenbrand and others (1977) described linear 

trends of positive magnetic and gravity anomalies that bound 

an area of low gravity and magnetic relief and delimit the 

seismicity patterns indicated by microearthquake epicenters in 

the New Madrid area. These anomalies are clearly seen on the 

maps (figs. 2, 3) in the northern Mississippi Embayment area 

(south of lat 37 ° N.). The linear trends of these anomalies can 

be traced to the northeast at least to lat 39° N. as shown on 

figures 4 and 5. The signature of the anomalies composing the 

trends can best be seen on a series of profiles perpendicular to 

trends (fig. 6). The anomaly patterns are less pronounced to 

the northeast, possibly because this area is the deepest part of 

the Illinois Basin and because of the interference of the

northwest-trending anomalies discussed previously. However, 

the trend (approximately northeast), the parallelism and 

width, and the correlative, positive, marginal character of the 

anomalies bounding the region of low gravity and magnetic 

relief continue to the northeast (fig. 6).

Hildenbrand and others (1977) have interpreted the 

generally circular, correlative, positive anomalies as originating from mafic plutons implaced within the crystalline basement and the structural feature defined by these anomalies as 

a buried rift. The earthquake activity associated with this 

feature is presumed by these authors to be due to reactivation 

of the rift structures or localization of stresses along this ancient zone of weakness. This interpretation rests heavily on the 

model of Ervin and McGinnis (1975), who postulated the existence of the Precambrian Reelfoot rift. Because the term 

"Rift' has a specific genetic meaning, or at least connotation, 

and because we have traced the structure associated with the 

New Madrid fault zone considerably to the northeast of where 

it has been described as a rift, we have chosen to term the 

structure evidenced by the marginal gravity and magnetic 

anomalies the New Madrid linear tectonic feature.

The New Madrid linear tectonic feature delineated by the 

gravity and magnetic anomalies (figs. 4, 5) also approximately 

delimits an area of maximum seismicity. Seismicity data consisting of historical events (from approximately 1600 to 1975 

exclusive of the 1811-12 New Madrid earthquake sequence) 

are shown compiled from Nuttli (1974), Varma (1975), and 

Kovacs and Murphy (1977) and from microearthquake 

epicenters recorded by the Saint Louis University seismic network (1974-76) (Hildenbrand and others, 1977). The correlation of seismicity with the New Madrid linear tectonic feature 

is clearly seen in a contour map of the numbers of epicenters 

per 104-km2 area in the midcontinent region (Hadley and 

Devine, 1974), which is shown in figure 7. The New Madrid 

linear tectonic feature is seen to approximately bound an area 

of high seismicity trending northeast. The northern and 

southern extent of this trend are unclear from the available 

seismicity and the gravity and magnetic anomaly data, but it is 

interesting to note the fact that the Anna, Ohio, earthquake 

zone is directly in line with the trend of the New Madrid linear 

tectonic feature. This correlation of the gravity and magnetic 

anomaly patterns with contemporary seismicity attains added 

significance when one considers the earthquake focal 

mechanisms reported by Herrmann and Canas (1978). These 

mechanisms indicate right-lateral movement along the 

northeast-trending line of epicenters, indicated by the microearthquake epicenters (south of lat 36°15 ' N.) and complex 

but probably dip-slip motion in the area of north-trending 

epicenters near lat 36°30' N. These mechanisms are consistent with right-lateral strike-slip motion in a northeasttrending fault zone north of the cluster of microearthquake 

epicenters (lat 36°45' N.). The northeast-trending gravity 

and magnetic anomaly patterns are direct evidence that the
180 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

40°,92° 90°

KENTUCKY

BOUGUER

ANOMALY

(MGAL)

A HISTORICAL EPICEN

EPICENTERS ..LDCATED

ST LOUIS UNIV. NETWORK

1974 ~I976

^INFERRED BOUNDARY OF 

* NEW MADRID LINEAR

TECTONIC FEATURE

0 100

FIGURE 4. Shaded gravity-anomaly contour map showing inferred boundaries of the New Madrid linear tectonic feature by the heavy dashed lines. South of 

Iat36° N., the boundaries are identical to those inferred by Hildenbrand and others (1977). Queries at north and south ends of the New Madrid linear 

tectonic feature indicate that the extent of the feature is presently unknown.

New Madrid feature extends north of the 38th parallel lineament into southern Indiana at least to lat 39° N. Although the 

seismicity north of lat 37 ° N. is less intense and the patterns of 

epicenters more diffuse than the seismicity associated with the 

New Madrid seismic zone, the northeastern extension of the 

New Madrid linear tectonic feature bounds a majority of the 

epicenters in the southern Indiana area (figs. 4, 5). It should 

be noted that the seismicity pattern in southern Indiana and 

Illinois are not as well known as those in the New Madrid area 

owing to the limited number of seismographs operating in the 

area and the difficulties of discriminating between small

earthquakes and strip-mine blasts, which are very frequent in 

the area. In fact, an excellent illustration of the inaccuracy of 

our knowledge of the epicentral patterns of the southern Indiana area from historical seismicity can be seen by analogy 

with the situation in the New Madrid area. The microearthquake data provided by the Saint Louis University seismograph array considerably refine the characteristics of the 

seismicity of the New Madrid seismic zone when compared 

with the historical data (figs. 4, 5).

Several additional features are evident in the northern extension of the New Madrid linear tectonic feature. First, a
THE NORTHEASTERN EXTENSION OF THE NEW MADRID SEISMIC ZONE 181

deflection in the New Madrid linear tectonic feature appears 

to exist at about lat 36°30' N. The deflection is suggested by 

the offset of the linear trends of the marginal gravity and 

magnetic anomalies when traced from the southwest (from 

Arkansas) and from the northeast (from Indiana) (figs. 4, 5). 

The deflection in the New Madrid linear tectonic feature occurs where the deflection in northeast-trending microearthquake epicenters occurs. The cluster of microearthquake 

epicenters located at the deflection of the New Madrid linear 

tectonic feature trends approximately N. 15° W. This trend is 

in line with a linear but diffuse trend of epicenters which 

begins at the cluster and extends northward to near St. Louis,

Mo. This trend of epicenters is particularly evident on figure 

7. Second, most of the seismicity associated with the northern 

extension of the New Madrid linear tectonic feature does not 

appear to be associated with the Wabash Valley fault zone 

(which is primarily to the west of the Indiana- Illinois border 

and trends approximately N. 20° E.) but rather with the New 

Madrid linear tectonic feature (which trends N. 45° E.). 

Third, the extension of the New Madrid feature into Indiana 

coincides with a basement trough superimposed on the Illinois 

Basin, as interpreted from magnetic anomaly data by Henderson and Zietz (1958). The precise limits and depth of this 

trough are unknown because of the paucity of deep drilling

TOTAL MAGNETIC

INTENSITY

(GAMMAS)

>1200 

I!00tol200 ;> ,"* 

1000 to 1100 K "l 

900 to 1000 [73 

800 to 900 _

HISTORICAL TOO to 800 EPICENTERS vmr~*

EPICENTERS LOCATED BY 600 J° 700 LJ

ST. LOUIS UNIV. NETWORK 500 to 600

1974-1976 400 to 500

INFERRED BOUNDARY OF 300 to 400

NEW MADRID LINEAR 200 to 300

TECTONIC FEATURE < 200

.___|

0 100 j i j

FIGURE 5. Shaded magnetic- anomaly contour map showing inferred boundaries of the New Madrid linear tectonic feature by the heavy dashed lines. South 

of lat 36 ° N., the boundaries are identical to those inferred by Hildenbrand and others (1977). Queries at north and south ends of the New Madrid linear 

tectonic feature indicate the extent of the feature is presently unknown.
182

40

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

90° 88° 7^ 86°

39'

38

t?

FIGURE 6. Selected profiles of gravity (dashed lines) and magnetic (solid lines) anomaly data across the New Madrid linear tectonic feature 

(heavy dashed lines). Profiles are numbered 1, 3, 4, 5, 6, and 7 for identification.

and the limited number and precision of the magnetically 

determined depths, but the feature is clearly evident in the 

deflection of the basement-configuration contours presented 

by Henderson and Zietz (1958).

The correlation of distinct gravity and magnetic anomaly 

patterns, including the deflection in those patterns, with 

earthquake epicenters suggests a basement structural control 

on contemporary seismicity. The gravity and magnetic 

anomalies are primarily an indication of an ancient basement 

geologic structure which extends from east-central Arkansas 

northeastward to southwestern Indiana. Simple linear continuation of this northeasterly trend extends to the Anna, Ohio, 

seismic area. However, it must be emphasized that, at present,

available data only permit delineation of the New Madrid 

linear tectonic feature northeastward to lat 39° N. In addition 

to the correlation of the seismicity with the New Madrid linear 

tectonic feature, it is significant that the inferred focal 

mechanisms indicate a fault-plane direction identical to the 

trend of the New Madrid linear tectonic feature and not an extensional pattern as anticipated in an active rift zone. That the 

number of historical earthquake epicenters decreases significantly north of lat 39° N. and that their pattern is more diffuse 

should also be noted. Whether this observation is an artifact of 

an inaccurate and incomplete data set may be determined by 

future seismicity investigations.

Any model that is proposed to explain the contemporary
THE NORTHEASTERN EXTENSION OF THE NEW MADRID SEISMIC ZONE

92° 90° 88° 86° 84°

183

/ EARTHQUAKES PER I04 KM2 AREA

/- {1800-1972, > MM IE) 

A EPICENTERS*

/ INFERRED BOUNDARY OF NEW MADRID 

/ LINEAR TECTONIC FEATURE (NMLTF)

200 KILOMETERS

37

35

FIGURE 7. Contour map of number of earthquakes per 104 -km2 area with intensity greater than or equal to three 

from 1800 to 1972 (modified from Hadley and Devine, 1974). Inferred boundaries of the New Madrid linear 

tectonic feature shown by dashed lines. Size of triangles is proportional to intensity.

seismicity in the midcontinent area should account for the 

spatial correlation of the microseismicity in the New Madrid 

area and the New Madrid linear tectonic feature, and the 

coincidence of the trend of this feature with results of focalplane solutions and the predominant direction of maximum 

compressive stress in eastern North America (Sbar and Sykes, 

1973; Haimson, 1977). These observations suggest that contemporary seismicity in the New Madrid area is derived from 

localization of stresses along an ancient crustal structure 

feature which is prone to reactivation by its orientation with 

respect to the current stress pattern and perhaps its general 

strength characteristics.

CONCLUSIONS

Interpretation of regional gravity and magnetic maps and 

the pattern of historical earthquake epicenters suggest that a 

basement structural feature is a primary control on seismicity

in the midcontinent area. The New Madrid linear tectonic 

feature is characterized by marginal, correlative, positive 

gravity and magnetic anomalies that bound a zone of low 

gravity and magnetic relief and can be traced northeastward 

from the New Madrid area into southern Indiana at least to lat 

39° N. The extension of the New Madrid linear tectonic 

feature to the northeast does not necessarily imply that this 

feature is or will be seismically active along its entire length. 

The frequency of occurrence of earthquakes to the northeast 

of the New Madrid area is clearly lower than along the New 

Madrid seismic zone. Thus, although the entire length of the 

New Madrid linear tectonic feature (lat 35°-39° N.) is here interpreted as a structural unit, whether it must be considered as 

a single unit for seismic regionalization is unclear. However, 

the presently available evidence, including the continuity of 

the New Madrid linear tectonic feature, its definite correlation 

with seismicity, and the possibility that the historical record is 

a statistically poor indication of the true seismicity of the area,
184 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

indicates to us that the entire structure (presently defined from 

lat 35 ° to 39 °) should be considered to be capable of significant earthquake activity.

We believe that the data and interpretations presented here 

demonstrate the importance of regional, integrated geological 

and geophysical studies in investigations of contemporary tectonics and seismicity in the midcontinent region. The data 

also provide striking evidence for basement structural control

Henderson, J. R., andZietz, Isidore, 1958, Interpretation of an aeromagnetic survey of Indiana: U.S. Geological Survey Professional Paper 316-B,

37 p. 

Herrmann, R. B., and Canas, J. A., 1978, Focal mechanism studies in the

New Madrid seismic zone: Seismological Society of America Bulletin, v.

68, p. 1095-1102. 

Heyl, A. V., 1972, The 38th parallel lineament and its relationship to ore

deposits: Economic Geology, v. 67, p. 879-894. 

Heyl, A. V., and McKeown, F. A., 1978, Preliminary seismotectonic map of

CONTENTS

Page

Abstract .................................................... 191

Introduction .................................................191

Phase I: Temporal variations ............................... 191

Phase II: Spatial distribution ............................... 192

Radon monitoring method ..................................... 192

Analysis of temporally varying factors ............................ 192

Radon-emanation variation ................................ 194

Water levels and radon emanation ........................... 194

Meteorologic factors and radon emanation .................... 194

Temperature ........................................ 194

Barometric pressure................................... 194

Rainfall............................................. 195

Earth tides and radon emanation ............................ 195

Seismicity and radon emanation ............................. 195

Amplitude of ground displacement ...................... 196

Number of seismic events. .............................. 196

Visual analysis of temporally varying factors ................... 197

Radon concentration in wells ............................... 198

Analysis of spatial distribution of radon emanation. ................. 198

Monthly contour maps..................................... 198

Suggested mechanisms influencing spatial distribution

of radon concentration .................................... 198

Conclusions ................................................. 200

References cited .............................................. 200

ILLUSTRATIONS

FIGURE 1. Map showing locations of seismographs and epicenters in the New Madrid seismic zone, July 1, 1974-March31, 1978 ..................

2. Map showing networks of radon monitoring stations for Phase I and Phase II ...................................................

3. Time series for radon data and other factors, April-December, 1977 ..........................................................

4. Time series and cross-correlation of radon data with water-level data, September 1977-February 1978 ..............................

5. Cross-correlations of radon data with meteorologic and Earth- tide effects .....................................................

6. Cross-correlations of seismicity with radon data ...........................................................................

7. Time series and cross-correlation of smoothed seismic-events data with smoothed radon data ......................................

8. Autocorrelation of smoothed radon-isotope data superimposed on autocorrelation of smoothed seismic-events data ...................

9. Time series of factors exhibiting significant increase in the 37th and 38th weeks, early December 1977 ...............................

10. Contour maps showing radon-isotope emanation survey data in the New Madrid seismic zone for months of June and November 1977, 

and February and March 1978.....................................................................................

11. Contour maps showing 13-month-average radon-isotope concentrations for the period June 1977 through June 1978 ...................

12. Map showing the 25 sites of greatest radon-isotope concentration. ............................................................

Page

.192

.192

.193

.193

.195

.196

.197

.197

.197

.199

.200

.200
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

HEAT FLOW 

IN THE UPPER MISSISSIPPI EMBAYMENT

By C. A. SWANBERG1 , B. J. MlTCHELL2 , R. L. LOHSE1 , and D. D. BLACKWELL3

ABSTRACT

We have analyzed heat-flow data from the New Madrid seismic zone and 

adjacent areas in Missouri, Kentucky, Arkansas, Illinois, and Tennessee. 

These data consist of two traditional heat-flow determinations based on 

detailed temperature logs and laboratory measurements of thermal conductivity, two approximate heat-flow values based on detailed temperature logs 

and estimated thermal-conductivity values, and several hundred bottomhole temperature measurements. The bottom-hole temperature data show a 

small, but well-defined, thermal anomaly associated with the most seismically active area. The heat-flow values are slightly higher in the seismic zone 

relative to the rest of the stable interior, but the absolute values (1.3-1.6 heat 

flow units) are not sufficiently above the midcontinent mean to verify the existence of a major heat-flow anomaly. Although several explanations for the 

New Madrid thermal anomaly are possible, we favor a small convective heatflow component resulting from deep ground-water circulation along fractures in the upper crust associated with active faulting.

INTRODUCTION

The New Madrid seismic zone is characterized by a high 

level of earthquake activity occurring in a relatively small 

region surrounded by the more stable crust of the Central 

United States. It is a region of practical interest because of the 

relatively high probability that earthquakes occurring there 

could cause damage to manmade structures (Algermissen and 

Perkins, 1976), and because the concentration of numerous 

earthquakes makes it a convenient area in which to pursue 

earthquake-prediction research under intraplate conditions 

(Mitchell, Stauder, and Cheng, 1977).

Although the region can be characterized, to a first approximation, by plane-layered seismic models of the crust (McCamy and Meyer, 1966; Mitchell and Hashim, 1977), recent 

observations of teleseismic P-wave residuals have indicated 

that compressional-wave velocities in the crust and upper 

mantle can vary laterally by as much as 5 percent (Mitchell,

'Dept. of Physics. New Mexico State University, Las Graces, NM 88003.

'Dept. of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO 63156.

'Institute for the Study of Earth and Man, Southern Methodist University, Dallas, TX 75275.

Cheng, and Stauder, 1977). The lowest velocities occur 

beneath the most seismically active portion of the New Madrid 

seismic zone.

It is interesting to speculate whether these lateral variations 

in elastic properties of the crust and upper mantle are related 

to earthquake activity and whether other geophysical properties also exhibit lateral variations. Heat flow is one geophysical 

property which could vary laterally and which is easily determined if suitable boreholes are available.

During the summer and fall of 1976, a search was conducted in the upper Mississippi Embayment for deep wells that 

might be suitable for heat-flow determinations. Most of the 

available wells turned out to be abandoned municipal water 

wells; they are adequate for temperature logging when in thermal equilibrium, but frequently lack the core material 

necessary for thermal-conductivity analysis. The one well logged for temperature in which a complete set of cuttings was 

preserved is a test well at Fort Pillow, Tenn. (Moore and 

Brown, 1969). We were also able to obtain cuttings from an oil 

test well near our well at Ridgely, Tenn., and from a test well 

located approximately 12 km north-northwest of the Ridgely 

well.

Two of the heat-flow values we obtained (Ridgely, Tenn., 

and Caruthersville, Mo.) are located within the zone of maximum seismicity as described by Stauder and others (1976). 

Two additional wells (Fort Pillow, Tenn., and Wilson, Ark.) 

are situated to the south of that zone. A fifth well, located at 

West Helena, Ark., yielded an unrealistically low heat-flow 

value and the data were rejected.

HEAT-FLOW DATA

The locations of the heat-flow sites are shown in figure 1 and 

the basic heat-flow data are summarized in table 1. The temperature gradients were obtained by making discrete temperature measurement (±0.01°C) at 5-m intervals through the 

wells (fig. 2) and applying the method of least-squares to 

various sections of the wells. Conductivity measurements 

(table 1) were made using a standard divided-bar apparatus

185
186 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

91°00' 90°30' 90W 89°30' 89°00'

36°00'

35°30'

35°00'

ARKANSAS

88°30' graphic facies changes will not significantly alter the thermal 

conductivity even though the formations are the same. The 

heat-flow values given for the Wilson and Caruthersville wells 

must therefore be regarded as estimates. When they are interpreted in combination with the more reliable data for the Fort 

Pillow and Ridgely wells (table 1) and the voluminous bottomhole temperature data, however, they add appreciably to our 

overall understanding of the thermal state of the New Madrid 

seismic zone.

Figure 3 compares heat-flow values obtained in the present 

study with those compiled for the stable interior of the United 

States (Sass and others, 1976). All values obtained in the upper 

Mississippi Embayment, except that at Fort Pillow (1.20), are 

higher than the mean value (also 1.20) for the stable interior. 

The value at Ridgely (1.32) is only slightly higher, whereas the 

values at Wilson (1.44) and Caruthersville (1.56) appear to be 

significantly higher than the stable-interior mean. These 

values indicate that a modest, but not major, thermal anomaly occurs in the region of the New Madrid seismic zone.

30 KILOMETERS

FIGURE 1. Location of heat-flow sites. Heat-flow values are given in parentheses in units of cal/cm2 -sec. The small solid circles denote earthquakes located by the Saint Louis University seismograph network between June 29, 1974, and December 31, 1977.

and a cylindrical cell containing water-saturated rock 

fragments (Sass and others, 1971). The conductivity of the 

nonporous rock (Kr) as determined in the laboratory has been 

converted to in situ conductivity (Kpr) using the expression of 

Sass and others (1971):

where Kw is the conductivity of water (1.46 meal/ cm-sec- °C 

at 25 °C) and ^ is the natural porosity of the rock. Porosity 

values of 30 percent and 40 percent have been taken for 

samples from Eocene and Pleistocene strata, respectively.

For the holes from which no cuttings were available, stratigraphic correlation was employed to estimate thermal conductivity. The Wilson, Ark., well was correlated with the Fort 

Pillow well (table 1). Because no lithologic logs were available 

for the Caruthersville well, a least-squares gradient was calculated for the bottom two- thirds of the well and combined with 

a conductivity value obtained by averaging 15 measured 

values representing the same depth interval in the Ridgely well 

and in a test well located 12 km northwest of the Ridgely well. 

This method yields only approximate conductivity values, 

however, because there is no guarantee that natural stratiQ_

LLJ

o

50

100

150

200

250

300

350

400

450

15 17 19 21 23 

Temperature, in °C

25 27

FIGURE 2. Temperature profiles for the heat-flow sites located in figure 1: 

1, Fort Pillow, Tenn.; (2) Ridgely, Tenn.; 3, Wilson, Ark.; and (4) 

Caruthersville, Mo.
HEAT FLOW IN THE UPPER MISSISSIPPI EMBAYMENT 

TABLE 1. Heat flow using measured thermal conductivity

187

Locality Lat N. Long W. Kiev Depth interval

(m) (m)

Stratigraphic

unit

(Eocene)

Temperature

gradient

(°C/kni)

Thermal

conductl

(10" cal/cm

vity

sec-deg)

Heat flow

(1CT cal/cm -sec)

Wells for which cuttings were available

Fort Pillow 35°38'39" 80°49'35" 135 130-140

140-190

190-225 

225-240

240-265

275-284

298-331 

349-386

404

414-423 

442-451

AfiQ.ASS

Cook Mountain Fm. 27.9 ±1.3

Cook Mountain Fm. 20.6 ± .1

Memphis Sand (upper) 16.2 ± .1 

Memphis Sand (upper) 21.3 ±1.8

Memphis Sand (lower) 15.9 ± .3

Memphis Sand (lower) n.d.

Memphis Sand (lower) n.d.

Memphis Sand (lower) n.d. 

Flour Island Fm. (upper) n.d.

Flour Island Fm. (lower) n.d. 

Transition zone n.d.

4.89

4. 88

6.45 

6.93

7.12

7.12

7.29 

7.55

5.40

5.88 

5.19

fi.QS

1. 36

1.00

1.04 

1.48

1.13

n.a

n.a

n.a

n.a

n.a

Ridgely 36°15'43" Memphis Sand (upper) 

average 1.20 ± 0.21 

1.32

Wells for which cuttings

Wilson 35°33'32" 90°02'14" 75 320-345

345-365

365-405

410-430

Caruthersville 36°11'38" 89°39'30" 75 125-370

Memphis Sand (lower)

Flour Island (upper)

Flour Island (lower)

Fort Pillow Sand 

average

Not available 

18.4 ± 0.

29.2 ± .

21.2 ± .

21.8 ± 1.

1.44 ± 0.

31.9 ± 0.

,4

,8

,4

,5

,15

.4

3 7.55

5.40

5.88

6.95

'*4.90

1.39

1.58

1.25

1.52

1.56

n.d. indicates no data.

n.a. indicates not applicable.

The conductivity for each Stratigraphic unit in the Wilson well is taken to be identical to the measured conductivity

Average of 5 values from Ridgely and 10 values from a test well located 12 km northwest of Ridgely, all from the 125-m

ralue for the same unit in the Fort Pillow well, 

to 370-m depth interval.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 

HEAT FLOW IN jjcal/cm'-sec

FIGURE 3. Comparison of heat-flow values from the New Madrid region 

with a histogram of heat-flow values for the stable interior of the Central United States. Data from Sass and others, 1976.

BOTTOM-HOLE TEMPERATURE DATA

In addition to the heat-flow data described above, several 

hundred botton-hole temperature measurements have been 

obtained from the USGS WATSTORE file for the general 

area surrounding the New Madrid seismic zone. These temperatures, when combined with well depth and an assumed 

mean annual air temperature, can be used to calculate the 

geotherrnal gradient; this approach is commonly applied to 

deep-oil-well data (Geothermal Survey of North America Subcommittee of the American Association of Petroleum Geologists' Research Committee, 1976). In the present case, 

however, the wells are generally shallow (50-500 m), and 

rather large errors can occur in the calculated gradients. 

Thus, rather than contouring the calculated temperature gradients, we have isolated the wells having abnormally high gradients and plotted them in figure 4 along with the wells having 

normal gradients. We have designated the gradients as being 

abnormal if they are more than 1.5 standard deviations above 

the calculated mean gradient for the region. For the New 

Madrid area, the cutoff gradient is 30°C/km. The data in 

figure 4 show that nearly all of the abnormally high gradients 

are located within or adjacent to the zone of maximum 

seismicity.

A second approach in interpreting the bottom-hole temperature data is to group the wells according to depth and examine each group for the highest bottom-hole temperatures. 

Individual groups consist of all the wells having bottom-hole 

depths within a 16-m interval centered at 90, 120, 150, 180, 

210, 240, 375, and 525 m. Figure 5 shows the locations of the 

hottest two or three wells from each group, along with the normal wells. This method of presentation has the advantage of 

not being dependent upon an assumed mean annual air 

temperature and avoiding the possibility that all of the high 

gradients may represent wells drilled into a particular aquifer 

that happens to be hot. The data in figure 5 clearly show that 

the hottest wells are consistently located within or adjacent to 

the zone of maximum seismicity.
188 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

o

o 

o°ooo

o 

o

o 

o

0

MISSOURI_8 

ARKANSAS

KENTUCKY

TENNESSEE

o 

o

FIGURE 4. Locations of wells having abnormally high temperature gradients as calculated from bottom-hole temperatures. Solid circles, 

triangles, and open circles denote gradients that are ">30°, 24° 30°, 

and< 24°C/km, respectively. The dark shading delineates the region 

of maximum seismicity (Stauder and others, 1976), and the light shading indicates the approximate extent of the Mississippi Embayment.

CONCLUSIONS AND DISCUSSION

All of our temperature data indicate the presence of a small 

but well- defined thermal anomaly associated with the New 

Madrid seismic zone. A new heat-flow map of the United 

States based on silica geothermometry also shows a small heatflow high located in the New Madrid region (Morgan and 

Swanberg, 1978).

Several possible origins exist for this thermal anomaly. The 

possibility of a mantle source is intriguing, because of the 

possible relationship of the thermal anomaly to the anomalously low velocities reported beneath the seismically active 

portion of the upper Mississippi embayment (Mitchell, Cheng, 

and Stauder, 1977). The limited spatial extent of the thermal 

anomaly, however, appears to argue against an upper mantle 

source.

The release of stored elastic strain or frictional heating during earthquake processes would lead to thermal anomalies of 

more limited extent than those produced by mantle sources. 

An evaluation of these possibilities will require a knowledge of 

the displacement along faults in the region, as well as of the 

seismic efficiency (the fraction of the total earthquake energy 

that goes into seismic waves), neither of which is known. The

small magnitudes of most New Madrid region earthquakes 

(and consequent low-energy release) seem, however, to argue 

against this mechanism as a major cause of the New Madrid 

thermal anomaly.

A component of convective heat flow due to ground water 

ascending along active faults provides another possible explanation for the anomaly. This mechanism, although difficult to evaluate, would explain the limited spatial extent of 

the anomaly, and the close association between elevated heat 

flow and seismicity; it would also be capable of producing the 

observed heat-flow values. We tentatively favor this convective 

mechanism as the causative process for the New Madrid heatflow anomaly.

ACKNOWLEDGMENTS

We are grateful to A. T. Statler of the Tennessee Division of 

Geology for providing rock samples from wells in western Tennessee and to David P. Russ for providing samples from the 

New Madrid test well. James Kohsmann and Wing- wah Chan 

assisted in the field measurements. This research was partially 

supported by the National Science Foundation, Division of 

Earth Science, under Grant EAR 77-27096.

FIGURE 5.- Locations of the hottest wells at depths of 90, 120, 150, 180, 210, 

240,375 and 525 ±16 m. Solid circles, triangles, and open circles 

denote the hottest wells, marginally hot wells, and normal wells, respectively. The dark shading delineates the region of maximum seismicity 

(Stauder and others, 1976), and the light shading indicates the approximate extent of the Mississippi Embayment.
HEAT FLOW IN THE UPPER MISSISSIPPI EMBAYMENT 189

REFERENCES CITED

Algermissen, S. T., and Perkins, D. M., 1976, A probabilistic estimate of 

maximum acceleration in rock in the conterminous United States: U.S. 

Geological Survey Open-File Report 76-416, 45 p.

Geothermal Survey of North America Subcommittee of the American Association of petroleum Geologists' Research Committee, 1976, Subsurface temperature map of North America: U.S. Geological Survey, scale 

1:5,000,000 (1 inch =80 miles).

McCamy, K., and Meyer, R. P., 1966, Crustal results of fixed multiple shots 

in the Mississippi embayment, in The earth beneath the continents: 

American Geophysical Union Monograph 10, p. 370-381.

Mitchell, B.J., andHashim, B. M., 1977, Seismic velocity determinations in 

the New Madrid seismic zone A new method using local earthquakes: 

Seis mological Society of America Bulletin, v. 67, p. 413-424.

Mitchell, B. J., Cheng, C. C., and Stauder, William, 1977, A three-dimensional velocity model of the lithosphere beneath the New Madrid 

seismic zone: Seismological Society of America Bulletin, v. 67, p. 

1061-1074.

Mitchell, B.J., Stauder, William, and Cheng, C. C., 1977, The New Madrid

seismic zone as a laboratory for earthquake prediction research: Journal 

of Physics of the Earth (Tokyo), v. 25, Supp., p. S43 S49.

Moore, G. K., and Brown, D. L., 1969, Stratigraphy of the Fort Pillow test 

well, Lauderdale County, Tennessee: Tennessee Division of Geology 

Report of Investigations 26, 1 sheet and tables.

Morgan, P., and Swanberg, C. A., 1978, Heat flow, topography, lithospheric thickness, and gravity anomalies in the United States: EOS 

(American Geo physical Union Transactions), v. 59, p. 120d.

Sass, J. H., Lachenbruch, A. H., and Munroe, R. J., 1971, Thermal conductivity of rocks from measurements on fragments and its application 

to heat-flow determinations: Journal of Geophysical Research, v. 76, p. 

3391 3401.

Sass, J. H., Diment W. H., Lachenbruch, A. H., Marshall, B. V., Munroe, 

R. J., Moses, T. H., Jr., and Urban, T. C., 1976, A new heat flow contour map of the conterminous United States: U.S. Geological Survey 

Open-File Report 76-756, 24 p.

Stauder, William, Kramer, M., Fischer, S., Schaefer, S., and Morrissey, S. 

T., 1976, Seismic characteristics of southeast Missouri as indicated by a 

regional telemetered microearthquake array: Seismological Society of 

America Bulletin, v. 66, p. 1953-1964.

Radon Emanation in the 

New Madrid Seismic Zone

By S. R. STEELE, W. C. HOOD, and]. L. SEXTON

INVESTIGATIONS OF THE NEW MADRID, MISSOURI, EARTHQUAKE REGION

GEOLOGICAL SURVEY PROFESSIONAL PAPER 123 6-N
CONTENTS

Page

Abstract ............................................................................... 185

Introduction............................................................................ 185

Heat-flow data .......................................................................... 185

Bottom-hole temperature data ............................................................. 187

Conclusions and discussion .................................................................188

Acknowledgments........................................................................ 188

References cited ......................................................................... 189

ILLUSTRATIONS

Page

FIGURE 1. Location of heat-flow sites ............................................................................................... 186

2. Temperature profiles for the heat-flow sites located in figure 1: (1) Fort Pillow, Tenn.; (2) Ridgely, Tenn.; (3) Wilson, Ark.;

and (4) Caruthersville, Mo. .......................................................................................... 186

3. Comparison of heat-flow values from the New Madrid region with a histogram of heat-flow values for the stable interior of the

Central United States ............................................................................................... 187

4. Locations of wells having abnormally high temperature gradients as calculated from bottom- hole temperatures ......................... 188

5. Locations of the hottest wells at depths of 90, 120, 150, 180,210,240,375, and 525 ± 16m ........................................... 188

TABLE

Page

TABLE 1. Heat flow using measured thermal conductivity .............................................................................. 187
INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

RADON EMANATION IN THE NEW MADRID SEISMIC ZONE

By S. R. SxEELE1 , W. C. Hooo2 , and J. L. SEXTONS

ABSTRACT

A study of radon emanation in the New Madrid seismic zone and adjacent areas, inferred from alpha-particle activity of soil gas and wells, has 

revealed that radon activity varies in both time and space, and that this 

variation appears to be related to several physical mechanisms. Seasonal as 

well as short-term variations in radon emanation were observed. The 

method used to determine alpha-particle activity made no distinction 

among the radon Rn-222 (radon), Rn-220 (thoron), and Rn-219 (actinon).

Using cross-correlation techniques and visual analyses, comparison of the 

temporal variations of the radon emanation with those of several other factors suggests that (1) periods of substantial variation in barometric pressure 

(undistinguished for positive and negative movements) are followed by increases in radon emanation; (2) periods of sharp increase in water levels are 

accompanied by similar increases in radon emanation, and periods in which 

water levels are signifiantly above the mean tend to be times of peak activity 

in radon emanation; (3) radon emanation increases significantly approximately 4-6 weeks prior to periods of increased seismicity (as defined by an increased number of events during a given weekly period); and (4) periods of 

increased ground motion are accompanied by relatively low concentrations 

of radon, which decrease further within 1-2 weeks after ground displacement occurs. Whereas a strong positive correlation was indicated from crosscorrelation of radon activity and temperature, visual analysis suggested that 

the observed high correlation may have resulted from the similar, though 

not identical, seasonal activity experienced by both factors.

Analysis of the spatial distribution of radon concentration revealed a 

linear zone of anomalously high radon emanation coinciding with the 

seismic trend, as well as a few isolated, small zones of high radon concentration that appeared to correspond to the peripheral regions of plutons. The 

spatial distribution of the near-surface radon concentration may be influenced by the following mechanisms: greater ease of emanation through 

existing fracture zones, increased fluid circulation enriching seismic-zone 

fractures in uranium and radium (precursors to radon), or increased 

outgassing of the thick sedimentary pile in the study area in response to tectonic stresses.

INTRODUCTION

A survey of radon emanation in the New Madrid seismic 

zone (fig. 1) and adjacent areas was initiated in February 1977 

in an effort to detect fluctuations in radon concentration oc-

'Earth Science Research and Consulting, Union Hill, Carbondale, IL 62901. 

"Geology Department, Southern Illinois University, Carbondale, IL 62901. 

'Department of Geosciences, Purdue University, W. Lafayette, IN 47907.

curring in both time and space, and to determine the physical 

mechanisms that might be influencing the variations. No 

distinction was made among the radon Rn-222 (radon), 

Rn-220 (thoron), and Rn-219 (actinon). This paper reports 

the results of the study for the period April 1977 through June 

1978. Monitoring of certain areas in the seismic zone has continued.

Recent research involving radon measurements along active 

faults in central California indicates that anomalous radon 

emanation occurred a few weeks prior to several earthquakes 

of 4.0-4.6 magnitude (King, 1977). Although King found correlations of radon activity with seismicity in central California, 

a plate-boundary region of frequent intermediate level 

tremors, the question remained whether seismically related 

variation in radon concentration existed in intraplate areas of 

lower level seismicity such as the New Madrid seismic zone. 

This zone is characterized by substantial thicknesses of unconsolidated sediments of the Mississippi Embayment, glacial outwash, and alluvial deposits (Buschbach, 1977).

Crustal microfracturing, a suggested mechanism of possible 

influence on deep-well radon activity in other seismic areas, 

was assumed to be an unlikely source of significant influence in 

the study area on the basis of the distance between the surface 

soil zone and the more competent rocks below the extensive 

embayment sediments. Numerous studies have shown that 

long-range migration of radon is not physically reasonable (A. 

B. Tanner, written commun., 1978), due in part to the 

relatively short half-life of 3.82 days for the longest lived 

isotope and to the slow process of diffusion. Thus consideration of other possible causes of the variation in radon emanation was a prime aspect of this study. The project was carried 

out in two separate phases: Phase I, which was an investigation 

of temporal variations; and Phase II, which was a study of 

spatial distribution.

PHASE I: TEMPORAL VARIATIONS

A nine-station network of radon monitoring stations was 

established in February 1977 within the zone of intense 

seismicity (fig. 2). These stations were located on significant 

geomorphic features such as lineaments, scarps, and sandblows, as it was felt that some of these features might relate to

191
192

 90°30 

3 7 Off 

INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

36 00

35 30

~~l 9000 89°00

MISSOURI

L £ __..

ARKANSAS

30 Kl LOMETERS

FIGURE 1. Locations of seismographs (triangles) and epicenters (dots) in the 

New Madrid seismic zone. Cumulative data of events from July 1,1974, to 

March 31, 1978, modified from Stauder and others (1978).

RADON MONITORING METHOD

Radon emanation at shallow levels in the soil was inferred 

from alpha- particle counts determined through the use of a 

commercially manufactured nitro-cellulose plastic film 

designed specifically for the detection of alpha radiation. Inverted plastic cups containing a strip of the film were put into 

shallow, plastic-lined holes, 0.31 m in depth, which were then 

covered for protection. Radon emanating from the soil 

entered the cup, giving off alpha radiation as it decayed. The 

film was removed at specified intervals and processed in a solution to enlarge the damaged zones, caused by alpha-particle 

interaction, to a size easily countable under low magnification. The number of enlarged tracks in a standard area of 14.5 

mm2 was used to infer relative radon emanation for a given 

period.

ANALYSIS OF TEMPORALLY VARYING 

FACTORS

The time series for all of the factors considered in temporal 

analysis are shown in figures 3 and 4A. Data sets for each factor were normalized to 1.0 and subsequently centered about 0 

mean by removing the long-term average from each of the 

weekly values. This procedure enabled comparisons between 

the variances from the mean for each factor. Specific datafaulting in the subsurface (F. A. McKeown, written commun., 

1977) and, hence, offer the best chance to detect any 

seismically related variation in radon concentration that 

might exist.

The stations in this local network were monitored at 7-day 

intervals, providing data for analysis of temporal variations. 

Factors which were considered as possible influences on temporal variations in radon emanation were meteorologic effects 

(temperature, barometric pressure, and rainfall); Earth-tide 

effects; water-level changes; and seismic activity that occurred 

in the study area during the period of observation.

The radon concentrations of two water wells in the seismic 

zone that became available later in the study were also 

monitored weekly (fig. 2).

PHASE II: SPATIAL DISTRIBUTION

A regional network of 51 stations, which encompassed the 

seismic zone as well as adjacent areas, was established in an effort to determine whether obvious differences in radon distribution occurred between the seismic zone and areas of less frequent seismicity. As the Mississippi River bisects the seismic 

trend, approximately half of the monitoring stations were in 

Missouri and half were in Tennessee (fig. 2). These stations 

were monitored once every 28 days.

90°00'

FIGURE 2. Networks of radon monitoring stations for phases I and II. Phase I 

stations (crosses) and wells (W) monitored weekly; Phase II stations (0) 

monitored monthly; A, seismographs; and dashed lines indicate axes of 

seismic trends.
RADON EMANATION IN THE NEW MADRID SEISMIC ZONE 193

MONTH OF 19// 

| APRIL i MAY i JUNE [JULY | AUG ,SEPT | OCT |

Weekly average 

of daily maximum

in Dyersburg, Tenn.

Weekly average o 

daily barometric

at Dyersburg, Te

Total weekly 

rainfall at 

New Madrid, Mo.

Earth-tide 

excursion 

recorded at 

St. Louis 

University, Mo.

Weekly average 

inferred radon

five Missouri 

stations

10 15 20 25 30 35 4

B

A

10 15 20 25 30 35 40

TV vAjV^

\J

10 15 20 25 30 35 40

> °- 5

<

(-

< -0.5 

0

A A . s\r^ ^ A AA AA A -"i A

/ v Vv ~ v N/ v vvy, XA/V V

0 5 10 15 20 25 30 35 4

at Lone Star, Mo.

Total amplitude of // 

ground displacement 

at Gratio, Tenn.

10 15 20 25 30 35 4C

porai variation in randon emanatidn using cross-correlation 

techniques, as well as visual analysis. Cross-correlation results 

are shown on correlograms (figs. 4-7). In the cross-correlation 

procedure, the time series for the first noted factor in each of

; the correlograms was held stationary while the series for the second factor was shifted past it; this procedure began with the 

second series completely offset to the left of the first. Shifting 

proceeded by 1-week increments, with subsequent determination of a correlation coefficient for each shift. The coefficient 

provides a measure of the overall similarity between the two 

series for the overlapped portions of the sequences. The factor 

noted first on each correlogram was generally assumed to be 

the dependent variable.

Peaks that exceed the 0.05 or 0.01 lines indicate a statistically significant probability of a relationship existing between the 

two factors being compared. Only those peaks occurring 

within a 10-week lag limit were considered here, as there is

L° some question of the validity of correlations at lags greater

W«t«r

Missouri radon 

emanation

Tennessee radon 

nut nation

10 15 20 25 30 35 4C 

WEEK OF STUDV PERIOD

FIGURE 3.- Time series for radon data and other factors during the period 

April-December 1977. All data normalized to 1.0 and subsequently 

centered about 0 mean by removing the long-term average from weekly 

values. A, Temperature is weekly average of the daily maximum temperature in Dyersburg, Tenn. B, Barometric-pressure excursion is the weekly 

average of the total daily pressure range recorded at Dyersburg, Tenn. C., 

Rainfall is the total weekly rainfall recorded at New Madrid, Mo. D, 

Earth-tide excursion is the weekly average of the total amplitudes of the 

two daily earth-tide peaks recorded at Saint Louis University, St. Louis, 

Mo. E, Missouri radon data is the total weekly radon concentration inferred from alpha-track count/standard area (14.5 mm^) based on a weekly 

average of the five Missouri stations. F, Number of seismic events is the 

total weekly number of significant events occurring in the study area. G, 

H, Amplitude of ground displacement/week at the seismographs at the 

indicated locations: G, Lone Star, Mo.; H, Gratio, Tenn.

reduction methods for each of the factors are discussed in the 

individual sections relating to each factor.

The temporal variations of water levels, meteorologic effects, Earth tides, and seismicity were compared with the tem20 25 30 35 40 45 50

Week of study period

0.01 probability level 

0.05

FIGURE 4. Comparison of radon-emanation data and water levels in a Ridgely, Tenn., well, September 1977 to February 1978. A, Time-series data 

for water levels, Missouri radon emanation, and Tennessee radon emanation. Data for the time series were normalized to 1.0 and subsequently 

centered about 0 mean by removing long-term average from weekly 

values. B, Cross-correlation graph for radon emanation and water levels.
194 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

than n/4 (Davis, 1973), where n is the number of weeks of observation. The amplitudes of peaks indicate the degree of 

similarity between the two time series; troughs represent corresponding dissimilarities.

To the left of the vertical axis, fluctuations in the first noted 

factor are precursory to those of the second by an amount of 

time indicated by the lag value. Conversely, to the right of the 

vertical axis, fluctuations in the second noted factor precede 

those of the first.

RADON-EMANATION VARIATION

In attempting to simplify the radon data collected from the 

nine stations monitored weekly, an average, weekly value of 

inferred radon emanation was computed for the Missouri stations and for the Tennessee stations. The range of the average 

values of radon concentration, expressed in terms of alpha 

track count per standard area of 14.5 mm2 , was 21-643 tracks 

/standard area of the detector. The two average, weekly radon 

concentration curves, which exhibited striking similarity, were 

then used in subsequent cross- correlations. A long-term 

seasonal variation as well as short-term fluctuations was 

observed.

WATER LEVELS AND RADON EMANATION

Water levels in a 200-m-deep well in Ridgely, Tenn., 

situated in an active portion of the seismic zone, exhibited variations similar to those of radon emanation (fig. 4). The water 

levels fluctuated between 5 and 7 m below the ground surface 

during the 29-week period of observation. Both radon emanation and water levels were below the mean during the first half 

of this period. A simultaneous sharp increase above the mean 

occurred in both factors in the 37th week. Water levels continued in a general upward trend that contained two periods 

of relatively greater increase, each of approximately 5 weeks' 

duration. Fluctuations in radon emanation were comparable 

in that two periods of increase above the mean were recorded 

for both Tennessee and Missouri. Current data for more than 

a year of observation suggest that sharp increases in water 

levels are generally accompanied by similar increases in radon 

emanation, and that periods in which water levels are significantly above the mean tend to be times of peak activity in 

radon emanation. Whereas water-level data reflect characteristically gradual changes a majority of the time, radon emanation is more sporadic.

If the relationship between water levels and radon emanation is real, it suggests that water levels may contribute to increased radon concentration at the surface. This increase may 

be due to outgassing of the sediments in response to rising 

water levels, or it may perhaps be due to upward transport of 

radon or their precursors from deeper levels by the rising 

water, causing an upward shift of the near-surface profile of

radon concentration. Further investigation of the water levelradon emanation relationship is needed before definitive conclusions are made. Water-level data were provided by Brian 

Mitchell of Saint Louis University.

METEOROLOGIC FACTORS 

AND RADON EMANATION

TEMPERATURE

The average, weekly maximum temperature was determined from data supplied by the weather service in Dyersburg, 

Tenn. A strong positive correlation was observed in cross-correlations between radon emanation and temperature, as 

shown in figure bA. However, visual analysis of the two series 

suggested that the strong correlation may have resulted from 

the similar, though not identical, seasonal variations exhibited 

by these two factors (Steele, 1978). The following observations 

seem to be evidence for a noncausal relationship between these 

factors: Both radon concentration and temperature values 

were relatively high in the first half of 1977 and dropped to significantly lower values in the latter half of the year, but the 

decrease in radon concentration preceded the seasonal 

decrease in temperature by several weeks and the radon 

emanation experienced a resurgence in early December (week 

37) which was not accompanied by a corresponding temperature rise. As further evidence in favor of a noncausal relationship, a deep trough in the radon time series in July 1977 (weeks 

14-16) was not accompanied by a corresponding temperature 

decrease.

It is noteworthy that King (1978) found an inverse relationship between radon emanation and temperature in the San 

Andreas region and that Gableman (1972) found a positive 

correlation between alpha-activity concentration and 

temperature.

BAROMETRIC PRESSURE

Reduction of barometric-pressure data involved determination of the maximum excursion of pressure for each daily 

period and subsequent determination of a weekly average of 

the daily pressure excursions. This provided a measure of the 

pumping effect which large changes in pressure may have initiated. In this treatment of the data, positive and negative 

pressure movements were not analyzed, only the relative range 

of the pressure from week to week. Earlier correlations using 

absolute, daily, noon pressure values to determine a weekly 

average pressure indicated no relationship between fluctuations in barometric pressure and radon emanation on a weekly 

basis.

Correlograms relating the weekly barometric-pressure excursions to variations in radon concentration indicated that 

large excursions in pressure bear some relationship to ensuing 

increases in radon emanation in the study area. The data 

shown in figure 5B, which indicate a significant correlation
RADON EMANATION IN THE NEW MADRID SEISMIC ZONE 195

0.01 probability levels

FIGURE 5. - Correlograms showing results of cross-correlations between radon 

data and meteorologic and Earth-tide factors. A, Correlogram for 

Missouri radon concentration (inferred from alpha-track count/standard 

area of 14.5 mm^, based on a weekly average of the five Missouri stations) 

and temperature (weekly average of daily maximum temperature recorded at Dyersburg, Tenn.) for the period Arpil 1977 to March 1978. B, Correlogram for Missouri radon data and barometric-pressure-excursion 

data recorded at Dyersburg, Tenn., April-December 1977. C, Correlogram for Missouri radon residual 1A and barometric-pressureexcursion data, April-December 1977. Residual 1A is the original 

Missouri radon-emanation data minus the temperaturelike effect determined through a least-squares fitting procedure. D, Correlogram for 

Missouri radon-emanation data and Earth-tide excursion data (determined from the continuous gravity-tide profile recorded at Saint Louis 

University).

between the time series for Missouri radon activity and 

barometric-pressure excursions, suggested a relatively large 

lag of 8 weeks between pressure effects and radon increases. 

This seemed an excessive lag for a causal relationship.

A shorter lag of 1 week and a more pronounced relationship 

were exhibited on the correlogram relating the pressure excursions to a radon residual (fig. 5C). This residual (IA) was the 

original radon time series minus the temperaturelike effect, 

approximated through a least-squares fitting procedure 

(Robinson and Treitel, 1967). The residual was determined in 

an effort to see if removal of the indicated temperaturelike effect from the radon time series would result in a better assessment of the influence of pressure on radon emanation.

Tanner (1978) noted that an inverse relationship exists between atmospheric pressure and exhalation of Rn-222 and 

causes the concentration profile in soil gas to shift upward. 

Numerous studies have considered the effects of atmospheric 

pressure on the variation in concentration of radon: Geudalia 

and others (1970) concluded that the exhalation of Rn-220 

was not influenced by variations in atmospheric pressure;

Gableman (1972) noted an inverse correlation between alpha 

activity of soil gas and pressure; Clements and Wilkening 

(1974) inferred that transport of Rn-222 at shallow depths 

only is influenced by daily changes in atmospheric pressure. 

Tanner further noted that Clements found, for sinusoidal 

pressure variations of 1- and 3-day periods, that the transport 

velocity at depths of 16.5 m and 28.5 m, respectively, would 

decrease by a factor of 1/e.

RAINFALL

Using local rainfall data for New Madrid, Mo., no relationship was found between total weekly rainfall and radon 

emanation. Using weekly data, King (1978) also found no correlation between rainfall and radon emanation in the central 

California region. However, Gableman (1972) found an inverse relationship beween rainfall and alpha activity of soil gas 

following a heavy rain; he noted a significant decrease in the 

concentration of alpha activity as downward percolation of the 

rain occurred.

EARTH TIDES AND RADON EMANATION

Earth-tide data were compiled from the continuous gravitytide profile produced by the tidal gravimeter at Saint Louis 

University. The average weekly tidal excursion was determined by measuring the amplitudes of the two daily Earth-tide 

peaks and calculating a weekly average of the daily tidal variation. This method of reducing the tidal data was felt to provide the most effective measurement of the biweekly Earthtide variation related to the lunar phases. A longer period, 

seasonal, Earth-tide component was also noticed.

The correlograms for Earth tides and radon emanation exhibited a nonrandom pattern of variation and an apparent 

biweekly fluctuation, although peaks did not exceed the 0.05 

probability level. The bi-weekly pattern suggests that Earth 

tides may have a subtle influence on radon activity (fig. bD). 

The use of residuals produced slightly greater peaks on the 

correlograms, but values still did not exceed the 0.05 probability level.

SEISMICITY AND RADON EMANATION

The two seismic parameters used in assessing the seismicity 

of the study area were (1) total weekly amplitude of ground 

displacement at two of the local seismographs, and (2) the 

number of significant events that occurred each week in the 

study area, as indicated in the quarterly reports of the 

'Southeast Missouri Regional Seismic Network Bulletin" 

(Stauder and others, 1977a, b, c) and the "Central Mississippi 

Valley Earthquake Bulletin" (Stauder and others, 1978). The 

significant events were those events which were strong enough 

to be detected at more than one seismograph and, thus, more 

accurately located.
196 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

AMPLITUDE OF GROUND DISPLACEMENT

A significant positive peak on the correlogram relating local 

New Madrid area values of radon concentration to the ground 

displacement at the Lone Star seismograph (near New 

Madrid) suggests that periods of increased radon emanation 

precede periods of increased ground displacement by several 

weeks (approximately 7-8 weeks, as indicated in fig. 6B). All 

correlograms relating ground displacement to radon emanation disclosed a characteristic pattern of inverse relationship at 

0 time lag, with continued decline in the coefficient to the left 

of the ordinate suggesting that a drop in radon concentration 

occurs shortly after periods of increased ground motion (figs. 

6A, B). Visual comparison of the ground displacement and 

radon time series seemed to support this relationship. The use 

of residuals did not significantly alter the correlation results.

One suggested explanation of the relationship between 

radon emanation and ground displacement may be that strain 

accrual, which precedes strain release by several weeks, may 

be effectively influencing sediments as well as crust. Perhaps 

this occurs through subtle deformation of the thick sediments 

in the study area, causing an outgassing of the sediments from 

slight shifting, compaction, or compression related to tectonic 

stresses. The drop in radon concentration values accompanying or shortly following periods of strain release (ground displacement) may thus relate to the stress drop accompanying 

release of strain. Crustal compression is the mechanism proposed by King (1978) as a possible cause of increased radon 

emanation along faults in central California. He suggested 

that compression may cause an increased rate of outgassing 

and contribute radon brought up from deeper soil gas in 

which increased radon concentration exists.

FIGURE 6. Correlograms showing results of cross-correlations between 

seismicity parameters and radon data, April-December 1977. A, Correlogram for amplitude of ground displacement at the Lone Star, Mo., 

seismograph and Missouri radon data. Amplitude is the total weekly 

amplitude of ground displacement. Missouri radon data are the weekly 

radon concentration inferred from total alpha-track count/standard 

area of 14.5 mm^, based on an average of the five Missouri stations. B, 

Correlogram for amplitude of ground displacement at the Lone Star, 

Mo., seismograph and New Madrid radon data. New Madrid radon 

data are the weekly concentration for the New Madrid, Mo., station 

only. This is the radon monitoring station closest to the Lone Star, 

Mo., seismograph. C, Correlogram for number of weekly events and 

Missouri radon data. Events are those seismic events detected at more 

than one seismograph and considered significant. D, Correlogram for 

number of weekly events and Missouri radon residual IB. Residual IB 

is the original Missouri radon data minus temperaturelike and pressure 

effects, determined through a least-squares fitting procedure.

NUMBER OF SEISMIC EVENTS

Cross-correlations indicated a positive correlation between 

radon activity and the periods of increased seismic activity (as 

defined by the increased number of weekly events) that occurred 4-5 weeks later. This positive correlation was also supported by visual analysis of the time series for radon emanation 

and seismic events. A significant positive relationship was 

observed in the majority of cross-correlations relating number 

of events to radon activity. Two graphs (figs. 6C, D) illustrate 

the significant increase in correlation which resulted when 

radon residual IB (original radon time series minus temperaturelike and pressure effects) was used.

Smoothing of the radon and events time series, by plotting 

only the periods of peak activity for both variables, resulted in 

signals that showed striking similarities when visually compared (fig. 7'A). Subsequent cross- correlation of these two 

smoothed signals (fig. IE) further substantiated the positive 

relationship found in the unsmoothed data, showing a strong 

positive correlation between radon emanation and periods of 

increased seismicity which occurred 4-5 weeks later. Of further interest on the correlogram of smoothed data is the 

periodic undulation and well-ordered decline of peaks. These 

may result from the periodic tendency exhibited by periods of 

peak seismic activity, combined (in the correlation procedure) 

with the seasonal periodic tendency exhibited by radon activity. Figure 8 shows the autocorrelation of the radon time series 

superimposed on the autocorrelation of the seismic- events 

data for comparison. The peaks on these graphs are an indication of the periodic tendencies of the two factors.

A similar mechanism to that given for the amplitude-radon 

relationship is suggested to explain the indicated positive relationship between radon emanation and ensuing seismic activi-
RADON EMANATION IN THE NEW MADRID SEISMIC ZONE 197

Smoothed Missouri 

radon emanation

Smoothed Tennessee 

radon emanation

Smoothed events

WEEK OF STUDY PERIOD

0.01 probability levels

FIGURE 7. Comparison of smoothed Missouri and Tennessee radonemanation data with smoothed seismic-events data, April 1977 to 

March 1978. A, Time-series data for smoothed Missouri and Tennessee radon concentration (inferred from alpha-track count/standard area of 14.5 mm^) and smoothed seismic events (number of 

events/week which occurred in the study area and were detected at 

more than one seismograph in the study area). Horizontal lines on time 

series are 0 mean lines for unsmoothed data. B, Correlogram for 

smoothed seismic events and smoothed radon data.

ty. Strain accrual several weeks before release of strain (in the 

form of increased number of seismic events) may impose an effect on the thick sediments in the study area through deformation of the sediments and subsequently increase the abundance of radon isotopes at the surface.

VISUAL ANALYSIS OF 

TEMPORALLY VARYING FACTORS

Viusal analysis of the time series for all factors being considered in this study revealed simultaneous sharp increases in 

several of these factors in the 37th-38th weeks, early 

December, of the study period (fig. 9).

The water levelin the Ridgely, Tenn., well exhibited the 

greatest increase in the 37th week and continued to rise significantly in the 38th week. A sharp increase in the Earth-tide excursion in the 37th week followed a 6-week low. The 

barometric-pressure excursion increased drastically to a maximum peak for the study period. Following a period of several 

weeks in which radon activity was at the lowest levels of the entire study period, sharp increases in radon concentration were

FIGURE 8. Autocorrelations of smoothed seismicity (solid line) and smoothed radon emanation (dashed line) superimposed for comparision of 

periodic tendencies exhibited by both factors.

fy -AAAAA/Vv . A V v V V V/x/v \

0 5 10 15 20 25 30 35 40 

WEEK OF STUDY PERIOD

FIGURE 9. Comparison of factors exhibiting significant increase in the 37th 

and 38th weeks (early December) of the study period, April-December 

1977. Data for each time series were normalized to 1.0 and subsequently 

centered about 0 mean by removing the long-term average from weekly 

values. Water-level data are for a well in Ridgely, Tenn., AugustDecember 1977. Earth-tide excursion is the weekly average of the total 

amplitudes of the two daily Earth-tide peaks. Barometric-pressure excursion is the weekly average of the daily pressure range recorded at 

Dyersburg, Tenn. Missouri radon emanation is the total weekly concentration inferred from alpha-track count/standard area of 14.5 mm^, 

based on a weekly average of the five Missouri stations. Seismic events 

are the number of events in the study area which are detected at more 

than one seismograph each week.
198 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

noticed at every station in the seismic zone (including wells). 

The number of seismic events during the 37th week also rose 

above the mean.

Although a causal relationship cannot be assumed from this 

one period of noticeable change in these numerous factors, the 

simultaneity of increased activity for all these factors would 

seem to be more than coincidental, and, therefore, suggestive 

of an interdependency. Perhaps the combined effects of these 

physical mechanisms served to initiate the rapid elevation of 

radon activity which occurred in the 37th week (Steele, 1978).

RADON CONCENTRATION IN WELLS

Weekly radon activity was monitored in two wells having 

bottom depths of 7 and 10 m. The radon concentration was 

measured at the bottom of the 7-m well and at a depth of 1 m 

in the 10-m well. The temporal fluctuations in radon activity 

in the wells exhibited close correspondence to the radon activity of soil gas, although the concentrations in the wells were, in 

general, slightly lower than simultaneous soil-gas concentrations.

ANALYSIS OF SPATIAL DISTRIBUTION 

OF RADON EMANATION

MONTHLY CONTOUR MAPS

Radon data from the regional network were collected monthly and plotted on maps to determine the spatial distribution 

of radon emanation. In general, individual monthly maps 

showed a characteristic pattern, with a linear zone of 

anomalous radon emanation coinciding with the seismic 

trend. This anomaly steadily diminished in size and intensity 

from June 1977 through November 1977 (figs. IQA, B). A significant resurgence was observed in February 1978 (fig. 10C). 

The anomaly then diminished again, and did not regain the 

size and intensity shown in June 1977 for the duration of the 

monthly monitoring, which continued through June 1978. 

Figure 10D illustrates the linear configuration of the main 

anomaly, which was characteristic of the radon distribution 

during the spring and early summer of 1978.

A composite of the monthly maps is exhibited in a map 

showing the 13- month average for radon concentration; a 

strong spatial correlation is shown between the region of most 

intense radon emanation and the zone of most frequent 

epicenters (fig. 11). This map further illustrates the linear 

nature of the anomalous radon distribution ( > 200) paralleling the seismic trend.

Sites of greatest radon concentration. Of the 25 stations 

exhibiting the greatest radon concentration (fig. 12), 18 were 

in the seismic zone and 5 were located near the edges of igneous intrusions which are inferred from gravity and aeromagnetic surveys of the Mississippi Ernbayment (Hildenbrand 

and others, 1977). The numbers in figure 12 indicate the 

descending order of the sites as determined from the 

13-month-average radon-concentration data. The duplication of several of the indicated order numbers resulted from 

identical 13-month-average values exhibited by a few of the 

sites.

SUGGESTED MECHANISMS INFLUENCING

SPATIAL DISTRIBUTION OF

RADON CONCENTRATION

The spatial correlation between surface radon anomalies 

and certain subsurface features, both in the present study and 

in numerous other studies, suggests that the near-surface 

radon-concentration profile is related to subsurface structures. In attempting to explain the spatial distribution of 

anomalous zones of radon emanation in the study area, the 

following mechanisms are suggested as factors that may influence radon concentrations in the near surface:

1. Faults in the subsurface may provide paths of least resistance along which gases can migrate upward with greater 

ease. Near-surface faulting has been shown to exist in the 

study area (Russ and others, 1978), and geophysical studies 

indicate the existence of faulting at depth in several locations. Other studies relating Rn-222 anomalies to faults 

have suggested migration over distances of 3 m or less 

(Israel and Bjornsson, 1967), and Gableman (1972) suggested migration from deep sources to account for Rn-222 

anomalies in soil gas over faults above a salt dome.

2. Deformation of the unconsolidated, thick sedimentary pile 

by imposed stresses may result in shifting and (or) compaction of sediments, causing outgassing and affecting a net 

upward shift of the near-surface profile of radon concentration.

3. Uranium localization in the seismic zone, due to increased 

fluid circulation facilitated by faults associated with the 

seismic zone, may be a source of radon enrichment. Accumulation of uranium and Ra-226 in the soils of fault 

zones, due to ion exchange with clays, was shown by Vogler 

(1960) to be the primary source of emanometric anomalies 

over faults.

4. Penetrative mantle convection related to suggested active 

rifting in the study area (Kumarapeli and Saull, 1966) may 

be an alternative source of radon precursors through 

crustal remobilization and subsequent injection of 

radiogenic fluids and gases into the crust beneath the embayment. Thermal gradients and crustal microfracturing 

may perhaps augment the rate of flow of such fluids and 

gases. The net effect of such upwelling may possibly have
RADON EMANATION IN THE NEW MADRID SEISMIC ZONE 199

B9°30'

Contour interval 200. 

Numbers indicate alphatrack count per standard 

area of detector.

Contour interval 100. 

Numbers indicate alphatrack count per standard 

area of detector.

MISSOURI

B Contour interval 100. 

Numbers indicate alphatrack count per standard

o

MISSOURI

Contour interval 100.

Numbers indicate alph; 

track count per standard area of detector.

FIGURE 10. Comparison of monthly radon-emanation-survey data for the New Madrid seismic zone. Radon monitoring stations (dots) and axes of 

seismic trends (dashed lines) shown. A, June 1977, contour interval, 200; B, November 1977, contour interval, 100 (the 50 isoline is also shown to illustrate the linear configuration of radon concentration at this lower level); C, February 1978, contour interval, 100; andD, Marchl978, contour interval, 100.

an ultimate influence on the surface profile of radon concentration. Long-range transport of radon itself is unlikely, but cannot be completely dismissed as a possible source. 

5. Regarding the zones of anomalous radon emanation which

appear to relate to igneous intrusive bodies in the study 

area, it is suggested that possible faulting on the periphery 

of the igneous bodies may provide zones of enrichment iu 

radon or their precursors through greater ease of emana-
200 INVESTIGATIONS OF THE NEW MADRID EARTHQUAKE REGION

V /

/KENTUCKY

 _:.

ENNESSEE

Contour interval 100. 

Numbers indicate alphandard 

rea of detector.

FIGURE 11. Thirteen-month-average radon-emanation map of the New 

Madrid seismic zone. Radon monitoring stations (dots), axes of seismic 

trends (dashed lines), and epicenters (crosses) shown. Average alphatrack count/standard area of detector isolines shown; contour interval, 

100.

tion and increased fluid migration, indirectly influencing 

the near- surface profile of radon concentration.

CONCLUSIONS

Analysis of the temporal variations in radon emanation in 

the New Madrid seismic zone suggests that radon activity in 

the soil gas is controlled by several physical mechanisms. The 

influences of barometric-pressure variations, water levels, and 

Earth tides were indicated, and a seasonal change in radon 

concentration was observed. However, the effects of these factors did not obscure an evident relationship between radon 

emanation and seismicity.

The fact that anomalous radon emanation was observed, 

which appeared to relate to ensuing seismicity of relatively low 

magnitudes (less than 3.0 in general), leads to the speculation 

that a proportionally larger increase in radon concentration 

might be expected to precede larger events. Continued monitoring of a few areas directly in the seismic zone could provide 

evidence to support this hypothesis (Steele, 1978).

Spatially, zones of anomalously high radon concentration 

coincide with the seismic zone and may result from a net upward shift of the near-surface profile of radon concentration, 

influenced by larger scale mechanisms originating at greater

depths. Some of these larger scale mechanisms may be (1) 

enrichment in radon or their precursors in subsurface faults 

due to increased fluid circulation, (2) greater ease of emanation of gases along faults (this may be particularly significant 

where faults approach the surface in the study area), (3) 

outgassing of sediments in response to deformation by tectonic 

stresses, and (4) continuous generation of radiogenic fluids 

and gases derived from crustal remobilization due to mantle 

convection and associated rifting.

Zones of high radon concentration which appear to correspond to the locations of igneous intrusives may result from 

uranium enrichment in the faulted peripheral zones of the igneous bodies or increased emanation of gases along faults 

related to these intrusive bodies. Such mechanisms may affect 

the near-surface profile of radon concentration.

REFERENCES CITED

Buschbach, T. C., 1977, Geologic setting of the New Madrid area: 

American Geophysical Union Third Annual Midwest Meeting, Program and Abstracts, Purdue University, W. Lafayette, Indiana, abs. 

T12.

Clements, W. E., and Wilkening, M. H., 1974, Atmospheric pressure effects 

on radon-222 transport across the earth-air interface. Journal of Geophysical Research, v. 79, no. 33, p. 5025-5029.

90°00' 89°30'

FIGURE 12. Locations of 25 sites of greatest radon concentration. Axes of 

seismic trends (dashed lines) and igneous intrusives (triangles) shown. 

Numbers represent the descending order of the sites of greatest radon 

emanation based on the 13-month averages for all stations in the networks. Duplication of several of the nummbers resulted from identical 

13-month-average values exhibited by some of the sites.
RADON EMANATION IN THE NEW MADRID SEISMIC ZONE 201

Davis J. C., 1973, Statistics and data analysis in geology: New York, John

Wiley and Sons, Inc.. 543 p. 

Gableman, J. W., 1972, Radon emanometry of Starks Salt Dome, Calcasiey

Parish, Louisiana: U.S. Atomic Energy Commission Report RME 414,

75 p. 

Guedalia, D., Laurent, J. L., Fontan, J., Blanc, D., andDruilhet, A., 1970,

A study of radon-220 emanation from soils: Journal of Geophysical

Research, v. 75, no.2, p. 357-369. 

Hildenbrand, T. G., Kane, M. F., and Stauder, William, 1977, Magnetic

and gravity anomalies in the northern Mississippi Embayment and their

spatial relation to seismicity: U.S. Geological Survey Miscellaneous

Field Studies Map MF-914, scale 1:1,000,000. 

Israel, H., and Bjornsson, S., 1967, Radon (Rn 222) and thoron (Rn 220)

in soil air over faults: Zeitschrift fur Geophysik, v. 33, no. 1, p. 44-64. 

King, C. Y., 1977, Temporal variations in radon emanation along active

faults [abs.]: EOS (American geophysical Union Transactions), v. 58,

no. 6, p. 434. 

______1978, Radon emanation on the San Andreas fault: Earthquake

Information Bulletin, v. 10, no. 4, p. 136-138. 

Kumarapeli, P. S., and Saull, V. A., 1966, The St. Lawrence valley system,

A North American equivalent of the East African rift valley system:

Canadian Journal of Earth Science, v. 3, p. 639 658.

Robinson, E. A., and Treitel, S., 1967, Principles of digital Wiener filtering: Geophysical Prospecting, v. 15, no. 3, p. 311 353. 

Russ, D. P., Stearns, R. G., and Herd, D. G., 1978, Map of exploratory

trench across Reelfoot scarp, northwestern Tennessee: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-985.

Stauder, William, Herrmann, R., Cheng, SI, Nicholson, C., Singh, S., 

Woods, M., Chen, W., and Morrissey, S. T., 1977, Southeast Missouri 

Regional Seismic Network quarterly bulletin: St. Louis, Mo., Saint 

Louis University, Department of Earth and Atmospheric Sciences, no. 

13, 26 p.

Stauder, William, Herrmann, R., Nicholson, c., Cheng, S., and Morrissey, 

S. T., 1977, Southeast Missouri Regional Seismic Network quarterly 

bulletin: St. Louis, Mo., Saint Louis University, Department of Earth 

and Atmospheric Sciences, no. 12, 24 p.

Stauder, William, Herrmann, R., Nicholson, C., Singh, S., Woods, M., 

Haug, E., and Morrissey, S. T., 1978, Southeast Missouri Regional 

Seismic Network quarterly bulletin: St. Louis, Mo., Saint Louis University, Department of Earth and atmospheric Sciences, no. 14, 24 p.

Stauder, William, Herrmann, R., Nicholson, C., Singh, S., Woods, M., 

Haug, E., and Morrissey, S. T., 1978, Central Mississippi valley earthquake bulletin, quarterly report: St. Louis, Mo., Saint Louis University, Department of Earth and Atmospheric Sciences, no. 15, 28 p.

Steele, S. R., 1978, Spatial and temporal radon variation in the New Madrid 

seismic zone: Southern Illinois University M.S. thesis, 112 p.

Tanner,, A. B., 1978, Radon migration in the ground- A supplementary 

review: U.S. Geological Survey Open-File Report 78 1050, 61 p.

Vogler, G., 1960, Origins of emanometric anomalies; Zeitschrift fur 

Geophysik, v. 26, no. 2, p. 57-71.

U.S. GOVERNMENT PRINTING OFFICE: 676-041/30 1