Introduction—Investigations of the 

New Madrid Seismic Zone

Summary and Discussion of

Crustal Stress Data in the Region of the

New Madrid Seismic Zone

Preliminary Seismic Reflection Study of 

Crowley's Ridge, Northeast Arkansas

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-A-C

MISSOURI

•Hk" I*. •"'Mk

ARKANSAS I*
Cover. Part of a gray, shaded-relief, reduced-to-pole magnetic anomaly map. Map area 

includes parts of Missouri, Illinois, Indiana, Kentucky, Tennessee, and Arkansas. 

Illumination is from the west. Figure is from Geophysical setting of the Reelfoot rift and 

relations between rift structures and the New Madrid seismic zone, by Thomas G. 

Hildenbrand and John D. Hendricks (chapter E in this series).
Investigations of the New Madrid Seismic Zone

Edited by Kaye M. Shedlock and Arch C. Johnston

A. Introduction—Investigations of the New Madrid Seismic Zone 

By Kaye M. Shedlock and Arch C. Johnston

B. Summary and Discussion of Crustal Stress Data in the Region of the 

New Madrid Seismic Zone 

By W. L. Ellis

C. Preliminary Seismic Reflection Study of Crowley's Ridge, 

Northeast Arkansas

By Roy B. VanArsdale, Robert A. Williams, Eugene S. Schweig III, 

Kaye M. Shedlock, Lisa R. Kanter, and Kenneth W. King

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-A-C

Chapters A, B, and C are issued as a single volume 

and are not available separately

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Robert M. Hirsch, Acting Director

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication July 22,1993 

Edited by Richard W. Scott, Jr. 

Graphics prepared by Wayne Hawkins 

Photocomposition by Shelly A. Fields

For Sale by U.S. Geological Survey, Map Distribution

Box 25286, MS 306, Federal Center

Denver, CO 80225

Any use of trade, product, or firm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-ln-Publication Data

Introduction—investigations of the New Madrid seismic zone / by Kaye M. Shedlock and 

Arch C. Johnston. Summary and discussion of crustal stress data in the region of the New 

Madrid seismic zone / by W.L. Ellis. Preliminary seismic reflection study of Crowley's 

Ridge, northeast Arkansas / by Roy B. VanArsdale... [et al.].

p. cm.—(Investigations of the New Madrid seismic zone: A-C) (U.S. Geological 

Survey professional paper; 1538)

Includes bibliographical references.

Supt.ofDocs.no.: I 19.16: 1538A-C

1. Seismology—Missouri—New Madrid Region. I. Title: Summary and discussion 

of crustal stress data in the region of the New Madrid seismic zone. 

II. Title: Preliminary seismic reflection study of Crowley's Ridge, northeast 

Arkansas. III. Series. IV. Series: U.S. Geological Survey professional paper; 1538. 

QE535.2.U6I59 1994 vol. A-C

551.2'2'09788985—dc20 93-39222 

[551.2'2'09788985] CIP
Introduction—Investigations of the 

New Madrid Seismic Zone

By Kaye M. Shedlock and Arch C. Johnston

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-A

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
CONTENTS

Tectonic Framework Studies............................................................................................... A3

Seismicity and Deformation Monitoring and Modeling...................................................... 4

Improved Seismic Hazard and Risk Assessments............................................................... 4

Cooperative Hazard Mitigation Studies............................................................................... 5

Summary.............................................................................................................................. 5

References Cited.................................................................................................................. 5

FIGURE

1. Index map showing the Reelfoot rift, plutons, the Blytheville arch, and epicenters in the New Madrid seismic

zone............................................................................................................................................................................ A2

IV
INTRODUCTION—INVESTIGATIONS OF THE 

NEW MADRID SEISMIC ZONE

By Kaye M. Shedlock1 and Arch C. Johnston2

The Mississippi Embayment, beginning near the Gulf 

of Mexico and extending north to the confluence of the Ohio 

and Mississippi Rivers, is surrounded by the Illinois Basin to 

the north, the Nashville dome and southern Appalachian Plateau to the east, and the Ouachita and Ozark uplifts to the 

west. The embayment contains the broad, flat Mississippi 

River flood plain that is incised into the gently rolling hills 

of the eastern embayment. Unconsolidated sediments, thin in 

the north but more than 1 km thick in the south, fill the 

embayment, an area of rich farmland. The seemingly infinite 

expanse of bucolic terrain belies the fact that three of the 

largest known intraplate earthquakes in the world occurred 

in the northern Mississippi Embayment in a 54-day period 

during the winter of 1811-12.

Geologically, the embayment is a reentrant into the 

North American craton that exhibits many subsurface geophysical characteristics of a failed triple junction (Ginzburg 

and others, 1983). A remnant of a Precambrian upper mantle 

plume is represented by a relatively high (7.3 km/sec) velocity layer in the lower crust (30-40 km deep). The most prominent buried structure in the northern Mississippi 

Embayment is the Reelfoot rift, originally defined using 

magnetic data (Hildenbrand and others, 1982; Hildenbrand 

and Hendricks, this volume). The Reelfoot rift, about 300 km 

long and 70 km wide, has 1.6-2.6 km of structural relief on 

the magnetic basement (fig. 1). The rift is roughly bounded 

along the northwest side by a series of buried plutons; there 

is a small buried pluton on the southeast rift margin and a 

larger pluton buried in the northern end of the rift. The subsurface Blytheville arch, a 10-15-km-wide and 110-km-long 

zone of arched strata and complex faulting (Hamilton and 

McKeown, 1988), trends northeast in the center of the rift 

(fig. 1). The New Madrid seismic zone (NMSZ), a clustered 

pattern of earthquake epicenters between 5 and 15 km deep, 

lies mostly within the Reelfoot rift (fig. 1). The NMSZ may

1 U.S. Geological Survey, Mail Stop 966, P.O. Box 25046, Denver 

Federal Center, Lakewood, CO 80225.

2 Center for Earthquake Research and Information (CERI), Memphis 

State University, Memphis, TN 38152.

be separated geographically into three major trends: a northeast-trending zone from Marked Tree, Ark., to Caruthersville, Mo. (ARK); a north-northwest-trending central zone of 

earthquakes between Ridgely, Term., and New Madrid, Mo. 

(CEN); and a northeast-trending zone (DWM) from New 

Madrid to Charleston, Mo. (Himes, and others, 1988). The 

large 1811-12 New Madrid earthquakes occurred within the 

NMSZ, although the exact epicenters are unknown and are 

only inferred from intensity data (fig. 1).

Prior to the 1990's, studies of the Mississippi Embayment and the NMSZ had proceeded largely independent of 

one another. The Precambrian through early Tertiary 

(600-30 Ma) formation and evolution of the Reelfoot rift, 

the Blytheville arch, the plutons, and the embayment were 

thoroughly mapped and understood to first order. The buried 

structures, all lying between about 1 km and 6 km deep, and 

the deformation associated with them tantalizingly overlay 

the diffusely linear trends of seismicity. By the mid-1980's, 

a wealth of information and interpretation of the region had 

been published, most notably U.S. Geological Survey Professional Paper 1236, "Investigations of the New Madrid, 

Missouri, Earthquake Region" (McKeown and Pakiser, eds., 

1982), and U.S. Geological Survey Open-File Report 

84-770, "Proceedings of the Symposium on 'The New 

Madrid Seismic Zone'" (Gori and Hays, eds., 1984). 

Researchers noted spatial associations between the axial 

trend of seismicity (between Marked Tree, Ark., and Caruthersville, Mo.) and the Blytheville arch (Zoback and others, 

1980; Hamilton and Zoback, 1982; Howe and Thompson, 

1984; Crone and others, 1985), as well as the north-northwest-trending central zone of earthquakes and a zone of 

localized uplift (Russ, 1982). Regional seismic monitoring, 

begun in 1974, resulted in studies of spatial relationships of 

earthquake hypocenters and fault-plane solutions that 

allowed for the interpretation of the ARK and DWM trends 

as nearly vertical strike-slip zones of faulting (Herrmann and 

Canas, 1978; Himes and others, 1988). But the physical relationships between the major subsurface structures and the 

historic and contemporary seismicity remained enigmatic.

Increasing national awareness and concern about the 

hazards and risks of earthquakes, dramatically emphasized

Al
A2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

50 KILOMETERS

Figure 1. Index map showing the Reelfoot rift, plutons, the Blytheville arch, and epicenters (+) in the New Madrid seismic 

zone (NMSZ). Epicentral locations for the large 1811-12 earthquakes (circles) are estimated from intensity data. Figure is 

modified from Luzietti and others (this volume).

by the October 17,1989, Loma Prieta, Calif., earthquake, led funds to begin implementation of the plan. Thus, in 1990, the

the U.S. Congress to direct the U.S. Geological Survey NMSZ/Central United States became the first seismically

(USGS) to prepare a plan for intensified study of the NMSZ active region east of the Rocky Mountains to be designated

(Hamilton and Johnston, eds., 1990) and to appropriate a priority research area within the National Earthquake
INTRODUCTION A3

Hazards Reduction Program (NEHRP). This USGS Professional Paper is a collection of papers presenting the newly 

intensified research program directed at the NMSZ/Central 

United States. Major components of this research program 

are:

• Tectonic framework studies.—The integration of geologic and geophysical studies to better understand the 

tectonic regime of the intraplate NMSZ, to define the 

geometry of the seismogenic structures in the NMSZ, 

and to determine the spatial, temporal, and source 

characteristics of damaging earthquakes in the Central 

United States.

• Seismicity and deformation monitoring and modeling.—The establishment of a modern seismic network 

in the NMSZ to both monitor the earthquakes and determine the characteristics of the ground motions that 

they generate, to establish geodetic baselines and continue to monitor crustal deformation in the region, and 

to model the seismicity and deformation.

• Improved seismic hazard and risk assessments.—The 

delineation of seismic sources, the identification of areas expected to experience strong ground shaking and 

(or) amplification from earthquakes, the identification 

of areas subject to ground failure (including liquefaction), and loss-estimation studies^

• Cooperative hazard mitigation studies.—The implementation of earthquake-hazard mitigation measures 

and of preparedness and response programs. 

The remainder of this paper briefly describes subsequent chapters of this Professional Paper within the conceptual framework of the revitalized NMSZ/Central United 

States research program.

TECTONIC FRAMEWORK STUDIES

Mapping of the Reelfoot rift and associated buried 

structures that was published during the late 1970's and 

1980's (Hildenbrand and others, 1977,1982; Braile and others, 1982a, 1982b) provided the broad tectonic framework of 

the NMSZ. Additional high-quality potential field, seismic 

refraction, and seismic reflection data, utilizing a decade of 

technological advances, have been collected.

Aeromagnetic data collected at lower altitudes and 

tighter grid spacing provide evidence that suggests the linear 

seismic trends of the NMSZ follow preferred directions of 

strain related to intrusions and older faults (Hildenbrand and 

Hendricks, this volume). Hildenbrand and Hendricks also 

propose that the Reelfoot rift extends southwest from western Kentucky to east-central Arkansas, where it is abruptly 

terminated at the Arkansas transform fault.

The northern extent of the Reelfoot rift is controversial. 

Hildenbrand and Hendricks argue that the rift, a large tectonic system with many related structures including an 

asymmetric graben, bends eastward and merges with the 

Rough Creek graben in western Kentucky. Potter and others

(this volume) examine seismic reflection data in the southern 

Illinois region where the Reelfoot rift and Rough Creek graben may intersect. Potter and others note that the smaller 

asymmetric graben formation in this region differs from the 

axially symmetric Reelfoot rift. Ren6 and Stanonis (this volume) present shallow reflection seismic profiles of the 

Wabash Valley fault system in southern Indiana and Illinois, 

just north of the Rough Creek graben. They interpret listric 

normal faulting throughout this system, again differing in 

style of faulting from the Reelfoot rift.

The first magnetotelluric (MT) surveys of the Reelfoot 

rift reveal an axial resistivity high near the Blytheville arch 

that may be a previously unrecognized horst (Rodriguez and 

Stanley, this volume). Gravity data collected over the Mississippi Embayment allow Langenheim (this volume) to 

infer that faulting in the region is not characterized by major 

vertical offsets.

Summaries of crustal-stress data in the New Madrid 

seismic zone (Ellis, this volume) and fluid pressure in the 

shallow crust (McKeown and Diehl, this volume) provide a 

wealth of information on the in situ stress and pore pressure 

in the shallow crust surrounding and overlying the NMSZ.

Five papers use reflection data to examine confirmed or 

suspected fault systems within the Mississippi Embayment. 

Both deep and shallow seismic reflection data were collected 

in the Crittenden County fault zone (CCFZ), a reactivated 

thrust fault system in the southeastern Reelfoot rift margin. 

Crone and Pratt (this volume) examine purchased proprietary seismic reflection data and interpret northwest-dipping 

thrust or oblique-slip faulting in Precambrian through Cretaceous reflectors. Luzietti and others (this volume) use highresolution Mini-Sosie data to demonstrate that faulting in the 

CCFZ extends from Cretaceous depths up to at least the 

Eocene-Quaternary unconformity. Taken together, these 

papers demonstrate that at least part of the southeastern 

Reelfoot rift boundary has been active since at least the Cretaceous and may have moved during the Quaternary, in particular during the Holocene.

The Bootheel lineament, first recognized from satellite 

imagery (Schweig and Marple, 1991), trends north-northeastward for about 135 km between Marked Tree, Ark., and 

New Madrid, Mo. The surficial morphological expression of 

the lineament includes linear bodies of liquefied sand, shallow depressions, and small scarps (Schweig and others, this 

volume). High-resolution Mini-Sosie reflection data collected across this lineament image subtle disruption and gentie warping in Paleozoic through Tertiary layers, interpreted 

as numerous "flower structures" (Sexton and others, this volume; Schweig and others, this volume). These data are consistent with, but not unambiguously demonstrative of, 

faulting and deformation in shallow sediments overlying a 

strike-slip fault.

High-resolution Mini-Sosie seismic reflection data 

were also collected across the flanks of the most prominent 

landform in the Mississippi Embayment, Crowley's Ridge 

(VanArsdale and others, this volume). These data image
A4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

ridge-margin faults from Paleozoic through at least Eocene 

sediments, leading to the interpretation that the topographic 

relief of Crowley's Ridge may be fault controlled rather than 

an erosional feature.

SEISMICITY AND DEFORMATION 

MONITORING AND MODELING

Seismicity in the NMSZ has been monitored since 1974 

by the Central Mississippi Valley Seismic Network 

(CMVSN), sponsored by the USGS and operated originally 

by Saint Louis University. In the late 1970's, the Nuclear 

Regulatory Commission joined the USGS as sponsor, and 

the CMVSN was extended, station density was increased, 

and Memphis State University joined Saint Louis University 

as a cooperator. The combined networks consisted of 

approximately 50 short-period vertical and four 3-component permanent seismograph stations. In 1990, Saint Louis 

and Memphis State Universities, with USGS sponsorship, 

began a multiyear effort to upgrade the networks to a single, 

digital, satellite-telemetered seismic network with at least 40 

3-component stations, named the Cooperative New Madrid 

Seismic Network (CNMSN).

Beginning in October 1989, the 3-component stations 

of the Portable Array for Numerical Data Acquisition 

(PANDA) were deployed in the central NMSZ. PANDA 

station spacing varied between 2 and 27 km, averaging about 

12 km overall and about 7 km in the CEN trend of seismicity 

(Chiu and others, this volume). The PANDA data have 

resulted in more tightly constrained hypocentral locations 

and focal mechanisms than are possible with the permanent 

CNMSN. Using these data, Chiu and others have resolved 

planar concentrations of hypocenters in the CEN trend that 

they interpret as southwest-dipping active faults. The surface projections of these inferred faults coincide with the 

eastern margin of the Lake County uplift (Russ, 1982), providing the first direct relationship between active faulting 

and surface-deformational features of the NMSZ.

Ellis and others (this volume) evaluate the historical 

and modern New Madrid region seismic catalog (1800 to 

1990) for completeness and annual periodicity. They find no 

unequivocal evidence of departures from a random distribution of earthquakes at annual periods, suggesting that any 

modulation of seismic energy release by seasonal influences 

such as ground water/pore pressure changes is slight to nonexistent.

Gomberg (this volume) employs 2-D boundary-element modeling of strain fields to try to replicate observed 

surface deformation associated with the 1811-12 earthquakes in order to place limits on the actual faulting process 

of this sequence. Although many simplifying assumptions 

are necessary in this modeling technique, Gomberg matches 

the observed 1811-12 subsidence of Reelfoot Lake and the 

St. Francis sunklands and the Lake County uplift with just

right-lateral strike-slip motion along the ARK and DWM 

seismicity trends.

Two studies have begun to address spatial and temporal 

variation in upper crustal strain rates in the NMSZ. Rates 

and spatial distribution of strain can be used to infer faultslip rates and constrain the mechanics of current tectonic 

deformation. Spatial and temporal strain patterns can be 

used to assess the earthquake hazard of faults that do not 

have surface expression. The lack of surface expression of 

faulting in intraplate regions means that defining active 

source zones may require a combination of seismic, structural, and geodetic information.

Liu and others (this volume) use Global Position System (GPS) methods to resurvey an existing triangulation network in the southern NMSZ near Caruthersville, Mo. They 

find a 0.1 ustrain/yr shear-strain rate for the period 1950 to 

1990, roughly one-third of the average strain rate in the San 

Andreas fault system. Visco-elastic modeling of post1811-12 strain appears to rule out post-1811-12 seismic 

relaxation as an explanation for this current strain accumulation. Liu and others note that the orientation and sense of 

shear is consistent with right-lateral strike-slip motion along 

a buried northeast-trending fault zone.

Snay and others (this volume) combine data from a 

1991 GPS survey with pre-existing GPS and triangulationtrilateration data to determine strain rates across the northern 

NMSZ, near the town of New Madrid, Mo. They find zero 

strain (at the 95 percent confidence level) across this region, 

in marked difference to the 0.1 (astrain/yr in the south. Liu 

and others (this volume) explain this difference as spatial 

variation due to the finite geometric length and complex 

seismicity of the NMSZ. Liu and others also note that the 

strain rate must vary temporally as well, pointing to the lack 

of cumulative surface expression that would have to exist if 

the 0.1 ustrain/yr persisted through geologic time.

IMPROVED SEISMIC HAZARD AND 

RISK ASSESSMENTS

Many of the studies previously discussed, in particular 

the seismic reflection surveys, the geodetic surveys, and the 

deformation modeling, also are key contributors to improved 

seismic hazard and risk assessments. The reflection studies 

help delineate faults and near-surface structure. The geodetic, deformation, and seismic monitoring studies help 

determine rates of occurrence of earthquakes and, hence, the 

rates of occurrence of hazards and risks associated with 

earthquakes.

Jibson and Keefer (this volume) examine possible trigger mechanisms for landslides along the eastern bluffs of the 

Mississippi River. They convincingly conclude that most of 

the currently visible bluff slides were triggered by the 

1811-12 earthquakes rather than any anomalous groundwater conditions.
INTRODUCTION A5

Paleoliquefaction studies (investigations of preserved 

liquefaction features) provide evidence for the occurrence of 

large earthquakes and can provide constraints of dates of 

occurrence and magnitude estimates. Wesnousky and Leffler (this volume) examined drainage ditches in the southern 

NMSZ. They noted pervasive liquefaction evidence for the 

1811-12 earthquakes but no evidence for widespread prehistoric liquefaction events for the previous 5,000 to 10,000 

years.

In contrast, Vaughn (this volume) presents data that he 

interprets as up to four episodes of liquefaction prior to 

1811-12 in the lowlands west of Crowley's Ridge. Vaughn 

cannot differentiate whether local Reelfoot rift (magnitude < 

6) or distant NMSZ (magnitude > 7) earthquakes generated 

the apparent liquefaction. Both of these paleoliquefaction 

studies establish the need for similar studies throughout the 

region.

In one of the more intriguing reports of this volume, 

measurements of seismic attenuation (0 by Catchings and 

Mooney indicate that the seismogenic crust in the New 

Madrid region attenuates seismic energy only about 25 percent as effectively as the crust in the Western United States. 

Therefore, damaging seismic wave amplitudes will travel 

much farther in the Central United States. Although this difference in Q is cause enough to worry, it may not be the 

major contributor to damage in several areas of the Central 

United States. Amplitude-distance curves show that seismic 

focusing due to crustal and Moho reflections occur near 

Memphis, Tenn., and St. Louis, Mo. This focused energy 

apparently increases seismic amplitudes by more than 1,000 

percent over background amplitudes at these sites. This 

increase in amplitude is roughly equivalent to an increase of 

at least three Modified Mercalli intensity units. Thus, Memphis and St. Louis will likely experience unusually high 

amplified ground motions during future large earthquakes.

Equally as important as determining where amplified 

ground motions are likely to occur is determining what 

man-made structures may be affected. In a series of maps, 

developed using new Geographic Information Systems 

(GIS) technology, Wheeler and others (this volume) 

present elements of the infrastructure in the Central United 

States and the seismic hazards associated with them. 

Their seismotectonic maps include surface faulting, 

ground shaking, landslides, liquefaction hazards, and 

deformation. Their infrastructure maps include lifelines, 

population centers, and critical facilities.

personnel to devise earthquake mitigation and preparedness 

programs. Olshansky (this volume) examines the potential 

for State-level seismic-hazard-mitigation policies in those 

States containing and surrounding the NMSZ. He examines 

both existing programs and the potential for new seismichazard-mitigation programs in seven States. He concludes 

with an 11-point strategy for State action, including major 

and minor legislative efforts, for seismic hazard mitigation.

SUMMARY

These Professional Paper chapters provide a wealth of 

new information about the hazards and risks associated with 

earthquakes in the NMSZ. The tectonic framework studies 

have provided a detailed subsurface "snapshot" of the Reelfoot rift and major internal and surrounding structures. For 

the first time ever in the Central United States, seismologic, 

deformation, and framework research results are being 

linked to explain historic and contemporary seismicity. Subsurface planar structures outlined by earthquake hypocenters 

project to observed surface deformation. The antithetic reactivation of at least one lesser Reelfoot rift boundary fault, 

through the Tertiary up until possibly as recently as the 

Holocene, has been unambiguously demonstrated. Near-surface faulting has been imaged along the boundaries of Crowley's Ridge and above the enigmatic Bootheel lineament. 

Regions of amplified ground motion potential have been 

mapped.

All of these studies represent significant progress in our 

ability to accurately assess the seismic hazards and associated risk in the NMSZ and Central United States. But a great 

deal of work remains to be done. For example, is the Crittenden County fault zone the only reactivated portion of the 

Reelfoot rift boundary? Is the Bootheel lineament actually a 

buried fault? What is the relationship between contemporary 

NMSZ seismicity and the 1811-12 earthquakes? What are 

the rate and spatial distribution of strain accumulation/relaxation throughout the NMSZ? Are there paleo-indicators that 

can unambiguously constrain earthquake recurrence and 

magnitude estimates? All of these questions and more need 

to be answered in order for us to understand the configuration and seismic potential of the NMSZ. These current 

studies provide a sound basis for approaching the remaining 

problems of understanding earthquake hazards and risk in 

the NMSZ for the 1990's and beyond.

COOPERATIVE HAZARD 

MITIGATION STUDIES

GIS-based hazard and infrastructure maps (Wheeler 

and others, this volume) contribute information necessary 

for city, county, and State planners and emergency-response

REFERENCES CITED

Braile, L.W., Hinze, W.J., Keller, G.R., and Lidiak, E.G., 1982a, 

The northeastern extension of the New Madrid seismic zone, in 

McKeown, F.A., and Pakiser, L.C., eds., Investigations of the 

New Madrid, Missouri, Earthquake Region: U.S. Geological 

Survey Professional Paper 1236, p. 175-184.
A6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Braile, L.W., Keller, G.R., Hinze, W.J., and Lidiak, E.G., 1982b, 

An ancient rift complex and its relation to contemporary seismicity in the New Madrid seismic zone: Tectonics, v. 1, 

p. 225-237.

Crone, A.J., McKeown, F.A., Harding, ST., Hamilton, R.M., Russ, 

D.P., and Zoback, M.D., 1985, Structure of the New Madrid 

seismic source zone in southeastern Missouri and northeastern 

Arkansas: Geology, v. 13, p. 547-550.

Ginzburg, A., Mooney, W.D., Walter, A.W., Lutter, W.J., and 

Healy, J.H., 1983, Deep structure of the northern Mississippi 

Embayment: American Association of Petroleum Geophysicists Bulletin, v. 67, no. 11, p. 2031-2046.

Gori, P.L., and Hays, W.W., 1984, Proceedings of the symposium 

on "The New Madrid seismic zone," O.W. Nuttli, convener 

and organizer: U.S. Geological Survey Open-File Report 

84-770,468 p.

Hamilton, R.M., and Johnston, AC., eds., 1990, Tecumseh's 

Prophecy—Preparing for the next New Madrid earthquake: 

U.S. Geological Survey Circular 1066,30 p.

Hamilton, R.M., and McKeown, F.A., 1988, Structure of the 

Blytheville arch in the New Madrid seismic zone: Seismological Research Letters, v. 59, no. 4, p. 117-121.

Hamilton, R.M., and Zoback, M.D., 1982, Tectonic features of the 

New Madrid seismic zone from seismic reflection profiles, in 

McKeown, F.A., and Pakiser, L.C., eds., Investigations of the 

New Madrid, Missouri, Earthquake Region: U.S. Geological 

Survey Professional Paper 1236, p. 55-82.

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, p. 1095-1102.

Hildenbrand, T.G., Kane, M.F., and Hendricks, J.D., 1982, Magnetic basement in the upper Mississippi Embayment region—

A preliminary report, in McKeown, F.A., and Pakiser, L.C., 

eds., Investigations of the New Madrid, Missouri, Earthquake 

Region: U.S. Geological Survey Professional Paper 1236, 

p. 39-53.

Hildenbrand, T.G., Kane, M.F., and Stauder, W., 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.

Himes, L., Stauder, W., and Herrmann, R.B., 1988, Indication of 

active faults in the New Madrid seismic zone from precise 

location of hypocenters: Seismological Research Letters, v. 59, 

no. 4, p. 123-131.

Howe, J.R., and Thompson, T.L., 1984, Tectonics, sedimentation, 

and hydrocarbon potential of the Reelfoot rift: Oil and Gas 

Journal, Nov. 12, p. 179-190.

McKeown, F.A., Pakiser, L.C., eds., 1982, Investigations of the 

New Madrid, Missouri, earthquake region: U.S. Geological 

Survey Professional Paper 1236, 201 p.

Russ, D.P., 1982, Style and significance of surface deformation in 

the vicinity of New Madrid, Missouri, in McKeown, F.A., and 

Pakiser, L.C., eds., Investigations of the New Madrid, Missouri, Earthquake Region: U.S. Geological Survey Professional 

Paper 1236, p. 95-114.

Schweig, E.S., III, and Marple, R.T., 1991, The Bootheel lineament—A possible coseismic fault of the great New Madrid 

earthquakes: Geology, v. 19, p. 1025-1028.

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.
Summary and Discussion of Crustal Stress Data 

in the Region of the New Madrid Seismic Zone

By W. L. Ellis

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-B

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
CONTENTS

Abstract................................................................................................................................ Bl

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

Types of Data....................................................................................................................... 3

Focal Mechanisms..................................................................................................... 3

Borehole Breakouts................................................................................................... 3

Shallow Stress Measurements................................................................................... 4

Data Quality......................................................................................................................... 5

Data Summary..................................................................................................................... 5

Discussion............................................................................................................................ 7

Stress Directions........................................................................................................ 7

Stress Regimes........................................................................................................... 10

Stress Distribution with Depth................................................................................... 11

Sununary.............................................................................................................................. 11

References Cited.................................................................................................................. 12

FIGURES

1. Map of New Madrid seismic zone region showing the distribution of contemporary seismicity, major tectonic

features, and inferred orientation of the maximum horizontal stress for all data....................................................... B2

2. Map of New Madrid seismic zone region showing the distribution of contemporary seismicity, major tectonic

features, and inferred orientation of the maximum horizontal stress for A- and B-quality data................................ 7

3. Map of region around the New Madrid seismic zone showing compiled earthquake focal-mechanism diagrams.... 8

4. Plot of maximum horizontal stress orientations.......................................................................................................... 9

5. Plot of maximum horizontal stress azimuth versus depth.......................................................................................... 11

TABLES

1. Quality-ranking criteria for stress data....................................................................................................................... B4

2. Maximum horizontal stress azimuths from earthquake focal-mechanism data.......................................................... 5

3. Summary of stress determination and borehole-breakout data................................................................................... 6
SUMMARY AND DISCUSSION OF

CRUSTAL STRESS DATA IN THE REGION OF

THE NEW MADRID SEISMIC ZONE

ByW.L. Ellis 1

ABSTRACT

A compilation of published stress indicators and stressmeasurement data from within and near the New Madrid 

seismic zone (NMSZ) provides some insights into the possible distribution of contemporary crustal stresses in the 

region. Most of the data are consistent with the east-west to 

east-northeast-west-southwest direction of maximum horizontal compression that is characteristic of the Midcontinent 

of the United States, but variations in horizontal stress orientation apparently exist near the left-stepping offset in the linear northeast-southwest trend of the NMSZ. Such variations 

could indicate a locally complex stress distribution associated with the termination or intersection of seismogenic 

structures. The data also indicate possible areal variations in 

stress regime in the region surrounding the NMSZ. In southern Illinois, two strike-slip-faulting and one deeper thrustfaulting earthquake focal-mechanism solutions suggest that 

the least horizontal stress is about equal to, and sometimes 

locally exceeds, the vertical stress. This inference is consistent with results of shallow stress measurements and the 

presence of small north-south-trending thrust faults in coal 

seams and adjacent strata of Pennsylvanian age in southern 

Illinois, southwestern Indiana, and western Kentucky. 

Nearer the NMSZ, focal mechanisms thus far indicate predominantly strike-slip faulting, with the only inferred thrust 

faulting being along the left-stepping offset in the northeasttrending epicentral alignment. Two normal-faulting focal 

mechanisms, 125 km northwest of the NMSZ in the St. Francois Mountains area, indicate extensional stress in that 

region. Areal variations in stress regime could indicate broad 

crustal deformation such as lithospheric flexure, in which an 

alternating pattern of compressional and extensional stresses 

would be expected.

1 U.S. Geological Survey, Mail Stop 966, P.O. Box 25046, Denver 

Federal Center, Lakewood, CO 80225.

The published stress data from this region also show 

that horizontal stress orientations are generally uniform over 

a wide depth range at many localities. In southern Illinois, 

shallow stress measurements, borehole breakouts, and earthquake focal mechanisms all provide generally consistent 

estimates for the maximum horizontal stress direction, suggesting that relatively inexpensive shallow stress measurements might be used to better define the lateral distribution 

of stress orientations over a broad region surrounding the 

NMSZ. Mapping of horizontal stress trajectories might contribute to a better understanding of the nature of contemporary crustal deformation and to an evaluation of potential 

seismic hazards from other faults in the region.

INTRODUCTION

The New Madrid seismic zone, located in the northern 

Mississippi Embayment (fig. 1), is the site of the great New 

Madrid earthquakes of 1811-12, the largest earthquakes 

known to have occurred in the United States east of the 

Rocky Mountains in historic time. The area remains seismically active and is the focus of ongoing research studies to 

better understand and define the regional seismic hazard. An 

improved knowledge of the distribution of crustal stresses in 

the vicinity of the NMSZ has been identified as an important 

step to understanding the physical mechanisms responsible 

for the seismicity of the region (Hamilton and Johnston, 

1990). The vertical and lateral distribution of stress magnitudes and areal variations in horizontal stress trajectories 

may offer clues to the nature of crustal deformation occurring at seismogenic depths, and detailed knowledge of the in 

situ stresses may be helpful in identifying and evaluating 

potential seismic hazards from other faults in the region.

Published information regarding crustal stresses in the 

region of the NMSZ is derived from three types of data: 

earthquake focal-mechanism solutions, borehole breakouts, 

and measurements of stress at shallow depths (<300 m). 

Most of the available stress data in the region are from the

Bl
B2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

39

Boundary of 

Reelfoot rift

Buried pluton

EXPLANATION

Fault

o Earthquake 

epicenter

SH orientation

• Focal mechanism

m Borehole breakout

• Stress measurement

Figure 1. Map of region around the New Madrid seismic zone showing the distribution of contemporary seismicity, major 

tectonic features (Braile and others, 1982; McKeown, 1982; Heyl and McKeown, 1978), and the inferred orientation of the 

maximum horizontal stress, SH, from all the compiled data (tables 2 and 3). Numbers refer to data entries in tables 2 and 3.

Illinois Basin, about 100-200 km northeast of the NMSZ. 

A limited amount of stress data are available from within 

and near the NMSZ and some areas west of the zone. No 

stress data have been reported from adjacent areas south 

and east of the zone. The quantity and spatial extent of 

stress data in the New Madrid region is insufficient to fully

characterize the lateral distribution of crustal stresses in and 

around the NMSZ. The fact that in situ stresses can be variable on a variety of scales also introduces uncertainties into 

any attempts at interpretation or comparative analyses of 

the sparse data. It is important to recognize, however, that 

variability in stress fields, no matter the scale, arises mainly
CRUSTAL STRESS DATA IN THE REGION OF THE NEW MADRID SEISMIC ZONE B3

from mechanical and structural heterogeneities in the Earth. 

Efforts to better characterize trends in stress distributions 

may therefore lead to an improved understanding of the 

mechanics and seismotectonics of intraplate seismic source 

zones. Although sparse for purposes of interpretation, the 

available data from around the NMSZ are more abundant 

than at most Eastern U.S. seismic source zones, and the 

data do provide some tentative insights into the distribution 

of contemporary stresses in the region.

The objective of this paper is to compile in one source 

the currently available published information concerning 

crustal stresses within a 200-km distance of the NMSZ. Previous compilations of stress data and stress indicators from 

the NMSZ region (Zoback and Zoback, 1980,1989; Nelson 

and Bauer, 1987; Hamburger and Rupp, 1988) either do not 

focus on the NMSZ in detail or do not contain all published 

information currently available. Consequently, some information and data relevant to crustal stress evaluations of this 

region are scattered among various publications, some of 

which receive limited distribution. Stress data from a relatively large region around the NMSZ is of potential interest 

because the zone of most active seismicity extends for a linear distance of about 200 km along the northeast-trending 

Reelfoot rift, a late Precambrian-early Paleozoic rift that 

underlies the northern Mississippi Embayment (fig. 1). Variations in regional stresses due to this zone of apparent major 

crustal weakness might extend for considerable distances 

away from the zone. Also, the 200-km range encompasses 

several regionally significant faults and fault zones adjacent 

to the NMSZ that may have an effect on, or be within the 

influence of, the stress distribution from this large tectonic 

feature.

TYPES OF DATA

generally correspond very well with stress directions 

inferred from other stress indicators or stress measurements 

(Zoback and Zoback, 1980).

A total of 18 published, single-event, earthquake focalmechanism solutions (Stauder and Nuttli, 1970; Herrmann 

and Canas, 1978; Herrmann, 1979; Taylor and others, 1989; 

Schweig and others, 1991) are compiled for the NMSZ 

region. In addition, a composite mechanism from the main 

shock and aftershocks from the magnitude 4.5 New Hamburg, Mo., earthquake of September 16, 1990, is included 

(Stauder and others, 1990). Most of these focal-mechanism 

solutions are from earthquakes located within or near the 

NMSZ and the Wabash Valley seismic zone in southern Illinois, where most of the larger earthquakes in this region 

have occurred. The hypocentral depth range represented by 

these focal-mechanism solutions is from 1.5 to 22 km, with 

most of the events in the 8-16-km depth range.

Single-event and composite focal-mechanism solutions 

from microearthquakes within the left-stepping offset in the 

northeasterly trend in seismicity in the central part of the 

NMSZ have been reported (O'Connell and others, 1982; 

Nicholson and others, 1984; Andrews and Mooney, 1985). 

The variability in P- and T-axis orientations and inferred 

mode of faulting indicated by these focal-mechanism data 

suggests a complex distribution of stresses within the small 

central portion of the NMSZ. Because focal-mechanism 

solutions from microearthquakes are more likely to reflect 

small-scale stress variations, and because the central leftstepping zone is small compared to the much larger region of 

study, these microearthquake focal-mechanism data are not 

included in this compilation. The stress distribution within 

the relatively small left-stepping zone of seismicity is therefore likely to be more complex than indicated from this compilation of P-axis orientations from the larger magnitude, 

single-event earthquake data.

FOCAL MECHANISMS

The P- and T-axis orientations from single-event earthquake focal-mechanism solutions in the New Madrid region 

are used to infer the approximate orientation of the maximum horizontal compressive stress, SH- The reliability of 

principal stress directions inferred from earthquake focalmechanism solutions has been discussed in the literature 

(McKenzie, 1969; Raleigh and others, 1972; Zoback and 

Zoback, 1980; Michael, 1987). In the general case of an 

earthquake on a preexisting fault, the P-axis may differ by as 

much as 35°-40° from the maximum principal stress direction (Raleigh and others, 1972; Zoback and Zoback, 1980). 

The orientation of SH inferred from any single focal 

mechanism provides only an approximation of the true horizontal stress orientation, especially when the fault plane is 

unknown. Experience has shown, however, that the average 

P- and T-axis orientations from earthquakes in a given area

BOREHOLE BREAKOUTS

Experience has shown that borehole breakouts, or 

stress-induced spoiling of borehole walls, is a generally reliable indicator of horizontal stress direction (Bell and Gough, 

1979; Plumb and Hickman, 1985; Plumb and Cox, 1987). 

Laboratory and theoretical studies have confirmed that borehole breakouts can result from shear failure due to the concentration of far-field stresses at the borehole wall and that 

the orientation of breakouts reflects the orientation of the 

least stress normal to the hole axis (Haimson and Herrick, 

1985; Zheng and others, 1989). Careful interpretation of 

wellbore logs to establish the existence and orientation of 

stress-induced breakouts can yield valuable information 

regarding horizontal stress trajectories. Breakouts in 16 

different boreholes in the NMSZ region have been reported 

(Dart, 1985; Dart and Zoback, 1989). Most of these data are 

from boreholes located in the southern part of the Illinois
B4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Table 1. Quality-ranking criteria for data in tables 2 and 3.

[Adapted from quality-ranking system for stress orientations of Zoback and Zoback (1991); M, magnitude; s.d., standard deviation]

Data-quality 

rank

Ranking criteria

Focal mechanism

A..................Average P-axis or formal inversion of four or more single-event solutions in close geographic proximity. At

least one event M £ 4.0; other events M £ 3.0. 

B ..................Well-constrained single-event solution (M £ 4.5) or average of two well-constrained, single-event solutions

(M £ 3.5) determined from first motions and other methods (e.g., moment tensor waveform modeling or 

inversion). 

C ..................Single-event solution (constrained by first motions only, often based on author's quality assignment). M > 2.5.

Average of several well-constrained composites (M > 2.0). 

D ..................Single composite solution.

Poorly constrained single-event solution.

_______Single-event solution for event with M < 2.5.__________________________________ 

______________________________Wellbore breakout___________________________________ 

A ..................Ten or more distinct breakout zones in a single well with s.d. < 12° and (or) combined length > 300 m.

Average of breakouts in two or more wells in close geographic proximity with combined length > 300 m and

s.d. < 12°. 

B ..................At least six distinct breakout zones in a single well with s.d. £ 20° and (or) combined length > 100 m.

C ..................At least four distinct breakouts with s.d. £ 25° and (or) combined length > 30 m.

D ..................Less than four consistently oriented breakouts or < 30 m combined length in a single well.

_______Breakouts in a single well with s.d. £ 25°.____________________________________ 

_______________________________Hydraulic fracture___________________________________ 

A ..................Four or more hydrofrac orientations in single well with s.d. £ 12°; depth > 300 m.

Average of hydrofrac orientations for two or more wells in close geographic proximity, s.d. < 12°. 

B ..................Three or more hydrofrac orientations in a single well with s.d. < 20°.

C ..................Hydrofrac orientations in a single well with 20° < s.d. < 25°; distinct hydrofrac orientation change with depth;

deepest measurements assumed valid. 

One or two hydrofrac orientations in a single well. 

D ..................Single hydrofrac measurement at < 100 m depth.________________________________

_________________________________Overcore______________________________________ 

A ..................Average of consistent (s.d. < 12°) measurements in two or more boreholes extending more that two excavation

radii from the excavation wall and far from any known local disturbances; depth > 300 m. 

B ..................Multiple, consistent (s.d. < 20°) measurements in one or more boreholes extending more than two excavation

radii from excavation wall; depth > 100 m. 

C ..................Average of multiple measurements made near surface (depth > 5-10 m) at two or more localities in close

proximity with s.d. < 25°.

Multiple measurements at depth > 100 m with 20° < s.d. < 25°. 

D ..................All near-surface measurements with s.d. > 15°; depth < 5 m.

All single measurements at depth. 

_______Multiple measurements at depth with s.d. > 25°. _________

Basin, but breakout orientations from one borehole within 

and one borehole southwest of the NMSZ, in northeastern 

Arkansas (Dart and Swolfs, 1991), are included in the compilation. The depth range represented by the breakout orientation data is 0.1-2.2 km.

SHALLOW STRESS MEASUREMENTS

The results of shallow stress measurements have been 

reported for eight locations within 200 km of the NMSZ. 

Haimson (1974) conducted hydraulic fracturing measurements in south-central Illinois, and Aggson and Hooker

(1982), in a summary of U.S. Bureau of Mines overcoring 

stress measurements made in the United States, included 

horizontal stress components determined at two locations in 

southeastern Missouri. It is noted that one of the overcoring 

stress measurements in Missouri was conducted in an underground mine at a site of locally complex geology. The horizontal stress orientations derived from this measurement 

deviate significantly from the regional trend and may reflect 

a locally complex stress field (J.R. Aggson, oral commun., 

1991). In a more recent study of thrust faulting in southern 

Illinois, Nelson and Bauer (1987) included some previously 

unpublished stress estimates obtained by strain-gage measurements in boreholes and strain relaxation measurements
CRUSTAL STRESS DATA IN THE REGION OF THE NEW MADRID SEISMIC ZONE B5

Table 2. Maximum horizontal stress azimuths inferred from earthquake focal-mechanism data in the region of the New Madrid seismic 

zone.

Data index 

number (State)

1 (Mo.)

2 (Mo.)

3 cm.)

4 (Mo.)

5 (Mo.)

6(111.)

7 (Ark.)

8(111.)

9 (Mo.)

10 (Ark.)d

11(111.)

12 (Ark.)c

13 (Mo.)

14 (Mo.)

15(Tenn.)

Magnitude

4.3

4.8

3.8

4.9

4.3

5.5

4.4

4.7

4.2

5.0

4.5

4.9

<4.5

4.5

4.6

3.8

Date

2/2/62

3/3/63

8/14/65

10/21/65

7/21/67

11/9/68

11/17/70

4/3/74

6/13/75

3/25/76

6/10/87

post 1982

9/26/90

5/4/91

8/29/90

Depth Inferred 

(km) stress regime8

7.5

15.0

1.5

5.0

15.0

22.0

16.0

15.0

9.0

12.0

16.0

10.0

3.0-6.0

13.9

7.1

13.9

SS

SS

SS

N

N

T

SS

SS

SS

SS

SS

SS

SS

SS

SS

Inferred 

SHmax 8*1TM*

N. 31° E.

N. 14° W.

N. 59° E.

N. 66° E.

N. 41° W.

N. 83° W.

N. 86° E.

N. 83° E.

N. 43° E.

N. 87° E.

N. 89° E.

N. 50° E.

N. 64° E

N. 45° E.

N. 88° E.

Data usedb

P&SW

P&SW

P&SW

P&SW

P&SW

P&SW

P&SW

P&SW

P&SW

P&SW

P&SW

?

P&WM

p

p

Qualityc

c

c

c

B

C

B

C

C

C

B

B

?

B

C

C

References 

(see footnote)

1,3,4

1

1,2

1,4

1

1,2,4,5

1,3,4

1,2

1,3,4

1,3,4

2

6

7

8

7

References: 1, Herrmann (1979); 2, Taylor and others (1989); 3, Herrmann and Canas (1978); 4, Zoback and Zoback (1980); 5, 

Stauder and Nuttli (1970); 6, Schweig and others (1991); 7, Stauder and others (1990); 8, Chiu and others (1991).

8 SS, predominantly strike-slip faulting; T, predominantly thrust faulting; N, predominantly normal faulting.

b P&SW, p-wave first arrivals and surface-wave data; P&WM, p-wave first arrivals and surface-wave modeling; P, p-wave first 

arrivals only.

c Quality ranking A through D using criteria of Zoback and Zoback (1991). See table 1.

d Average P-axis azimuth from two earthquakes with very similar focal mechanisms.

c Average P-axis azimuth from four earthquakes with similar focal mechanisms.

on core samples from three locations in southern Illinois. 

Well-documented and internally consistent sets of stress data 

obtained by the U.S. Bureau of Mines overcoring technique 

have been reported from recent measurements in two coal 

mines in southern Illinois (Ingram and Molinda, 1988). The 

depth range of the shallow stress measurements compiled in 

the NMSZ region is approximately 1.5-300 m.

DATA QUALITY

The quality ranking system for stress-orientation data 

that was developed and published by Zoback and Zoback 

(1989, 1991) is applied here to provide an estimate of the 

reliability with which the data are thought to represent the 

tectonic stress. The question of assigning a measure of quality to stress-determination results or to data used to infer 

some property of the stress field is, by nature, subjective. 

Methods of data collection, analysis, and reporting vary from 

author to author, and the results of different studies may be 

supported by varying quantities and coherence of input data. 

In many cases, the original data upon which a result is 

reported is often not published, and it is therefore necessary 

to rely on the author's judgment of quality.

The quality-ranking method of Zoback and Zoback 

has been developed over a number of years. The method

attempts to indicate both the reliability of the data, as gaged 

by its quantity, extent, and internal consistency, and the 

degree to which it is believed to be an indicator of tectonic 

stress, as judged by the volume of rock represented by the 

stress indicator and its depth in the crust. This method does 

not guarantee that all available data that may serve as reliable indicators of regional tectonic stress are included or 

that poor indicators are eliminated. It does, however, provide a reasonable approach to filtering widely disparate 

types of published stress data and stress indicators such that 

the potential significance of each can be judged. Table 1 

presents a part of the Zoback and Zoback (1989, 1991) 

quality-ranking system appropriate to the data summarized 

here for the NMSZ.

DATA SUMMARY

A summary of the currently available stress data in the 

NMSZ region is shown in table 2, as inferred from earthquake focal-mechanism data, and in table 3, as inferred from 

borehole-breakout data and determined from shallow stress 

measurements. Figure 1 is a map of the NMSZ region 

showing the distribution of seismicity, major tectonic features, and inferred SH orientation for all of the data listed in 

tables 2 and 3, and figure 2 is the same map showing SH orientations as inferred from only data of quality A and B (see
B6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Table 3. Summary of stress determination and borehole-breakout data in the region of the New 

Madrid seismic zone.

[Leaders (-) indicate no data available]

Index number 

and State

Depth 

(km)

Stress magnitudes 

(MPa)

^Hmax Data-quality 

azimuth estimate8

References 

(see footnote)

Borehole-breakout measurements

16 (111.) 

17 (111.) 

18 (Ky.) 

19(111.) 

20 (Ky.) 

21 (111.) 

22 (111.) 

23 (Ind.) 

24 (111.) 

25 (Ind.) 

26 (111.) 

27 (111.) 

28 (HI.) 

29 (111.) 

30 (Ark.) 

31 (Ark.)

0.1-0.4 

0.8-1.1 

0.5-0.9 

0.8-1.5 

0.2-0.7 

0.3-0.8 

1.4-2.2 

0.5-0.7 

0.8-0.9 

0.5-0.8 

0.9-1.3 

0.7-0.9 

0.8-0.9 

0.5-0.6 

1.3-2.9 

1.3-2.9

-

N. 57° E. 

N. 65° E. 

N. 88° W. 

N. 86° E. 

N. 87° W. 

N. 78° W. 

N. 67° E. 

N. 88° E. 

N. 50° E. 

N. 81° W. 

N. 35° E. 

N. 74° E. 

N. 90° E. 

N. 86° E. 

N. 73° E. 

N. 75° E.

B 

B 

A 

B 

A 

B 

B 

B 

B 

B 

B 

B 

D 

D 

A 

A

1 

1 

1 

1 

1 

1

1 

1 

2,8,9 

2,8,9

Hydraulic-fracture measurements

32 (111.) 0.1.. .......... c — 0 0 ........£>vert -/./

SHmax — ~ 1 Q '- 8

.......SHmin = 2.4

N.62°E. B 3,7

Stress-relief, overcore, and strain-gage measurements

33(111.) 

34 (111.)

35 (111.) 

36 (111.)

37(111.)

38 (Mo.) 

39 (Mo.)b

<0.3 

<0.3. ...........

<0.3 

0.3 ........... .

0.2............

surface.......

0.3............

........Svert = 5.3

SHmax — ~ 22oo n -u

C — ft /I ••••••• SHmin- 9-°

........8^ = 6.1

niiioA 

S— C C Brain--*--*

.......8^ =3.6

......SHmax iUlMA = 10-3

S

— -I C Hmin -•>•->

......SHmax = 22.0

C — n fi ••••••• ^Hmin-y-o

......SHmin =11.0

N. 67° W. 

N. 76° E.

N. 87° E. 

N. 88° E.

N. 78° E.

N. 77° E. 

N. 17° W.

NA 

NA

NA 

B

B

D 

D

4 

4

4

5

5

6,10 

6,10

References: 1, Dart (1985); 2,DartandSwolfs(1991); 3,Haimson(1974); 4, Nelson and 

Bauer (1987); 5, Ingram and Molinda (1988); 6, Aggson and Hooker (1982); 7, Zoback and 

Zoback (1980); 8, Dart (1987); 9, Dart and Zoback (1989); 10, Hooker and Johnson (1969).

a Quality ranking A through D using criteria of Zoback and Zoback 1991 (see table 1). NA 

indicates data type not included in Zoback and Zoback quality-ranking scheme or insufficient 

information available.

b Possible local stress anomaly at geologically complex site (J.R. Aggson, oral commun., 

1991).

table 1). Figure 3 is a map of the region showing the compiled focal-mechanism solutions plotted at their approximate epicentral locations. The SH orientations and focalmechanism plots shown in figures 1,2, and 3 are indexed by 

number to the data points listed in tables 2 and 3.

It is noted that data entry no. 10 in table 2 is the average of very similar focal-mechanism solutions from two 

earthquakes that occurred at the same epicentral location. 

These two earthquakes had hypocentral depths of 12 km 

and 16 km, respectively, and were separated in time by 20
CRUSTAL STRESS DATA IN THE REGION OF THE NEW MADRID SEISMIC ZONE

92° qn° 88°

B7

Boundary of 

Rcclfoot rift

Buried pluton

EXPLANATION

Fault

° Earthquake 

epicenter

H orientation 

Focal mechanism

Borehole breakout 

Stress measurement

Figure 2. Map of region around the New Madrid seismic zone showing the distribution of contemporary seismicity, major 

tectonic features (Braile and others, 1982; McKeown, 1982; Heyl and McKeown, 1978), and the inferred orientation of the 

maximum horizontal stress, SH, from the compiled A- and B-quality data (tables 2 and 3). Numbers refer to data entries in 

tables 2 and 3.

minutes (Herrmann and Canas, 1978; Herrmann, 1979). 

Also, data entry no. 12 is the average of nearly identical 

focal-mechanism solutions from four closely grouped 

earthquakes that occurred between depths of 3 and 6 km in 

the Enola, Ark., earthquake swarm (Schweig and others, 

1991).

DISCUSSION 

STRESS DIRECTIONS

The data from stress indicators and shallow stress measurements in the NMSZ region are generally consistent with
B8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

39

EXPLANATION

Boundary of 

Reelfoot rift

Buried pluton

Fault

° Earthquake 

epicenter

Figure 3. Map of region around the New Madrid seismic zone showing compiled earthquake focal-mechanism diagrams 

listed in table 2 and plotted at their approximate epicentral locations. Compressional quadrants in the focal-mechanism diagrams are shaded. Numbers refer to data entries in table 2.

the east-west to northeast-southwest direction of maximum 

horizontal compression that is characteristic of the Midcontinent of the United States (Zoback and Zoback, 1980,1989). 

Figure 4 is a plot of the SH orientations inferred from this 

data compilation and shows the semicircular vector means 

(Agterberg, 1974) for each data type. The semicircular vector mean directions of SH for all of the data is N. 80° E.

Local variations in the regional stress orientations may be 

present, however, near some tectonic and structural features. 

Indications of such potential variations in stress orientation 

come primarily from the focal-mechanism data, which 

unfortunately may be the least accurate indicator of stress 

directions. It is therefore not clear whether the observed 

variations in P- and T-axis orientations represent actual
CRUSTAL STRESS DATA IN THE REGION OF THE NEW MADRID SEISMIC ZONE B9

changes in stress orientations or whether they simply result 

from uncertainty in how well the P- and T-axes correspond 

with the principal stress directions. The observation, however, that stress directions inferred from focal mechanisms in 

this region in most cases agree quite well with other nearby 

stress indicators (fig. 1) of different types increases confidence that significant departures from the regional trend in 

P-axis orientation for any single focal-mechanism solution 

may indicate a true local stress variation. Also, the most pronounced change in P-axis directions in the NMSZ region are 

near the left-stepping offset in the northeasterly alignment of 

earthquake epicenters in the central part of the NMSZ (fig. 

1), an area where a complex distribution of stresses might be 

expected.

The earthquake focal-mechanism data that indicate a 

possible rotation in stress direction near the left-stepping offset in seismicity (data points 1,2,9, and 14 in fig. 1) are rated 

C quality, and no A- and B-quality data are available from 

this area. Also, the map of A- and B-quality data (fig. 2) 

shows a more consistent orientation of SH azimuth for the 

region than when the C-quality data are included (fig. 1). 

This observation raises the question that perhaps the apparent stress rotation is not real and is only a consequence of 

less reliable, and possibly less accurate, data. Although this 

possibility cannot be dismissed, there are reasons to believe 

that the focal mechanisms in the area likely do reflect an 

actual change in stress direction. First, all four focal-mechanism solutions within the area indicate a northward rotation 

of SH azimuth relative to the regional trend. Second, one of 

the mechanisms (data point 14, fig. 1) is from a recent earthquake that was surrounded and recorded by a modem 

regional seismic network and a local network of three-component, high-dynamic-range seismometers, providing a 

robust data set for analysis of the event (Chiu and others, 

1991). The C quality assigned to this data is because of the 

fact that the mechanism was determined from first-motion 

data only (table 1).

In actuality, the quality of the data from this particular 

area is likely to be more reliable than indicated by the quality-ranking criteria in table 1. The agreement in P-axis orientation between the recent earthquake and focalmechanism solutions from the earlier earthquakes that 

occurred prior to installation of the local network and before 

the current level of understanding of the velocity structure in 

the region reinforces the reliability of the earlier data. Also, 

two of the earlier earthquakes (data points 1 and 9, fig. 1) are 

ranked C quality only because their magnitudes of 4.3 and 

4.2, respectively, are slightly less than the magnitude 4.5 cutoff for B-quality data (table 1). Thus, the focal-mechanism 

data that indicate a possible rotation in stress direction near 

the left-stepping offset in seismicity in the central part of the 

NMSZ may actually be of better quality than implied by the 

quality-ranking scheme. It is noteworthy that the average Paxis orientation from these four data points would also

Figure 4. Plot of maximum horizontal stress orientations as inferred from the three data types in tables 2 and 3. FM, focal mechanisms; BO, borehole breakouts; MS, shallow stress measurements. 

Arrows indicate the semicircular vector mean of orientations for each 

data type.

indicate a northward rotation of SH azimuth, and this average 

orientation would rank as an A-quality data point using the 

widely accepted criteria of table 1. Thus, although the individual data points from near the left-stepping offset in seismicity are only ranked as C quality, they may actually be 

more reliable than indicated, and, taken together, they may 

provide a strong indication of a local and perhaps significant 

variation in the regional stress trend.

A notable variation from the regional trend in horizontal stress directions is also indicated by focal-mechanism 

solutions in the St. Francois Mountains area of southeast 

Missouri. There, the inferred SH directions from two focalmechanism solutions from earthquakes about 50 km apart 

differ by nearly 90°. Both of these mechanisms, one of 

which is ranked C quality, also indicate normal-faulting 

stress conditions, in contrast to the predominantly strike-slip 

and thrust-faulting mechanisms for other earthquakes in the 

region. The difference in SH orientations inferred from these 

two focal mechanisms could be interpreted as indicating that 

the horizontal stress components in this area are approximately equal to each other, but less than the vertical stress, 

with the result that normal faulting on preexisting faults with 

a wide range of strikes is possible. The two shallow stress 

measurements in this same area also give opposing results 

for the SH direction (fig. 1), but, as noted earlier, the one 

measurement result that contrasts with the regional trend in 

stress orientation may reflect a localized stress variation 

associated with complex geologic structure at the measurement site. Both shallow stress measurements, however, indicate horizontal stress magnitudes that greatly exceed the 

vertical stress, contrary to the stress state indicated by the 

deeper normal-faulting focal mechanisms.
BIO INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

STRESS REGIMES

The types of earthquake focal mechanisms and their 

distribution (fig. 3) suggest the possibility of broad areal 

variations of stress regime in the region around the NMSZ. 

In the Illinois Basin, north of the Mississippi Embayment, 

the mixture of one thrust-faulting and two strike-slip-faulting focal mechanisms implies that the stress field there could 

be transitional between strike-slip and thrust faulting, 

wherein the least horizontal stress is about equal to and 

locally exceeds the vertical stress. This possibility is consistent with the existence of minor thrust faults in shallow coal 

seams and adjacent strata of Pennsylvania age in southern 

Illinois, southwestern Indiana, and western Kentucky 

(Nelson and Bauer, 1987). These thrust faults, which formed 

in response to an approximately east-west-directed maximum horizontal compression, cannot be precisely dated, but 

because no known past tectonic stress field or nontectonic 

process was found to have produced them, Nelson and Bauer 

(1987) believe that they formed in response to the contemporary tectonic stress field. The shallow stress measurements in 

southern Illinois (table 3) also indicate that the contemporary 

stress regime within the upper few hundred meters is transitional between thrust-faulting and strike-slip faulting, with 

the maximum horizontal stress aligned in a generally eastwest direction. Whether this transitional stress regime in the 

Illinois Basin is confined only to shallow depths is uncertain, 

but the presence of at least one deep thrust-faulting earthquake suggests the possibility that it may exist throughout 

the upper several kilometers of crust.

To the south of the Illinois Basin, focal-mechanism 

solutions from earthquakes in and near the most active part 

of the NMSZ indicate that the predominant component of 

fault movement there is strike-slip. The only indications of 

earthquakes with a predominantly thrust-faulting component 

of displacement in this area come from focal-mechanism 

solutions from microearthquakes in the vicinity of the leftstepping offset in seismicity in the central part of the NMSZ 

(O'Connell and others, 1982; Nicholson and others, 1984; 

Andrews and Mooney, 1985). As noted earlier, this area also 

produces some normal-faulting and strike-slip-faulting composite focal mechanisms and is probably an area of complex 

stress distribution associated with the intersection of seisrnically active structures.

Southwest of the NMSZ, in northeastern Arkansas, 

focal-mechanism solutions from four earthquakes in the 

tightly clustered Enola, Ark., earthquake swarm (only the 

average focal mechanism is shown on fig. 3) are interpreted 

as representing strike-slip movement on east-west-trending 

faults (Schweig and others, 1991). Herrmann (1979) 

reported a thrust-faulting focal mechanism with a northwesterly oriented P-axis for an earthquake that occurred 

approximately 60 km to the southwest of this earthquake 

swarm (outside the area included in this compilation). The 

apparent absence of thrust-faulting stress conditions in and

immediately around the most active part of the NMSZ 

(with the exception of the small area within the central leftstepping offset) suggests that the least horizontal-stress in 

the areas near the seismic zone may be somewhat reduced 

in magnitude relative to the area to the north and possibly 

to the south. Conversely, the prevailing stress regime 

throughout the above areas could be strike slip, with the 

thrust-fault focal mechanisms only representing very local 

stress anomalies and with the high lateral stresses in the 

Illinois Basin confined to the near-surface.

As noted earlier, the two focal-mechanism solutions 

from earthquakes located in the St. Francois Mountains area 

of southeastern Missouri both indicate normal-faulting stress 

conditions (Herrmann, 1979), a highly unusual circumstance 

for the Midcontinent. These normal-faulting mechanisms 

imply that an extensional stress regime may exist in the 

Ozark uplift northwest of the NMSZ, although the areal 

extent of such a stress regime cannot be defined with the limited data available. The shallow stress measurements made 

in this same area indicate that large-magnitude horizontal 

stresses are present near the surface, suggesting that the 

stress regime varies with depth in this area (from one of 

thrust faulting near the surface to normal faulting at seismogenic depths).

Although sparse, the focal-mechanism data suggest the 

possibility of lateral variations in stress regime in the NMSZ 

region. A transitional stress regime between thrust and 

strike-slip faulting may be present in southern Illinois; a predominantly strike-slip stress regime may exist in the area 

immediately around the NMSZ; and an extensional stress 

regime may exist at depth in the area of the Ozark uplift in 

southeast Missouri. Such areal variations in stress regime, 

should they prove to be real, could result from broad-scale 

geologic or tectonic features. Lateral variations in crustal 

strength and density contrasts in the crust could give rise to 

broad-scale variations in regional stress. Lithospheric flexure, perhaps resulting from sediment loading or lateral density contrasts in the lower crust, could also result in broad 

areal variations in stress regime. Crustal flexure, if present, 

would be expected to produce alternating patterns of compression and extension over large areas that would be superimposed upon the regional stresses thought to originate from 

plate-driving forces. Because the largest flexural stresses 

would develop near the outer margins of the flexing elastic 

plate, their influences would be more pronounced at relatively shallow depths. Such a conceptual model has been 

proposed by Dewey (1988) as a possible explanation for 

seismicity exterior to former rift basins in intraplate settings.

If, on the other hand, the most active part of the NMSZ 

simply represents a narrow zone of crustal weakness within 

a relatively uniform regional stress, then there should be 

some evidence of redistribution of the regional stresses 

around the weakened core zone. Maximum horizontal stress 

orientations in the Illinois Basin, north of the NMSZ, do tend 

to align in a more east-west orientation than is typical for
CRUSTAL STRESS DATA IN THE REGION OF THE NEW MADRID SEISMIC ZONE Bll

most of the midcontinent region. Insufficient data in other 

areas surrounding the NMSZ, however, make it unclear 

whether this change in orientation may be related to the 

NMSZ or whether it represents a general change to a more 

east-west orientation of the maximum horizontal stress trajectories in the central Midcontinent. More data on stress 

orientation and relative stress magnitudes are needed to adequately characterize the details and spatial extent of the 

stress distribution around the NMSZ.

STRESS DISTRIBUTION WITH DEPTH

The distribution of horizontal stress orientations in the 

region of the NMSZ shows little apparent variation with 

depth. The semicircular vector-mean directions of SH for 

each of the three data types, which correspond generally with 

depth, differ only by a maximum of 11° (fig. 4). Figure 5 is 

a plot of the SH azimuths as a function of depth for the compiled data. Note that the scatter in orientations of SH for the 

shallow stress measurements is comparable to that inferred 

from the intermediate-depth borehole breakouts and the 

deeper focal-mechanism solutions. The similar means and 

range in orientations for each of the three sets of data implies 

approximately equal uncertainty in the reliability of data 

from each set to reflect the mean SH direction. This observation also suggests that a carefully planned and implemented 

program of shallow stress measurements may offer a viable 

means of mapping the horizontal stress orientations in a wide 

region surrounding the NMSZ. The level of precision in horizontal-stress trajectories determined from shallow stress 

measurements may be comparable to that of borehole-breakout orientations, and it is probably better than for horizontalstress trajectories determined from earthquake focal-mechanism P-axis orientations. It is noted, however, that relative 

stress magnitudes determined from shallow stress measurements may not be indicative of those at depth. The lateral 

distribution of shallow stress magnitudes could, however, be 

useful as a possible indicator of crustal flexure and for distinguishing between various seismotectonic models for the 

NMSZ region.

SUMMARY

A compilation of shallow stress measurements and 

deeper stress indicators in the region of the NMSZ shows 

that, whereas most of the horizontal-stress trajectories are 

consistent with the east-west to east-northeast-west-southwest direction of maximum horizontal compression characteristic of the midcontinent region, local variations in stress 

directions apparently exist. These variations may be related 

to tectonic structures and the intersections of seismically 

active fault zones. Earthquake focal-mechanism solutions 

also suggest areal variations in the stress regime that could 

possibly be indicative of the type of broad crustal deformation occurring in the region. Unfortunately, the available

0

2

4

6

CO

a: LU 8

O 10

? 12

I

a_ 14

a

16

18

20

AZIMUTH, IN DEGREES 

30 60 90 120 150 180

22 L-

— o

Figure 5. Maximum horizontal stress azimuth as a function of 

depth for the data in tables 2 and 3, distinguished by data type. FM, 

focal mechanisms; BO, borehole breakouts; MS, shallow stress 

measurements. Shaded area represents range of ±40° of the semicircular vector mean of N. 80° E. for all of the data. This range is 

an estimate of the uncertainty in how well the P-axis orientation 

from any single focal-mechanism diagram corresponds to the maximum compressive stress direction (Raleigh and others, 1972) 

when the actual fault plane is unknown.

database of stress measurements and stress indicators is not 

of sufficient quantity or lateral extent to adequately define 

the stress distribution in the region of the NMSZ or to firmly 

establish relationships between stress, structure, and seismicity. Additional stress data are needed, especially to the 

west and east of the NMSZ. A better characterization of the 

stress distribution, in association with the results of geodetic 

strain determinations and deformation modeling studies, 

should lead to an improved understanding of the seismotectonics of the region. The consistency between horizontal 

stress directions derived from the various types of data over 

a wide range of depths suggests the possibility that shallow 

stress measurements may offer a viable and economic means 

of contributing to an improved understanding of contemporary stresses in the NMSZ region.
B12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

REFERENCES CITED

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Methods Handbook: New York, Society of Mining Engineers 

of American Institute of Mining, Metallurgical, and Petroleum 

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Agterberg, P.P., 1974, Geomathematics: Amsterdam, Elsevier, 

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McKeown, F.A., and Pakiser, L.C., eds., Investigations of the 

New Madrid, Missouri, Earthquake Region: U.S. Geological 

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Chiu, S.C., Hwang, H., Chiu, J.M., and Johnston, A.C., 1991, The 

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Geological Survey Open-File Report 85-733,41 p.

———1987, South-central United States well-bore breakout-data 

catalog: U.S. Geological Survey Open-File Report 87-405, 

95 p.

Dart, R.L., and Swolfs, H.S., 1991, Contemporary stress in northeastern Arkansas [abs.]: Transactions of the American Geophysical Union (EOS), v. 72, no. 17, p. 264.

Dart, R.L., and Zoback, M.L., 1989, Wellbore breakout stress analysis within the central and eastern continental United States: 

The Log Analyst Journal, v. 30, no. 1, p. 12-25.

Dewey, J.W., 1988, Midplate seismicity exterior to former rift-basins: Seismological Research Letters, v. 59, p. 213-218.

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p. 156-182.

Haimson, B.C., and Herrick, C.G., 1985, In situ stress evaluation 

from borehole breakouts, experimental studies, in Research 

and Engineering Applications in Rock Masses—Symposium 

on Rock Mechanics, [26th, Rapid City, S. Dak.]: Rotterdam, 

A.A. Balkema, p. 1207-1218.

Hamburger, M.W., and Rupp, J.A., 1988, The June 1987 southeastern Illinois earthquake: Possible tectonism associated with the 

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prophecy: Preparing for the next New Madrid earthquake: U.S. 

Geological Survey Circular 1066,30 p.

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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, 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, scale 

1:250,000.

Hooker, V.E., and Johnson, C.F., 1969, Near-surface horizontal 

stresses including the effects of rock anisotropy: U.S. Bureau 

of Mines Report of Investigations 7224,29 p.

Ingram, O.K., and Molinda, G.M., 1988, Relationship between horizontal stresses and geologic anomalies in two coal mines in 

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McKenzie, D.P., 1969, The relationship between fault plane solutions for earthquakes and the directions of the principal stresses: Bulletin of the Seismological Society of America, v. 59, 

p. 591-601.

McKeown, F.A., 1982, Overview and discussion, in McKeown, 

F.A., and Pakiser, L.C., eds., Investigations of the New 

Madrid, Missouri, Earthquake Region: U.S. Geological Survey 

Professional Paper 1236, p. 1-4.

Michael, A.J., 1987, Use of focal mechanisms to determine stress— 

A control study: Journal of Geophysical Research, v. 92, 

p. 357-368.

Nelson, W.J., and Bauer, R.A., 1987, Thrust faults in the Illinois 

Basin—Result of contemporary stress?: Geological Society of 

America Bulletin, v. 98, p. 302-307.

Nicholson, C, Simpson, D.W., Singh, S., and Zollweg, J.E., 1984, 

Crustal studies, velocity inversions, and fault tectonics; results 

from a microearthquake survey in the New Madrid seismic 

zone: Journal of Geophysical Research, V. 89, p. 4545-4558.

O'Connell, D.R., Bufe, C.G., and Zoback, M.D., 1982, Microearthquakes and faulting in the area of the New Madrid, 

Missouri-Reelfoot Lake, Tennessee, in McKeown, F.A., and 

Pakiser, L.C., eds., Investigations of the New Madrid, Missouri, Earthquake Region: U.S. Geological Survey Professional 

Paper 1236, p. 31-38.

Plumb, R.A., and Cox, J.W., 1987, Stress directions in eastern 

North America determined to 4.5 km from borehole elongation 

measurements: Journal of Geophysical Research, v. 92, 

p. 4805-4816.

Plumb, R.A., and Hickman, S.H., 1985, Stress-induced borehole 

elongation—A comparison between the four-arm dipmeter and 

borehole televiewer in the Auburn geothermal well: Journal of 

Geophysical Research, v. 90, p. 5513-5521.

Raleigh, C.B., Healy, J.H., and Bredehoeft, J.D., 1972, Faulting and 

crustal stress at Rangely, Colorado, in Flow and Fracture of 

Rocks: American Geophysical Union Geophysical Monograph 

16, p. 275-284.

Schweig, E.S., VanArsdale, R.B., and Burroughs, R.K., 1991, Subsurface structure in the vicinity of an intraplate earthquake 

swarm, central Arkansas: Tectonophysics, v. 186, p. 107-114.

Stauder, W.J., Herrmann, R., Wuenscher, M., Taylor, K., Mitchel, 

S., and Whittington, M., 1990, Central Mississippi Valley 

earthquake bulletin: Saint Louis University, Quarterly Bulletin 

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Stauder, W.J., and Nuttli, O.W., 1970, Seismic studies: south central Illinois earthquake of November 9, 1968: Bulletin of the 

Seismological Society of America, v. 60, p. 973-981.
CRUSTAL STRESS DATA IN THE REGION OF THE NEW MADRID SEISMIC ZONE B13

Taylor, K.B., Herrmann, R.B., Hamburger, M.W., Pavlis, G.L., 

Johnston, A., Longer, C., and Lam, C., 1989, The southeastern 

Illinois earthquake of 10 June 1987: Seismological Research 

Letters, v. 60, p. 101-110.

Zheng, Z., Kemeny, J., and Cook, N.G., 1989, Analysis of borehole 

breakouts: Journal of Geophysical Research, v. 94, 

p. 7171-7182.

Zoback, M.D., and Zoback, M.L., 1991, Tectonic stress field of 

North America and relative plate motions, in Slemmons, D.B., 

Engdahl, E.R., Zoback, M.D., and Blackwell, D.D., eds., Neotectonics of North America: Boulder, Colorado, Geological 

Society of America, Decade Map, v.l, p. 339-366.

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.

Zoback, M.L., and Zoback, M.D., 1989, Tectonic stress field of the 

continental United States, in Pakiser, L.C., and Mooney, W.D., 

eds., Geophysical Framework of the Continental United States: 

Boulder, Colorado, Geological Society of America Memoir 

172, p. 523-539.

Preliminary Seismic Reflection Study of 

Crowley's Ridge, Northeast Arkansas

By Roy B. VanArsdale, Robert A. Williams, Eugene S. Schweig III, 

Kaye M. Shedlock, Lisa R. Kanter, and Kenneth W. King

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-C

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994
CONTENTS

Abstract...............................................................^ Cl

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

Geologic Setting............................................................................................................... 3

Surface Geology of Crowley's Ridge and Vicinity................................................ 3

Geologic History of the Mississippi Embayment................................................... 3

Seismicity and Neotectonics of Crowley's Ridge and Vicinity............................. 4

Data Acquisition and Processing...................................................................................... 4

Interpretation of the Mini-Sosie Reflection Lines............................................................ 7

Mini-Sosie Line 1................................................................................................... 7

Mini-Sosie Line 2................................................................................................... 7

Mini-Sosie Line 3................................................................................................... 7

Mini-Sosie Line 4................................................................................................... 11

Mini-Sosie Line 5................................................................................................... 11

Similarities Between Crowley's Ridge and the New Madrid Seismic Zone.................... 14

Discussion and Conclusions............................................................................................. 14

Acknowledgments............................................................................................................ 15

References Cited............................................................................................................... 15

FIGURES

1. Map of northeast Arkansas...................................................................................................................................... C2

2. Cretaceous-Quaternary stratigraphic section for northeast Arkansas...................................................................... 3

3. Map showing Crowley's Ridge in Arkansas and epicenters of microearthquakes in the upper

Mississippi Embayment........................................................................................................................................... 5

4-8. Figures showing topographic profile, reflection profile, and line drawing for:

4. Mini-Sosie line 1.............................................................................................................................................. 8

5. Mini-Sosie line 2.............................................................................................................................................. 9

6. Mini-Sosie line 3.............................................................................................................................................. 10

7. Mini-Sosie line 4.............................................................................................................................................. 12

8. Mini-Sosie line 5.............................................................................................................................................. 13

TABLES

1. Data-acquisition parameters and data-processing sequence.................................................................................... C6

2. Elevations of geologic units..................................................................................................................................... 6
PRELIMINARY SEISMIC REFLECTION STUDY OF 

CROWLEY'S RIDGE, NORTHEAST ARKANSAS

By Roy B. VanArsdale, 1 Robert A. Williams,2 Eugene S. Schweig III,3 

Kaye M. Shedlock,2 Lisa R. Kanter,4 and Kenneth W. King 2

ABSTRACT

Five high-resolution seismic reflection lines (24-channel Mini-Sosie data with 1-s record length) were acquired 

across the margins of Crowley's Ridge in northeast Arkansas 

to test the hypothesis that Crowley's Ridge is fault bounded. 

Three of the lines traversed the margins of the northern segment of the ridge near Jonesboro, Ark., and the other two 

were located south of Jonesboro, across the margins of the 

southern segment. Subsurface faulting of Paleozoic strata 

through the Eocene Wilcox Group was interpreted in the 

lines beneath the ridge margins at all five sites. The data did 

not resolve the uppermost 0.15 s (150 m) of two-way travel 

time, so it is not possible to determine if the PliocenePleistocene sediments have been faulted. Thus, we can not 

determine if the ridge truly owes its topographic relief to 

fault displacement or if Tertiary faulting controlled subsequent Quaternary river incision. However, at one site, the 

ridge margin is interpreted as a fault scarp because the ridge 

height is the same as the structural relief on the ridge-bounding fault and fault displacement can be traced into the shallowest reflectors (0.15 s).

The Mini-Sosie data reveal a rather complex Tertiary 

structural history. Paleocene and possible early Eocene 

faulting is evident in thickened Midway Group and lower 

Wilcox Group strata across some faults. Post-Wilcox (early 

Eocene) faulting is present in all of the lines, and postClaiborne Group (middle Eocene) folding and faulting are 

suggested in some of the lines. The steepness of the faults 

and the normal and reverse displacements occurring within a

i

72701.

Department of Geology, University of Arkansas, Fayetteville, AR

2 U.S. Geological Survey, Mail Stop 966, P.O. Box 25046, Denver 

Federal Center, Lakewood, CO 80225.

^ U.S. Geological Survey, Center for Earthquake Research and Information (CERI), Memphis State University, Memphis, TN 38152.

4 Center for Earthquake Research and Information (CERI), Memphis 

State University, Memphis, TN 38152.

small area and over a relatively short length of time suggests 

that these faults may be the upper portions of strike-slip 

flower structures.

INTRODUCTION

Crowley's Ridge, in northeast Arkansas (fig. 1), has 

been interpreted to be an erosional remnant formed during 

Quaternary incision of the ancestral Mississippi and Ohio 

Rivers (Call, 1889; Stephenson and Crider, 1916; Fisk, 

1944; Guccione and others, 1986). However, the Reelfoot 

rift COCORP line (AR-6 of Nelson and Zhang, 1991) has 

revealed a down-to-the-west fault beneath the western margin of the southern segment of the ridge. The pronounced 

linearity and declivity of the ridge margins throughout its 

length suggests that the ridge may also be fault bounded at 

other locations.

Crowley's Ridge consists of two discrete segments with 

geomorphic characteristics that further suggest the ridge is 

structurally controlled. At lat 35°45/N., Crowley's Ridge 

abruptly changes width, geology, bearing, and symmetry 

(Cox, 1988a) (fig. 1). North of lat SS^S'N., the axis of the 

northern ridge segment is straight, bearing N. 30° E., 

whereas south of this latitude, the southern ridge segment is 

straight and trends N. 5° E. The northern segment is asymmetrical in transverse profile, having a steep west margin relative to the southern segment (Cox, 1988a). Drainage-basin 

analyses of streams on the two segments suggest that the 

northern segment has undergone southeast tilting during the 

Quaternary (Cox, 1988a).

The geomorphology of Crowley's Ridge and the interpreted fault beneath the western margin of the southern ridge 

segment suggest that the entire ridge may be bounded by 

faults. In order to test the hypothesis that Crowley's Ridge 

has been structurally uplifted, we collected a total of 12 km 

of high-resolution seismic reflection data (Mini-Sosie) at 

five locations across the ridge margins (fig. 1).

Ci
C2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

EXPLANATION

Pleistocene loess and 

Pliocene-Pleistocene 

sand, gravel, and clay 

(undifferentiated)

Pliocene-Pleistocene 

sand, gravel, and clay

Eocene sand and clay

Figure 1. Map of northeast Arkansas (modified from Cox, 1988a). Dashed lines represent margins of the Reelfoot rift (Hildenbrand and 

others, 1982). Numbered lines (1-5) are Mini-Sosie lines. Lowercase letters are petroleum exploration wells: q, Quintin Little No. 1 Little; 

n, J.F. Scott Trustees No. 2-A Nelson; h, Henley-Lanagren Co. No. 1 E.E. Hogue; j, C.E. Walters and others No. 1 H. Johnson; t, Tennark 

Inc. R.M. Martin and others; m, Volcanic Oil and Gas Co. No. 1-A McDaniel; f, W. Adams No. 1 Fee; s> Houston Mineral and Oil No. 1 

Singer. P, Paragould; J, Jonesboro; H, Harrisburg; MT, Marked Tree; M, Memphis.
SEISMIC REFLECTION STUDY OF CROWLEY'S RIDGE, NORTHEAST ARKANSAS C3

GEOLOGIC SETTING

SURFACE GEOLOGY OF 

CROWLEY'S RIDGE AND VICINITY

Crowley's Ridge is a 320-km-long topographic ridge 

that extends from Thebes, 111., to Helena, Ark. (Arkansas 

portion shown in fig. 1). The ridge is from 1.6 to 19 km wide, 

averages 60 m above the adjacent lowlands, and is stratigraphically composed of Eocene and Pliocene-Pleistocene 

unconsolidated sediments (fig. 2). The Eocene sequence is 

dominated by continental and deltaic sands and clays with 

minor intervals of lignite (Murray, 1961). Eocene units dip 

approximately 0.5° southeast (Meissner, 1984) and are 

unconformably overlain by Pliocene-Pleistocene fluvial 

sands, gravels, and clays that locally exceed 50 m in thickness (Holbrook, 1980). These Pliocene-Pleistocene fluvial 

deposits have been correlated with the Citronelle (Lafayette) 

Formation of the Gulf Coast (Potter, 1955; Saucier, 1974; 

Guccione and others, 1986). Above these fluvial deposits, up 

to 10 m of Pleistocene loess is locally preserved with major 

accumulation confined to the southern portion of Crowley's 

Ridge (Saucier, 1974; Guccione and others, 1986; Autin and 

others, 1991).

Flanking the ridge at our reflection-line sites is an early 

Wisconsin braided-stream terrace that is 80,000 to 35,000 

years old (Royall and others, 1991). This terrace level is 

underlain by glacial outwash and valley-train deposits of the 

Mississippi and Ohio Rivers; the terrace is capped by loess 

(Saucier, 1974; Rutledge and others, 1990).

GEOLOGIC HISTORY OF THE 

MISSISSIPPI EMBAYMENT

Crowley's Ridge lies within the northwestern portion of 

the Mississippi Embayment, abroad and gentle south-southwest-plunging syncline of Cretaceous and Tertiary age 

(Stearns, 1957; Stearns and Marcher, 1962). In order to better understand the structural setting of Crowley's Ridge, we 

herein briefly review the structural history of the Mississippi 

Embayment area.

Drill hole data and exposures in the Ozarks of southeastern Missouri indicate that, during late Precambrian time, 

the upper Mississippi Embayment area was a subaerial landscape with from 150 to 450 m of topographic relief 

(Schwalb, 1982; Buschbach and Schwalb, 1984) cut into 

Middle Proterozoic granites and rhyolites (Bickford, 1988).

Figure 2 (facing column). Cretaceous-Quaternary stratigraphic 

section for northeast Arkansas with principal seismic reflectors 

(modified from Renfroe, 1949). P, Paleozoic; K, Cretaceous; M, 

Midway Group; W, Wilcox Group; C, Claiborne Group.

FORMATION

CITRONELLE

COCKFIELDCOOK MTNSPARTA

SAND

CANE 

RIVER

PORTERS 

CREEK 

CLAY

SARATOGA

CHALKMARLBROOK

MARL

OZAN

PRE-OZAN

EXPLANATION 

Sandstone 

Shale or clay 

Sandy shale or siltstone 

Calcareous shale or marl 

Limestone or chalk 

Sandy limestone 

Glauconite 

Lignite 

Silt

Siderite 

Phosphate 

Sand and gravel

Locally unconformable 

Regional nonconformity
C4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

This landscape was dramatically altered by the initiation of 

the northeast-trending Reelfoot rift in late Precambrian or 

Early Cambrian time (Ervin and McGinnis, 1975; Kane and 

others, 1981; Hildenbrand and others, 1982). The Reelfoot 

rift underlies and is essentially coincident with the present 

Mississippi Embayment syncline (Ervin and McGinnis, 

1975). Middle Ordovician through Early Devonian unconformities mark marine transgressions and regressions that 

may have been accompanied by reactivation of rift-bounding 

faults. Late Devonian deformation resulted in Upper Devonian black shales lying with angular unconformity over 

strata as old as Ordovician. Uplift of the Pascola arch in the 

upper embayment area, between late Paleozoic and middle 

Cretaceous time, ultimately resulted in the erosion of most of 

the Paleozoic section to as deep as Cambrian Lamotte Sandstone strata across the arch (Stearns, 1957; Stearns and 

Marcher, 1962). Beneath Crowley's Ridge at Jonesboro, 

Upper Cretaceous sedimentary rocks overlie Ordovician carbonates (Caplan, 1954).

Formation of the Mississippi Embayment initiated in 

Late Cretaceous time with subsidence and deposition of 

Upper Cretaceous marine sediments (Stearns, 1957; Pryor, 

1960) and intrusion of plutons along the rift-bounding faults. 

The axis of the embayment is nearly coincident with the 

underlying Reelfoot rift and appears to reflect Cretaceous 

reactivation of the ancient rift (Braile and others, 1982). 

Embayment subsidence continued into early Tertiary time, 

as reflected in Paleocene to middle Eocene marine and terrestrial sedimentary rocks (Stearns, 1957). Late Eocene 

compression resulted in minor faulting and folding within 

the embayment (Howe and Thompson, 1984; Luzietti and 

others, 1992). Absence of Oligocene and Miocene strata suggests that, during this time, the embayment area was subaerially exposed. In Pliocene time, the embayment underwent 

terrestrial deposition as reflected in the Pliocene-Pleistocene 

fluvial gravels preserved in Crowley's Ridge and the embayment perimeter (Potter, 1955). The Pleistocene was dominated by erosion of most of the Pliocene-Pleistocene section, 

deposition of glacial outwash and loess, and the establishment of the current drainage system within the embayment. 

Sedimentation and tectonism within the Mississippi Embayment continues to the present with Mississippi River sedimentation and the New Madrid seismicity (Nuttli, 1973, 

1982; Johnston, 1982; Braile and others, 1982).

SEISMICITY AND NEOTECTONICS OF 

CROWLEY'S RIDGE AND VICINITY

Current seismicity in the Mississippi Embayment primarily occurs east of Crowley's Ridge and is coincident with 

structural highs within the Reelfoot rift (fig. 3). Specifically, 

the northeast-trending Blytheville arch and the northwesttrending Lake County uplift are the most seismically active 

areas in the embayment (Russ, 1979, 1982; Hamilton and

Zoback, 1982; Crone and others, 1985; Hamilton and 

McKeown, 1988; McKeown and others, 1990). Although 

the area has fewer events than the Blytheville arch and Lake 

County uplift areas, there is also seismicity along the eastern 

margin of the rift north of Memphis, Tenn. (fig. 3). This eastem-rift-margin seismicity is of interest because of the recent 

identification of Eocene and probable post-Eocene deformation along the Crittenden County fault, located 10 km west 

of Memphis (Luzietti and others, 1992).

Crowley's Ridge overlies and crosses the western margin of the Reelfoot rift near Jonesboro, Ark. (fig. 1). The 

intersection area of the ridge and the western margin of the 

rift also coincides with the Jonesboro pluton, the change 

from northern to southern ridge segments, and a proposed 

northwest-trending fault line (Fisk, 1944; Krinitzsky, 1950; 

O'Leary and Hildenbrand, 1978; Cox, 1988a, 1988b). One 

of the most striking geomorphic features of the entire ridge 

is this 19-km-long, very straight, southwestern termination 

of the northern ridge segment near Jonesboro (fig. 1).

Seismicity in the Crowley's Ridge area is very low. 

However, diffuse seismicity northeast of the ridge may be 

associated with the bounding faults on the west side of the 

rift (Howe, 1985). These same faults pass beneath the ridge 

near Jonesboro (fig. 3).

DATA ACQUISITION AND 

PROCESSING

Five Mini-Sosie seismic reflection profiles were 

recorded across margins of Crowley's Ridge at five different 

locations (fig. 1). The field data were acquired using common-midpoint seismic reflection techniques (Mayne, 1962). 

Line lengths varied from 1.5 km at line 4 to 5 km at line 5. 

Acquisition parameters are listed in table 1.

The Mini-Sosie system is a high-resolution, shallowreflection seismic technique that uses multiple impacts from 

gasoline-powered earth tampers (Wackers) as a pseudo-random seismic source (Barbier and others, 1976). The impact 

moments from each Wacker are identified by a sensor 

attached to the base plate and sent by radio (carried on the 

tamper operator's backpack) as a time-break sequence in real 

time to the Input/Output DHR 2400 seismograph. For each 

station, three Wackers operated simultaneously and 

advanced as a tight group along the line of the survey, generating 2,000 impulses. The seismograph correlates the timebreak sequence in real time with the seismic signal received 

by the linear array of geophones to produce a 1-s seismic 

record from the 2,000 impacts. Source and geophone spacing 

were selected to image the upper 1 s of two-way travel time 

(1.2 km) so that the Cretaceous-through-Tertiary section 

would be recorded. In most of the lines, the top of the Paleozoic was also clearly imaged. This configuration allowed 

determination of displacement at the top of the Cretaceous 

and provided resolution to as shallow as 0.15 s (150 m). In
SEISMIC REFLECTION STUDY OF CROWLEY'S RIDGE, NORTHEAST ARKANSAS

91° 90°

C5

50 KILOMETERS

Figure 3. Map showing Crowley's Ridge and epicenters of microearthquakes (+) in the upper Mississippi Embayment 

(modified from Luzietti and others, 1992).

figures 4-8, the deepest portions of the seismic lines are not roads (lines 3 and 4). These variations in surface material 

reproduced because there are no continuous reflectors in the may have affected seismic data quality because the data

data. acquired on the sand-gravel roads tends to have more reflecThe surface material along the lines varied from sand- tion energy, possibly because the elevated shoulders along 

gravel roads (lines 1,2, and 5) to sand-silt shoulders of paved the paved roads absorb more energy than the hard-pack
C6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Table 1. Data-acquisition parameters and data-processing 

sequence.

___________Data-acquisition parameters___________

Source type:................ 3 earth tampers (Wackers)

Source array:...............3-m spacing parallel to profile line

Impacts per station:.....2,000

Source point interval:.. 15.2 m (50 ft)

Geophone array:.......... 12,28-Hz natural-frequency geophones

in a cluster 

Geophone spacing:......15.2 m (50 ft)

Spread geometry: ........24 channels, in-line, off-end;

chl: 30.5-m offsetch24: 381-m offset

Recording pass band: ..20-240 Hz; 60-Hz notch filter 

Recording system:.......I/O DHR 2400,24-channel Mini-Sosie

Sampling interval:...... 1 ms

Record length: ............1 s___________________

____________Data-processing sequence____________

1. Trace edit

2. Common midpoint sort

3. Elevation static timing corrections

4. Surface-wave surgical mute

5. Normal moveout velocity correction

6. Band-pass frequency filter (roughly 3040-120-150 Hz)

7. Surface-consistent automatic statics

8. Refraction mute

9. Stack (nominally 12-fold)

10. F-K filter1

11. 2nd zero-crossing predictive deconvolution filter

12. Band-pass frequency filter

13. Automatic gain control amplitude scaling________

1 Post-stack frequency-wave number.

gravel roads. Tests using various geophone group arrays did 

not significantly alter the signal from a well-developed surface wave enough to warrant the extra time required to handle spreadout geophone arrays. Interference from overhead 

power lines and other unknown sources of electrical noise 

required use of a 60-Hz notch filter on all lines. These acquisition parameters resulted in dominant reflection frequencies 

of about 50 Hz. Despite the attempts to improve data quality 

using the parameters described above, the overall signal-tonoise (S/N) ratio is quite low (except for line 5) for these 

data. Evidently, a high water table and compact sands and 

clays along the fluvial-terrace portions of the lines resulted 

in relatively good continuity and resolution of reflectors 

compared to the portions of the h'nes on Crowley's Ridge 

where a lower water table and unconsolidated gravel resulted 

in poorer source and geophone coupling and, therefore, 

poorer resolution of reflectors beneath the ridge.

The processing parameters are summarized in table 1. 

Field data were processed on a VAX 780 computer using 

Digicon's DISCO software at the U.S. Geological Survey 

facilities in Denver, Colo. The processing flow was relatively standard except for a surgical mute applied prior to

Table 2. Elevations of geologic units.

[Elevations in meters; minus sign indicates elevation below sea level. Well identification: t, Tennark R.M. Martin and others; q, Quintin Little No. 1 Little; n, J.F. Scott 

Trustees No. 2-A Nelson]

Sosie line 

or well

Top of 

Wilcox Gp.

Top of 

Midway Gp.

Top of 

Cretaceous

Top of 

Paleozoic

1............. -180........... -231........... -685........... -823

2............. -142........... -206........... -601........... -778

3................85...........-156...........-368...........-508

4....................................-8...........-268...........-375

5...............-30.............-75...........-297...........-410

t...............-30.............-98...........-262...........-404

q........................................ ........... -276........... -421

n........................................................... ............594

normal-moveout-velocity correction that deleted all data 

arriving at or below the speed of the surface wave. This mute 

greatly improved the data S/N ratio, although it also lowered 

the fold coverage to about 6 or 8 below 0.5 s.

Elevation static corrections were applied using a velocity calculated from refraction energy on near-offset traces 

displayed in common-shot gathers. These static corrections 

resulted in a zero time for these profiles that corresponds 

topographically to the lowest elevation point on each profile. 

Stacking velocities were determined from common-midpoint gathers and generally ranged from about 1,600 m/s 

from 0.0 to 0.2 s to 2,000 m/s at 0.7 s. We have good velocity 

control on lines 2 and 5 (±5 percent), and similar velocity 

profiles were determined for lines 1, 3, and 4. A post-stack 

frequency-wave number (F-K) filter also improved the data 

by removing much of the remaining, dipping, coherent 

energy not removed by the surgical mute. Because of the 

generally low S/N ratio on lines 1,2,3, and 4, only line 5 was 

migrated using a post-stack finite-differenced algorithm.

The stratigraphy of the area is detailed in figure 2. 

Stratigraphic interpretation of the Mini-Sosie lines was 

undertaken using petroleum test wells located on or near 

Crowley's Ridge (fig. 1) (Renfroe, 1949; Dart, 1990). Additional data used in interpreting the seismic lines include the 

New Madrid test well 1-X in New Madrid County, Missouri 

(Crone, 198 l;Frederiksen and others, 1982)^ the Fort Pillow 

test well in Lauderdale County, Tennessee (Moore and 

Brown, 1969), and published seismic lines that pass over 

(Nelson and Zhang, 1991) or near Crowley's Ridge (Hamilton and Zoback, 1982; Howe and Thompson, 1984; Howe, 

1985; Luzietti and others, 1992). As shown in table 2, the 

elevations of tops of the Eocene Wilcox Group (base of Wilcox may be Paleocene, e.g., Frederiksen and others, 1982), 

Paleocene Midway Group, Cretaceous section, and Paleozoic section have been picked (Renfroe, 1949) from the Tennark well, located southwest of Jonesboro near the middle of 

our five lines (fig. 1). Strong reflectors exist at or near the 

tops of these Stratigraphic units in all five seismic lines (table 

2 and figs. 4-8).
SEISMIC REFLECTION STUDY OF CROWLEY'S RIDGE, NORTHEAST ARKANSAS C7

INTERPRETATION OF 

MINI-SOSffi REFLECTION LINES

Seismic signatures of the geologic units identified in 

figures 4-8 are consistent from line to line. The Claiborne 

Group has discontinuous reflectors; the Wilcox Group has 

continuous reflectors; the Midway Group has almost no 

reflectors; the Cretaceous section has continuous reflectors; 

and the Paleozoic section changes from discontinuous 

reflectors to no reflectors with depth. The major difference 

among the five lines is that the reflectors are deeper in lines 

1 and 2 because lines 1 and 2 are closer to the MississippiEmbayment axis.

MINI-SOSIE LINE 1

Line 1 is a 2-km-long line on the western side of the 

southern ridge segment (figs. 1 and 4). As shown in the topographic profile (fig. 4), the ridge is approximately 30 m high 

at this location. Line 1 coincides with a portion of the Reelfoot rift COCORP AR-6 line. Although resolution of the top 

of the Paleozoic is poor in line 1, we believe that it is at 0.8 

s at the western end of the line, which corresponds with the 

0.8 s time for the top of the Paleozoic in the COCORP line 

(Nelson and Zhang, 1991). The COCORP line also reveals a 

near-vertical fault that displaces the Paleozoic Knox through 

Tertiary sections down to the west beneath the western margin of the ridge (Nelson and Zhang, 1991).

The Wilcox is flat-lying beneath the ridge from station 

numbers (STAT) 104 to 180. At STAT 180, the Wilcox is 

displaced approximately 0.04 s (42 m) down to the west and 

dips gently westward from STAT 180 to the western end of 

the line. Although not as evident, it appears that the Cretaceous is also displaced 0.03 s (42 m) down to the west at 

STAT 180. Within the Claiborne, the fault is manifested as 

truncated reflectors and relatively steep westerly dipping 

reflectors from STAT 175 to 180. This steep fault can be 

traced to within 0.15 s (90 m) of the surface, above which 

there are no reflection data (fig. 4). The fault is located 

directly beneath the topographic base of Crowley's Ridge 

and appears to correspond with the fault identified on the 

COCORP line. Lack of surface strata exposure did not allow 

us to determine if the fault cuts to the ground surface.

The Nelson No. 2 well is believed to have been drilled 

near the western end of line 1 (fig. 1 and table 2). However, 

this well was drilled in 1921, and its exact location is not 

known. In comparing the depth of the top of the Paleozoic in 

the well with the calculated depth from line 1, it is apparent 

that the Paleozoic is 229 m deeper in line 1. If the Nelson 

No. 2 well is indeed near the western end of line 1, then our 

calculated depths would be too deep for line 1, or alternatively, there may be a down-to-the-east fault between the 

well and the seismic line.

MINI-SOSIE LINE 2

Line 2 is a 2-km-long line that descends eastward off 

Crowley's Ridge, which is 30 m high at this location (fig. 5). 

Coincident with line 2 is a portion of the Reelfoot rift 

COCORP line (Nelson and Zhang, 1991). The top of the 

Paleozoic is at 0.77 s in the eastern end of line 2 and approximately 0.8 s in the COCORP line.

The tops and bases of the Wilcox and Cretaceous sections are well imaged over most of the profile. From STAT 

345 to 375, Paleozoic strata through Eocene Wilcox Group 

strata have been folded and faulted into an anticline with 

approximately 0.025 s (38 m) of structural relief at the top of 

the Cretaceous. This structure appears to persist through to 

the top of the Wilcox. Upturned Wilcox strata from STAT 

350 to 375 appear to be truncated by an overlying reflector, 

which may be an unconformity near the top of the Wilcox. 

Within the anticline, at STAT 375, a steep west-dipping normal fault displaces the Wilcox 0.02 s (24 m). Also within the 

anticline, an east-dipping fault has been interpreted at STAT 

355 primarily on the evidence of the upturned Wilcox reflectors east of the fault and the upturned Cretaceous reflectors 

on the west side of the fault. The elevations of the mapped 

units are the same at the eastern and western ends of the seismic line. Thus, there is no net vertical displacement across 

this structure. The structure appears to be a transpressional 

strike-slip fault whose anticlinal core later subsided.

In the deep section, a west-dipping reverse fault at 

STAT 390 displaces the top of the Cretaceous by approximately 0.02 s (30 m).

At STAT 370, a west-dipping reverse fault displaces 

Claiborne strata by 0.01 s (7 m). Reflectors within the Claiborne from STAT 345 to 355 dip steeply to the west over the 

underlying west-sloping unconformity. This steep dip above 

the unconformity may be depositional dip, but we believe the 

high declivity suggests post-Claiborne reactivation of the 

underlying structure.

The faulting and folding in line 2 lie directly beneath 

the eastern margin of Crowley's Ridge.

MINI-SOSIE LINE 3

Line 3 is oriented north to south and was located across 

the southwest termination of the northern ridge segment 

(figs. 1 and 6). A data gap exists between STAT 565 and 575 

where the line crosses a highway. The line is 2 km long and 

the ridge is 40 m high at this location. The Tennark well (fig. 

1) is located on top of the ridge, 3.2 km north of line 3. Comparison of elevations of the stratigraphic units reveals that 

the Paleozoic and Cretaceous tops are 100 m higher and the 

Midway and Wilcox tops are 50 m higher in the Tennark 

well that is located on top of the ridge than in the southern 

end of line 3, which is south of the ridge (table 2). The data 

in table 2 also reveal that the Midway Group is approximately 50 m thicker in line 3 than in the Tennark well.
C8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

WEST 

120 

110

EAST

< tH 100

|1 90 

80

Mini-Sosie profile line 1: Crowley's Ridge, Arkansas

+87

-150 -

-180-

-231 —

-685-

-823 —

C

~1TL_——

W

M

300 600 METERS 

_J

227 220 210

C

— 0.5

— 0.9

200 190 180 170 160 150 

STATION NUMBER

140 130 120 110 104

Figure 4. Mini-Sosie line 1: A, topographic profile; B, reflection profile; C, line drawing. See figure 1 for location. The 

topographic profile has a 7x vertical exaggeration. P, Paleozoic; K, Cretaceous; M, Paleocene Midway Group; W, Eocene 

Wilcox Group; C, Eocene Claiborne Group.
T

p p

o io 

TWO-WAY TRAVEL TIME, IN SECONDS
CIO INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

SOUTH 

120

100 

90 

80 [ 

70

NORTH

Mini-Sosie profile line 3: Crowley's Ridge, Arkansas

0.0

^^,!^^^t^^5^^

-368

-508 —

0.9

676 670

c

660 650 640 630 620 610 600 590 580 

STATION NUMBER

560 550 540 530 520 510 503

Figure 6. Mini-Sosie line 3: A, topographic profile; B, reflection profile; C, line drawing. See figure 1 for location. The topographic 

profile has a 5x vertical exaggeration. P, Paleozoic; K, Cretaceous; M, Paleocene Midway Group; W, Eocene Wilcox Group; 

C, Eocene Claiborne Group.

Very few reflectors were imaged beneath the ridge from 

STAT 503 to 600; however, reflection characteristics from 

0.1 to 0.15 s are similar to the Claiborne in the southern portion of the line. Although reflection continuity is poor, it 

does appear that the upper 0.15 s of reflectors dip north from 

STAT 600 to 580. Reflectors south of the ridge are continuous and identifiable. From STAT 600 to 635, the reflectors 

dip southerly, descending 0.02 s (16 m); from STAT 635, 

they remain horizontal to the southern end of the line.

Perhaps most striking about this line is the absence of 

Wilcox or lower reflectors north of STAT 600 beneath the 

ridge. Reflectors can be traced beneath Crowley's Ridge at 

the other four Mini-Sosie sites; however, apparently local 

geologic conditions do not permit imaging of strata beneath 

the ridge at this location with the field and processing methods used in this study.

The upturned and truncated reflectors suggest that a 

fault lies beneath STAT 600 at the base of Crowley's Ridge.
SEISMIC REFLECTION STUDY OF CROWLEY'S RIDGE, NORTHEAST ARKANSAS Cll

We believe that either this seismic line has crossed a northwest-trending ridge-bounding fault or the line has intersected, at a low angle, one of the northeast-trending margin 

faults of the Reelfoot rift (fig. 1). In either case, faulting at 

this location appears to have occurred both during Midway 

deposition, because the Midway is thicker in seismic line 3 

than in the Tennark well, and after Wilcox deposition.

MINI-SOSIE LINE 4

Line 4 is a 1.5-km-long line located on the west side of 

the northern segment of Crow ley's Ridge (figs. 1 and 7). 

Topographic relief at this location is 20 m, with three small 

ridges paralleling the margin of Crowley's Ridge. There are 

no wells near line 4; however, the calculated elevations of 

the Paleozoic and Cretaceous in line 4 are reasonably close 

to those in line 5 and in the Quintin Little and Tennark wells.

All the reflectors dip gently to the east, with five faults 

displacing the top of the Cretaceous and two of the five displacing the base of the Wilcox. The fault at STAT 715 displaces the Cretaceous 0.06 s (61 m) down to the west. 

Reflectors are displaced in the Midway Group, but the base 

of the Wilcox is not disrupted above this fault. At STAT 

730, a second fault displaces the Cretaceous 0.015 s (22 m) 

and basal Wilcox down to the west. The fault at STAT 755 

also displaces the Cretaceous 0.02 s (30 m) and basal Wilcox 

down to the west. A small, west-dipping fault at STAT 770 

displaces the Cretaceous 0.01 s (12.5 m). The fault at STAT 

785 is a west-dipping fault with 0.01 s (12.5 m) of displacement. The net vertical separation across the entire fault zone 

is less than the sum of the fault displacements because the 

strata dip eastward.

There are two, perhaps three, faulting events revealed in 

line 4. The faulting at STAT 715 occurred during Paleocene 

Midway deposition but ceased before Wilcox deposition. 

This down-to-the-west normal faulting resulted in a thicker 

Midway section west of the fault. Normal faulting also 

occurred post lower Wilcox, as indicated by the displaced 

basal Wilcox at STAT 730 and 755. A possible compressional third faulting event is indicated by the small reverse 

fault at STAT 785. Absence of continuous reflectors in the 

overlying Midway Group at STAT 785 does not allow us to 

determine if the Midway Group has also been faulted, and so 

the timing of this event is indeterminable.

MINI-SOSIE LINE 5

Line 5 is a 5-km-long line located on the east side of the 

northern segment of Crowley's Ridge (figs. 1 and 8). The 

data gap between STAT 225 and 235 is where a highway and 

railroad track were crossed. Ridge topography is subdued in

this area, and the relief of the ridge is 15 m. The western portion of the profile was shot along a gravel road that followed 

a stream valley, so there is no sharp topographic break at the 

ridge margin located at STAT 240. Quality of the data in line 

5 is excellent with good continuity of reflectors beneath the 

ridge. The Quintin Little well is located approximately 2.5 

km north of the western end of line 5, and the Tennark well 

is located approximately 15 km southwest of line 5 (fig. 1). 

As illustrated in table 2, the elevations of the tops of the 

stratigraphic units in the wells are reasonably close to the 

calculated depths in line 5. However, we are uncertain 

whether the uppermost picked reflector is truly the top of the 

Wilcox or just a strong reflector near the top of the Wilcox.

From STAT 125 to 185, a broad anticline with 0.02 s 

(25 m) of amplitude is present in the Paleozoic and Cretaceous sections. The fold cannot be identified in the Midway 

Group, but a much shorter wavelength fold with 0.02 s (14 

m) of amplitude occurs in the Wilcox and Claiborne from 

STAT 150 to 175. When reflectors are projected into the data 

gap at STAT 230 from the east and west, there appears to be 

a down-to-the-east fault that displaces the Paleozoic through 

Wilcox by 0.04 s (56 m). This proposed fault lies beneath the 

ridge margin. A small horst with 0.01 s (9 m) of displacement is present between STAT 330 and 345. The fault that 

bounds the west side of the horst appears to fold strata as 

stratigraphically high as the top of the Wilcox.

A graben is clearly evident at the level of the Paleozoic 

and Cretaceous sections between STAT 380 and 415. The 

west bounding fault of the graben displaces the Paleozoic 

and Cretaceous sections 0.07 s (90 m). Normal-fault displacement of 0.035 s (28 m) at the base of the Wilcox and 

thickening of the Wilcox indicates that the fault was active 

during basal Wilcox deposition. The faulting can be traced to 

the top of the Wilcox, where it appears that there has been 

reverse movement after Wilcox deposition. This fault comes 

to within 0.1 s (75 m) of the ground surface and appears to 

be truncated by flat-lying Claiborne, but better resolution of 

the uppermost 0.2 s is necessary to determine if faulting 

reaches the surface. A shallow, antithetic, normal fault at 

STAT 390 displaces the Wilcox and Claiborne down to the 

west. The east bounding fault of the deep graben at STAT 

420 has 0.025 s (40 m) of displacement at the top of the Cretaceous section, and there is no evidence that this fault displaces overlying Tertiary sediments.

Strata from the western end of the line to STAT 380 are 

gently easterly dipping to horizontal except adjacent to the 

structures discussed above. From STAT 380 to the eastern 

end of the line, the Claiborne and Wilcox dip westerly 

(opposite to the regional dip) as do the underlying Cretaceous and Paleozoic strata east of the deep graben. Thus, it 

appears that, during post-Claiborne reactivation of the fault 

at STAT 380, there was minor westward tilting of the hanging-wall block.
C12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

WEST 

120

110

90 u

A Mini-Sosie profile line 4: Crowley's Ridge, Arkansas

EAST

^^S^S^^S^S^^K^^^S^Si^t *^-"H»J-^.L<!-.•**»icirr... \ > *f fcwi**Kftrft*a1'•»*;?• tyT^gj^^m ***^» •'• •• ^>'' • • \ *'• • • •• — t- nmtti C". LI IT fr>i ""L T"nDilTiViir—fT ti!

^^S^r^K^S^S^S^r^^^^^i^^^^: I!!

802800

C

790 780 770 760 750 740 

STATION NUMBER

730 720 710 702

Figure 7. Mini-Sosie line 4: A, topographic profile; B, reflection profile; C, line drawing. See figure 1 for location. The topographic profile has a lOx vertical exaggeration. P, Paleozoic; K, Cretaceous; M, Paleocene Midway Group; W, Eocene Wilcox 

Group.
a

2. 5? 

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8- I 

ho

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> 8 

5

l

8. ll 

n x II

T3

3

p

EL 

n

I

-a

SP

o O 

METERS IN DEPTH, SUBSEA 

l|

T

Q - 

m

SURFACE 

ELEVATION, 

METERS IN 

SECONDS IN TIME, TRAVEL TWO-WAY 

DMSIHS NOI1DH1JHH AQH1S JO .AH1AVOUD ' ISVHHl^ON SVSNV^^V 
C14 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

SIMILARITIES BETWEEN

CROWLEY'S RIDGE AND

THE NEW MADRID SEISMIC ZONE

A comparison of the shape and dimensions of 

Crowley's Ridge and earthquake epicenter distribution in the 

New Madrid seismic zone reveals striking similarities (fig. 

3). The southern portion of Crowley's Ridge is similar to the 

southern arm of seismicity (Blytheville arch); the left step of 

Crowley's Ridge at Jonesboro is similar to the northwesttrending zone of seismicity (Pascola arch and overlying Lake 

County uplift); and the northern segment of Crowley's Ridge 

is similar to the northeast-trending northwestern arm of New 

Madrid seismicity. In addition, the Reelfoot rift margin 

passes beneath both structures where major changes in orientation occur.

Focal mechanisms along the southern arm of New 

Madrid seismicity reveal right-lateral strike-slip faulting 

within the Blytheville arch (Herrmann and Canas, 1978; 

Stauder, 1982). Reverse faulting is recorded in the northwest-trending zone of seismicity within the Pascola arch, 

and right-lateral faulting is predominant in the northwestern 

zone of New Madrid seismicity. Like the New Madrid seismic zone, we believe that the bounding faults of the northern 

and southern segments of Crowley's Ridge may be strikeslip faults and that the bend at Jonesboro may be a left-stepping bend, perhaps with an underlying reverse fault.

The Mini-Sosie lines suggest that much of the faulting 

along Crowley's Ridge may be of Paleocene and Eocene 

age. We believe that Crowley's Ridge may primarily be a 

Tertiary structure and that current seismicity may now 

include, or has shifted to, the New Madrid seismic zone. 

Although most of the faulting of Crowley's Ridge may be 

Tertiary in age, it is important to note that the faults bounding the ridge are favorably oriented for strike-slip and 

reverse movement in the current Midcontinent stress field 

(Zoback and Zoback, 1989).

DISCUSSION AND CONCLUSIONS

A rather complex Tertiary structural history is revealed 

in the Mini-Sosie lines of Crowley's Ridge. Paleocene normal faulting is evident by the thickening of the Midway 

Group in line 4. Eocene normal faulting is also revealed by 

thickening of the Wilcox Group in line 5. Post-Wilcox (early 

Eocene) normal faulting is evident in all of the lines. The 

faulted anticline and post-Wilcox erosion in line 2 suggests 

that the area of line 2 was subjected to transpression and was 

subaerially exposed just after early Eocene Wilcox time. 

Post-Claiborne compression is evident in lines 2 and 5, and 

post-Claiborne normal faulting appears to have occurred in 

lines 1,3, and 5.

The steepness of the faults, the normal and reverse-fault 

movements that occur within a small area and over a relatively short length of time (Paleocene and Eocene), and the 

marked strike-slip appearance of the major faults in line 2 

suggest that most of these faults may be the upper portions 

of flower structures as has been suggested for other areas of 

the Mississippi Embayment (Luzietti and Harding, 1991; 

Schweig and others, 1992).

At line 1, the ridge height is the same as the structural 

relief on the ridge-bounding fault, and fault displacement 

can be traced through the shallowest resolvable reflectors 

(fig. 4). Thus, at this location, we believe the ridge margin is 

a fault scarp.

Mini-Sosie line 2 displays folding and faulting beneath 

the eastern margin of the ridge; however, the elevation of the 

Paleozoic and Cretaceous sections on both sides of the fold 

are the same (fig. 5). Ten m of post-Claiborne displacement 

on the small reverse fault at STAT 370 may have lifted 

Crowley's Ridge to provide some of the 30 m of local ridge 

relief.

We believe that the ridge margin at line 3 (fig. 6) is 

probably a fault line because of the upturned and sharply 

truncated seismic reflectors beneath the ridge margin, the 

declivity and linearity of the ridge margin, and the difference 

in elevations of the stratigraphic units in line 3 as compared 

to the Tennark well on top of the ridge. In the subsurface, a 

fault through this area appears to define the southwestern 

limit of artesian flow from the Upper Cretaceous Nacatoch 

Sand (Boswell and others, 1965); the fault separates two 

areas of different crustal thickness (Mooney and others, 

1983); and the southeastern projection of the fault defines 

the southern limit of intense liquefaction in 1811-12 and 

modern New Madrid seismicity near Marked Tree, Ark. 

(Cox, 1988a, 1988b).

Line 4 reveals a number of faults near the ridge margin 

(fig. 7). Of the five lines, this line has the greatest amount of 

topography, and we propose that this topography may be due 

to displacement on the underlying faults because the faults 

appear to project to the surface at the base of each small 

ridge.

Folding and faulting are revealed in line 5 (fig. 8). PostClaiborne folding is well expressed in this line in addition to 

post-Wilcox movement on a large graben at the eastern end 

of the line. The eastern end of the line intersects the western 

margin of the Reelfoot rift (fig. 1); thus, we believe that the 

graben indicates Tertiary reactivation of the west bounding 

faults of the Reelfoot rift.

Crowley's Ridge has faults beneath its margins at all 

five Mini-Sosie line locations. Because the seismic surveys 

did not image the uppermost 0.15 s (150 m), it is not possible 

to determine if these faults reach the ground surface. Thus, 

we cannot determine if the ridge truly owes its topographic 

relief to fault displacement or if Tertiary faulting controlled 

subsequent Quaternary river incision. However, we do
SEISMIC REFLECTION STUDY OF CROWLEY'S RIDGE, NORTHEAST ARKANSAS C15

conclude that Crowley's Ridge is not a simple erosional remnant but is structurally controlled.

The seismic potential of the Crowley's Ridge faults is 

difficult to assess. Crowley's Ridge is seismically quiet. 

However, the fact that the faults imaged in this study lie at 

the base of the ridge margins suggests that they have been 

active within the Quaternary. We believe it is geomorphically unreasonable to maintain topographic scarps above 

Tertiary faults, particularly when Crowley's Ridge is made 

of poorly consolidated sands, clays, and gravel.

Most of the faults identified in this study cut the top of 

the Paleozoic section and have been traced into the Eocene 

section. However, until we can determine if these faults displace the overlying Wisconsin terrace (Saucier, 1974; Royall 

and others, 1991) we cannot ascertain the seismic hazard of 

these faults (Reiter, 1990). A follow-up study is in progress 

to seismically image the uppermost 0.2 s above the faults 

identified in this study to determine if the Pliocene and Pleistocene sections have been faulted.

ACKNOWLEDGMENTS

The contents of this report were developed under a 

grant from the Department of the Interior, U.S. Geological 

Survey (USGS) (14-08-0001-G1926). However, the contents of this report do not necessarily represent the policy of 

the USGS, and one should not assume endorsement by the 

Federal Government. The project could not have been 

accomplished without the dedicated work of the U.S. Geological Survey's Mini-Sosie crew and students from the University of Arkansas and Memphis State University.

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