Gravity of the New Madrid Seismic ZoneA Preliminary Study

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-L

MISSOURI

"IGL'si: H'i 

ARKANSAS

TENNESSEE

Jiff*. ir>
Cover. Gray, shaded-relief map of magnetic anomaly data. Map area includes parts of 

Missouri, Illinois, Indiana, Kentucky, Tennessee, and Arkansas, fllumination 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).
Gravity of the New Madrid Seismic ZoneA Preliminary Study

By V.E. Langenheim

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-L

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

For sale by U.S. Geological Survey, Information Services

Box 25286, 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-in-Publication Data

Langenheim, Victoria E.

Gravity of the New Madrid seismic zone: a preliminary study / by V. E. Langenheim.

p. cm. — (Investigations of the New Madrid seismic zone) 

(Geological Survey professional paper : 1538-L)

Includes bibliographical references.

1. Gravity anomalies—New Madrid Region—Measurement 2. Seismicity— 

Mississippi Embayment. I. Title. II. Series. III. Series: Geological Survey 

professional paper ; 1538-L. 

QB337.5.U5L36 1996

551.2'2'09788985 s—dc20 94-24610 

[526'.7'09788985] CIP
CONTENTS

Abstract........................................................................................................................... LI

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

Geologic Setting............................................................................................................. 1

Data................................................................................................................................. 3

Geophysical Features...................................................................................................... 10

Regional Gravity..................................................................................................... 10

Detailed Gravity Profiles........................................................................................ 11

Discussion....................................................................................................................... 15

Conclusions.................................................................................................................... 16

Acknowledgments.......................................................................................................... 17

References Cited............................................................................................................. 17

	FIGURES

1. Map showing seismicity and inferred structures in the northern Mississippi Embayment......................................... L2

2. Shaded-relief complete Bouguer gravity map of the northern Mississippi Embayment............................................ 4

3. Shaded-relief pseudomagnetic map of northern Mississippi Embayment.................................................................. 5

4. Magnetic map of the northern Mississippi Embayment.............................................................................................. 6

5. Upward-continued gravity map of the northern Mississippi Embayment.................................................................. 7

6. Shaded-relief residual gravity map of the northern Mississippi Embayment............................................................. 8

7. Complete Bouguer gravity map showing locations of detailed gravity profiles......................................................... 9

8. Gravity and magnetic profiles along lines 1,2, and 3................................................................................................. 12

9. Simple two-dimensional model of gravity data on line 1........................................................................................... 14

III

GRAVITY OF THE NEW MADRID SEISMIC ZONEA PRELIMINARY STUDY

By V.E. Langenheim

ABSTRACT

The gravity field over the northern Mississippi 

Embayment indicates no vertical offsets of the unconformity 

between Paleozoic and Cretaceous rocks of greater than 0.25 

km. Using the maximum-horizontal-gravity-gradientmethod, 

it is possible to locate density boundaries within the subsurface 

that delineate small, post-Cretaceous faults or igneous intrusions. Detailed gravity profiles display small-amplitude (less 

than 2 mGal), short-wavelength anomalies that are caused by 

shallow sources (less than 1 km deep). These anomalies may 

result from density variations caused by (1) post-Cretaceous 

faults, (2) igneous bodies, or (3) fill in ancient stream channels. 

Specific sources for these anomalies can be distinguished by 

the anomaly shape (linearity, amplitude, width), as well as by 

using other geophysical data (e.g., magneticand seismic data).

INTRODUCTION

Three devastating earthquakes shook the northern 

Mississippi Embayment in the winter of 1811 and 1812, 

destroying the town of New Madrid, Mo. (Nuttli, 1973). The 

New Madrid area is currently the most seismically active 

region of the Central and Eastern United States. Because 

Tertiary and Cretaceous sedimentary rocks and unconsolidated Quaternary sediments cover the New Madrid seismic 

zone, potential-field geophysics has played a crucial role in 

understanding the geologic framework of the Mississippi 

Embayment region (Ervin and McGinnis, 1975; 

Hildenbrand and others, 1977,1982; Kane and others, 1981; 

Hildenbrand, 1985). Potential-field studies have outlined 

buried plutons and an ancient rift system that play a key role 

in influencing the distribution of seismicity. In general, 

though, these previous studies have been regional in scope 

and have not investigated the small-amplitude, 

short-wavelength anomalies that result from shallow 

sources. Such sources may provide information on specific 

structures that control the release of seismic energy in the

region. One of the few studies addressing these 

short-wavelength anomalies is the detailed gravity and 

ground-magnetic data collected across the Reelfoot scarp 

(Stearns, 1980). In this study, I discuss the gravity field over 

the seismically active zones in the northern Mississippi 

Embayment in an attempt to identify possible shallow 

gravity sources related to faulting over a broader area. 

Existing regional gravity data are filtered to enhance certain 

anomaly characteristics such as wavelength or trend and thus 

provide new interpretive information about the distribution 

and nature of shallow structures.

GEOLOGIC SETTING

The northern Mississippi Embayment is characterized 

by a broad, southwest-plunging syncline filled with 

unconsolidated Upper Cretaceous and Tertiary sedimentary 

deposits, which unconformably overlie lower Paleozoic 

limestone, shale, and sandstone (Stearns and Marcher, 

1962). Precambrian crystalline rocks, which underlie the 

Paleozoic rocks and form the basement, may be equivalent 

to rocks exposed in the St. Francois Mountains in Missouri. 

The northeast-trending Reelfoot graben, imaged primarily 

by magnetic methods (Hildenbrand and others, 1982; 

Hildenbrand, 1985), developed during rifting in 

Precambrian or Cambrian time (fig. 1). The graben is 70 km 

wide, with a structural relief of about 2 km, and is flanked by 

several large, dense, magnetic plutons. These mafic 

intrusions could have formed during the Precambrian or 

Cambrian rifting episode or as recently as the Cretaceous 

during an episode of rift reactivation (Glick, 1982; 

Hildenbrand and others, 1982). One well has penetrated 

Precambrian granitic gneiss within the graben (Denison, 

1984). Regional subsidence and deposition during the early 

Paleozoic resulted in the thick accumulation of sedimentary 

rocks in the Reelfoot Basin. These rocks were then uplifted 

and eroded to form the Pascola arch during late Paleozoic 

and possibly Mesozoic time (Grohskopf, 1955). Late 

Cretaceous subsidence and deposition of marine and

Ll
L2 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

91 C 90° 89°

37C

ST. FRANCOIS 

MTNS

• *f

• MISSOURI 

ARKANSAS

7 _*/

, - , BLOOMFIELD >-v^;^^ , • • V > V ' • . ^-*?

\ PLUTON / , JA ,^{..-s.^ ---I ^~

+

Olderrocks } " ^^^ / . 4.] ''[/(*

/z^ ' / , - f'irS- ' /\ * Cretaceous and. ^<» / :> v •p^../ 

'ertiary sediments >"/• . ^\^^Ci/t*^- .^•a^K^r.^•/ ' ^-- cvy 's/xx*'" 

,!/ S VXA ^/KENTUCKY t-^-.yy.—St/J&ST^-^-----------

'• I ^ ^ ^^TENNESSEE r-. 1 -^o^ /^

__ : ;:

Cretaceous and 

Tertiary sediments

35

n r

0 5 10 15 KILOMETERS

Figure 1. Map showing seismicity and inferred structures in the northern Mississippi Embayment and surrounding regions. 

Circles indicate earthquakes. Areas with diagonal-line pattern denote inferred mafic plutons whose boundaries are based on 

maximum-horizontal-gravity-gradient method. For some plutons, more than one boundary may be necessary to delineate the areal extent of the pluton (i.e., Covington). Thin lines indicate density boundaries calculated from residual gravity field. Stippled 

area outlines the Blytheville arch. Stars show locations of drill wells that bottomed in Precambrian basement, and lines A-A' 

and B-B' delineate the graben margins as interpreted from magnetic data (Hildenbrand and others, 1979). Thick line outlines extent of Cretaceous and Tertiary sedimentary deposits. MT, Marked Tree, Ark.; C, Caruthersville, Mo.; NM, New Madrid, Mo.; 

R, Ridgely, Tenn.; CGF, Cottonwood Grove fault; RF, Reelfoot fault.
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY L3

nonmarine clastic sediments in the Mississippi Embayment 

continued during the Tertiary. These deposits are covered by 

a thin blanket of Quaternary fluvial deposits.

Earthquakes occur in the New Madrid seismic zone 

along three well-developed linear zones: (1) a 

northeast-trending linear zone extending from Marked Tree, 

Ark., to Caruthersville, Mo., (2) a north-northwest-trending 

zone from Ridgely, Tenn., to New Madrid, Mo., and (3) a 

northeast-trending zone west of New Madrid (fig. 1). Earthquakes are characterized by both right-lateral and thrust 

focal mechanisms (Herrmann and Canas, 1978) at depths of 

3 to 14 km (Andrews and others, 1985). Seismic reflection 

data have revealed a major disrupted zone and antiform 

within the Paleozoic sedimentary sequence that coincides 

geographically with the Marked Tree-Caruthersville seismic 

zone (Howe and Thompson, 1984; Crone and others, 1985; 

Hamilton and McKeown, 1988). This disrupted zone, named 

the Blytheville arch, is about 10-15 km wide and 110 km 

long. Various explanations have been suggested for the 

origin of the disrupted zone, including felsic igneous intrusions (Crone and others, 1985), shale diapirs (McKeown and 

others, 1990), and fault zones (Howe and Thompson, 1984; 

Hamilton and McKeown, 1988). Seismic reflection data 

have also imaged faults that displace Upper Cretaceous and 

Tertiary rocks (Zoback and others, 1980; Hamilton and 

Zoback, 1982; Sexton and Jones, 1986), but individual faults 

with vertical offsets greater than 100 m have not been 

identified.

Using remote sensing data, Schweig and Marple (1991) 

have found an intriguing lineament that may be the surface 

expression of one of the coseismic faults of the 1811-12 

New Madrid earthquakes. The Bootheel lineament is a 

135-km-long zone of liquefied sand erupted through 

fissures, but it does not coincide exactly with any of the 

major trends in seismicity.

DATA

The gravity map of the New Madrid region (fig. 2) was 

compiled from several data sets obtained from the Defense 

Mapping Agency and incorporates more than 14,800 gravity 

stations. Within the northern Mississippi Embayment, the 

average station spacing is about 2 km, but gaps are present 

along the Mississippi River and over wildlife refuges and 

lakes. The data have been reduced to free-air gravity values 

using standard formulas (Telford and others, 1976). 

Bouguer, curvature, and terrain corrections were applied to 

the free-air value at each station to determine the complete 

Bouguer gravity anomalies using a standard reduction 

density of 2.67 g/cm3 (Plouff, 1977). The data were edited to 

remove single-station anomalies and then gridded using a 

1-km interval.

In order to enhance short-wavelength features due to 

shallow sources, the gridded data were filtered using two

different techniques. The term "shallow sources" refers to 

rock bodies or structures that are present within the upper 3 

to 4 km; "deep" refers to rock bodies that are more than 3 to 

4 km deep. The first technique used the first vertical derivative of the gravity field, which suppresses longer wavelength 

regional trends. The method also has a physical significance 

in that both the force of gravity and the magnetic scalar 

potential are proportional to 1/r2, where r is the distance 

from the causative body. In other words, the first vertical 

derivative of the gravity field is equal to the magnetic field 

in the case where the density of the causative body is 

replaced in constant proportion with a magnetization of 

known direction (Dobrin and Savit, 1988). Except for a 

multiplicative constant, the first-vertical-derivative anomalies can be considered to be pseudomagnetic anomalies. A 

low-pass filter was then applied to the first-vertical-derivative map in order to smooth small, spotty features that are the 

result of noise in the data set. Figure 3 shows the first vertical 

derivative of the gravity field, and figure 4 shows the 

magnetic field. Features on these maps are discussed later.

Another method for separation of residual from 

regional anomalies involves subtracting the upward 

continuation of the gravity field from the actual data. 

Upward continuation is the transformation of gravity data 

measured on one surface to a higher surface; this operation 

tends to smooth the data by attenuation of short-wavelength 

anomalies (Dobrin and Savit, 1988). Subtraction of the 

upward-continued field from the original data results in 

high-pass-filtered, residual anomalies. Figure 5 shows the 

gravity field continued upward to 3 km, and figure 6 shows 

the residual gravity field after subtracting the 

upward-continued gravity field from the complete Bouguer 

gravity field.

To help delineate trends and gradients in the gravity 

field, maximum-horizontal-gradient values were calculated 

using a computer algorithm (Blakely and Simpson, 1986). 

Gradient maxima occur directly over vertical or near-vertical contacts that separate rocks of contrasting densities. 

These density boundaries were calculated for both the residual gravity data (fig. 6) and the upward-continued gravity 

(fig. 5).

To supplement the regional gravity coverage, the U.S. 

Geological Survey collected three detailed gravity profiles 

during the summer of 1990 near preexisting seismic reflection lines (fig. 7). The data were collected with a LaCoste 

and Romberg gravimeter at 300- to 600-m intervals using 

surveyed elevations and locations, although spacing may be 

greater locally because of access problems (i.e., the 

Mississippi River). The observed gravity values were referenced to the IGSN71 datum as described by Morelli (1974) 

and were then reduced to complete Bouguer anomaly values 

using standard formulas (Telford and others, 1976). No 

regional correction has been applied to the data. The 

measurements are considered accurate to 0.2 mGal.
L4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

37°

36°

35°

10 40 50 KILOMETERS

MILLIGALS

Figure 2. Shaded-relief map of complete Bouguer gravity of the northern Mississippi Embayment. Illumination is from the 

west. Contour interval is 5 mGal. Dots indicate positions of maximum horizontal gravity gradient (density boundaries). B, Bloomfield pluton; C, Covington pluton; J, Jonesboro pluton; M, Maiden pluton; P, Paragould pluton. State boundaries are shown as 

solid lines.
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY L5

91

37°

36C

35C

-3,000 -1,500 1,500 3,000

50 MILES 

I

4,500 NANOTESLAS

10

I 

50 KILOMETERS

Figure 3. Pseudomagnetic shaded-relief map of the northern Mississippi Embayment computed from Bouguer gravity map. 

Illumination is from the west. Contour interval is 500 nT. Shading process enhances subtle structures as well as limitations in the 

quality of the data. B, Bloomfield pluton; C, Covington pluton; J, Jonesboro pluton; M, Maiden pluton; P, Paragould pluton. X-X 

marks a narrow, discontinuous high along the center of the Reelfoot rift. State boundaries are shown as solid lines.
L6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

90° 89°

36°

35

-300 -200

%&m&x?

NANOTESLAS

10

40 50 MILES

I 

50 KILOMETERS

Figure 4. Magnetic field map of the northern Mississippi Embayment. Contour interval is 50 nT. Merged survey at 1,000 ft above 

ground (Hildenbrand and others, 1979). Magnetic data have been reduced to the pole, assuming the direction of the total magnetization 

vector of 66° N. inclination and 1.5° E. declination. The area of the Reelfoot graben is depicted by the belt of relatively smooth 

magnetic field that trends northeast across the map between plutons P and C. B, Bloomfield pluton; C, Covington pluton; J, Jonesboro 

pluton; M, Maiden pluton; P, Paragould pluton. State boundaries are shown as solid lines.
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY

90° 89°

L7

35

-28 -21 -14

0

-7

20

14 21

50 MILES

28 MILLIGALS

20 40 50 KILOMETERS

Figure 5. Gravity map of the northern Mississippi Embayment continued upward to an altitude of 3 km. Contour interval is 5 

mGal. Dots indicate positions of maximum horizontal gravity gradient. B, Bloomfield pluton; C, Covington pluton; J, Jonesboro 

pluton; M, Maiden pluton; P, Paragould pluton. Anomalies reflect long-wavelength component of gravity field and thus reflect 

sources with considerable vertical extent. State boundaries are shown as solid lines.
L8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

37°

36°

35C

i$sSf8^»*

* *&'"£."* '-f9- .*' ~~ i ••* "i »- ••-'iwA- *.' **»«.J 

J^^JJ'^-* .^sKjA.- :^- /U "/J '

>V J»' J. 3 x* vv^rr. •- ••**« *.%. • *.* '.^ «•

IM^MP

^^-j^v^'j^N^,^,; ,3:K J^i^^

t.H-^,-A%f,^r^

-5 -3

10 20

5

40

MILLIGALS

50 MILES

_J

10

I 

50 KILOMETERS

Figure 6. Shaded-relief map of residual gravity of the northern Mississippi Embayment. Residual gravity field is obtained by 

subtracting gravity field upward-continued to 3 km from the complete Bouguer gravity field. Illumination is from the west. 

Anomalies reflect short-wavelength component of gravity field and thus emphasize shallow gravity sources. Contour interval is 

1 mGal. Dots indicate positions of maximum horizontal gravity gradient. B, Bloomfield pluton; C, Covington pluton; J, Jonesboro 

pluton; M, Maiden pluton; P, Paragould pluton. State boundaries are shown as solid lines.
L10 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

GEOPHYSICAL FEATURES

REGIONAL GRAVITY

Bouguer gravity values range from -50 mGal in the 

northwestern part of the map (over the low-density St. 

Francois Mountains, a 1.2- to 1.5-Ga rhyolitic and granitic 

terrane (Bickford and others, 1981)) to almost 40 mGal in 

the north-central part of the map (fig. 2). Rocks in the St. 

Francois Mountains have an apparent average density of 

2.67 g/cm3 . The largest positive gravity anomalies occur 

over the unconsolidated sediments of the Mississippi 

Embayment and coincide with high-amplitude magnetic 

anomalies (fig. 4). The causative bodies are considered to be 

dense, highly magnetic mafic plutons that are as shallow as 

1.5 to 3.0 km below the ground surface (Hildenbrand and 

others, 1982; Ravat and others, 1987). Only one such anomaly has been drilled: a well near the Covington pluton in 

southwestern Tennessee (fig. 7) encountered nepheline 

syenite directly beneath Upper Cretaceous sedimentary 

rocks (Kidwell, 1951); the age of the pluton can only be 

constrained as post-Ordovician and pre-Late Cretaceous. A 

prominent northwest-trending gravity low extending from 

the northwest corner of the map (fig. 2) is the Missouri 

gravity low, which Guinness and others (1982) interpreted 

as the expression of a Precambrian rift and which Hildenbrand and Hendricks (chapter E, this series) interpret as a 

batholith. It intersects a somewhat more subdued, northeast-trending gravity trough in the center of the map (fig. 2). 

This northeast-trending feature is the gravity expression of 

the Reelfoot graben, which is more strikingly imaged by the 

magnetic anomaly map and pseudomagnetic and 

residual-gravity-anomaly maps (compare fig. 1 with figs. 3, 

4, and 6). Most of the seismicity in the New Madrid seismic 

zone occurs within the area at the intersection of the two 

gravity lows (Guinness and others, 1982; Hildenbrand and 

Hendricks, chapter E, this series; compare figs. 1 and 2). The 

gravity field within the Reelfoot graben is relatively nondescript, but several northeast-trending gravity highs with 

amplitudes of 5 to 15 mGal are present along the axis of the 

graben (fig. 2). These gravity highs coincide geographically 

with magnetic highs, suggesting that the causative bodies 

are buried igneous intrusions of intermediate to mafic composition.

The pseudomagnetic and residual gravity fields (figs. 3 

and 6) more closely resemble the magnetic field. For 

example, the Covington pluton is a broad (more than 50 km 

wide), somewhat blocky high on the unfiltered gravity. On 

the filtered gravity maps (figs. 3 and 6), this broad high 

separates into two to three isolated anomalies; the central 

high most closely resembles the magnetic anomaly of the 

Covington pluton, a small anomaly elongated in both 

east-west and northeast-southwest directions. The gravity 

high 25 km northwest of Covington still appears to be 

associated with the main anomaly, contrary to more 

completely separated anomalies on the magnetic map; the

shape of this gravity high northwest of Covington, though, 

is more arcuate, as is the magnetic anomaly. Most of the 

gravity highs along the margins of the graben are characterized by blocky northeast- or northwest-trending edges (fig. 

2), suggesting either that older structures provided pathways 

for ascending magmas or that younger faults have truncated 

pluton boundaries. Considering the long and complex 

geologic history of the Mississippi Embayment, the rising 

magmas that formed the plutons probably took advantage of 

older or contemporaneous zones of weakness within the 

crust. The Maiden pluton (fig. 2) is an exception to the 

blocky texture of the gravity field; it is a circular, isolated, 

gravity high but is also smaller in size than the other major 

plutons.

Both northwest and northeast trends on the Bouguer 

gravity map are enhanced on the filtered-gravity-anomaly 

maps (figs. 3 and 6). Many of these trends are also present in 

the magnetic field, suggesting that these gradients reflect 

density and magnetic susceptibility boundaries in crystalline 

rocks (Precambrian basement rocks or igneous rocks of 

younger age). However, the pseudomagnetic map shows a 

10- to 20-km-wide, discontinuous feature (X-X', fig. 3) 

along the center of the Reelfoot graben that is not present on 

the magnetic map. This northeast-trending feature is also 

visible on the residual map (fig. 6) as a gravity high of 1-2 

mGal. The southwestern part of the high corresponds 

geographically to the Blytheville arch and the Marked 

Tree-Caruthersville zone of earthquakes; the northeastern 

part of the high, separated from the southeastern trend by a 

gravity low, trends more northerly than does the zone of 

earthquakes and appears to be associated with a 

horseshoe-shaped magnetic anomaly (fig. 4). Thus, the 

source for the northeastern part of the high is most likely an 

igneous intrusion, whose linear edges, expressed in the 

gravity field, suggest faulting at shallow depths. The source 

for the southwestern part of the feature is essentially 

nonmagnetic; slightly denser sedimentary rocks forming the 

core of the anticlinal structure of the Blytheville arch could 

explain the presence of the narrow, discontinuous gravity 

high at the southwestern end of X-X'.

The largest amplitudes of horizontal gravity gradient 

occur along the boundaries of the plutons and over the northeastern part of the Reelfoot graben, where values reach 6 

mGal/km. A comparison of the upward-continued and 

residual gravity fields shows that some features, such as the 

plutons and the southeastern graben boundary, contain both 

short- and long-wavelength components (figs. 5 and 6). 

Many other features, however, only appear on the residual 

map, implying that they do not extend to any great depth 

(i.e., more than a few kilometers). For example, most of the 

density boundaries within the graben are visible only on the 

residual map. The density boundaries derived from the 

residual gravity field are aligned along a predominantly 

northeasterly direction within the graben and along 

northwesterly directions in the area west of the graben.
90° 15' 

36°30'

GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY

90°00' 89°45' 89°30'

L9

36°15'

36°00'F MISSOURI __+_ 

ARKANSAS

35°45'

89°15'

15 KILOMETERS

Figure 7. Complete Bouguer gravity map showing locations of detailed gravity profiles. Contour interval is 2 mGal. 'X" indicates 

location of gravity measurement; star, location of selected well penetrating igneous rocks—see discussion in text (1, Strake Petroleum 

Incorporated, T.P. Russell No. 1; 2, Henderson Oil Markham No. 1; 3, O.W. Killiam, K. Pattinson No. 1 (Grohskopf, 1955); 4, Pure 

Oil Co. R.R. McGregor No. 1 (Dart, 1992)); triangle, selected seismic-reflection-profile shotpoint. Gravity profile of Sexton and Jones 

(1988) was conducted along the six easternmost shotpoints of seismic profile S-6. Stippled area outlines Blytheville arch. CGF, 

Cottonwood Grove fault; RL, Ridgely lineament. Anomalies A and B are also shown on figures 8 and 9.
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY Lll

Some of the northeast-trending boundaries occur in approximately the same location as faults that have been imaged on 

seismic reflection profiles. For example, density boundaries 

coincide with the Reelfoot fault and the Cottonwood Grove 

fault (Hamilton and Zoback, 1982). The magnitude of the 

horizontal gradient across these faults is relatively small, 

usually less than 1 mGal/km. Figure 1 shows selected 

density boundaries digitized from the residual gravity map; 

these boundaries were chosen only where station coverage 

was adequate for determining the locations of the horizontal 

gravity gradients. Pluton boundaries were derived from a 

combination of residual and upward-continued density 

boundaries (fig. 1). The significance of these boundaries is 

discussed later.

DETAILED GRAVITY PROFILES

Three profiles collected by the U.S. Geological Survey 

were located to examine small-amplitude anomalies that 

might result from shallow structures in the New Madrid 

seismic zone (fig. 7). Line 1 (fig. 8A) is an 87-km-long, 

east-west line that crosses both the Marked Tree-Caruthersville and the Ridgely-New Madrid zones of seismicity. The 

westernmost gradient reflects the presence of the Paragould 

pluton (figs. 1,8A). Although the magnitude of gravity variation is small along line 1 (less than 5 mGal), the abrupt 

changes in gradient indicate very shallow sources (at most 

about 1 km deep; anomalies A and B on figs. 8A, 9). The 

depth to the top of the Paleozoic rocks in this area ranges 

from 450 to 730 m, based on well and seismic data (Dart, 

1992). Thus, the sources are possibly at or above the top of 

the Paleozoic section, which is Cambrian to Ordovician in 

age along line 1 (Grohskopf, 1955). Possible sources for 

these small-amplitude, high-frequency gravity anomalies 

include (1) lateral density variations within the 

post-Paleozoic sedimentary rocks, (2) the presence of small 

igneous intrusions, or (3) undulations or faulted offsets in the 

Paleozoic surface.

Lateral density variations within the post-Paleozoic 

sedimentary rocks may be related to low-density fill in 

ancient stream channels. Densities measured in well logs 

within the Cretaceous and Cenozoic sedimentary sequence 

range from 1.42 to 2.65 g/cm3 (Crone and Russ, 1979). 

Sexton and Jones (1988) imaged Eocene stream channels on 

high-resolution seismic reflection lines and modeled the 

gravitational effects of the fill using a density contrast of 

0.10 g/cm3 to fit the observed gravity field. Thus, density 

contrasts exist within the post-Paleozoic section that could 

cause small-amplitude and small-wavelength anomalies. In 

the particular case of line 1, the widths of anomalies A and 

B (greater than 10 km) are larger than those of very wide 

stream channels, implying a non-fluvial source for the 

anomalies. The gravity anomalies are the southernmost

expression of two regional gravity anomalies that have 

straight, northeast-trending edges that are about 20 km in 

length (fig. 7); such long, straight edges for stream channels 

seem improbable. The gravity anomalies also appear to be 

spatially related to magnetic anomalies (fig. 8A), which is 

suggestive of an igneous origin. Thus, it seems unlikely that 

the cause of the gravity anomalies on line 1 is lateral density 

variations associated with stream channels within 

Cretaceous-Cenozoic sedimentary rocks, although 

channel-fill deposits are potential candidate sources for 

gravity anomalies on other profiles.

The second possibility, small igneous intrusions, is 

certainly intriguing in that such intrusions, at depths of less 

than 1 km (and thus possibly younger than the Cretaceous 

unconformity) could suggest Tertiary igneous activity 

within the Mississippi Embayment. A Tertiary igneous event 

has been inferred from on the basis of deformation of 

shallow reflectors (Zoback and others, 1980) and from 

doming of pre-Eocene sediments over a pluton along the 

graben margin (Glick, 1982). However, if igneous rock 

intrudes only the Paleozoic rocks, the age of the intrusion is 

only constrained as post-Middle Ordovician. In the case of 

the anomalies on line 1, the spatial relationship of the gravity 

and magnetic anomalies would support this interpretation of 

a pluton intruding at least the Paleozoic rocks. However, the 

magnetic signature along line 1 does not reveal the 

high-frequency anomaly expected for a magnetic body at 

depths of about 1 km. Instead, the magnetic gradients 

indicate maximum depths of 2 to 3 km; such depths would 

place the intrusion entirely within Paleozoic sedimentary 

rocks or at the top of the Precambrian basement in this area. 

A seismic reflection line along the western two-thirds of line 

1 (west of the Mississippi River) does image disrupted and 

arched reflections in the area of anomaly A, which have been 

interpreted as an intrusion at depths of about 1.5 km (F.A. 

McKeown, written commun., 1992). Superimposed on 

anomaly B are two very small anomalies of about 0.5 mGal, 

which correspond in location to very small magnetic 

anomalies of about 6-8 nT (arrows on fig. 8A). These features could be caused by small dikes at very shallow depths. 

Part of the magnitude of the gravity anomalies on line 1 is 

caused by igneous intrusions, but this source alone cannot 

explain both the magnetic and gravity signature of the 

anomalies.

The third possible explanation of the source of the 

high-frequency gravity anomalies is undulations or vertical 

offsets in the unconformity on the Paleozoic rocks. A substantial average density contrast of about 0.5 g/cm3 exists 

between the Paleozoic rocks (average density about 2.7 

g/cm3) and the post-Paleozoic sedimentary rocks (average 

density of about 2.2 g/cm3). Crone and Brockman (1982) 

have argued that little topographic relief on Paleozoic bedrock existed by Late Cretaceous time, based on geologic 

considerations and seismic reflection data. Seismic line S-6, 

which is parallel to but about 8 km north of the eastern part
L12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

CD W

150

CD

w 100

Z 50

2 I-H o

Q_I -50 

W

E

QJ -100

I-H

H

w2-15°

o

-200

10 20 30 40 50 60 70 80

en

-12

-13

-14

-15

-16

-17 

G -18

W 

E

PARAGOULD

PLUTON

10 20 30 40 50 60 70 80

DISTANCE, IN KILOMETERS

Figure 8 (above and facing page). Locations of gravity-profile lines 1, 2, and 3 are shown in figure 7. A, Gravity and magnetic-field 

profile along line 1. Cross indicates gravity measurement; circle, gridded magnetic-field point; line on magnetic profile, data along flight 

line at 90-m altitude. Gridded data have not been adjusted relative to the arbitrary datum of flight-line data. Anomalies A and B are discussed 

in text as bounded by gradients that indicate sources at depths of less than 1 km. Dashed lines between gravity measurements indicate offset 

or gap in sampling. CGF shows the projection of the Cottonwood Grove fault onto line 1, and "eq" indicates where line 1 crosses zones of 

seismicity. Arrows point to small-amplitude, short-wavelength anomalies that may indicate buried igneous dikes. B, Gravity and 

magnetic-field profile along line 2. Cross indicates gravity measurement; circle, gridded magnetic-field point; solid line on magnetic profile, 

data along flight line at 90-m altitude. Gridded data have not been adjusted relative to the flight-line data. Gravity and magnetic profiles 

projected onto east-west line that bends to the northeast at about 47 km. Arrows indicate coincident magnetic and gravity anomalies that 

may indicate buried igneous bodies. C, Gravity and magnetic-field profile along line 3. Cross indicates gravity measurement Magnetic-field 

data from grid. Gravity and magnetic high on southern part of profile indicates an igneous intrusion that may be associated with the 

Covington pluton. Arrows point out small inflections on gradient that may reflect the presence of faults, igneous bodies, or stream channels.
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY

3 300

O 200 

Z

z 10°

M

i °

M -100

u z

O -200

-6

-7

-8

-9

-10

-11 

fc -12

S-.3

O

-14

-15

0 10 20 30 40 50

10 20 30 40 50 

DISTANCE, IN KILOMETERS

NE

60

60

100

g -50

Q

Sj -loo

C ~15° 

Q -200

-250

0

L13

N

10 20 30 40 50 60 70

10 20 30 40 50 

DISTANCE, IN KILOMETERS

70

B

of line 1, images the Cotton wood Grove fault (CGF) at the 

approximate location of anomaly B (Zoback and others, 

1980; Hamilton and Zoback, 1982). The amount of vertical 

offset on the CGF is about 80 m. Using the infinite Bouguer 

slab approximation, the maximum gravity anomaly resulting 

from an 80-m offset of the Cretaceous unconformity (and 

assuming a density contrast of 0.5 g/cm3 ) would be about 1.7 

mGal, which is about half of the 3 mGal anomaly observed 

on line 1 (fig. 8A). The maximum amount of offset predicted 

from gravity data is thus about 160 m, double that imaged on 

the seismic reflection line. Seismic line S-7 images another 

small fault with an offset of 20 m, that lies along trend with 

anomaly A (Hamilton and Zoback, 1982); the predicted 

offset from the gravity data is about 100 m.

The discrepancy between the fault-offset estimates may 

disappear when one considers the gap in sampling at the 

exact location of the CGF and that igneous intrusions are 

present beneath line 1. Both anomalies A and B are located 

on the southernmost tip of two regional gravity highs (fig. 7) 

and their corresponding regional magnetic highs. For this 

reason, the two-dimensional model in figure 9 is an 

oversimplification because the anomalies are clearly 

three-dimensional in nature. Nonetheless, all of the lower 

amplitude gravity anomalies have sources no deeper, and

probably shallower, than the body producing anomaly A. 

Although the character of the anomalies along line 1 indicate 

shallow depths, both the regional gravity and magnetic 

anomalies imply deeper sources that are probably igneous 

intrusions. Three drill holes intersect mica peridotite and 

lamprophyric dikes at depths of less than 1 km in the area of 

line 1 (fig. 7; Grohskopf, 1955); one of these dikes has been 

dated as Permian in age (Zartman, 1977). These dikes could 

be associated with larger intrusions at depth. The magnitude 

of anomalies A and B may thus be a combination of both 

deeper igneous bodies and shallower faults. Hamilton and 

Zoback (1982) suggested that shallow faults may be related 

to igneous intrusions at depth. Zoback and others (1980) 

interpreted arched reflectors below the Paleozoic-Upper 

Cretaceous unconformity on seismic line S-6 as evidence of 

possible shallow intrusions (depths of about 1-2 km), but a 

detailed gravity profile directly over the eastern half of 

seismic line S-6 does not indicate a subsurface density 

contrast that would be expected from such a body (Sexton 

and Jones, 1988). Unfortunately, this detailed gravity profile 

is short (5 km) and obliquely crosses the gravity feature (fig. 

7), but it does show a small gravity step associated with the 

CGF. In summary, the abrupt changes in gradient along line 

1 indicate sources at 1-km depth that could be caused by
INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

O

g °

w

H 

W

§ O ^ ^

i-J

2

o, observed; line, calculated

w 

Q p

CO

2.20 KT

2.85

2.70

A1

BODY A

BODYB

2.70

0.0 20.0 40.0 60.0 80.0

DISTANCE, IN KILOMETERS

Figure 9. Simple two-dimensional model of gravity data across anomalies A and B on line 1. KT refers to Cretaceous and Tertiary 

sedimentary rocks; Pz, Paleozoic sedimentary rocks; pC, Precambrian crystalline basement. Control on depth of Paleozoic-Precambrian 

contact from maximum depths to magnetic basement (Hildenbrand and others, 1979); control on depth of Paleozoic-Cretaceous 

unconformity taken from gridded drill-hole data and seismic reflection data (Dart, 1992). Numbers within model are densities in grams per 

cubic centimeter (g/cm3 ). The model is oversimplified in that anomalies A and B are the southernmost tips of a pair of three-dimensional 

anomalies. Source of A must be shallower than 1 km in order to match abrupt changes in gradient bounding anomaly A (shown by arrow). 

All the smaller and more abrupt gravity features have sources no deeper, and probably shallower, than body A.

either (1) faults offsetting the Paleozoic surface possibly 

related to deeper igneous intrusions, (2) shallow intrusions, 

or (3) both, especially because the CGF seems to correlate 

with the edge of a gravity high.

Line 2, the northernmost profile, trends east-west for 

most of its length, but bends abruptly to the northeast at its 

eastern end, skirting the southern flank of the Maiden pluton 

(fig. 7). The profile shows several small-amplitude gravity 

anomalies (fig. 85). The total variation in gravity values is 

about 9 mGal along the 56-km-long profile. The western 

part of the profile crosses the northern tip of anomaly A (fig. 

7), which is expressed here by a broad 3- to 4-mGal anomaly. Superimposed on this gentle high are smaller anomalies 

(less than 1 mGal), some of which have relatively abrupt 

gradients. These smaller anomalies may be caused by stream

channels, small faults, or small igneous intrusions. Because 

little seismic reflection data are available in this area and 

high-resolution aeromagnetic data cover only the eastern 

half of line 2, it is difficult to determine whether these 

anomalies are caused by igneous rock. Nonetheless, two 

anomalies on the gridded magnetic data coincide somewhat 

with the gravity signature, suggesting that these gravity 

features are caused by possible igneous intrusions (arrows 

on fig. 85).

Line 3, a north-south profile measured along a 

proprietary seismic reflection line that crosses the 

Blytheville arch, shows a large gravity high at its southern 

end (fig. 8C). The 3-mGal high on the northern part of the 

profile is the southwesternmost tip of anomaly A (fig. 7). 

The major gravity high at the southern end of line 3
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY L15

corresponds geographically with a magnetic high of about 

50 nT, which probably indicates the existence of an igneous 

body at depth. The northern gradient of this gravity high 

coincides with the location of the Blytheville arch. The 

steep, fairly uniform northern gradient of the anomaly suggests that no significant lateral density contrast exists 

between the zone of incoherent reflected energy that characterizes the arch and the coherent, generally undeformed 

strata of the adjacent Paleozoic rock. These results are consistent with previous work that suggested no gravity expression of the Blytheville arch (Crone and others, 1985). 

However, small inflections in the gravity field are present 

along line 3; these small-amplitude (less than 1 mGal) 

gravity anomalies along line 3 may indicate small offsets on 

post-Cretaceous faults, small igneous intrusions, or density 

variations within the Cretaceous and Tertiary sedimentary 

deposits. Some of the small inflections in the magnetic 

anomalies may not be real but instead may be artifacts 

resulting from small datum shifts between the east-west 

flight lines used to create the gridded data; these small 

changes do not correlate well with small gravity anomalies 

(fig. 8Q, suggesting that small igneous bodies are not a 

likely source for the gravity anomalies. Although the disrupted zone in the Paleozoic rocks in the arch is blanketed by 

Cretaceous sedimentary rock (Crone and others, 1985), and 

McKeown and others (1990) argue that the arch formed by 

shale diapirism between Pennsylvanian or Permian and Late 

Cretaceous time, present-day seismicity appears to correlate 

with the location of the Blytheville arch. The small gravity 

anomalies on line 3 may be caused by small, vertical offsets 

on faults caused by reactivation of deeper, older structures 

associated with the arch; these faults may be continuous, as 

suggested by density boundaries calculated on residual 

gravity anomalies. These density boundaries correlate with 

and are parallel to a 20- to 25-km segment of the southern 

part of the Blytheville arch (fig. 1).

DISCUSSION

All three detailed gravity profiles show relatively flat 

gravity fields within the Reelfoot graben, except where the 

profiles cross inferred igneous intrusions. Nonetheless, 

small-amplitude, short-wavelength gravity anomalies indicate shallow masses of relatively dense rock within the 

Reelfoot graben. Some of these anomalies could be caused 

by small igneous intrusions because the gravity features 

coincide with very small magnetic features. Other gravity 

anomalies could be caused by small vertical displacements 

on faults that offset the Cretaceous unconformity, such as 

along line 1. Other anomalies could be caused by ancient 

stream channels, that is, very near surface density contrasts, 

but additional data are needed to demonstrate this 

possibility. These profiles demonstrate that the gravity

method, in conjunction with other geophysical and geologic 

data, can be an important tool in delineating shallow 

features and determining their structural significance with 

respect to the cause and distribution of earthquakes in the 

Mississippi Embayment.

The residual gravity field images shallow gravity 

sources. The accuracy of this statement depends greatly on 

the assumption that long-wavelength features are the result 

of deeply buried bodies. However, shallow bodies are 

capable of generating long-wavelength anomalies, given the 

non-uniqueness of potential-field data; a thin sheet near the 

surface (if characterized by a complex density distribution) 

may produce an anomaly similar to that of an 

equidimensional object at depth. As a result, some shallow, 

broad structures may not appear on the residual map. Nonetheless, narrow fault zones that displace shallow horizons 

should be visible on the residual map.

The maximum-horizontal-gradient method should be 

successful in locating faults that offset the 

Paleozoic-Cretaceous unconformity. The method fails in 

circumstances where the gravity source is characterized by 

gently dipping sides or non-uniform densities. Most of the 

faults imaged by seismic reflection techniques (Zoback and 

others, 1980; Hamilton and Zoback, 1982; Sexton and 

Jones, 1986) dip steeply, and, therefore, the locations of 

these faults will be adequately located by the 

maximum-horizontal-gravity-gradient method if the faults 

juxtapose rocks of different densities. A large density 

contrast exists between the Cretaceous and Tertiary 

sedimentary rocks (average density of about 2.20 g/cm3) 

and the Paleozoic sedimentary rocks (densities ranging 

between 2.40 to 2.80 g/cm3, averaging about 2.70 g/cm3). 

Assuming a density contrast of 0.50 g/cm3 and an infinite 

Bouguer slab, the maximum gravity anomaly associated 

with this unconformity is estimated to be 0.02 mGal per 

meter of vertical fault offset. If this typically flat 

unconformity is offset by only 50 m, a detectable gravity 

anomaly of 1 mGal would result. Because the density of 

Paleozoic sedimentary rocks is slightly less than or 

comparable to the density of the Precambrian basement 

(densities ranging from 2.60 to 2.75 g/cm3), the gravity 

method will probably be of limited use in locating deeper 

faults that offset Paleozoic and basement rocks.

The maximum-horizontal-gravity-gradient method can 

locate possible fault offsets within the embayment. The 

horizontal displacement of a gradient maximum from the 

top edge of an offset horizontal layer is always less than or 

equal to the depth to the top of the source for moderate to 

steep dips (45° to vertical; Grauch and Cordell, 1987). Thus, 

for faults that offset the Cretaceous unconformity within the 

northern Mississippi Embayment, where the thickness of 

Cretaceous and Tertiary sedimentary deposits rarely 

exceeds 1 km (Dart, 1992), the maximum location error is 

on the order of 1 km.
L16 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

The maximum-horizontal-gravity-gradient method also 

assumes that gravity sources are characterized by 

homogeneous densities. However, drill-hole data show that 

the Covington pluton violates this assumption because 

gravity data indicate the pluton is composed of dense rock 

(Hildenbrand and others, 1982), yet the top of the pluton is 

composed of low-density nepheline syenite. Hie maximum 

horizontal gravity gradients outlining the extent of the pluton 

may delineate density variations within the pluton or abrupt 

changes in depth of burial. For this reason, more than one 

boundary may be necessary to delineate the area! extent of 

the buried plutons (fig. 1). In addition, the wide range of 

densities present within the Cretaceous and Tertiary 

sedimentary rocks means that some of the density 

boundaries that are interpreted as possible faults may be the 

edges of stream-channel deposits. Anomaly linearity, 

amplitude, and width may help to distinguish between 

anomalies caused by faulting or stream fill.

The gravity field over the northern Mississippi Embayment does not reveal any large gravity anomalies within the 

boundaries of the Reelfoot graben except over major igneous 

intrusions that are inferred from associated gravity and 

magnetic anomalies. The absence of large gravity anomalies 

within the graben provides a maximum limit to the amount 

of vertical displacement on faults that offset the 

Palozoic-Cretaceous unconformity. For example, a vertical 

offset of 1 km on the unconformity would produce an 

estimated maximum gravity anomaly of 21 mGal. On the 

residual gravity map, the maximum amplitude of gravity 

highs present within the graben ranges between 3 to 5 mGal 

(fig. 6), which corresponds to vertical offsets of about 140 to 

240 m on the Cretaceous unconformity. Large, 

post-Cretaceous offsets are expected because of the 

large-magnitude 1811-12 earthquakes and the well-defined 

zones of seismicity. One explanation for the lack of large 

vertical offsets is the alternative of right-lateral strike-slip 

motion similar to that based on focal mechanisms for earthquakes along the graben's axis. This right-lateral shear is 

consistent with studies showing that the regional stress field 

is dominated by east-west compression, which would 

produce lateral slip on the northeast-trending structures in 

the northern Mississippi Embayment (Zoback and Zoback, 

1981). The present orientation of the stress field may be old, 

perhaps dating back to the beginning of rifting along the 

eastern North American margin during the Jurassic and 

Cretaceous. The cumulative amount of right-lateral motion 

is not known, although gravity and magnetic data do not 

indicate any significant right-lateral motion (greater than 

about 10 km) that disrupts the continuation of the Missouri 

gravity low or other northwest-trending geophysical features 

across the Reelfoot graben. Perhaps the activation of these 

faults is very episodic. Zoback and Zoback (1981) identified 

several intraplate regions that are characterized by long 

periods of quiescence separated by large historical

earthquakes. Trenching in the Reelfoot Lake area suggests 

that recurrence times for large earthquakes are on the order 

of 600-1,000 years and that most of the uplift seen along the 

northwest-trending zone of seismicity occurred during the 

last 2,000 years (Russ, 1982). Schweig and Marple (1991) 

have also suggested that the 1811-12 earthquakes occurred 

along different planes of weakness than those outlined by 

current seismicity. Another, less likely, possible explanation 

for small vertical offsets is that the amount of energy 

released by the Mississippi Embayment earthquakes has 

been overestimated because of the lack of attenuation of 

seismic waves through old, competent Precambrian crust. 

Nonetheless, the maximum-horizontal-gravity-gradient 

method can locate density boundaries that coincide 

geographically with faults imaged by seismic reflection data. 

Thus, inexpensive gravity data can help pinpoint potential 

faults that warrant further investigation by other more 

expensive geophysical data.

CONCLUSIONS

Despite well-defined linear zones of seismicity and the 

occurrence of large historical earthquakes in the New 

Madrid seismic zone, the analysis of the gravity field 

indicates that, in general, faults here are not characterized by 

major vertical offsets, confirming the results of previous, 

more localized, seismic reflection work. Because of the large 

density contrast between the Paleozoic rocks and overlying 

Cretaceous and Tertiary sedimentary rocks, any offset of this 

unconformity would produce a gravity anomaly of 

significant magnitude; such anomalies are not found. In the 

northern Mississippi Embayment, where the thickness of 

post-Paleozoic rocks ranges from near zero to about 1 km, 

the change in gradient that results from a vertical offset of the 

Paleozoic-Cretaceous unconformity would also be abrupt 

and local because of the shallow depth of burial. Three 

detailed gravity profiles in the Reelfoot graben show 

unexpected local variation in gravity values and identify 

small-amplitude anomalies characterized by abrupt changes 

in gradient. These anomalies may be caused by either small 

igneous intrusions (where the anomalies correlate with small 

magnetic anomalies) or post-unconformity faulting. Gravity 

data filtered to enhance short-wavelength features reveal the 

locations of density contrasts that correspond to known 

subsurface faults cutting the Cretaceous unconformity. The 

method of analyzing maximum horizontal gravity gradients 

can be used to define the extent of faults imaged on seismic 

reflection profiles—the method thus provides important 

information that will contribute to better seismic risk 

assessments, such as the amount of post-Cretaceous offset 

and the lengths of faults.
GRAVITY OF THE NEW MADRID SEISMIC ZONE—A PRELIMINARY STUDY L17

ACKNOWLEDGMENTS

John Mariano and Robert Sikora of the U.S. Geological 

Survey collected the detailed gravity profiles. Eleanor Dixon 

assisted in the reduction of the gravity data. This paper benefited greatly from discussions with Tom Hildenbrand and 

reviews by Andrew Griscom and AJ. Crone of the U.S. 

Geological Survey.

REFERENCES CITED

Andrews, M.C., Mooney, W.D., and Meyer, R.P., 1985, The 

relocation of microearthquakes in the northern Mississippi 

Embaymenfc Journal of Geophysical Research, v. 90, no. B12, 

p. 10223-10236.

Bickford, M.E., Harrower, K.L., Hoppe, W.J., Nelson, B.K., 

Nussbaum, R.K., and Thomas, J.J., 1981, Rb-Sr and U-Pb 

geochronology and distribution of rock types in the 

Precambrian basement of Missouri and Kansas: Geological 

Society of America Bulletin, pt. 1, v. 92, p. 323-341.

Blakely, R.J., and Simpson, R.W., 1986, Approximating edges of 

source bodies from magnetic or gravity anomalies: 

Geophysics, v. 51, p. 1494-1498.

Crone, A.J., and Brockman, S.R., 1982, Configuration and 

deformation of the Paleozoic bedrock surface in 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 1236-1, p. 

115-135.

Crone, A.J., McKeown, F.A., Harding, S.T., 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.

Crone, A.J., and Russ, D.P., 1979, Preliminary report on an 

exploratory drill hole—New Madrid Test Well 1-X in 

southeast Missouri: U.S. Geological Survey Open-File Report 

79-1216,12 p.

Dart, R.L., 1992, Catalog of pre-Cretaceous geologic drill-hole data 

from the upper Mississippi Embayment: A revision and update 

of Open-File Report 90-260: U.S. Geological Survey 

Open-File Report 92-685, 253 p.

Denison, R.E., 1984, Basement rocks in northern Arkansas, in 

McFarland, J.D., and Bush, W.V., eds., Contributions to the 

Geology of Arkansas—Volume II: Arkansas Geologic 

Commission Miscellaneous Publication 18-B, p. 33-49.

Dobrin, M.B., and Savit, C.H., 1988, Introduction to Geophysical 

Prospecting: New York, McGraw-Hill Book Company, 867 p.

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

precursor to the Mississippi Embayment: Geological Society 

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

Click, E.E., 1982, Stratigraphy and structure of sediments above 

the Newport pluton of northeastern Arkansas, in McKeown, 

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

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

Professional Paper 1236-K, p. 151-174.

Grauch, V.J.S., and Cordell, Lindrith, 1987, Limitations of 

determining density or magnetic boundaries from the

horizontal gradient of gravity or pseudogravity data: 

Geophysics, v. 52, no. 1, p. 118-121.

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

Embayment of southeast Missouri: Missouri Division of 

Geological Survey and Water Resources, v. 37, 2nd sen, 

133 p.

Guinness, E.A., Arvidson, R.E., Strebeck, J.W., Schulz, K.J., 

Davies, G.F., and Leff, C.E., 1982, Identification of a 

Precambrian rift through Missouri by digital image processing 

of geophysical and geological data: Journal of Geophysical 

Research, v. 87, no. BIO, p. 8529-8545.

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-F, p. 55-82.

Herrmann, R.M., and Canas, J.A., 1978, Focal mechanisms studies 

in the New Madrid seismic zone: Bulletin of Seismological 

Society of America, v. 68, p. 1095-1102.

Hildenbrand, T.G., 1985, Rift structures of the northern Mississippi 

Embayment from the analysis of gravity and magnetic data: 

Journal of Geophysical Research, v. 90, no. B14, p. 

12607-12622.

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-E, 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.

HUdenbrand, T.G., Kucks, R.P., Kane, M.F., and Hendricks, J.D., 

1979, Aeromagnetic map and associated depth map of the 

upper Mississippi Embayment region: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1158, scale 

1:1,000,000.

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

and hydrocarbon potential of the Reelfoot rift: Oil and Gas 

Journal, v. 82, no. 46, p. 179-190.

Kane, M.F., Hildenbrand, T.G., and Hendricks, J.P., 1981, A model for the tectonic evolution of the Mississippi Embayment and 

its contemporary seismicity: Geology, v. 9, p. 563-567.

Kidwell, A.K., 1951, Mesozoic igneous activity in the northern 

Gulf Coast Plain: Transactions Gulf Coast Association 

Geologic Society, v. 1, p. 182-199.

McKeown, F.A., Hamilton, R.M., Diehl, S.F., and Click, E.E., 

1990, Diapiric origin of the Blytheville and Pascola arches in 

the Reelfoot rift, East-Central United States: Relation to New 

Madrid seismicity: Geology, v. 18, p. 1158-1162.

Morelli, Carlo, ed., 1974, The International Gravity Standardization Net 1971: International Association of Geodesy Special 

Publication No. 4,194 p.

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

and 1812—Intensities, ground motion, and magnitudes: Bulletin of the Seismological Society of America, v. 63, p. 227-248.
L18 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Plouff, Donald, 1977, Preliminary documentation of a FORTRAN 

program to compute gravity terrain corrections based on 

topography digitized on a geographic grid: U.S. Geological 

Survey Open-File Report 77-535, 45 p.

Ravat, D.N., Braile, L.W., and Hinze, W.J., 1987, Earthquakes and 

plutons in the Midcontinent—Evidence from the Bloomfield 

pluton, New Madrid rift complex: Seismological Research 

Letters, v. 58, no. 2, p. 41-52.

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-H, p. 95-114.

Schweig, E.S., and Marple, R.T., 1991, Bootheel lineament: A 

possible coseismic fault of the great New Madrid earthquakes: 

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

Sexton, J.L., and Jones, P.B., 1986, Evidence for recurrent faulting 

in the New Madrid seismic zone from Mini-Sosie 

high-resolution reflection data: Geophysics, v. 51, no. 9, p. 

1760-1788.

Sexton, J.L., and Jones. P.B., 1988, Mini-Sosie high-resolution 

reflection survey of the Cottonwood Grove fault in

northwestern Tennessee: Bulletin of the Seismological Society

of America, v. 78, no. 2, p. 838-854. 

Stearns, R.G., 1980, Near-surface geology of the Reelfoot Lake

district of the New Madrid earthquake region, in New Madrid

Seismotectonic Study: U.S. Nuclear Regulatory Commission

NUREG/CR-2129 R6, RA, p. 97-101. 

Stearns, R.G., and Marcher, M.V., 1962, Late Cretaceous and

subsequent structural development of the northern

Mississippi Embayment area: Geological Society of America

Bulletin, v. 73, p. 1387-1394. 

Telford, W.M., Geldart, L.P., Sheriff, R.E., and Keys, D.A., 1976,

Applied Geophysics: Cambridge, England, Cambridge

University Press, 860 p. 

Zartman, R.E., 1977, Geochronology of some alkalic rock

provinces in Eastern and Central United States: Earth and

Planetary Science Letters Annual Review, v. 5, p. 257-286. 

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

McKeown, F.A., and Brockman, S.R., 1980, Recurrent

intraplate tectonism in the New Madrid seismic zone:

Science, v. 209, p. 971-976. 

Zoback, M.D., and Zoback, M.L., 1981, State of stress and

intraplate earthquakes in the United States: Science, v. 213,

p. 96-104.

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication August 23, 1994 

Edited by Richard W. Scott, Jr. 

Graphics preparation and photocomposition by 

Gayle Dumonceaux

* U.S. GOVERNMENT PRINTING OFFICE: 1995-673-046/86104