Axial Structures Within the Reelfoot Rift 

Delineated With Magnetotelluric Surveys

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-K
Cover. Gray, shaded-relief map of magnetic anomaly data. 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).
Axial Structures Within the Reelfoot Rift 

Delineated With Magnetotelluric Surveys

By Brian D. Rodriguez, William D. Stanley, and Jackie M. Williams

INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE 

Edited by Kaye M. Shedlock and Arch C. Johnston

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1538-K

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1996
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

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-in-Publication Data

Rodriguez, Brian D.

Axial structures within the Reelfoot rift delineated with magnetotelluric surveys / by 

Brian D. Rodriguez, William D. Stanley, and Jackie M. Williams.

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

(U.S. Geological Survey professional paper ; 1538-K) 

Includes bibliographical references. 

Supt.ofDocs.no. ; I 19.16: 1538K

1. Geology, Structural—Missouri—New Madrid Region. 2. Magnetotelluric 

prospecting. I. Stanley, William D. II. Williams, Jackie M. III. Title. 

IV. Series. V. Series: U.S. Geological Survey professional paper ; 1538-K. 

QE535.2.U6I59 1995vol. K 

[QE627.5.M8] 

551.2'2'09788985 s—dc20

[551.8'09788985] 94-45670

CIP
CONTENTS

Abstract........................................................................................................................... Kl

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

MT Surveys.................................................................................................................... 3

MT Models..................................................................................................................... 4

Interpretation of MT Models and Well Logs.................................................................. 5

Seismic Reflection and Refraction Results .................................................................... 12

Relationship to Seismicity.............................................................................................. 13

Tectonic Model............................................................................................................... 13

Summary and Conclusions............................................................................................. 15

Acknowledgments.......................................................................................................... 16

References Cited............................................................................................................. 16

Appendix A..................................................................................................................... 18

Magnetotelluric Method......................................................................................... 18

Interpretation of MT and Seismic Data.................................................................. 19

Appendix B..................................................................................................................... 20

FIGURES

1. Index map for the Reelfoot rift-New Madrid region with key geologic features and locations

of seismic and MT profiles................................................................................................................................. K2

2. Sample MT sounding data set............................................................................................................................ 3

3. Skew and rotation parameters for example MT soundings................................................................................ 3

4. Pseudosection plots of MT data from profile A-A'............................................................................................ 5

5. 2-D models for MT profiles A-A', B-B', C-C', D-D', and E-E'......................................................................... 6

6. Generalized stratigraphy of Dow Chemical No. 1 Garrigan and No. 1 Wilson drill holes................................ 7

7. Simplified resistivity logs and MT models for Dow Chemical No. 1 and No. 1 Wilson wells ......................... 8

8. Seismic reflection data and line drawing for seismic profile 2; line drawing for seismic profile 2

with MT model for profile E-E'; and line drawing for seismic profile 3 with MT model for profile B-B'........ 10

9. Tectonic reconstruction for a proposed axial horst in the Reelfoot rift.............................................................. 14

10. Drawing showing possible generation of warped reflections by simple uplift.................................................. 15

B1-B10. Plots of observed data input to the 2-D inversion program and computed data for the derived model 

for each individual sounding on the five magnetotelluric profiles shown in figure 5: 

Bl. Resistivity plots for 23 stations along profile A-A'.................................................................................. 20

B2. Phase plots for 23 stations along profile A-A'.......................................................................................... 21

B3. Resistivity plots for 19 stations along profile B-B'.................................................................................. 23

B4. Phase plots for 19 stations along profile B-B'.......................................................................................... 24

B5. Resistivity plots for 20 stations along profile C-C'.................................................................................. 25

B6. Phase plots for 20 stations along profile C-C'.......................................................................................... 26

B7. Resistivity plots for 16 stations along profile D-D'.................................................................................. 27

B8. Phase plots for 16 stations along profile D-D'.......................................................................,.................. 28

B9. Resistivity plots for 17 stations along profile E-E'................................................................................... 29

BIO. Phase plots for 17 stations along profile E-E'........................................................................................... 30

III

AXIAL STRUCTURES WITHIN THE REELFOOT RIFT 

DELINEATED WITH MAGNETOTELLURIC SURVEYS

By Brian D. Rodriguez, 1 William D. Stanley, 1 and Jackie M. Williams 1

ABSTRACT

The results of magnetotelluric (MT) surveys reveal 

structures associated with the Reelfoot rift, including an 

axial region of high resistivity that may indicate a previously 

unrecognized horst. Two-dimensional inversion methods 

were used to model the MT data, and three deep drill holes 

and seismic reflection data were used to help geologically 

constrain the MT models. The proposed horst occurs in the 

axial part of the Reelfoot rift and is generally coincident with 

a major, northeast-trending zone of seismicity. A linear 

region of upwarped and disrupted seismic reflections called 

the "Blytheville arch" is located near the proposed axial 

horst. The MT axial structure appears to terminate in the area 

of a northwest-trending belt of seismicity that extends southsoutheastward from New Madrid, Mo. However, the nature 

of the axial resistivity structure is uncertain in the New 

Madrid region because of the influence of a major axial 

intrusion that has been mapped in aeromagnetic surveys. 

This axial intrusion is interpreted from the MT models to 

have intruded into conductive, rift sedimentary rocks, resulting in three-dimensional geometries that complicate the MT 

analysis of the local rift structures.

INTRODUCTION

The Reelfoot rift is a possible failed rift arm, or aulacogen (Milanovsky, 1981; Howe, 1984) that probably formed 

in Late Proterozoic to Early Paleozoic time (Ervin and 

McGinnis, 1975; Thomas, 1991). Such rifts generally contain thick sedimentary sequences in the downdropped portion, and most are believed to have formed though horizontal 

extension that may extend into the middle and lower crust. A 

rift "pillow" of upwelled mafic material at deep crustal levels has been mapped in the Reelfoot rift and other rifts

1 U.S. Geological Survey, Mail Stop 964. P.O. Box 25046, Denver 

Federal Center, Denver, CO 80225.

(Mooney and others, 1983; Hildenbrand, 1985). In this 

paper, we refer to the "rift" as the sedimentary-filled trough, 

recognizing that there are deeper structures required to compensate for the regional extension that caused the downdropping. The term graben is avoided because the nature of 

bounding faults has not been established in detail. We follow 

J.H. lilies (1981), who reserves the term "graben" in its original sense to denote only those linear depressions with parallel normal faults along which both central downdropping and 

shoulder upwarping have occurred.

The aeromagnetic data that first indicated the margins 

of the rift (Hildenbrand, 1985) also reveal plutons flanking 

the rift (fig. 1) and, at some places, within the rift. The rift is 

filled with basal Upper Precambrian to Cambrian clastic 

rocks. Upper Cambrian carbonate rocks (Bonneterre Formation) and shale (Elvins Group), Cambrian to Ordovician carbonate rocks (Knox-Arbuckle Group), and sedimentary 

rocks of Upper Cretaceous to Recent age that fill the Mississippi Embayment. The Blytheville arch is a 5- to 10-kmwide structural high inferred from upwarped reflections 

flanking a zone of incoherent acoustic energy on seismic 

reflection records (Hamilton and McKeown, 1988). The arch 

is truncated by a post-Ordovician erosional surface. In the 

Reelfoot rift between Marked Tree, Ark., and Caruthersville, 

Mo., the modern seismicity coincides with this disturbed 

zone (fig. 1). McKeown and others (1990) have ascribed the 

arch's formation to diapirism. In addition, evidence for 

ancient and recent overpressure within the rift has been 

found (Diehl and McKeown, 1991) and may be another factor in formation of the arch.

The northeast-trending seismic zone (fig. 1) between 

Marked Tree, Ark., and Caruthersville, Mo., intersects a 

broader, northwest-trending zone of more intense seismicity 

that extends between Dyersburg, Tenn., and New Madrid, 

Mo. Another seismicity trend continues subparallel to the 

northwestern margin of the Reelfoot rift from New Madrid 

northeastward (fig. 1). The large earthquakes of 1811-12 

are believed to have been located in the region between 

Marked Tree and Blytheville, Ark., and near New Madrid, 

Mo. (Nuttli, 1973).

Kl
INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

MSSOURI 

ARKANSAS

EXPLANATION

Reelfoot rift margin 

Pluton

Figure 1. Index map for the Reelfoot rift and New Madrid region, adapted from McKeown and others (1990). Shown 

are rift margins and plutons determined from magnetic data by Hildenbrand (1985); Blytheville arch from Hamilton and 

McKeown (1988); epicenter density of 1973-88 seismicity (from Hamilton and Johnston, 1990); seismic reflection profiles from Hamilton and McKeown (1988); and selected drill holes. Magnetotelluric (MT) soundings are denoted by the 

inverted triangles; solid triangles are soundings with low three-dimensionality indicators; open triangles represent soundings with moderate to high three-dimensionality indicators (see text and fig. 3). Five MT profiles are discussed in the text, 

A-A', B-B', C-C', D-D', and E-E'. Earthquake densities for 1973-88 are denoted by stippled pattern where event densities 

were greater than one per square kilometer; cross-hatch pattern indicates densities of greater than two events per square 

kilometer; and no pattern indicates densities of less than one event per square kilometer.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K3

MT SURVEYS

One hundred and twenty magnetotelluric (MT) soundings were completed in the Reelfoot rift region to provide a 

geologic image of the subsurface through the study of 

changes in rock resistivity. Of particular interest were structural details of the Reelfoot rift and the deep crust and mantle 

beneath the sedimentary fill. The MT studies were part of a 

concentrated study of the New Madrid seismic zone that also 

included seismic refraction and wide-angle seismic profiling; high-resolution seismic reflection profiling; paleomagnetic studies; gravity, airborne magnetic and 

electromagnetic surveys; and seismicity, stress, geochemical, and geological studies.

The MT method employs natural electric and magnetic 

signals from ionospheric sources and lightning energy to 

determine tensor resistivities in the Earth's subsurface. In 

our MT surveys, signals at frequencies from 0.001 to 300 Hz 

were measured with U.S. Geological Survey-designed 

instrumentation and processed in real time with a computer 

that controlled the MT system. The recorded time-series data 

were transformed into the frequency domain using Fourier 

analysis and cross-spectral techniques. These techniques 

were used to determine a resistivity and phase tensor at each 

of the 120 measurement sites. Assuming a two-dimensional 

(2-D) Earth, the tensor can be rotated to maximum and minimum resistivity directions, and propagation modes for the 

signals are then decoupled into transverse magnetic (TM) 

and transverse electric (TE) modes that are equivalent to 

measurement of resistivity with the electric field directed 

across the strike and along the strike of the geologic structures, respectively. The MT method utilizes the fact that 

attenuation of electromagnetic fields in the Earth (or other 

conductors) is proportional to the square root of frequency 

times the conductivity (reciprocal of resistivity). This means 

that high-frequency signals are attenuated at shallow depth 

and lower frequency signals penetrate deeper, with the result 

that information on shallows rocks is contained in the highfrequency part of the MT sounding curves and information 

on deeper rocks is in the lower frequency parts of the sounding curves. This frequency-dependence of signal penetration 

is known as the "skin depth" effect.

Both resistivity and phase values are used in the modeling process. Sample plots of tensor resistivity and phases for 

the Reelfoot rift data are shown in figure 2. If data are 

obtained over 3-D structures, then the skew (fig. 3) of the 

tensor will have significant values, whereas for an exactly 2- 

D structure, the skew will be zero. Most of the data from the 

survey had low skew values, typically 0.1-0.3, but, in the 

northeastern part of the survey area, skew values were much 

larger, from 0.3 to 1. Associated with these high skew values 

is a departure from the typical northeast rotation azimuth of 

the tensor principal direction with respect to a north azimuth, 

as indicated in figure 3. The significance of the high skew

100

o 0

00000

001 100

Qj 70

(D

LLJ

20

001 01 1 1 10 100 1,000 

FREQUENCY(LOG), IN HERTZ

EXPLANATION 

* TMmode 

o TEmode

Figure 2. Sample magnetotelluric (MT) sounding data set from 

profile A-A' of figure 1. Resistivity values are plotted in the upper 

graph and phase values in the lower graph. TM, transverse magnetic; TE, transverse electric.

001 .01 1 1 10 100 1,000 

FREQUENCY(LOG), IN HERTZ

Figure 3. Skew of the impedance tensor and principal rotation direction of the tensor for typical magnetotelluric (MT) soundings on 

the eastern half of profiles B-B' and D-D'.
K4 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

values and change in rotation directions is discussed in a subsequent section.

Silicate minerals, which make up most rocks in the 

Earth's crust, are highly resistive, and conduction paths usually occur through the conductive fluids, clay minerals, or 

metallic-ore minerals in the rocks. Increased temperatures 

can raise the activation energy of minerals in rocks and 

enhance ionic conduction, thereby decreasing resistivities. 

Temperature is not a controlling factor in the upper 10 km of 

the Reelfoot rift due to low heat-flow values, and resistivities 

are largely controlled by the porosity and salinity of the 

rocks, as well as by their clay content. In general, the 

younger sedimentary rocks in the rift region have high clay 

content, large porosity, and moderate to high salinities, making them quite low in resistivity (2-30 ohm-m). Older sedimentary rocks have reduced porosity and minor clay content; 

thus, they are more resistive (30-500 ohm-m). A more 

extensive introduction to the MT method and a discussion of 

rock properties in included in Appendix A of this report.

MT MODELS

The MT data were modeled with a 2-D inversion algorithm (Smith and Booker, 1991) called the rapid relaxation 

inverse (RRI) because of similarities to classical relaxation 

solutions and because of its great operational speed. This 

method, which is one to two orders of magnitude faster than 

more conventional methods of 2-D inversion, utilizes 

approximate partial derivatives of MT data with respect to 

the conductivity directly beneath the measurement site and 

ignores cross-terms of horizontal field-gradients and conductivity gradients. Effectively, the iteration process 

involves intermediate 1-D inversions that are used to construct a 2-D model needed to obtain new sets of model residuals from 2-D forward calculations for subsequent iterations. 

The algorithm has been tested with numerous syntheticmodel data sets and has proven to converge to the correct 

geometry of these models. In our experience with data from 

this survey and several others, we have found that the RRI 

method provides a rapid means of 2-D modeling that 

achieves excellent fits to the observed data. The other advantage to the RRI inversion method stems from the use of a 

minimum-structure criteria (Smith and Booker, 1988), in 

which the algorithm applies a selected number of final iteration steps in which the complexity of the model is reduced 

while maintaining an overall good fit to the data. These latter 

steps substantially reduce model artifacts in the final solution. In addition, the effects of near-surface resistivity anomalies that cause "static shifts" (Sternberg and others, 1988) of 

the data can be reduced by utilizing a distortion matrix in the 

program. This shift was manually corrected in five of the 

more severely shifted soundings by using comparisons with 

unshifted data nearby. For the remainder of the data, we used 

the technique of Booker (1991), which involves determining

the distortion coefficients using a joint TE-TM inversion 

first, followed by a TE inversion only. No resolution statistics are provided on the uniqueness of the derived models, 

but, with the use of minimum-structure constraints, one can 

be assured that the totality of models that fit the data is 

reduced to a small set of simplest models. The program can 

be run with different starting models and error assumptions 

to more thoroughly investigate the range of possible models.

For our data, the inversion used a model grid with typically 70 columns and 35 rows of conductivity cells. Separate 

inversions of the TE- and TM-mode data were done, as well 

as joint inversion of both modes. For the structure of the 

Reelfoot rift, with a highly conductive linear channel composed of rift sedimentary fill, it was found that inversions of 

the TE data yielded models that were more consistent 

between MT profiles and with well logs than did inversions 

of the TM data. The TM mode involves currents that flow 

across the strike of the structures, and the inversion process 

is required to fit these currents at all stations along the profile. In contrast, the TE mode inversions can work more 

exclusively on the structures beneath an individual site. The 

result is that TE inversions are more sensitive to structures 

directly beneath the measurement site, whereas the TM 

inversions greatly average details across the structure. However, inversion of the TE-mode data has one key disadvantage: limited strike length of the rift affects low-frequency 

data; hence, the resolution of deeper structures (below the 

rift) is poor. The fits to the observed data were generally 

within measurement errors in the data, typically 3-10 percent. A comparison of the computed and observed data for 

profile A-A' is shown in figure 4. The agreement between 

the observed and model data is excellent and is typical of the 

fits for the other profiles. The observed and computed data 

for individual stations on all five MT profiles are included in 

Appendix B of this report. Three-dimensional models of the 

rift using forward-modeling methods should be useful for 

refining details of the structure in the northeastern part of the 

study area, where the data clearly show severe departures 

from 2-D character.

Models for the four east-west profiles and the northsouth profile located in figure 1 are shown in figure 5. The 

starting model for the inversions was a simple, pseudo-horizontally layered Earth designed for each individual profile 

based upon a 1-D model for each sounding; this starting 

model can be observed unchanged on the ends of the display of figure 5. The derived conductivity model has been 

simplified by using five ranges as shown in figure 5. The 

MT method is very insensitive to high resistivities beneath a 

conductive section such as the Cretaceous and Tertiary sedimentary rocks that fill the Mississippi Embayment. Sensitivity analysis for 1-D and 2-D models in our study has 

shown that it is very difficult to accurately resolve resistivities greater than about 500 ohm-m in the upper crust; thus, 

the resistivities shown as greater than 500 ohm-m may be 

much larger. The MT data used in the models should have
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K5

(Observed)

100 150 200 250

HORIZONTAL SCALE, IN KILOMETERS

EXPLANATION

Resistivity (in ohm-meters)

100-500

>500

Figure 4. Pseudosection plots of observed magnetotelluric (MT) data from profile A-A' and computed data for the transverse electric (TE) 

inversion of this profile. The vertical axes of the plots are logarithmic frequency, which reflects the "skin depth" relationship of MT soundings, wherein the highest frequency data contains information about shallow rocks and the lower frequency data contains information on 

deeper rocks. The resistivity patterns are the same as those in the models of figure 5, where the data have been mapped to depths and layer 

resistivities by the inversion algorithm.

delineated crust-mantle structures to depths greater than 40 

km, but our investigation of the resulting models indicates 

that the overpowering effects of the conductive sedimentary 

rocks in the Reelfoot rift and Mississippi Embayment make 

meaningful interpretation of middle-lower crust/mantle rift 

structures difficult. Based upon study of the models of figure 5, but plotted to depths of 40 km, we interpret that the 

crust has resistivities greater than 500 ohm-m at depths 

greater than a few kilometers and that no conductive zones 

that might be caused by magma or other fluids exist in significant thicknesses (probably less than 500 m) in the deep 

crust. A combination of 3-D and newly acquired ultra-lowfrequency (as low as 0.0002 Hz) MT data are being utilized 

to further investigate structures deeper than those shown in 

figure 5.

INTERPRETATION OF MT MODELS 

AND WELL LOGS

The geological interpretation of the MT profile models 

in figure 5 has relied heavily on well logs from the Dow 

Chemical No. 1 Wilson and No. 1 Garrigan drill holes (fig. 

6) and seismic reflection data. These two wells have been 

studied intensively by McKeown and others (1990), Diehl 

and McKeown (1991), and Collins and others (1992). The 

Dow Wilson well is located just east of the Blytheville arch, 

close to MT profile A-A' (fig. 1), and the Dow Garrigan is 

actually located on the arch, just north of MT profile A-A' 

(fig. 1). These two wells exhibit the stratigraphy of the region 

in detail, as shown in the chart of figure 6 (modified from 

F.A. McKeown, written commun., 1991). Approximately
K6 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

NORTH 

E 

0

SOUTH

E1 

i HI nil mi nil i

WEST 

D

EAST 

D'

North

A

°1 m -

KILOMETER

z _

fr

Q _

m

i ii i i i i n i i i i i i i "

l mLmMMnnn;m!ni!MHn!uj|fff:i^

R *

*

*

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*

i i i i ii i M i i i it 11 i i i i

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EXPLANATION

Resistivity (in ohm-meters)

1

>500

* Earthquakes

RB

• Rift margins

11 l MT soundings

50 100 KILOMETERS 

I

Figure 5. Final models for two-dimensional inversions of magnetotelluric (MT) data from profiles A-A', BB', C-C', D-D', and E-E'. Most, but not all, of the soundings along the profiles were used (indicated by heavy 

ticks). Vertical exaggeration is 8:1. The combined view of all five profile models is shown in an approximately 

relative spatial position (except for E-E'). The approximate surface spatial relationship of the Blytheville arch to 

the MT profiles is shown on the models for A-A' and B-B'. Line drawings of reflection data from seismic reflection profiles 1, 2, and 3 using a time-to-depth conversion velocity of 4 km/s (see text) are superimposed on the 

models for MT profiles A-A', B-B', and E-E', respectively. Heavy, vertical lines on profile A-A' labeled "G" and 

"W" show the location and depths of the Dow Garrigan and Dow Wilson deep drill holes, respectively. Edges of 

the Reelfoot rift are interpreted from MT models. Pluton location, indicated on MT models C-C' and D-D', is 

sketched from the location noted in figure 1. The approximate location and attitude of zones of seismicity (from 

Chiu and others, 1991) are indicated by asterisks.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K7

METERS BELOW 

SEA LEVEL 

OH

Dow No.1 Garrigan Dow No.1 Wilson

3000-

4000-

5000-

Quat. sediments and 

Upper Cretaceous 

sedimentary rocks 

of Mississippi Embayment

Paleozoic rocks

Knox or Arbuckle Group 

equivalents (Lower 

Ordivician and Upper 

Cambrian)

Elvins Group (Upper Cambrian)

Bonneterre Formation 

(Upper Cambrian)

-Gray arkose

Basal clastic rocks (Upper and 

Middle)?) Cambrian)

—Red arkose

Crystalline rocks 

(Precambrian)

EXPLANATION

Sandstone and shale Shale Limestone Dolomite Arkose Crystalline rocks

Figure 6. Generalized stratigraphy in the Dow Chemical No. 1 Garrigan and No. 1 Wilson drill holes. Adapted 

from F.A. McKeown (written commun., 1991).

600-800 m of Quaternary sediments and Upper Cretaceous 

sedimentary rocks of the Mississippi Embayment occur on 

top of an erosional surface in the Lower Ordovician KnoxArbuckle Group-equivalent limestones. Elvins Group rocks 

are largely shales in the Garrigan well but have more limestone content in the Dow Wilson well. Upper Cambrian dolomitic rocks of the Bonneterre Formation occur on top of basal 

arkoses of Upper and Middle Cambrian age. The Precambrian gneisses encountered in the Dow Wilson well form the 

basement for the Reelfoot rift.

The electrical resistivity logs from the Wilson and Garrigan drill holes are shown in figure 7. Ranges of resistivity 

from the well logs are indicated by the bold boxes at various 

depth intervals. The electrical resistivity logs indicate that 

the resistivities of the Cenozoic sedimentary rocks are 7-40 

ohm-m and resistivities of the Cretaceous mudstones and 

sandstones that extend to depths of slightly less than 1 km are 

3-20 ohm-m. The thick Knox-Arbuckle Group carbonate 

rocks in the Wilson drill hole have resistivities of 100-500 

ohm-m, but a red clay unit at 2,500-m depth has a resistivity

of 1-3 ohm-m. This red clay section generates key seismic 

reflections (fig. 8A) that are mappable throughout much of 

the area (McKeown and others, 1990). The resistivity distribution in the Wilson well from 2,700 m to 3,900 m is highly 

variable. The resistivities range from 30-900 ohm-m within 

the basal part of the Knox or Arbuckle Group equivalents, 

the Elvins Group, the Bonneterre Formation, and the upper 

part of the basal clastic sequence. The lower part of the basal 

clastic rocks and the Precambrian crystalline rocks have 

resistivities greater than 1,000 ohm-m in the Wilson well; 

thus, the MT data cannot be used to distinguish basal elastics 

from crystalline basement. This is unfortunate because interpretations of seismic reflection data are thought to possibly 

show several kilometers of these clastic rocks in parts of the 

rift (F.A. McKeown, oral, commun., 1991; Howe, 1984).

The upper two-thirds of the Elvins Group shales (from 

800-m to 1,900-m depth) in the Garrigan drill hole has resistivities that range from 5-20 ohm-m, but the lower one-third 

(from 1,900 m to 2,400 m) has significantly higher resistivities of 15-80 ohm-m. The general resistivity values of
K8 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Wilson drill hole 

LOG 10

Qarrigan drill hole 

RESISTIVITY, IN OHM-METERS

LJ ? t

CO 

DC 

LLJ

t;

Q. 

LJJ 

Q

Basal elastics 

Upper and Middle Cambrian

Precambrian 

crystalline rocks

2000

2500 —

3000

3500

Figure 7. Simplified resistivity logs (bold, blank boxes) for ranges of resistivity at various depth intervals. 

Vertical slices from magnetotelluric (MT) models on profile A-A' at the projected location of the wells are 

superimposed on the well logs. Key geological formations are shown between electrical logs.

marine shales ranges from less than 1 ohm-m for clay-rich 

black shales with high carbon content to tens of ohm-meters 

in shales with small amounts of carbon (or carbon that has 

been oxidized). Based upon the higher resistivity character of

the lower one-third of the Elvins Group in the Garrigan drill 

hole, we infer that this portion of the Elvins Group may have 

been deposited in shallow-water, oxidizing conditions. If the 

lower part of the Elvins Group represents a shallow-water,
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K9

limy equivalent of the upper two-thirds of the Elvins in the 

Garrigan hole, then the vertical change in the resistivity of 

the Elvins Group shales may be evidence for progressive 

transition from limestones to deep-water shales in the Upper 

Cambrian. The much larger shale thickness in the Garrigan 

hole suggests that this drill hole is located closer to an Upper 

Cambrian depocenter than the Wilson hole. Basal elastics 

encountered in the Garrigan hole have resistivities of 

500-1,000 ohm-m.

We have projected the Wilson and Garrigan holes along 

strike (N. 45° E.) onto MT profile A-A' as indicated in figure 

5. MT soundings could not be done at each individual well 

because of the influence of nearby power lines. Vertical 

slices from the models at the projected well locations indicated in figure 5 are plotted on the resistivity logs (fig. 7), 

with the same resistivity patterns used as in the model cross 

sections (fig. 5). Considering the low resolving power of the 

MT method, the agreement between the model and the well 

logs is very reasonable. This composite plot can be understood more easily if it is remembered that both plots represent ranges; thus, there is general agreement when the two 

plots touch each other. It is not expected that the ranges will 

completely coincide because the MT method measures a 

large volume of rock, whereas the well log measures only the 

variable fluid/rock environment in the drill hole. The MT 

method has relatively poor resolution, usually with a minimum error of 10 percent in the depth to a given layer; comparison with coincident MT-seismic refraction surveys in 

areas such as the Cascades in the Western United States 

(Stanley and others, 1990) show that the resolving power of 

the MT method for thick geologic units is generally similar 

to that of seismic refraction surveys.

A difference of about 400 m occurs between the well 

log and the MT models for the Wilson hole in regard to the 

thickness of the uppermost conductive section. This section 

corresponds to units younger than Paleozoic (0- to 800-m 

depth). The difference in thickness of the conductive sections on the resistivity log and MT model for the Wilson hole 

may be due to noncoincidence of the MT model profile and 

the drill hole location, but it may also be due to differences 

in measurement characteristics of the electrical-log tools and 

the MT sounding. The MT method measures an integrated 

value of the reciprocal of resistivity (conductivity) for individual beds multiplied by the thickness of the beds. Thus, a 

sequence of thin, low-resistivity beds in a formation can 

completely dominate the response in the MT data. For the 

Garrigan hole, the thick conductive section from 0-1,700 m 

on the MT model corresponds to both the post-Paleozoic 

Mississippi Embayment rocks and the upper part of the 

Elvins Group shales; there is better correspondence between 

the MT model and he resistivity log for this section than in 

the Wilson hole. For the remainder of the depth of the two 

drill holes, we consider that there is agreement between the 

MT models and the well logs, except for minor complexities 

from 3,000 m to 3,800 m in the Wilson hole and higher

resistivities in the MT model (compared to the resistivity 

log) for the 2,700- to 3,500-m interval in the Garrigan hole. 

This latter disagreement may be due to relief on the highresistivity, basement structure indicated between the projected locations of the Wilson and Garrigan holes.

The east-west MT profile models in figure 5 clearly 

define the margins of the rift. The conductive section of less 

than 30 ohm-m represents the poorly consolidated Quaternary-Cretaceous clay-rich sandstones and shales, KnoxArbuckle carbonates with highly saline pore fluids, and the 

upper parts of the Elvins. Units of 30-100 and 100-500 ohmm probably represent the remainder of the Knox-Arbuckle 

and Elvins Group rocks, the Bonneterre Formation, and the 

upper part of the basal elastics (based upon correlations of 

such resistivity units at the Dow Wilson well). Crystalline 

basement occurs about at the >500-ohm-m zone on profile 

A-A' at the location where we have projected the Dow Wilson well (fig. 5). We infer that this >500-ohm-m level in the 

models represents the approximate depth to crystalline basement elsewhere in the survey area.

Seismic reflection data show that the interface between 

the Cretaceous and Paleozoic units has much less relief than 

indicated by the MT models (fig. 5). The greater relief in the 

upper conductive section in the MT models is caused by several factors, including "static-shift" effects of near-surface 

resistivity anomalies, inversion and data artifacts, and, lastly, 

the variability of resistivity within formations caused by lateral changes in clay and salinity content. Because of the 

nature of the MT method, some of these effects propagate 

downward into the models below their depth of occurrence. 

Any direct correlation of the shape of anomalous zones in the 

upper part of the section with the shape of the basement units 

(>500 ohm-m) may be artifacts of the static-shift effects on 

the model. However, the general shape of the "basement" 

horizon on the models does not correlate well with the shape 

of the upper conductive section. Therefore, we interpret that 

the MT models represent a valid representation of the true 

structure of Reelfoot rift, at least in the gross sense. The 

intrusive bodies along the margins of the rift that have been 

mapped with aeromagnetic data (fig. 1) were not detectable 

in the MT data because their resistivities were similar to that 

of the Precambrian gneisses that form the basement.

The most significant aspect of the models, apart from 

detailed information on the location of the rift margins, is the 

resistivity high at basement depths in the axial part of the rift. 

The general thickness of the sedimentary rocks that have 

resistivities less than 500 ohm-m along the four profiles varies from about 3 km on D-D' to as much as 7 km on profile 

A-A'. The resistive high along the center of the models for 

profiles A-A', B-B', and C-C' has a relief of about 3 km. This 

axial resistivity high clearly follows the northeast-trending 

axis of the rift up to profile C-C', but its location on profile 

D-D' is ambiguous. The eastern ends of both profiles C-C' 

and D-D' apparently cross a pluton that was interpreted from 

magnetic data (fig. 1) by Hildenbrand (1985). We believe
NORTH 

iii/^

Figure 8 (above and facing page). A, Coherency-filtered, seismic reflection data (upper) and line drawing (lower) for seismic profile 2 from M

Tertiary-Upper Cretaceous contact; Pz, Upper Cretaceous-Paleozoic contact; RC, top of red clay unit; BC, top of basal clastic unit; B, possible cry

ing of seismic reflection data from profile 2 (see fig. 1 for location) with coincident portion of magnetotelluric (MT) model E-E' underneath. A v

scale the reflection data to the MT model (see text for further explanation). C, Line drawing of seismic reflection data from profile 3 (see fig. 1 for l

of MT model B-B' underneath. Refer to figure 5 for comparisons with complete MT models.
:-;;:=-=//> '.m^.

m m

i00

H X> Cn

H 

Ca m§ m

EXPLANATION

Resistivity (in ohm-meters)

10-30 

30-100 

100-500 

>500

25 KILOMETERS 

I

o 

zm

mr 

c x>

o00

C 50 

<m
K12 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

that the pluton covers a larger area than that indicated from 

figure 1. Thus, the shallower depth to the >500-ohm-m units 

on the eastern end of profiles C-C' and D-D' may be caused 

by the pluton and not by the eastern rift boundary. Along 

profile D-D', the effects of the pluton may mask the expression of the axial structure that appears to have been delineated on MT profiles A-A', B-B', and C-C' or it may 

terminate near D-D'. The apparent continuity of an axial 

structure from A-A' to C-C' may also be misleading, because 

an east-west seismic profile 7 km south of seismic profile 3 

(fig. 1) shows very little evidence for the disruption of reflections that typifies the Blytheville arch. In addition, another 

east-west seismic reflection profile 8 km south of seismic 

profile 1 (fig. 1) reveals that, although there is structure with 

the characteristics of the Blytheville arch, its amplitude is 

only several hundreds of meters, rather than several kilometers as on other key seismic profiles that were instrumental 

in discovering the arch. These observations imply that the 

Blytheville arch and the axial resistivity structure may have 

considerable amplitude variation along strike and may not be 

as continuous in character as suggested by the three MT profiles and several seismic reflection profiles. An alternative 

interpretation for the MT axial structure is one of several 

uplifts within the axis of the rift, possibly related primarily to 

intrusions. With all available evidence considered, however, 

we prefer the interpretation of a continuous axial structure as 

suggested to us by the MT and seismic reflection data.

SEISMIC REFLECTION AND 

REFRACTION RESULTS

The axial structure identified on the MT models was 

largely unknown prior to this study, although Howe (1984) 

interpreted an axial-fault system using seismic reflection that 

was connected with the original version of the Blytheville 

arch (called "Charlies Ridge" by Howe, 1984). Seismic 

refraction models (Mooney and others, 1983) from profiles 

across and along the rift contain a low-velocity zone (1.8 km/ 

s) corresponding to the Tertiary and Cretaceous sedimentary 

rocks and a high-velocity zone with 6.1 km/s velocities that 

represents the Knox-Arbuckle carbonates. The refraction 

model used a velocity of 4.9 km/s for a low-velocity zone 

interpreted by Mooney and others (1983) to occur beneath 

the 6.1 km/s Knox-Arbuckle carbonates. Neither the thickness nor velocity of this latter low-velocity zone could be 

determined from the refraction data, but it corresponds to 

some portion of the Elvins Group, Bonneterre Formation, 

and basal clastic rocks. It is unlikely that the axial structure 

interpreted from the MT surveys would have been detected 

in the refraction travel-time curves because of the effects of 

this low-velocity zone, which introduces an unresolved layer 

in the refraction models, and because of the broad spacing of 

the recording systems in the refraction survey. In a

reexamination of the refraction data set, Hamilton and 

Mooney (1990) interpret that low amplitudes on some of the 

refraction recording traces are due to attenuation in rocks of 

the Blytheville arch, rather than to other factors, such as poor 

coupling of geophones at the surface. They interpret the trace 

attenuation to be due to (1) defocusing of seismic energy by 

velocity structures in the arch, and (2) seismic absorption by 

rocks in the arch. Seismic models have not been developed 

to simulate the amplitude characteristics of the refraction 

data; thus, the actual causes of amplitude variability are still 

poorly understood but could be related to the arch and (or) 

structures associated with the axial high-resistivity zone.

A line drawing of seismic profile 2 (fig. 8A, used by 

McKeown and others, 1990, in their discussion of the 

Blytheville arch) shows that the Blytheville arch is defined 

by the set of warped reflections in the central part of the seismic profile beneath horizontal reflections that were interpreted to mark the top of Cretaceous and Paleozoic rocks. 

The vertical scaling of the reflection horizons in figure 5 

used a velocity of 4 km/s. In the survey area, the velocity of 

the upper 0.6- to 1.0-s section (representing Quaternary to 

Upper Cretaceous units) is about 2 km/s and the section from 

about 1.0 to 2.0 s (Knox-Arbuckle carbonates and part of 

Elvins-Bonneterre sequences) has velocities of 4.9-6.1 km/s 

(Andrews and others, 1985); thus, we use a value of 4 km/s 

as an approximate integrated velocity. This conversion 

allows a qualitative comparison of the MT models and 

reflection results without requiring a detailed knowledge of 

the depth-velocity function.

The Blytheville arch appears to be unconformable with 

the post-Late Paleozoic erosion surface in the reflection 

records (figs. 8A, 85); thus, the arch must be older than this 

period. A portion of the MT model for profile E-E' (fig. 5) is 

shown underneath the reflection line drawing (fig. 85). The 

base of the Cretaceous and younger rocks on the MT model 

corresponds to resistivities of >30 ohm-m; this boundary 

agrees well with the reflection data for the depth-conversion 

velocity used. As discussed above, the top of the Precambrian clastic rocks may coincide with the >500-ohm-m zone 

in the MT models; this is certainly suggested by the correspondence of the reflections picked as basal elastics to the 

>500-ohm-m depth in figure 8#.

Another key reflection profile coincides with the central part of MT profile B-B' (seismic profile 3, fig. 1). The 

Killam No. 1 Pattinson drill hole is located on this seismic 

profile; the stratigraphy in this well was helpful in interpreting the seismic reflection and MT data. A line drawing of the 

seismic data from profile 3 (from F.A. McKeown, written 

commun., 1991) has been superimposed on the coincident 

portion of MT model B-B' in figure 8C. The base of the Cretaceous and younger units again corresponds to >30 ohm-m 

resistivities, and agreement with the reflection data is excellent. The crystalline basement horizon, tentatively picked by 

F.A. McKeown (written commun., 1991) from the reflection 

data has much less structure than is indicated on the MT
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K13

model for the western part of the profile. The Blytheville 

arch disturbed zone occurs on the flank of the axial resistivity high that we interpret to be a horst as denoted by the fault 

symbols in figure 8C.

The reflection line drawings on the full MT models in 

figure 5 show that the relatively narrow (10 km) zone of 

disrupted and arched reflections on the seismic records are 

located near the center of the much broader (30-50 km) 

resistivity axial high on profile B-B' (fig. 5), but on A-A' it 

appears to be displaced toward the west flank of the resistivity structure. The zone of disruption of seismic reflections extends to depths that are apparently greater than the 

500-ohm-m level in the MT models to depths that are or 

greater than approximate depths to crystalline basement. 

However, this deeper disruption of reflections may be due 

to velocity and geometric effects at shallower levels.

RELATIONSHIP TO SEISMICITY

Seismicity along the northeast trend from Marked Tree 

to Caruthersville (fig. 1) occurs in near-vertical fault zones at 

depths of 3-15 km, and focal mechanisms indicate mainly 

right-lateral slip (Andrews and others, 1985; Chiu and others, 1991). In the northwest-trending seismicity zone 

through New Madrid, the earthquakes occur at similar 

depths (Andrews and others, 1985) but along planes that dip 

32°-55° SW. that exhibit both normal and thrust motion 

(Chiu and others, 1991). The general attitude and depth 

range of these earthquakes is indicated by the asterisks in figure 5. We interpret that earthquakes in the northeast-trending 

zone are associated with sets of near-vertical faults bounding 

and within the axial horst. These seismogenic faults involve 

both basement rocks and overlying Cambrian to Upper Cretaceous sedimentary rocks; earthquakes may be further concentrated along fault segments that have overpressured 

conditions (F.A. McKeown and S.F. Diehl, written commun., 1992).

The nature of the axial structure as it approaches the 

northwest-trending zone of seismicity is unclear. As stated 

above, it may terminate in the area of MT profile D-D' or, 

alternately, it may bend to merge with the western rift boundary. The MT strike directions bend from N. 45° E. to northsouth in the area between sections C-C' and D-D'. This 

change in direction may be related to the axial pluton noted 

in figure 1 on the east ends of these profiles. However, highfrequency MT data also indicate a similar change in strike in 

numerous locations, suggesting that a strong structural grain 

in the Quaternary sedimentary rocks controls shallow MT 

strike directions in the region between C-C' and D-D'. This 

shallow structural grain may be related to sand blows or to 

faults that extend into the Quaternary fill, but the actual 

causes of this shallow structural grain is still unclear and their 

determination will require analysis of detailed, shallow geophysical studies (T.G. Hildenbrand, 1992, oral commun.).

TECTONIC MODEL

The structural high along the central part of the Reelfoot rift on the MT models (fig. 5) may be similar to axial 

horsts recognized in numerous aulacogens (Milanovsky, 

1981). The formation of these axial horsts is related to a process called "inversion" by Milanovsky (1981); this process 

of intra-rift horst development may be related to a combination of post-rift intrusions, regional compression, or isostatic 

response to thick depocenters along the axial zones. The 

inversion process is commonly repeated several times during 

an aulacogen's long geological history (Milanovsky, 1981), 

leading to complex sets of faults within and bounding the 

axial uplifts.

As depicted in figure 9, the early stages of the Reelfoot 

rift included deposition of thick basal clastic rocks (fig. 9), 

sometimes referred to as the Lamotte Sandstone (Howe, 

1984). In late Cambrian time, a thick sequence of anoxic 

shales in the Elvins Group and carbonates of the Bonneterre 

Formation were deposited (fig. 9). We show the axial horst 

as developing during this period. Strong variations in resistivity of the Elvins Group in the Garrigan well and complexity of the electrical signatures of the Bonneterre Formation in 

the Wilson well suggests rapid facies changes that may have 

been related to formation of the proposed horst. We interpret 

that the horst may have subdivided the rift into two, deep, 

shale basins in Middle to Late Cambrian time, with a carbonate platform developing over the horst axis. Regional downwarping in Late Cambrian to Middle Ordovician time led to 

deposition of the thick Arbuckle-Knox carbonates, possibly 

initiating an episode of overpressuring documented by Diehl 

and McKeown (1991). Uplift that began in late Paleozoic 

and possibly continuing into Mesozoic time led to partial 

erosion of the Arbuckle-Knox carbonates and preservation 

of the arch structure delineated in the seismic reflection data 

of figure 8. Intrusions were localized along some rift structures, possibly including the axial horst, in the Permian and 

in Cretaceous time (F.A. McKeown, oral commun., 1992). 

Subsidence of the region surrounding the rift during Late 

Cretaceous led to deposition of Upper Cretaceous to Lower 

Tertiary units of the Mississippi Embayment (fig. 9). Faults 

are interpreted to occur in higher concentrations along the 

axial horst and, as in other inversion structures within rifts 

worldwide (Milanovsky, 1981), many of the faults are thrust 

faults. Overpressuring of the thick Elvins shale may have 

continued to the present time, as suggested by Diehl and 

McKeown (1991) and may be a selective factor in determining which of the axial faults associated with the proposed 

horst actually generate earthquakes.

The Blytheville arch has been interpreted as a diapiric 

structure caused by gravitationally unstable (lower density) 

sedimentary rocks rising upward into higher density rocks 

(McKeown and others, 1990). In addition, it has been suggested that overpressured fluids have played a role in formation of the arch (Diehl and McKeown, 1991). The seismic
K14 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Elvms Gp., 

Bonneterre Fm. 

shales!

Early Cambrian

Arbuckle-Knox equiv. carbonates 

ml Overpressure HI

Late Cambrian to Middle Ordovician

Arbuckle-Knox equiv. carbonates

Arbuckle-Knox equiv. carbonates

Late Cretaceous

Figure 9. Tectonic reconstruction of development of a proposed axial horst in the Reelfoot rift. Arrows 

represent direction of crustal motions.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K15

Figure 10. Drawing showing possible upwarping of reflection 

horizons by simple uplift (arrows) rather than diapiric or intrusive 

processes. Dotted lines are original horizons. Solid lines show final 

position of horizons after uplift.

reflection features used to define the arch may simply be 

related to differential uplift on faults that the axial structure 

mapped in the MT surveys. It is possible to produce 

upwarped reflections from a layered sequence by invoking 

differential uplift over a horizontal distance approximately 

equal to the scale length of the anomalous feature. This is 

illustrated in figure 10, where an original sequence of sedimentary layers (shown by the dotted lines) has been uplifted 

by a bell-shaped uplift vector (arrows in the lower part of figure). The resultant interfaces suggest internal "pillowing" 

that might be attributed to intrusive activity or diapirism, but 

are only the result of differential uplift on beds of varying 

thickness. In addition, if a bell-shaped high velocity occurred 

over the region of distorted reflectors, then the distortion of 

arrival times for reflectors could mimic the patterns of 

observed reflections. For this latter exercise, the bell-shaped 

uplift vector in figure 10 could be replaced by a bell-shaped 

positive velocity anomaly occurring near the Paleozoic 

boundary. Shales are generally lower velocity than carbonate 

rocks, so the latter possibility is probably not realistic in 

terms of any proposed models and well evidence.

The divergence of reflector dips in the seismic reflection drawing of figure 8 have been cited (A.J. Crone, written 

commun., 1992) as evidence for intrusion of material 

between crystalline rocks below and sedimentary rocks 

above. However, the uplift model of figure 10 can clearly 

cause divergence in reflection horizons, as shown by the 

solid lines indicating the final state of the rocks in this model. 

To have "introduction of material between the crystalline 

basement and overlying sedimentary rocks," one would 

expect to have had diapirism involving units such as shale or, 

alternately, igneous activity. It is clear from study of aeromagnetic data (in Hildenbrand, 1985) that there is no linear 

magnetic anomaly that might be expected from a linear zone 

of intrusions at the depths of the arched reflections; thus, it is 

not likely that igneous activity is the cause of the arched

reflections. Release of high pore pressures in the thick Elvins 

shales in the area of the Blytheville arch upon erosion of the 

overlying Knox-Arbuckle rocks (fig. 9) may have caused 

volume expansion of these shales and formation of the arch. 

This would have required local thinning of the compressing 

medium (made up of the Knox-Arbuckle rocks over the axis 

of the arch), which would have required localized uplift. If 

this localized uplift did occur, then both the release of fluid 

pressures and the effect of differential uplift indicated in figure 10 may have contributed to the appearance of reflection 

horizons in figure 8. The Elvins Group shale rocks have relatively high densities of >2.7 gm/cm3 (T.G. Hildenbrand, 

oral commun., 1992), and significant volume expansion in 

the area of the Blytheville arch should give rise to a linear 

gravity low. Such a linear gravity low has not been detected 

as yet; in fact, recently compiled residual gravity maps indicate a linear gravity high that appears to coincide with the 

Blytheville arch (V. Langenheim and T.G.'Hildenbrand, 

written commun., 1992). This gravity high also is negative 

evidence for diapirism involving lower density shales intruding higher density rocks above.

In summary, four factors may have led to formation of 

the arch: (1) igneous intrusions, (2) diapirism involving lowdensity shales, (3) volume expansion due to release of highpore-pressure fluids, and (4) vertical uplift of formations 

with varying thickness. Because of the absence of linear 

magnetic anomalies, explanation (1) must be discounted; 

because of the existence of a linear gravity high over the arch 

(2) and (3) must be somewhat discounted. Because the 

Elvins Group shales are so dense (2.7 gm/cm3), vertical 

uplift of these shales as in explanation (4) and represented in 

figure 10 would give a linear gravity high. However, the 

Blytheville arch is related to quite complex stress and geological factors that we only attempt to categorize, and we do 

not claim to have narrowly constrained its origin.

SUMMARY AND CONCLUSIONS

We have described the results of the first MT surveys 

done in the Reelfoot rift region. The data was interpreted with 

two-dimensional models to show details of structure within 

the rift such as the high-resistivity structure that may represent an axial horst. Comparison of seismic reflection data 

with the MT models shows that the Blytheville arch is a considerably more narrow structure than the axial, high-resistivity structure, and we interpret that the Blytheville arch may 

be related to differential uplift on faults that flank the axial 

horst. These flanking faults may be responsible for localizing 

seismicity in the northeast-trending zone of seismicity from 

Marked Tree, Ark., to Caruthersville, Mo. The margins of the 

rift-fill sedimentary sequences were well defined by the MT 

models, but because of the large effect of these conductive 

sedimentary sequences, we were unable to detect any subupper-crustal structures of the rift (such as the "rift-pillow"
K16 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

interpreted from seismic refraction models). With the present 

data set, we were only able to establish that deep crustal (>10 

km) resistivities must be greater than 100 ohm-m, ruling out 

the possibility of large magma chambers or pervasive deep 

crustal fluids. Actual depth to crystalline basement in the 

main part of the rift is poorly resolved due to the high resistivity in the lower part of the rift fill and the low resistivities 

of the overlying Mississippi Embayment sequences. Intrusive bodies along the margins of the rift that have been 

mapped with aeromagnetic data were not detected in the MT 

surveys, largely because the intrusive rocks have resistivities 

close to those of the Precambrian gneisses that form the basement. Although the MT method has much poorer resolution 

with depth than seismic reflection techniques, the combination of the two geophysical tools provides a refined view of 

the subsurface in the Reelfoot rift region.

ACKNOWLEDGMENTS

The authors wish to thank Frank McKeown, Tom 

Hildenbrand, and Anthony Crone, for providing extremely 

useful reviews of the manuscript and much of the insight, as 

well as geologic and seismic data, that we needed to understand the setting of the Reelfoot rift. We are especially 

indebted to Torquil Smith and John Booker, University of 

Washington, for providing their code for the RRI algorithm 

which allowed us to efficiently interpret the large set of MT 

data. Their software represents a major advance in the field 

of MT interpretation. This research was done under auspices 

of the U.S. Geological Survey National Earthquake Hazards 

Reduction Program.

REFERENCES CITED

Acuna, M.H., Scarce, C.S., Seek, J.B., and Scheifele, I, 1973, The 

MAGSAT vector magnetometer—A precision fluxgate magnetometer for the measurement of the geomagnetic field: 

Greenbelt, Md., NASA Goddard Space Flight Center, Technical Memorandum No. 79656, 18 p.

Andrews, M.C., Mooney, W.D., and Meyer, R.P., 1985, The relocation of microearthquakes in the northern Mississippi Embayment: Journal of Geophysical Research, v. 90, p. 

10223-10236.

Bendat, J.S., and Piersol, A.G., 1971, Random Data: Analysis and 

Measurement Procedures: New York, Wiley Interscience, 

407 p.

Booker, J.R., 1991, Removal of static distortion from 2-D inversions of MT data [abs.]: American Geophysical Union Fall 

Meeting, Program and Abstracts, p. 129.

Chiu, J.M., Yang, Y.T., Chen, K.C., Liaw, Z.S., Chiu, S.C., 

Johnston, A.C., and the PANDA Group, 1991, 3-dimensional 

fault zone geometry and sedimentary basin configuration in the 

central New Madrid seismic zone determined from PANDA 

array data [abs.]: Memphis, Tenn., Proceedings of the Eastern

Section of the Seismological Society of America, 63rd Annual 

Meeting, p. 44.

Clarke, J., Gamble, T.D., Goubau, W.M., Koch, R.H., and Miracky, 

R.F., 1983, Remote-reference magnetotellurics: Equipment 

and procedures: Geophysical Prospecting, v. 31, p. 149-170.

Collins, D.S., Taylor, M.E., Repetski, I.E., and Palmer, A.R., 1992, 

New sedimentologic and paleontologic data for the Dow Chemical No. 1 B.L. Garrigan drill hole, Mississippi County, Arkansas: U.S. Geological Survey Open-File Report 92-6, 38 p.

Diehl, S.F., and McKeown, F.A., 1991, Evidence of paleo- and 

recent fluid overpressure in the New Madrid seismic zone of the 

Reelfoot rift, southeast and northeast Arkansas [abs.]: Memphis, Tenn., Proceedings of the Eastern Section of the Seismological Society of America, 63rd Annual Meeting, p. 68.

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

Prospecting (4th ed.): New York, McGraw-Hill, 867 p.

Ervin, C.P., and McGinnis, L.D., 1975, Reelfoot rift—A reactivated precursor to the Mississippi Embayment: Geological 

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

Gamble, T.D., Goubau, W.M., and Clarke, John, 1979, Magnetotellurics with a remote reference: Geophysics, v. 44, p. 53-68.

Hamilton, R.M., and Johnston, A.C., 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 Mooney, W.D., 1990, Attenuation of seismic 

waves in the upper crust of the New Madrid seismic zone: Science, v. 248, p. 351-354.

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

Embayment from the analysis of gravity and magnetic data: 

Journal of Geophysical Research, v. 90, p. 12607-12622.

Howe, J.R., 1984, Tectonics, sedimentation, and hydrocarbon 

potential of the Reelfoot aulacogen: University of Oklahoma, 

unpub. M.S. thesis, 75 p.

lilies, J.H., 1981, Mechanism of Graben Formation: Amsterdam, 

Elsevier Scientific Publishing Company, 266 p.

McKeown, F.A., Hamilton, R.M., Diehl, S.F., and Glick, 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.

Milanovsky, E.E., 1981, Aulacogens of ancient platforms: Problems of their origin and tectonic development: Tectonophysics, 

v. 73, p. 213-248.

Mooney, W.D., Andrews, M.C., Ginzburg, A., Peters, D.A., and 

Hamilton, R.M., 1983, Crustal structure of the northern Mississippi Embayment and a comparison with other continental rift 

zones: Tectonophysics, v. 94, p. 327-348.

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

1812—Intensities, ground motion and magnitudes: Bulletin of 

the Seismological Society America, v. 63, p. 227-248.

Olhoeft, G.R., 1981, Electrical properties of granite with implications for the lower crust: Journal of Geophysical Research, v. 

86, p. 931-936.

Smith, J.T., and Booker, J.R., 1988, Magnetotelluric inversion for 

minimum structure: Geophysics, v. 53, p. 1565-1576.

————1991, Rapid inversion of two- and three-dimensional magnetotelluric data: Journal of Geophysical Research, v. 96, p. 

3905-3922.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K17

Stanley, W.D., Mooney, W.D., and Fuis, G.S., 1990, Deep crustal 

structure of the Cascade Range and surrounding region from 

seismic refraction and magnetotelluric data: Journal of Geophysical Research, v. 95, no. B12, p. 19419-19438.

Stanley, W.D., and Tinkler, R.D., 1983, A practical, low-noise coil 

system for magnetotellurics: U.S. Geological Survey OpenFile Report 83-85, 18 p.

Sternberg, B.K., Washburne, J.C., and Pellerin, L., 1988, Correction for the static shift in magnetotellurics using transient electromagnetic soundings: Geophysics, v. 53, p. 1459-1468.

Thomas, W.A., 1991, The Appalachian-Ouachita rifted margin of 

southeastern North America: Geological Society of America 

Bulletin, v. 103, p. 415-131.

Vozoff, Keeva, 1972, The magnetotelluric method in the exploration of sedimentary basins: Geophysics, v. 37, p. 98-141.

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication November 9, 1994 

Edited by Richard W. Scott, Jr. 

Graphics prepared by Norma J. Maes; use made of

author-drafted material 

Photocomposition by Mari L. Kauffmann
K18 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

APPENDIX A

MAGNETOTELLURIC METHOD

MT data consist of measurements of the orthogonal 

components of time-varying surface magnetic fields and 

their associated electric fields. These fields occur naturally 

as a result of electromagnetic induction in the Earth from 

magnetic variations originating in the ionosphere and atmosphere. In our surveys, horizontal electric fields were sensed 

using an L-shaped, three-electrode array with dipole lengths 

of 37.5 m. The orthogonal, horizontal magnetic fields in the 

direction of the electric-field-measurement array were 

sensed using permalloy-cored induction coils (Stanley and 

Tinkler, 1983). The vertical magnetic field was measured 

only at selected sounding sites when a ring-core fluxgate 

magnetometer (Acuna and others, 1973) was deployed for 

magnetic-field sampling instead of the induction coils. 

Time-series recordings of the electrical- and magnetic-field 

data were acquired using a computer-controlled data-acquisition system, with the computer and other electronics powered by marine storage batteries. The acquisition system 

used an analog signal-conditioning system for pre-whitening 

of the input spectrum and post-amplification, combined with 

a 12-bit analog-to-digital converter. Data were recorded to 

disk for pass-bands at the three frequency ranges of 

0.002-0.2 Hz, 0.05-3 Hz, and 2-300 Hz. The measurements 

spanned a period of about 2-3 hours and included three to six 

recordings of the higher frequency bands and one to four 

recordings of the lowest frequency band. At selected sounding sites, the ring-core fluxgate magnetometer was used 

instead of the induction coils for magnetic-field measurement to obtain vertical magnetic-field data and to extend the 

MT data to frequencies of 0.0002 Hz (one decade lower than 

that obtainable with the three standard bands above). An 

automated, battery-powered data-acquisition system 

designed by V.F. Labson (written commun., 1989) was used 

with the ring-core-magnetometer system and associated 

electric-field sensors (identical to those used for the three 

standard bands). The automated system was deployed for at 

least 9 hours to record the lower frequency data.

The time-series data were converted to complex crossspectra using FFT (fast-Fourier-transform) techniques. 

Least-squares, cross-spectral analysis (Bendat and Piersol, 

1971) were used to solve for a tensor-transfer function relating the observed electric fields to the magnetic fields under 

the assumption that the Earth consists of a two-input, twooutput, linear system with the magnetic fields as input and 

the electric fields as outputs:

(1)

where

£/ are the two horizontal electric fields,

HJ are the two horizontal magnetic fields, and

all quantities are a function of frequency.

When the vertical magnetic field was measured, the 

vertical-field-transfer function (or "tipper") was solved for, 

with the assumption that the vertical field is the output of a 

two-input, one-output system (with the vertical field as the 

output):

H.= AHX + BHV (2)

where

Hx and Hy are the horizontal magnetic fields, 

H-, is the vertical magnetic field, and

A and B are the transfer-function elements.

The impedance tensor, Zy-, is treated as amplitude and 

phase quantities. The amplitude of the tensor is converted to 

"apparent resistivities" using the relationship between the 

resistivity of a homogeneous Earth to the tensor-impedance 

relationship between surface electric and magnetic fields:

P, = ri (3)

where

pa is the "apparent resistivity,"

Ej and HJ are the orthogonal electric- and magnetic-field

pairs,

co is the radian frequency, and

jig is the magnetic permeability (assumed to be the same 

as free space at MT frequencies). 

The phase of the impedance tensor, 0, is given by the 

arctangent of the amplitude of the real part of the tensor to 

the imaginary part:

= arctan(Zrea/ /ZimaKinary ) (4)

is the impedance tensor,

The measured resistivity and phase values are plotted 

versus frequency for computer modeling. Both the resistivity 

and phase data are normally used in the modeling, although 

it can be demonstrated that they are not independent by 

means of the Hilbert transform (Bendat and Piersol, 1971). 

Prior to conversion to resistivity and phase, the tensor is normally rotated into principal directions that usually correspond to the direction of maximum and minimum resistivity. 

For a two-dimensional (2-D) Earth, the MT fields can be 

decoupled into transverse electric (TE) and transverse magnetic (TM) modes; two-dimensional modeling is generally 

done to fit both modes. When the geology satisfies the 2-D 

assumption, the MT data for the TE mode is assumed to represent the situation when the electric field is along the geologic strike, and the data for the TM mode is assumed to 

represent the situation when the electric field is across strike.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K19

A measure of the correctness of the 2-D assumption for MT 

data is provided by the skew of the impedance tensor, 

defined as the ratio of the magnitude of the diagonal elements over the off-diagonal elements (Vozoff, 1972). If the 

geology is truly 2-D, then the skew will be zero. Remote or 

local reference sensors can be used to help reduce bias in the 

impedance determinations due to instrument or environmental noise (Gamble and others, 1979; Clarke and others, 

1983). Although reference techniques were not used in our 

survey, we did sort cross-power files to select optimal signalto-noise data sets.

Man-made electrical noise, such as power lines and irrigation pumps, can have a negative effect on MT data quality. 

In the survey area, there are numerous small power lines that 

did not affect data quality as long as the soundings were done 

about 0.4 km from them. Large sprinkler systems driven by 

electric irrigation pumps were more of a problem and were 

not approached closer than 1.5 km for soundings. Local 

lightning, wind, and rainstorms can also degrade data quality, but these can be generally avoided by planning surveys 

to miss the most active thunderstorm periods. The MT signals that originate in the ionosphere and from equatorial 

lightning belts vary in spectral densities (amplitudes) from 

several factors. The lower frequency ionospheric signals 

(0.0001 to 1 Hz) are related to sunspot activity, and the last 

3 years have been ones of high sunspot activity and good signal levels, but, even in low sunspot activity, the MT signals 

are adequate for year-round MT surveys. Higher frequency 

MT signals (1 Hz to 10,000 Hz) that are generated largely 

from equatorial lightning storms that propagate in the Earthionospheric waveguide have minimums in amplitude in the 

Northern Hemisphere winter; this amplitude minimum does 

cause problems with some wintertime MT surveys using the 

higher frequency ranges. An excellent introduction to the 

MT method and references for a more advanced understanding are contained in Dobrin and Savit (1988).

INTERPRETATION OF MT AND 

SEISMIC DATA

Resistivities in rocks are largely controlled by ionic 

conduction in pore fluids and clay minerals in rocks, rather 

than by electronic conduction through the rock-matrix resistivity itself. Archie's law (Olhoeft, 1981) expresses a relationship between pore-fluid resistivity, rock-matrix 

resistivity, and actual rock resistivity.

Pr = Ap w O" (5)

where

pr is bulk resistivity of the rock with a water resistivity of 

pM, and a porosity of O (raised to the empirical value

-m), and 

A is an empirical constant.

The constant, A, varies from slightly less than 1 with 

rocks having mainly intergranular porosity to slightly more 

than 1 for rocks with mainly joint porosity. The exponent, m, 

is somewhat larger than 2 for cemented and well-sorted 

granular rocks and less than 2 for poorly sorted and poorly 

cemented granular rocks.

Resistivities in most rock-forming minerals are quite 

high, typically greater than 1,000 ohm-m, but typical values 

for basin-filling sedimentary rocks or marine sedimentary 

rocks are low (1-100 ohm-m) due to the low resistivity of 

pore fluid. As porous rocks are buried deeper than a few kilometers, porosities are decreased due to lithostatic loading 

until the rocks normally become highly resistive. In shales, 

ionic conduction also occurs in water trapped in clays and 

zeolites; as a result, the resistivity of shales will be low (1-20 

ohm-m) and vary less than other lithologies as porosity 

decreases. As shales are metamorphosed, both porosity and 

layered clays are destroyed, but low resistivities can still be 

maintained due to the formation of carbonaceous or iron 

mineral films along bedding planes in the metashales. Intrusive rocks have minor porosity and, thus, are normally very 

resistive, typically in the range of 500-20,000 ohm-m. Fracture porosity and intense alteration of intrusive rocks can 

lower their resistivity well below this range. Unaltered volcanic rocks have very high resistivities when pore waters are 

fresh; however, as volcanic rocks increase in age, the formation of clays and zeolites dramatically decreases their resistivity. In our MT surveys of the Reelfoot rift, the variety of 

rocks and physical conditions mean that resistivities were 

expected to vary from 1 to 10,000 ohm-m, with conductive 

shales at the low end of the spectrum and crystalline rocks at 

the high end.

Seismic compressional-wave velocities for rocks in the 

Reelfoot rift vary from about 1.8 to greater than 6 km/s, as 

determined from well logs and seismic refraction surveys 

(Mooney and others, 1983). The lowest velocities are in 

Cenozoic and Cretaceous sedimentary rocks. High-dolomite-content carbonate rocks of the Knox or Arbuckle Group 

equivalents have velocities of about 6.1 km/s, and Elvins 

Group shale and Bonneterre Formation carbonate rocks 

beneath the Knox-Arbuckle Group equivalents have velocities of 4.9-5.5 km/s. The velocity of intrusive rocks is influenced by the proportion of quartz to mafic minerals in the 

rocks, with felsic rocks such as granite having velocities of 

about 5-6 km/s and intermediate to mafic intrusive rocks 

ranging higher than 6.3 km/s. The recording of reflections 

from geologic formations is dependent upon sufficient 

acoustic contrast and favorable geometry of the reflecting 

surfaces. Because of the different controls over resistivity 

and compressional velocities, we do not expect complete 

agreement between MT and seismic interpretation.
K20 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

APPENDIX B fr°m west to east- Increasing station numbers on profile E-E'

	represent positions along the profile from north to south. On

The following pages show plots of the observed data the plots, x's indicate the digitized values of the transverseinput to the 2-D inversion program and the computed data electric (TE) resistivity data from the MT soundings after 

for the derived model for each individual sounding on the hand-smoothing of the curves. Solid lines represent cornfive magnetotelluric (MT) profiles shown in figure 5. The puted TE data for the models shown in figure 5. The plots are 

station numbers for profiles A-A', B-B', C-C', and D-D' rep- arranged by profile (A-A', B-B', C-C', D-D', and E-E'), and, 

resent, with increasing numbers, positions along the profile for each profile, resistivity plots are followed by phase plots.

Sta t ton 2 Station 3 Station A

Sto t Ion 5 Station 6 Station 7

Station 9 Station 10

i mm i nun i nun i nun 

Station 11

Station B

Station 12

Station 13 Station 14 Slat ton 15 Station 16

0.8ei 0.01 0.1 i 10 use 1000 0.001 0.01 0.1 1 10 100 1000 0.001 0.01 0.1 i 10 use 1000 0.001 0.01 0.1 1 10 iee 1000

FREQUENCY, IN HERTZ

Figure Bl (above and facing page). Resistivity plots for 23 stations along profile A-A' (see fig. 5). See Appendix A for explanation of 

"apparent resistivity." Increasing station numbers represent positions along the profile from west to east.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K21

Station 17 Station 18 Station 19 Station 20

Station 21 Station 22 Station 23

0.001 0.01 0.1 t 10 100 1000 0.00t 0.01 0.1 I 10 100 1000 0.001 0.01 0.1 I 10 100 1«

90.0- 

80.0-

78.8- 

6e.e60.0-

te.e38.8- 

20.0-

ie.eSte t Ion 1

' ""'q ' ""•< ' mil

Station 5

g 70.0H 

Q 68.0H

LU 40.0-

CO

< 38.0-

X

Q- «

Station 9

78. e6e. e58.8-

•n.e30.8- 

28.0-

18.eSte t Ion 2

Station 6

Station 10

Stotlon 3

Station 7

Station 11

Station 4

Station B

Station 12

0.81 e.i 1 10 100 ieee e.rai 0.01 0.1 i 10 tee 1000 0.001 0.01 0.1 1 10 tee 10

FREQUENCY, IN HERTZ

Figure B2 (above and following page). Phase plots for 23 stations along profile A-A' (see fig. 5). Increasing station numbers represent 

positions along the profile from west to east.
K22 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

99.0-

BB.B78.8-

66.6-

68.8- 

48.0- 

38.8- 

20.0-

10.8-

Statlon 13 Slot Ion Station 15 Station 16

Station 17

^ -18.0-

LLJ 38.8-

<C 28.8-

X

Q- 10.0H

Sto t Ion 21

Station 18

Station 22

Station 19

Station 23

Station 20

01 0.01 0.1 t 10 100 1080 0.001 0.01 0.1 1 10 100 1000 8.001 0.01 0.1 1 10 100

FREQUENCY, IN HERTZ
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K23

Station 1 Station 2 Station 3 Station 4

Stotlon 5

I

O 1Bae i

Station 9

LLJ e.iDC

<

0- Stotlon 13

Q_ i«

<

Station 17

Stotlon 6

Station 10

Stotlon 14

Station 18

e.eei e.ei e.\ i 10 100 iaoe e.een e.ei e.t 1 ie

Stotlon 7

Station 11

Stotlon 15

Station 19

Stotlon B

Station 12

Stotlon

01 0.01 e.1 1 10 106 10

FREQUENCY, IN HERTZ

Figure B3. Resistivity plots for 19 stations along profile B-B' (see fig. 5). See Appendix A for explanation of "apparent resistivity. 

Increasing station numbers represent positions along the profile from west to east.
K24 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Station 1 Station 2 Station 3

78.8 

60.0 

E0.0 

48.0 

38.0

Stetlon

Station 5

78.e60.0- 

se.e- 

*e.e -

30.8-

28.BSletlon 9

CO 78.8-

38.8 

LU

CO 2e.e

Station 13

79.9- 

68.6

Station 17

78.8-

60. ese.eSta t Ion 6

Station 10

Station 14

Station 18

Station 7

Station 11

Station 15

Station 19

Station 8

Station 12

Station 16

e.eei e.et e.1 t 10 iee 1000 e.eei e.ei e. 1 1 le iee 1000 0.001 0.01 0.1 i 10 100 1000

FREQUENCY, IN HERTZ

Figure B4. Phase plots for 19 stations along profile B-B' (see fig. 5). Increasing station numbers represent positions along the profile from 

west to east.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K25

Station 1 Station 2 Station 3 Station

Stotlon 5 Stotton 6

Station 10

Stotlon 7

Station 11

Stotlon 8

Station 12

Stotlon 13 Stotlon 14

Station 17 Station 18

Stotlon 15

Stotlon 19

Station 16

Stotlon 20

e.0ei 0.01 0.1 i 10 im ieee e.sen 0.01 0.1 i 10 iae iaee e.eei 0.01 0.1 i 10 100 iaee 0.001 0.01 0.1 1 10 iae 10(

FREQUENCY, IN HERTZ

Figure B5. Resistivity plots for 20 stations along profile C-C' (see fig. 5). See Appendix A for explanation of "apparent resistivity. 

Increasing station numbers represent positions along the profile from west to east.
K26 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Station 1 Station 2 Station 3 Station 4

79.0

60.0

28.0

18.8

Sto t Ion 5 Station 6

•W.B38. BStatlon 9 Stetlon 10

CO 90.8-

X 78.8-

D_

60.0-

50.B- 

•W.8- 

30.0- 

20.0-

18.8-

Stotlon 13 Stotlon

7B.B 

68.6

38. B 

38.8

Station 17 Stetlon 18

78.8-

60.0-

50.B- 

•0.8- 

30.0- 

28.B18.8-

0.01 e. 1 i 10 ie

Stotlon 7 Stotlon 8

i niin i mm i nun i nun i i

Station 11 Station 12

Stotlon 15 Stotlon 16

Stotlon 19 Stotlon 20

e.eei 0.01 0.1 1 le ie ei 0.01 0.1 i 10 lee iem e.eei 0.01 0.1 1 10 100 ie

FREQUENCY, IN HERTZ 

Figure B6. Phase plots for 20 stations along profile C-C' (see fig. 5). Increasing station numbers represent positions along the profile from

west to east.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K27

Station 1 Station 2 Station 3 Station 4

Cfl

cc

LU 

LU

I 0000-

Stotlon 5

Station 9

Station 13

Sta t Ion 6

Station 10

Station 14

Station 7

Station 11

Station 15

Station B

Station 12

Station 16

0.001 0.01 0.1 1 10 100 1000 0.001 0.01 0.1 1 10 100 1000 0.001 0.01 0.1 1 10 100 1000 0.001 0.0| 0.1 1 10 100 1000

FREQUENCY, IN HERTZ

Figure B7. Resistivity plots for 16 stations along profile D-D' (see fig. 5). See Appendix A for explanation of "apparent 

resistivity." Increasing station numbers represent positions along the profile from west to east.
K28 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

96.8

ae.e

79.9

49.9

38. e

28.0

19.9

Station 2 Station 3 Station 4

Stotlon 5

79.9- 

60. B

40.0

38.0

CO

II I 20.9

LU

DC ie.e

CD

LU e.e

Q

Station 9

68. e • 

se.e-

-W.9-

Stotlon 13

7B.0- 

68.6- 

5e. e- 

•«.e3e.e20.8-

ie.eStotlon 6

Station 10

Stotlon

Stotlon 7

Station 11

Station 15

Stotlon 8

Station 12

Station 16

e.eei e.ei e. 1 t ie 100 ieee 9.MI e.ei e.i i 10 180 ieae 0.001 e.ei e.i

FREQUENCY, IN HERTZ

100 ieee 0.001 e.ei 0.1 i ie 10« 1000

Figure B8. Phase plots for 16 stations along profile D-D' (see fig. 5). Increasing station numbers represent positions along the profile from 

west to east.
REELFOOT RIFT STRUCTURES DELINEATED WITH MAGNETOTELLURIC SURVEYS K29

Station 1 Station 2 Station 3 Station 4

Sto t Ion 5

Station 9

Station 13

Station 17

Station 6

Station 10

Stollon 14

e.081 0.01 0.1 t

e.001 0.01 0.1 i

Sto t Ion 7

Station 11

Sto t Ion 15

StotIon 8

Station 12

Sto t Ion 16

10M B.B01 0.01 0.1 1 10 108 1890 0.001 0.01 0.1 1 10 10«

FREQUENCY, IN HERTZ

Figure B9. Resistivity plots for 17 stations along profile E-E' (see fig. 5). See Appendix A for explanation of "apparent resistivity. 

Increasing station numbers represent positions along the profile from north to south.
K30 INVESTIGATIONS OF THE NEW MADRID SEISMIC ZONE

Station 1 Station 2 Station 3

70.0 

60.0 

BB.B

•«.0 

30.0

ZB.B

90.0- 

80.0-

•W.B3B.0- 

20.0- 

10.0 

0.9

Sto t Ion 5

Station 9

70.0- 

60.0- 

58.B- 

•40.0- 

30.0- 

2B.BStation 13

28.Bia.8-

Stetlon 17

01 0.01 0.1 1 10 100 18

Station 6 Station 7 Station 8

Station 10 Station 11 Station 12

Station Station 15 Station 16

.001 0.01 0.1 I 10 100 1000 0.001 0.01 0.1 1 10 100 1090 0.001 0.01 0.1 1

FREQUENCY, IN HERTZ

Figure BIO. Phase plots for 17 stations along profile E-E' (see fig. 5). Increasing station numbers represent positions along the profile from 

north to south.