GEOPHYSICS, VOL. 61, NO. 6 (NOVEMBER-DECEMBER 1996); P. 1871–1882, 10 FIGS., 2 TABLES.

Shear-wave splitting in Quaternary sediments: Neotectonic implications in the central

James B. Harris

ABSTRACT 8 ms at a two-way traveltime of 375 ms produced an Determining the extent and location of surface/near- average azimuthal anisotropy of 2% between the tar- surface structural deformation in the New Madrid get reflector (top of Quaternary gravel at a depth of seismic zone (NMSZ) is very important for evaluating 35 m) and the surface. Based on the shear-wave polariza- earthquake hazards. A shallow shear-wave splitting ex- tion data, two explanations for the azimuthal anisotropy periment, located near the crest of the Lake County in the study area are (1) fractures/cracks aligned in re- uplift (LCU) in the central NMSZ, shows the presence sponse to near-surface tensional stress produced by uplift of near-surface azimuthal anisotropy believed to be as- of the LCU, and (2) faults/fractures oriented parallel to sociated with neotectonic deformation. A shallow four- the Bend scarp, a recently identified surface component data set, recorded using a hammer and mass deformation feature believed to be associated with con- source, displayed abundant shallow reflection energy on temporary seismicity in the central NMSZ. In addition records made with orthogonal source-receiver orienta- to increased seismic resolution by the use of shear-wave tions, an indicator of shear-wave splitting. Following ro- methods in unconsolidated, water-saturated sediments, tation of the data matrix by 40 , the S1 and S2 sections measurement of near-surface directional polarizations, (principal components of the data matrix) were aligned produced by shear-wave splitting, may provide valuable with the natural coordinate system at orientations of information for identifying neotectonic deformation and N35W and N55E, respectively. A dynamic mis-tie of evaluating associated earthquake hazards.

INTRODUCTION of tectonic deformation (e.g., Zoback et al., 1980; Sexton and Jones, 1986; Odum et al., 1994). Although recent high- The relationship between geologic structure and contempo- resolution P-wave (compressional wave) investigations in the rary seismicity is one of the most important topics of research region have successfully imaged faults in poorly consolidated related to earthquake hazard evaluation in the New Madrid Tertiary sediments (Luzietti et al., 1992; Schweig et al., 1992; seismic zone (NMSZ), the most active earthquake zone in the Sexton et al., 1992; Van Arsdale et al., 1992), the Quaternary central and eastern . Determining the association section has been generally unresolvable. Consequently, studies between seismicity and structural deformation in the NMSZ is emphasizing the characteristics of S-wave (shear wave) prop- hindered by the presence of thick, unconsolidated sediments agation, which include increased seismic resolution in water- that cover the region. The upward continuation of basement saturated sediment sequences (i.e., Woolery et al., 1993) and faults into Quaternary strata is often masked by the inability of the phenomenon of shear-wave splitting (discussed in this pa- soft sediments to propagate large fractures, making identifica- per), may provide more suitable methods of investigating near- tion and age determination of near-surface geologic structures surface structure in the NMSZ and areas with similar geology. a difficult problem. Shear-wave splitting, induced by stress-aligned inclusions Seismic reflection methods have been used for many years (fractures, cracks/microcracks, pore spaces), causes shear within the NMSZ to determine the style, extent, and age waves to exhibit directional polarizations in response to

Manuscript received by the Editor February 13, 1995; revised manuscript received January 31, 1996. Formerly Kentucky Geological Survey, University of Kentucky, Lexington, KY 40506-0107; presently Millsaps College, Department of Geology, 1701 N. State St., Jackson, MS 39210. c 1996 Society of Exploration Geophysicists. All rights reserved.

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1872 Harris propagation through azimuthally anisotropic media (Crampin, image near-surface geologic structure in the central NMSZ 1985; Thomsen, 1988; Tatham and McCormack, 1991). This (Harris et al., 1994). A four-component reflection test was phenomenon is manifested by differences in shear-wave veloc- completed using a hammer and mass energy source (i.e., ity between waves traveling parallel (S1—fast shear wave) and Hasbrouck, 1987). The objectives of the experiment addressed perpendicular (S2—slow shear wave) to the trend of the inclu- two questions: (1) could shear-wave splitting be identified on sions. The difference in arrival time between the two waves, reflection records in near-surface unconsolidated sediments? measured for a particular event, can be used to estimate the and (2) if shear-wave splitting was identifiable, could its pres- amount of anisotropy that exists along the path of the wave. ence be used as an additional tool in the evaluation of shallow When shear-wave splitting observations are made at the surface deformation associated with NMSZ seismicity? (i.e., reflection surveys), the last anisotropic horizon encoun- tered is responsible for the recorded polarizations. Reflection GEOLOGIC SETTING AND SEISMICITY experiments designed to evaluate shear-wave splitting typically record an orthogonal pair of S-waves, generally designated The location of the experiment was near the center of a SH-waves (horizontally polarized shear waves) and SV-waves region called Kentucky Bend, which lies inside a meander (shear waves polarized in a vertical plane). However, unless the loop of the in extreme western Kentucky seismic line is exactly parallel or perpendicular to the trend of (Figure 1). Geologically, the study area is situated in the upper the inclusions, the S-wave designations are only useful in de- Mississippi embayment, where Paleozoic bedrock is overlain scribing the acquisition geometry. Most commonly, the S-wave by approximately 600 m of Cretaceous to Recent unconsol- components must be rotated from the acquisition coordinate idated sediments, with local accumulations of 50 to 60 m of system into the natural coordinate system. Quaternary sand, silt, and gravel. The area is located in the A shear-wave splitting experiment was carried out as part central NMSZ near the crest of the Lake County uplift (LCU), of a shallow SH-wave seismic reflection program designed to the most prominent known example of surficial deformation in

FIG. 1. The pattern of contemporary seismicity and regional tectonic setting of the New Madrid seismic zone (modified after Luzietti et al. 1992). Inset shows the location of the study area in relation to the Lake County uplift (modified after Russ 1982).

Shallow Shear-wave Splitting: NMSZ 1873 the NMSZ. Russ (1982, 97) described the LCU as “...a gently and radial source-radial receiver (SV-SV) reflection records sloping, irregularly shaped topographic bulge whose surface taken along a 75-m-long test line oriented N85W (see Figure 1 has been upwarped as much as 10 m above the general level for location). The data were recorded on a 12-channel engi- of the Mississippi River Valley.” Along the eastern edge of the neering seismograph using a 5.4-kg sledgehammer/10-kg steel LCU lies the Reelfoot Lake Basin. Separating the LCU from I-beam energy source and single-component 30-Hz horizontal the basin is the north-south–trending Reelfoot scarp, which geophones. The geophones were first oriented transverse to the Russ (1979) described as a complex monoclinal fold of 3 to 9 m seismic line (SH-mode), and recordings were made of horizon- in height. Recent paleoseismologic trenching on the central tally directed transverse and radial hammer blows (five blows Reelfoot scarp (Kelson et al., 1992; 1994) has found evidence on each side of the I-beam were stacked using the polarity re- for at least three moderate to large earthquakes within the past versal feature of the seismograph). The line was then reshot 2200 years, including the great earthquakes of 1811–1812. with radial geophone orientation (SV-mode), and both trans- The four major earthquakes that occurred in the cen- verse and radial hammer blows were repeated. Table 1 details tral Mississippi River Valley between December 1811 and the seismic data acquisition parameters used in the experiment. February 1812 constitute the largest sequence of earthquakes Examination of the field records showed that records made in recorded North American history. The magnitudes of the when the receivers were oriented perpendicular to the source events were estimated to range from mb,Lg 7.0 to mb,Lg 7.3 motion contained as much or more S-wave reflection en- (Nuttli, 1973; Street, 1982). The earthquakes, centered near ergy than the recordings made with coincident source-receiver New Madrid, , caused ground failure over an area of orientations (Figure 3). This observation is consistent with in- 48 000 km2 (Fuller, 1912) and damaged structures as far away terpreted shear-wave splitting from previous multicomponent as Cincinnati, Ohio, and St. Louis, Missouri. In the 3 1/2-month reflection surveys (Alford, 1986; Willis et al., 1986; Squires period following the first earthquake, more than 200 after- et al., 1989; Mueller, 1991), and suggested that the near-surface shocks greater than mb,Lg 5.0 (with at least six greater than structure in the study area was azimuthally anisotropic, possi- mb,Lg 6.0) are believed to have occurred (Street and Nuttli, bly a result of neotectonic activity in the central NMSZ. 1990). Stauder et al. (1976) first recognized the three linear trends PROCESSING AND ROTATION ANALYSIS of seismicity that make up the NMSZ (Figure 1). The south- Each of the data sets from the experiment were processed ern, northeast-trending segment extends from northeastern identically (Table 2a) to produce a matrix of four stacked re- Arkansas to northwestern ; the central segment flection sections (Figure 4), and the individual components extends to the northwest from northwestern Tennessee to were named 100 (SH-SH), 200 (SV-SH), 300 (SH-SV), and southeastern Missouri; and the northern segment, whose seis- 400 (SV-SV). Alford’s (1986) rotation was used to determine micity becomes more diffuse, extends northeast from south- the orientation of the natural coordinate system by maximiz- eastern Missouri to near the confluence of the Mississippi and ing the reflection energy on the elements of the main diagonal Ohio rivers. First-motion studies of earthquakes in the NMSZ (100 and 400) of the matrix and minimizing the reflection en- (Herrmann and Canas, 1978; Herrmann, 1979) indicate that ergy on the elements of the cross diagonal (200 and 300) of the dominant focal mechanism is a right-lateral strike slip on the matrix. The rotation approximates physical alignment of the southern and northern segments and that the central seg- the sources and receivers with the natural axes. An initial es- ment displays a significant reverse component. Recent high- timate of the correct rotation angle was determined by visual resolution monitoring of microearthquakes in the central seg- inspection of the sections plotted at 15 rotation increments. ment of the NMSZ (Chiu et al., 1992) has delineated clusters of A5 rotation increment was then used to determine the final earthquake hypocenters that have been interpreted to repre- orientation more precisely (i.e., Squires et al., 1989). Figure 5 sent active fault planes. The imaged fault plane that lies beneath presents the data matrix aligned (following a 40 counterclock- the wide part of the LCU (where the study area is located) dips wise rotation) with the natural coordinate system, showing to the southwest at an angle of 31 and has a surface projection the focused reflection energy on the main-diagonal elements along the eastern margin of the LCU. Chiu et al. (1992) con- and the reduced reflection energy on the cross-diagonal el- tended that if the fault plane represents a thrust fault, which ements. The field data were then rotated to the appropriate is consistent with the northeast-southwest compressive stress orientation and the 100 and 400 sections (main diagonal of the regime in the area (Zoback and Zoback, 1980; Ellis, 1994), then four-component data matrix) were extracted and processed it could explain the existence and geometry of the LCU. independently. Table 2b presents the post-rotation processing SEISMIC DATA ACQUISITION sequence. Following identification of a strong shallow reflection on a Table 1. Acquisition parameters. nearby S-wave seismic reflection profile (Harris et al., 1994) and correlation of walk-away tests with the log from a lo- Source 5.4 kg sledgehammer/10 kg steel cal borehole (Figure 2), four-component common-depth-point I-beam (horizontal impact) (CDP) reflection data, targeting the top of a basal Quaternary Source interval 3 m gravel layer (approximately 350 ms to 400 ms two-way trav- Receivers 30 Hz horizontal geophones Receiver interval 3 m eltime), were collected in an attempt to identify, and inter- Spread configuration Off end, 3 m offset pret the significance of, shear-wave splitting in overlying Qua- Recording system EG&G ES-2401 (12-channel) ternary sands and silts. The data set consisted of transverse Fold 600% source-transverse receiver (SH-SH), radial source-transverse Field filters 25–250 Hz; 18 db/octave slope receiver (SV-SH), transverse source-radial receiver (SH-SV), Record length 1024 ms (0.5 ms sampling interval)

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Table 2. Data processing sequence. To improve confidence in the results, alternate sections were processed without AGC to ensure that its application in a) Prerotation prerotation processing did not adversely affect the rotational 1 Reformat to SEGY; resample to 1 ms analysis. The results of an amplitude versus rotation angle sampling interval 2 Gain recovery (exponential) comparison for data with and without AGC applied (Figure 6) 3 Bandpass filter (18/20–100/105 Hz) suggest that the AGC window (250 ms) used in this study was 4 AGC (250 ms window) sufficiently large as not to detrimentally influence the rota- 5 Elevation statics tional analysis. 6 Trace editing (first-arrival and surgical mutes) 7 Velocity analysis The shear-wave window in unconsolidated sediments 8 NMO correction 9 CDP Sort (6-fold) One of the difficulties in interpreting shear-wave splitting 10 CDP Stack on reflection data is the influence of the Earth’s surface. At 11 AGC (250 ms window) incident angles greater than critical [sin 1(V /V )1], ampli- 12 Coherency filter P S 13 Alford rotation analysis tude, phase, and frequency changes, caused by interactions with 14 Rotate field data with determined angle the free surface, can distort the arriving waveform, while ar- b) Postrotation rivals within the “shear-wave window” are generally undis- 15 Extract S1 and S2 data sets turbed (Evans, 1984; Booth and Crampin, 1985; Crampin, 16 Repeat 2–6 1985). VP /VS ratios in near-surface unconsolidated sediments 17 Velocity analysis (independently for S1 and S2) are almost always greater than 2 (Dohr and Janle, 1980) and 18 Repeat 8–12 commonly exceed 5 (Stumpel ¨ et al., 1984). High VP /VS ratios,

FIG. 2. S-wave walk-away test and velocity-depth model for the central Kentucky Bend compared to the gen- eralized log for a local borehole (see Figure 1 for locations). The shaded arrows mark the top and bottom of a basal Quaternary gravel unit. The top-of-gravel reflection (375 ms) was chosen as the target for the shear-wave splitting experiment.

Shallow Shear-wave Splitting: NMSZ 1875

FIG. 3. Selected field records (with bandpass filter and AGC applied) from the four-component shear-wave splitting experiment: 100, 200, 300, and 400 show the response of SH-SH, SV-SH, SH-SV, and SV-SV source- receiver pairs, respectively. The near offset and trace spacings are 3 m.

1876 Harris corresponding to high Poisson’s ratios, reduce the width of the before the CDP stack. Although the fold on the target reflec- shear-wave window and can cause distortions on shallow re- tor was reduced by as much as half, the mutes improved the flection data recorded at offsets that demand incident angles frequency content and signal-to-noise ratio of the data. beyond critical. For this experiment, S-wave velocities at the surface (calcu- Influence of shallow anisotropic structure lated from first-arrival data on the field records) ranged from 100 m/s to 130 m/s along the seismic line and corresponding In addition to waveform distortions caused by interactions P-wave velocities ranged from 275 m/s to 335 m/s. VP /VS with the free surface outside the shear-wave window, another ratios ranged between 2.1 and 3.3, thereby resulting in a shear- potential difficulty with shear-wave reflection survey in az- wave window with a maximum width ranging from 17 to 28. imuthally anisotropic material was suggested in Crampin and Based on the velocity ratios and source-receiver offsets (3 m to Lovell (1991): because shear-wave polarizations are deter- 36 m) used to image the target reflection (approximately 35 m mined by anisotropy close to the receiver, recorded polariza- deep), incident angles of 2 to 27 were calculated, suggesting tions often represent the near-surface structure and not the that some of the longer offset reflection data might have been structure of deeper targets. However, since the primary in- altered by recording outside the shear-wave window (Figure 7). tent of this experiment was to evaluate the anisotropic struc- With this problem recognized, very careful muting (front-end ture of the near surface (<35 m deep) and attempt to relate it and surgical) was performed to eliminate the distorted data to deformation associated with active tectonics in the central

FIG. 4. Two-by-two shear-wave splitting data matrix. The individual elements (100, 200, 300, and 400) represent stacked reflection sections (1.5 m trace spacing) from the recorded data set. Note the abundant reflection energy on the cross-diagonal (200 and 300) elements of the matrix, an indication that shear-wave splitting has occurred.

Shallow Shear-wave Splitting: NMSZ 1877 NMSZ, shallow anisotropic conditions were viewed as a tar- reflection. After rotation, the S1 section is oriented N35 W get rather than an obstacle. Nevertheless, the complex nature and the S2 section is oriented N55 E. of the shallow stress field, weathering and erosional features, Another display commonly used to compare S1 and S2 seis- and depositional processes are all possible contributors to ob- mic profiles is a spliced section showing the time shift, or dy- served near-surface anisotropy. Indeed, Lynn (1991) suggested namic mis-tie, between the two shear polarizations. Figure 9 a combination of circumstances, including unequal horizon- is a spliced section for this experiment. The section displays a tal stresses, sediment grains aligned by method of deposition, mis-tie of 8 ms at a two-way time of 375 ms. Using the time- and oriented cracks/pore spaces induced by stress along the shift data, an estimate of the average azimuthal anisotropy was San Andreas Fault system, to explain large near-surface az- made (i.e., Martin and Davis, 1987), that is, approximately 2%. imuthal anisotropy measurements in the San Francisco Bay Values of measured shear-wave anisotropy are regularly be- area. tween 0.5 and 5% (Crampin and Lovell, 1991), and this study’s value is within that range. S1 AND S2 PROFILES INTERPRETATION OF SHEAR-WAVE SPLITTING: Figure 8 displays the main diagonal components (100 and NEOTECTONIC SIGNIFICANCE 400) of the rotated data matrix shown in Figure 5. The po- larizations were renamed S1 (fast) and S2 (slow) based on the Stress-aligned inclusions, called EDA (extensive-dilatency velocities (S1 = 205 m/s; S2 = 185 m/s) used to stack the target anisotropy) cracks in Crampin (1994), are a generally accepted

FIG. 5. Two-by-two shear-wave splitting data matrix. The individual elements (100, 200, 300, and 400) represent stacked reflection sections (1.5 m trace spacing) following a 40 counterclockwise rotation. Note the increased reflection energy on the main-diagonal elements of the matrix (100 and 400) and the reduction in reflection strength on the cross-diagonal elements of the matrix (200 and 300) compared to Figure 4, an indication that the data are aligned with the natural coordinate system.

1878 Harris

geologic cause for widely observed shear-wave splitting in the Earth’s crust. Among the arguments presented for azimuthal anisotropy caused by EDA cracks are shear-wave splitting in diverse earth materials ranging from poorly consolidated sediments to igneous rocks, the consistent alignment of frac- tures/cracks parallel or subparallel to the maximum horizontal compressive stress in an area, and the success of shear-wave splitting in determining the orientation and extent of frac- tures in hydrocarbon reservoirs (Crampin and Lovell, 1991; Crampin, 1993). Based on the orientations determined for the S1 and S2 seismic sections, two possible explanations for the observed azimuthal anisotropy in Kentucky Bend are: (1) fractures and cracks/microcracks aligned in response to the near-surface stress field, and (2) faults and fractures associated with Kentucky Bend scarp, a recently recognized surficial defor- mation feature (Van Arsdale el al., 1995).

Near-surface stress field

Zoback and Zoback (1980) indicated that the direction of FIG. 6. Plot of amplitude ratio (rotated/initial) difference maximum stress in the central United States is approximately (main-diagonal elements minus cross-diagonal elements) east-west compressive, but acknowledged that in the central versus rotation angle for seismic sections processed with and part of the NMSZ (the area surrounding Kentucky Bend), the without AGC. When the data are aligned with the natural coor- stress-direction data indicate a rotation to northeast-southwest dinate system the difference in amplitudes between the main- compression. However, Zoback and Zoback (1980) ques- diagonal and cross-diagonal should be at a maximum. Both curves suggest that a 40 rotation maximizes the amplitude tioned the accuracy of this orientation and suggested that it may difference and is therefore the optimum rotation angle for the be caused by uncertainties associated with the calculation of data set. stress orientations from earthquake focal mechanism solutions.

FIG. 7. Example field record showing distorted reflection arrivals recorded outside the shear-wave window (see Figure 3 for additional examples).

Shallow Shear-wave Splitting: NMSZ 1879

After reexamination of the existing stress data and incorpora- tion of new results, Ellis (1994) suggested that the northward rotation of the stress orientation in the central NMSZ indi- cated by focal mechanism data likely does reflect an actual stress direction change. Because the stress direction change coincides with a change in orientation of the seismicity pattern (from northeast-trending to northwest-trending), a complex local stress distribution is likely. A study of shear-wave splitting from microearthquakes in the NMSZ in Rowlands et al. (1993) showed a preferred polar- ization direction aligned with the northeast-southwest maxi- mum compressive stress direction. The average S1 polarization azimuth was calculated to be N62E which correlates with the S2 polarization direction (N55 E) determined from the shallow shear-wave splitting experiment described in this paper. Upon first examination, these observations appear to be contradic- tory. However, if the seismicity underlying Kentucky Bend represents motion on a southwest-dipping thrust fault (Chiu et al., 1992), then although the stress field would be compres- sive at seismogenic depths (5 to 15 km), it would likely be reversed near the surface because of extension, produced by uplift, across the crest of the LCU (Figure 10a). In response to the shallow tensional stress field, near-surface stress-aligned in- FIG. 9. Spliced section (1.5 m trace spacing) showing dynamic mis-tie between S1 and S2 data sets. The gray arrows represent clusions (i.e., fractures and cracks/microcracks) could cause a shift of 8 ms at a time of 375 ms, indicating an average az- the azimuthal anisotropy. In support, Woolery et al. (1993) sug- imuthal anisotropy of 2% between the reflector (35 m deep) gested that small-offset normal faults interpreted on a shallow and the surface.

IG F . 8. S1 and S2 sections (1.5 m trace spacing) extracted from the rotated data matrix. Following rotation, the S1 (fast) section is oriented N35 W and the S2 (slow) section is oriented N55 E.

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SH-wave seismic reflection profile, located south of Kentucky Kentucky Bend area are an indicator of a near-surface differ- Bend also along the crest of the LCU, probably developed in ential horizontal stress field. response to tensional stress associated with the uplift of the LCU. Additionally, Crampin et al. (1986) found S1 polariza- Kentucky Bend scarp tions perpendicular to the maximum horizontal compressive stress direction from observations of small earthquakes in a re- Another explanation for the cause and orientation of the gion of compressional tectonics in Tadzhikistan. The reversed anisotropy in Kentucky Bend is northwest-oriented faults/ anisotropy was attributed to vertical cracks, oriented perpen- fractures/cracks related to the formation and periodic reactiva- dicular to the compressive stress direction, that formed in a tion of Kentucky Bend scarp, the probable northwest extension tensional stress field above the level of the earthquakes. of the Reelfoot scarp (Van Arsdale et al., 1995). The Kentucky Evaluation of shallow anisotropy is often difficult because Bend scarp (Figure 10b) is oriented approximately N40W and of the presence of complex near-surface stress fields (Crampin, is nearly coincident with the S1 direction determined from the 1990). Crack alignments change from vertical (at depth) to shear-wave splitting experiment. horizontal (near the free surface) as the magnitude of verti- The Reelfoot scarp (Russ, 1979) is an east-facing monocline cal stress decreases to approximately zero. However, in ar- located at the eastern margin of the LCU and represents a eas where the minimum horizontal stress is tensional (i.e., the fault-propagation fold that formed above a west-dipping re- LCU), the vertical stress may always be greater than the min- verse fault (Kelson et al., 1994). The reverse fault has been im- imum horizontal stress and vertical fractures may continue to aged on shallow seismic data (Sexton and Jones, 1986; Kelson et the surface (Crampin, 1990). Therefore, it is likely that the al., 1994) and is thought to be the near-surface expression of the azimuthal orientations of the split shear waves observed in the deeper seismogenic fault-plane imaged in Chiu et al. (1992).

a)

b)

FIG. 10. Two possible explanations for the observed azimuthal anisotropy in the LCU (modified after Russ, 1982): (a) oriented inclusions aligned in response to tensional stress developed across the crest of the LCU, and (b) faults/fractures aligned parallel to the Kentucky Bend scarp, the probable northern extension of the Reelfoot scarp.

Shallow Shear-wave Splitting: NMSZ 1881

In addition to the primary reverse fault, Sexton and Jones in the NMSZ and other seismically active areas are likely to (1986) mapped several other parallel/subparallel faults coin- improve assessments of the orientation and significance of the ciding with the Reelfoot scarp. If a similar pattern is present in near-surface stress field and the geometry and extent of shallow the vicinity of the Kentucky Bend scarp, then the polarization structural deformation. orientations are likely influenced by faults/fractures paralleling the trend of the scarp. In fact, recent shallow seismic reflection ACKNOWLEDGMENTS profiling (Harris et al., 1994) has identified near-surface faults I thank Ron Street, Todd Mullins, Zhenming Wang, and within 0.5 km of the Kentucky Bend scarp, very near the site Ed Woolery (Department of Geological Sciences, University of the shear-wave splitting experiment. of Kentucky) for field assistance and other contributions to this The dissimilar reflection amplitudes between the S1 and project; Meg Smath (Kentucky Geological Survey) for edito- S2 sections can be explained by azimuthal anisotropy caused rial review; and Stuart Crampin (Department of Geology and by parallel northwest-striking faults/fractures associated with Geophysics, University of Edinburgh) and Roy Van Arsdale the Kentucky Bend scarp. A qualitative comparison of the (Department of Geological Sciences, University of Memphis) S1 and S2 sections (Figure 7) shows that the amplitude of the for technical reviews of the manuscript. I also thank Bob target reflection on the S1 section is consistent along the profile, Tatham, Daniel Ebrom, and Heloise Lynn for their helpful while the target reflection on the S2 section has zones of weak reviews. The research was supported by the U.S. Geologi- or nonexistent reflection strength. Mueller (1991) showed that cal Survey (USGS), Department of the Interior, under USGS similar amplitude “fade-outs” on an S2 line in the Austin Chalk award number 1434-93-G-2361. The views and conclusions represent sensitivity to lateral changes in fracture intensity. contained in this document are those of the author and should not be interpreted as necessarily representing the official poli- CONCLUSIONS cies, either expressed or implied, for the U.S. government.

Shallow seismic (both P- and S-wave) reflection profiling REFERENCES (e.g., Sexton and Jones, 1986; Woolery et al., 1993) and pale- Alford, R. M., 1986, Shear data in the presence of azimuthal anisotropy: oseismologic trenching (e.g., Kelson et al., 1992, 1994) have Dilley, Texas: 56th Ann. Internat. Mtg., Soc. Expl. Geophys., Ex- been the primary methods used to identify, characterize, and panded Abstracts, 476–479. evaluate the significance of surface/near-surface tectonic defor- Booth, D. C., and Crampin, S., 1985, Shear-wave polarizations on a curved wavefront at an isotropic free-surface: Geophys. J. Roy. Astr. mation in the NMSZ. As the results and data interpretations in Soc., 83, 31–45. this study show,analysis of directional shear-wave polarizations Chiu, J. M., Johnston, A. C., and Yang, Y. T., 1992, Imaging the active faults of the central New Madrid seismic zone using PANDA array caused by shear-wave splitting may provide another tool to in- data: Seis. Res. Lett., 63, 375–393. vestigate shallow subsurface conditions in seismically active Crampin, S., 1985, Evaluation of anisotropy by shear-wave splitting: areas. Geophysics, 50, 142–152. ———1990, Alignment of near-surface inclusions and appropriate Although the exact cause of the azimuthal anisotropy in crack geometries for geothermal hot-dry-rock experiments: Geo- Kentucky Bend is equivocal at present, it is almost certainly phys. Prosp., 38, 621–631. related to neotectonic deformation associated with contempo- ———1993, Arguments for EDA: Can. J. Expl. Geophys., 29, 18–30. ———1994, The fracture criticality of crustal rocks: Geophys. J. Int., rary seismicity in the central NMSZ. The actual cause of the 118, 428–438. anisotropy is probably a combination of the two explanations Crampin, S., Booth, D. C., Krasnova, M. A., Chesnokov, E. M., Maximov, A. B., and Tarasov, N. T., 1986, Shear-wave polarizations presented above; however, the questions remain: (1) is the in the Peter the First range indicating crack-induced anisotropy in a anisotropy essentially caused by shallow tensional stress pro- thrust-fault regime: Geophys. J. Roy. Astr. Soc., 84, 401–412. duced by uplift of the LCU? or (2) is the anisotropy isolated in Crampin, S., and Lovell, J. H., 1991, A decade of shear-wave splitting in the Earth’s crust: What does it mean? What use can we make of the vicinity of the Kentucky Bend scarp and primarily caused by it? and what should we do next?: Geophys. J. Int., 107, 387–407. oriented faults/fractures associated with the emplacement and Crone, A. J., 1992, Structural relations and earthquake hazards of the periodic reactivation of the scarp resulting from local compres- Crittenden County Fault zone, northeastern Arkansas: Seis. Res. Lett., 63, 249–262. sional stresses (reverse faulting)? 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