Geophysical and Geological Evidence of Neotectonic Deformation ...

Bulletin of the Seismological Society of America, Vol. 95, No. 3, pp. 1193–1201, June 2005, doi: 10.1785/0120040142

Geophysical and Geological Evidence of Neotectonic Deformation along the Hovey Lake Fault, Lower Wabash Valley Fault System, Central United States by Edward W. Woolery

Abstract High-resolution seismic (shear-wave) reflection profiles were collected over a segment of the Hovey Lake fault, a known Paleozoic fault within a system of faults in the southernmost Wabash River valley of the central United States. Although the fault zone, called the Wabash Valley fault system, lies in an area of recognized prehistoric and contemporary seismicity, the seismogenic potential of this system remains poorly defined. Consequently, the objectives of this study are to assess the Hovey Lake fault, one of the more prominent fault strands in the system, for neotectonic reactivation and if present, collect sediment samples for dateable material from the disrupted horizons to provide an age constraint for the movement. The resultant stacked profiles show high-angle deformation extending above the Paleozoic bedrock and into Upper Quaternary sediment. Time-displacement calculations from the data show approximately 10.5 m of offset on the top-of-bedrock horizon and 2 m of inverted displacement along the earliest-arriving Quaternary soil reflector. Preliminary correlative coring and carbon-14 age dating of a disrupted soil horizon, located 7.7 m below ground surface, suggest fault movement at this site as late as approximately 37,000 years before present. Introduction The Wabash Valley fault system (WVFS) is a linear, northeast–southwest-trending band of narrow graben structures that lie in the Wabash River valley of southern Indiana and Illinois (Fig. 1) (Bristol and Treworgy, 1979; Treworgy, 1981; Ault and Sullivan, 1982; Ault et al., 1985; Rene and Stanonis, 1995). Paleoseismological evidence, historical earthquake accounts, and contemporary earthquake records also indicate that the Wabash Valley has a significant seismic hazard (Street et al., 1980; Obermeier et al., 1991; Pond and Martin, 1997; Munson et al., 1997; Pavlis et al., 2002; Kim, 2003). The preinstrumental and instrumental evidence has shown that small to moderate earthquakes occur in an area roughly coincident with the WVFS (Fig. 2). The lower rate of seismicity, relative to the central New Madrid seismic zone, and insufficient seismic network coverage have made correlating seismicity with geological structure problematic, however. Consequently, two shallow, high-resolution, shear-wave reflection profiles were acquired across the Hovey Lake fault, a prominent well-defined Paleozoic structure in the southernmost WVFS, to consider potential neotectonism (Fig. 3). The initial wide-aperture profile (HL-A) targeted the top of bedrock, with the objective of locating the top-of-bedrock fault manifestation. The subsequent survey (HL-B) was shot coincident with HL-A, but at a much

smaller group interval (1 m) to target overlying Quaternary sediment. The objective of HL-B addressed two questions: (1) do the older geological structures exhibit neotectonism that extends into the Quaternary cover? (2) If neotectonic deformation is present, can dateable materials be recovered from the disrupted horizons to provide an age constraint for the fault activation?

Geological and Seismological Setting The faults that comprise the lower WVFS were initially recognized during petroleum exploration drilling in the early part of the twentieth century (Fig. 1). Since then, continued drilling and deep geophysical imaging have characterized the fault system as a series of high-angle, normal faults that are rooted in a series of basement-penetrating faults at seismogenic depths (Sexton et al., 1986; Rene and Stanonis, 1995; Bear et al., 1997). McBride et al. (1997) interpreted industry lines to show that the Paleozoic faults did not penetrate basement rocks west of the Inman East fault (Fig. 1). Scientific consensus is also lacking (Braile et al. 1982, 1986, 1997; Bear et al., 1997; Hildenbrand and Ravat, 1997) regarding the complex seismotectonic relationship of the WVFS to other major regional geological structures that ex-

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Figure 2. Location map that shows approximate location of historical and contemporary earthquakes in relationship to WVFS (modified from Bear et al., 1997, and Street et al., 2002). Dashed circles indicate uncertainty from instrumentally derived epicenters (filled circles). Shaded circles represent historical epicenters from individual investigators’ interpretation of intensity reports.

Figure 1. Major structural features located in the central Mississippi Valley (modified from Kolata and Nelson, 1991). Shaded circles represent the locations of trends 1 and 2 seismicity (Wheeler 1997) in relation to the New Madrid seismic zone. Inset is an expanded map of individual fault strands of the lower Wabash Valley fault system relative to the Hovey Lake fault study location (modified from Ault et al., 1982). Shaded study location box approximates the boundaries of Figure 3.

hibit both aseismic (i.e., Rough Creek Fault Zone) and seismic (i.e., Reelfoot Rift) characteristics (Fig. 1). Despite these controversies, researchers agree that the southern halves of Indiana and Illinois are exposed to a significant level of earthquake hazard (Wheeler, 1997). Widespread Holocene paleoliquefaction features (Obermeier et al., 1991; Munson et al., 1997) and numerous small to moderate historical and contemporary seismic events during the past 200 years (Street et al., 1980; Pavlis et al., 2002; Kim 2003) support this notion (Fig. 2). The largest event to have occurred since regional seismic networks were established was the 9 November 1968 magnitude 5.5 (mb,Lg) earthquake, located near the deep seismic profile of McBride et al. (1997), and approximately along the WVFS’s southwestern boundary (Fig. 2). McBride and colleagues presented con-

vincing evidence that this earthquake was associated with a blind thrust fault in crystalline basement. The most recent earthquakes of significance are the magnitude 3.9 (mb) event that occurred on 7 December 2000 near the Heusner fault and the magnitude 5.0 (mb) earthquake of 18 June 2002 that was located in proximity to the Caborn and Hovey Lake faults (Fig. 2). Kim (2003) described the source mechanism for the 18 June 2002 event as predominantly strike-slip along near-vertical nodal planes and suggested a seismogenic relationship to the Caborn fault. The orientation of the northeast-oriented nodal plane is coincident with the regional strike of the WVFS, and the Caborn and Hovey Lake faults, in particular. The bedrock in the study area consists of predominantly Pennsylvanian sandstone, shale, coal (with clays), and occasional interbedded carbonates. Johnson and Norris (1976) reported that as much as 45 m of unlithified Quaternary alluvial and lacustrine sediment overlies the bedrock. They also stated that the sediment overburden, in general, consists of Pleistocene to Holocene clays underlain by a finingupward sand sequence. Our four continuously sampled boreholes in the immediate area corroborate these findings (Fig. 3). The depth to bedrock (shale) measured in three boreholes varied between approximately 10 and 34 m. One uncased borehole, HL-1, was drilled coincident with the southeastern end of seismic profile HL-A. We found the sediment overburden immediately above bedrock to consist of approximately 5 m of well (SW) to poorly (SP) graded sands. These sands were overlain by an additional 5.5 m of low-plasticity (CL) silty clays (lacustrine?) (Fig. 3).

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1195 Figure 3. Site location map that shows the two seismic profiles (bracketed circles) and soil borings (shaded circles) in relation to the Hovey Lake fault. Bold dashed lines are deep seismic profiles performed by Rene and Stanonis (1995). Inset shows a detailed soil description (Unified Soil Classification System), depth log, and carbon-14 results for borehole HL-1. The CL-SP boundary correlates with the R1 reflector on the HL-B profile.

Shear-Wave Reflection Data Acquisition and Processing The SH wave is amenable for imaging neotectonic features in near-surface sediment (⬍100 m) because it is a framework wave that samples the geological medium more accurately than the fluid-sensitive P wave (i.e., not affected by the “masking” effect of water in very-low-velocity sediment). Our experience also shows that although S waves commonly have frequencies only one-half to one-third that of P waves, the P waves have velocities five to ten times higher than S waves (Woolery and Street 2002). Consequently, we estimate that resolution can often be improved by a factor of 2 to 3 through the use of S waves. These

geophysical characteristics are considerable when investigating relatively thin low-velocity media. For example, the major reflection horizons observed in our profiles have an average velocity range between 240 and 360 m/sec, and a dominant frequency of approximately 40 Hz. This yields a temporal resolvable limit (i.e., calculated by the one-quarter wavelength criteria of Sheriff and Geldart [1989]), ranging between 1.5 m in the very near surface and 2.2 m at the top of bedrock. The detectable limits are considerably smaller (i.e., 1/10 k to 1/20 k) (D. Steeples, personal comm., 2002). The spatial resolution of the subject-reflecting horizons is constrained between approximately two and four shotpoints, based on the radius of the first Fresnel zone. These imaging characteristics are especially useful for

1196 accurate identification and characterization of near-surface geological structures in the expansive river valleys of the seismically active central United States. The sequences of soft, unlithified, water-saturated sediment cover, in general, possess inherently weak mechanical properties that commonly fail to transform near-surface propagated faults and folds into significant or noticeable surface geomorphic features. In addition, the long recurrence intervals for large earthquakes in the Wabash Valley that are capable of creating significant geomorphic deformation features (Munson et al., 1997; Pond and Martin, 1997) can be eroded relatively quickly. Consequently, geophysical methodology, and Swave reflection imaging, in particular, is an effective initial means of near-surface geological structure location and interrogation. Two coincident surveys, HL-A and HL-B, were acquired with an active spread of 48 horizontally polarized geophones oriented perpendicular to the seismic lines. The initial profile (HL-A) was collected as a reconnaissance survey to locate the Hovey Lake fault. Representative optimalwindowed field files from the beginning and near the end of the line are shown in Figure 4. A subsequent small-aperture profile (HL-B) was acquired to image the approximately 10 to 30 m of Quaternary sediment overburden immediately above the fault’s bedrock expression. The symmetrical shots from HL-B were used to better identify very-near-surface reflectors (i.e., as early as ⬃60 msec two-way travel time). Although all SH-wave reflection data were collected with a 24-bit, 48-channel engineering seismograph, conservative 12 (6-fold) and 6 (3-fold) trace near-offset windows were used in the final reconnaissance and Quaternary survey stacks, respectively. This small optimal window was chosen to minimize any wide-angle static and normal moveout frequency-distortion effects. The narrow data window (i.e., near-offset space-time window outside the ground-roll dispersion) also minimized surface-wave interference with shallow reflections (Figs. 4, 5, and 6). The seismic energy was generated by three to five horizontal impacts of a 0.9-kg hammer on a 3-kg modified H-pile section oriented perpendicular to the spread. The pile flanges were placed in prepared slit trenches to improve the energy couple. To ensure the accurate identification of SH events, double-sided impacts and polarity reversals were recorded on initial test records to attenuate P-wave contamination; however, impacts were only recorded on one side during general production because of the good data quality. Detailed acquisition parameters are given in Table 1. All seismic data were processed on a personal computer using commercial signal-processing software. The signal processing applied to the shallow common midpoint reflection data is also shown in Table 1. Coherent noise muting, digital filtering, trace editing, appropriate trace balancing, and careful correlation statics were the primary processes in improving the prestack quality of the events seen on the raw field file. This is an acceptable, routine, processing sequence for most shallow high-resolution seismic-reflection work

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(Steeples and Miller, 1990; Baker, 1999). These standard near-surface data-processing procedures are similar to those used in the petroleum industry but have been scaled down and conservatively applied. Other processing methods (i.e., deconvolution, migration, etc.) were considered but not applied because of the lack of significant improvement in the signal quality and the small, but inherent, resolution degradation associated with any data manipulation.

Results and Discussion Line HL-A is an 864-m-long northwest–southeastoriented reconnaissance survey collected in the vicinity of the well-log–derived projection of the Hovey Lake fault. The primary objective for the HL-A survey was to locate the topof-bedrock signature of the Hovey Lake fault. The overlying Quaternary sediment was targeted in a subsequent 300-mlong profile, HL-B, acquired at a reduced group interval (i.e., 1 m). HL-B was shot coincident with the northwestern segment of HL-A, across the identified fault signature. Structural interpretations from both profiles were defined by: (1) offset reflectors, (2) abrupt termination of relatively strong reflection horizons, and (3) changes in reflector apparent dip. Noticeable sediment thickening on the downthrow of interpreted faults can also be a structural indicator. Temporal continuity of an anomaly was used to discriminate between structure and erosion or soft-sediment deformation features. Except for the last six field files, the data quality along seismic line HL-A is good (Fig. 4). A selected field file (SP6) is also shown in Figure 6a as an example of the upper range for the data quality. The most prominent reflector on the stacked section, varying between approximately 100 and 200 msec two-way travel time (TWTT), was interpreted as the top of Paleozoic bedrock, based on a calculated depth correlation to borehole HL-1 (i.e., 10 m calculated bedrock depth and 10.05 m drilled bedrock depth) located near trace 410 (Figs. 3 and 7). The horizon is relatively strong and coherent across the profile. The Hovey Lake fault is identified as an approximately 300-m-wide zone between trace numbers 1 and 150. The abrupt change in reflector orientation and wavelet character across this zone is interpreted as evidence of the structure. The continuous nature of the reflector across the fault zone (i.e., the lack of a pronounced discontinuity) is most likely the result of horizontal oversampling within the first Fresnel radius. The location of the imaged fault is approximately 0.5 km northwest of the mapprojected fault location determined from numerous gas wells in the area. This is further indication that the imaged anomaly represents the Hovey Lake fault. The fault is near vertical, with a relative down-to-the-northwest throw of approximately 10.5 m. A smaller zone of deformation is also exhibited between trace numbers 250 and 340. No reflecting horizons above the sediment–bedrock interface can be confidently interpreted in this profile.

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Figure 4.

Representative HL-A optimal-windowed field files selected from the beginning and near the end of the survey. The data are shown as raw (a), bandpass filtered (30/80 Hz) with AGC (75 msec) (b), and muted (c). A significant rise in the TWTT arrival of the bedrock reflector is exhibited over the horizontal distance of the survey. Bedrock refraction on the far traces of SP 200 and 201 is clearly visible.

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Figure 5. Representative HL-B field files selected evenly across the survey. The data are shown as raw 48-channel records (a), bandpass filtered (30/80 Hz) with automatic gain control (50 msec) (b), 12-channel optimal window (c), and muted optimal window (d). Progressive rise in the top of bedrock is clearly seen across the survey. Intra-alluvial reflections are discernable as high as 60 ms TWTT. Note a double-hammer impact in SP-48 (c) and at ⬃80 msec TWTT (d); recognition and careful surgical muting of these types of coherent noise is essential for high-quality stacks.

The relatively small-aperture HL-B survey was collected coincident with the first 300 m of the northwestern end of HL-A, across the Hovey Lake fault, to image horizon characteristics within the soil overburden. The data quality for HL-B is good; moreover, the top-of-rock (varying between 100 and 200 msec) and earliest-arriving intra-alluvial reflector (⬃60 msec) is separable from the coherent noise events (Figs. 5 and 6b). The Hovey Lake fault is seen on the bedrock reflector at approximately trace 150. The larger zone is exhibited between traces 1 and 400 as the nearly 100-msec variation (Fig. 7). Approximately 5 m of the total 10.5 m of displacement is accumulated along the bedrock horizon’s steepest gradient (i.e., between traces 150 and 230). Overlying the bedrock reflector between traces 1 and 200 is as many as four relatively low-impedance intraalluvial horizons. In this area, two relatively steep northwestdipping reflections above bedrock unconformably underlie gentle apparently southeast-dipping reflectors at approximately 120 msec TWTT. The evidenced extension of the bedrock structure across these overlying sediment horizons

is the most significant characteristic, however. The deformation affects the entire profile, including the shallowest resolvable reflector (R1) at 60 msec. R1 is calculated at approximately 5 m below ground surface and correlates to the drilled clay-sand (CL-SP) interface (i.e., 5.7 m) found in borehole HL-1 (Fig. 3). A hinge point for a monoclinal to asymmetrical antiformal fold along R1 is defined near trace 300. An apparent northwestern dip in the reflector is evident from trace 300 to 150; the relief along this part of the horizon is approximately 3 m. An abrupt loss of coherency occurs at trace 150, which is interpreted as a truncation by the primary strand of the Hovey Lake fault. The R1 incoherency extends laterally between traces 150 and 75; coherency is regained at trace 75, but with a 2-m inverted sense of throw (relative to bedrock throw). The southeastern side of the hinge point (trace 300) exhibits a very subtle apparent southeastern dip along R1 to trace 380. Approximately 2 m of structural relief is also noted between traces 380 and 440. Between traces 440 and 510 R1 generally appears coherent and undisturbed. R1 is again displaced 2 m down to the

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Table 1 Acquisition Parameters and Processing Procedures Field Parameters

Source Acquisition, low-cut Acquisition, high-cut Geophone Near offset Group/shot interval Sample interval

HL-A

HL-B

0.9-kg hammer 25 Hz Out 30 Hz 4m 4m 0.5 msec

0.9-kg hammer 25 Hz Out 30 Hz 1m 1m 0.25 msec

General Processing Procedures 1. Reformat 2. Spherical divergence gain 3. Bandpass filter 4. Automatic gain control 5. Elevation statics 6. Geometry and sort gathers 7. Velocity analysis 8. First-break and surgical mutes; trace kills 9. Normal moveout correction 10. Residual statics (10 msec maximum shift) 11. f-k filter

Figure 6. Selected high-quality field files from the northwest part of each survey serve as indicators for the upper range of data quality. Field files include a 24-trace window from HL-A SP-12 (a) and a 12-trace window from HL-B SP-6 (b). Data were viewed in various windows before the final 12- and 6-trace optimal windows for HL-A and HL-B, respectively, were selected.

southeast near trace 510. The displacement is visible across all reflecting horizons on the stacked seismogram. The remainder of the profile to the southeastern terminus appears undisturbed. The angular unconformity above the footwall, inverted offsets in the R1 deformation near trace 75, the subtle antiformal force folds in the hanging wall, and the secondary small-scale deformation zones suggest a complex structural evolution that has undergone at least one inversion from the original extensional episode. Observation of these indicators temporally through the seismic profile suggests that the features are not derived from erosion or soft-sediment deformation. In addition, refraction data along the line were analyzed; results show the uppermost “weathered” zone had a 1.6 m (Ⳳ0.8 m) average thickness with an average velocity of 160 m/sec (Ⳳ16 m/sec). The relatively uniform “weathered” zone, narrow near-offset acquisition window, and surface-consistent procedures minimized the potential for statics-induced artifacts. The borehole, HL-1, drilled and sampled at the southeastern terminus of HL-A (Fig. 3), recovered two small organic deposits from the lower sand unit at depths of approximately 7.7 and 9.0 m below the ground surface. The samples were shipped to Beta Analytic Inc. for carbon-14 age determinations by use of the accelerator mass spectrometry method. Dates of 36,980 (Ⳳ450) years before present (YBP) and 39,480 (Ⳳ620) YBP were derived for the 7.7-m and 9.0-m samples, respectively.

Conclusions Complex Paleozoic geological structures associated with the Wabash Valley fault system have been well known for several years; however, the two resultant stacked seismic profiles in this study offer the first evidence for possible primary coseismic neotectonism extending above the Paleozoic bedrock and into Upper Quaternary sediment. Timedisplacement calculations from the data show approximately 10.5 m of offset on the top-of-bedrock horizon, and 2 m of inverted high-angle displacement along the earliest-arriving Quaternary soil reflector at approximately 5 m depth. Preliminary correlative coring and carbon-14 dating of the disrupted basal soil horizon, located 7.7 m below ground surface, suggests a maximum age of fault movement at this site of approximately 37,000 YBP. The age and extent of the earliest deformation remains unknown; however, the potential for later deformation generates the speculative question: Does first-order neotectonism cross cut horizons that correlate temporally to the published (Obermeier et al., 1991; Munson et al., 1997) and unpublished (N. Hester, personal comm., 2002) paleoliquefaction features in the region? Answers will require higher-resolution analysis (i.e., paleoseismology trenching) of these structures.

Acknowledgments This research was supported by USGS-NEHRP award number 04HQGR0094. Additional support came from the Kentucky Geological Survey (KGS) and U.S. Army Corps of Engineers. The author wishes to thank Frederick A. Rutledge for his efforts in the difficult task of data acquisition. The author wishes to thank Dr. L. Wolf and an anonymous reviewer for their helpful comments. I also thank Meg Smith (KGS) and Collie Rulo (KGS) for their editorial and graphic arts assistance. The views and conclusions contained in this document are those of the author, and

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Figure 7. The stacked profiles HL-A (6-fold) and HL-B (3-fold) acquired across the Hovey Lake fault. The location of borehole HL-1 relative to the profiles is shown on profile HL-A. Note multiple later in the HL-A section.

they do not necessarily represent those of the U.S. Government or the Commonwealth of Kentucky.

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Johnson, W. D., and R. L. Norris (1976). Geologic map of parts of the Uniontown and Wabash Island quadrancles, Union and Henderson Counties, Kentucky, U.S. Geol. Surv. Geological Quadrangle Map GQ-1291. Kim, W-Y (2003). The 18 June 2002 Caborn, Inidana, Earthquake: Reactivation of Ancient Rift in the Wabash Valley Seismic Zone, Bull. Seism. Soc. Am. 93, 2201–2211. Kolata, D., and J. Nelson (1991). Tectonic history of the Illinois Basin, in American Assoc. Petroleum Geologists, Memoir 51, M. Leighton, D. Kolata, D. Oltz, and J. Eidel (Editors), 263–285. McBride, J. H., M. L. Sargent, and C. J. Potter (1997). Investigating possible earthquake-related structure beneath the southern Illinois Basin from seismic reflection, Seism. Res. Lett. 68, 641–649. Munson, P. J., S. F. Obermeier, C. A. Munson, and E. R. Hajic (1997). Liquefaction evidence for Holocene and latest Pleistocene seismicity in the southern halves of Indiana and Illinois: A preliminary overview, Seism. Res. Lett. 68, 521–536. Obermeier, S. F., N. K. Bleuer, C. A. Munson, P. J. Munson, W. S. Martin, K. M. McWilliams, D. A. Tabaczynski, J. K. Odum, M. Rubin, and D. L. Eggert (1991). Evidence of strong earthquake shaking in the lower Wabash Valley from prehistoric liquefaction features, Science 251, 1061–1063. Pavlis, G. L., A. L. Rudman, B. M. Pope, M. W. Hamburger, G. W. Bear, and A. S. Haydar (2002). Seismicity of the Wabash Valley seismic zone based on a temporary seismic array equipment, Seism. Res. Lett. 73, 751–761. Pond, E. C., and J. R. Martin (1997). Estimated magnitudes and accelerations associated with prehistoric earthquakes in the Wabash Valley region of the central United States, Seism. Res. Lett. 68, 611–623.

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Short Notes Rene, R. M., and F. L. Stanonis (1995). Reflection seismic profiling of the Wabash Valley fault system in the Illinois Basin, U.S. Geol. Surv. Profess. Pap. 1538-O. Sexton, J. L., L. Braile, W. Hinze, and M. Campbell (1986). Seismicreflection profiling studies of a buried Precambrian rift beneath the Wabash Valley fault zone, Geophysics 51, 640–660. Sheriff, R. E., and L. P. Geldart (1989). Exploration Seismology: History, Theory, and Data Acquisition, Cambridge University Press, New York, 253. Steeples, D. W., and R. D. Miller (1990). Seismic-reflection methods applied to engineering, environmental, and ground-water problems, Society of Exploration Geophysicists, Investigations in Geophysics No. 5, S. Ward (Editor), Vol. 1, 1–30. Street, R. (1980). The southern Illinois earthquake of September 27, 1891, Bull. Seism. Soc. Am. 70, 915–920. Street, R., G. Bollinger, and E. Woolery (2002). Blasting and other miningrelated activities in Kentucky—a source of earthquake misidentification, Seism. Res. Lett. 73, 739–750.

Treworgy, J. D. (1981). Structural Features in Illinois: a Compendium, Illinois St. Geol. Surv. Circular 509. Wheeler, R. L. (1997). Boundary separating the seismically active Reelfoot Rift from the sparsely seismic Rough Creek Graben, Kentucky and Illinois, Seism. Res. Lett. 68, 586–598. Woolery, E., and R. Street (2002). Quaternary fault reactivation in the Fluorspar Area Fault Complex of western Kentucky—Evidence from shallow SH-wave reflection profiles, Seism. Res. Lett. 73, 628–639.

Department of Geological Sciences University of Kentucky 101 Slone Research Building Lexington, Kentucky 40506-0053 [email protected]

Manuscript received 27 July 2004.