Wabash Valley Fault Research Articles

An Ongoing 13.5 Yr Hiatus in M ≥4 New Madrid/Wabash Valley Earthquakes: Could People Be the Cause of the “Pause?”

Editorial| November 10, 2021 An Ongoing 13.5 Yr Hiatus in M ≥4 New Madrid/Wabash Valley Earthquakes: Could People Be the Cause of the “Pause?” David N. Lumsden; David N. Lumsden 1Emeritus, Department of Earth Sciences, The University of Memphis, Memphis, Tennessee, U.S.A. Search for other works by this author on: GSW Google Scholar Randel T. Cox Randel T. Cox * 2Department of Earth Sciences, The University of Memphis, Memphis, Tennessee, U.S.A. *Corresponding author: randycox@memphis.edu https://orcid.org/0000-0002-1066-0024 Search for other works by this author on: GSW Google Scholar Author and Article Information David N. Lumsden 1Emeritus, Department of Earth Sciences, The University of Memphis, Memphis, Tennessee, U.S.A. Randel T. Cox https://orcid.org/0000-0002-1066-0024 * 2Department of Earth Sciences, The University of Memphis, Memphis, Tennessee, U.S.A. *Corresponding author: randycox@memphis.edu Publisher: Seismological Society of America First Online: 10 Nov 2021 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © Seismological Society of America Seismological Research Letters (2022) 93 (1): 1–4. https://doi.org/10.1785/0220210236 Article history First Online: 10 Nov 2021 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David N. Lumsden, Randel T. Cox; An Ongoing 13.5 Yr Hiatus in M ≥4 New Madrid/Wabash Valley Earthquakes: Could People Be the Cause of the “Pause?”. Seismological Research Letters 2021;; 93 (1): 1–4. doi: https://doi.org/10.1785/0220210236 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The vast majority of earthquakes in the United States occur along the west margin where North America impinges on the Pacific and Juan de Fuca plates. The major exceptions to this pattern occur in the central United States, the New Madrid and Wabash Valley (NMWV) seismic zones (Fig. 1a). These two seismic zones appear to be linked (Mueller et al., 2004; Li et al., 2005) and are herein called the NMWV seismic zone. Because the NMWV region has a large population and is identified on U.S. Geological Survey (USGS) seismic hazard maps as having... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Shear Velocity Structure Beneath the Central United States From the Inversion of Rayleigh Wave Phase Velocities

AbstractA three‐dimensional shear velocity model for the crust and upper mantle beneath the central United States is presented by inverting Rayleigh wave phase velocities from 20 s to 100 s periods. These phase velocities were determined using regional and teleseismic earthquakes recorded by the Northern Embayment Lithospheric Experiment stations, the CERI New Madrid Seismic Network, the Earthscope Transportable Array, and the Ozark Illinois INdiana Kentucky Flexible Array. A low Vs anomaly is imaged in the mantle below the Reelfoot Rift, which is the uppermost portion of connected low‐velocity zones dipping toward the southwest below the rift and to the northwest below the Illinois Basin. According to the analysis in previous tomographic studies using both Vp and Vs anomalies, the elevation of temperature and the enrichment of iron, water, and orthopyroxene contents are required factors to explain the reduced seismic velocities. These low‐velocity zones are produced by silica‐rich fluids rising from the stalled Farallon slab. Two areas with low shear strength characterized by low Vs are imaged below the Ste. Genevieve and the Wabash Valley seismic zones. The low‐velocity, weak areas may be responsible for stress concentration and thus the generation of intraplate seismicity.

Velocity and azimuthal anisotropy structure underneath the Reelfoot Rift region from Rayleigh wave phase velocity dispersion curves

SUMMARYPhase velocity and azimuthal anisotropy maps for fundamental mode Rayleigh waves are determined for a portion of the central United States including the seismically active Reelfoot Rift (RFR) and the enigmatic Illinois Basin. Dense seismic array installations of the Northern Embayment Lithosphere Experiment, the EarthScope transportable array and the Ozarks Illinois Indiana Kentucky array allow a detailed investigation of phase velocity and anisotropy in a broad period range (20–100s).We obtain more than 12 000 well-constrained, unique two-station paths from teleseismic events. The two-station method is used to determine dispersion curves and these are inverted for isotropic phase velocity maps and azimuthal anisotropy maps for each period. The presence of fast phase velocities at lower crustal and uppermost mantle depths is found below the RFR, and Ste. Genevieve and Wabash Valley fault zones. At periods of 30s and higher, the RFR is underlain by slow phase velocities and is flanked to the NW and SE by regions of fast velocity. Fast phase velocities are present below the centre of the Illinois Basin in the period range 75–100s. Anisotropy fast axis orientations display complex patterns for each period and do not trend parallel to the direction of absolute plate motion. Anisotropy fast directions are consistently parallel to the trend of the RFR from 50s to higher periods, suggesting the presence of either frozen-in anisotropic fabric or fabric related to material transport from a recently discovered, pronounced low velocity zone below the Mississippi Embayment.

Mesoscale structures in the Rough Creek Graben, U.S. Midcontinent craton: An integrated DEM analysis, structural modeling, and field-based approach

The Rough Creek Graben (RCG) within the U.S. Midcontinent craton possesses some of the world's longest cave and karst landforms such as the Mammoth Cave. Despite the graben's position at the junction of two active seismic zones, the New Madrid Seismic Zone (NMSZ) and the Wabash Valley Seismic Zone (WVSZ), the RCG is seismically inactive. Here, we map mesoscale geologic structures within the RCG for understanding structural deformation and styles. We integrated DEM (Digital Elevation Model)-based structural analysis, field-based structural documentation, 3-d models, and GIS-based spatial analysis. Results of this integrated study reveal the presence of dominantly E-W-striking mesoscale structures with secondary NW-SE- and NE-SW-strikes. Many of these structures are documented in the field as high-angle normal faults with distinct fault cores, displacements, and damage zones. Most of the mapped structures are likely originated from extensional stresses associated with the development of the RCG. The presence of less frequent contractional structures (reverse and thrust faults, and folds) suggests subsequent strain localization within this graben. Although most of these structures are seismically inactive, understanding their deformation and kinematics is crucial for synthesizing any reactivation in response to a large magnitude earthquake originating from either the NMSZ or the WVSZ. The findings of this study not only advance our knowledge of the mesoscale structures in a midcontinent cratonic rift but also help evaluate potential earthquake hazards and risks. Moreover, positive correlations between these mesoscale structures and the karst systems suggest a relationship between these structures and the ultimate morphology of conduit systems that assisted in developing these karst landforms.

Joint inversion for 1-D crustal seismicS- andP-wave velocity structures with interfaces and its application to the Wabash Valley Seismic Zone

SUMMARYInterfaces are important part of Earth’s layering structure. Here, we developed a new model parametrization and iterative linearized inversion method that determines 1-D crustal velocity structure using surface wave dispersion, teleseismic P-wave receiver functions and Ps and PmP traveltimes. Unlike previous joint inversion methods, the new model parametrization includes interface depths and layer Vp/Vs ratios so that smoothness constraint can be conveniently applied to velocities of individual layers without affecting the velocity discontinuity across the interfaces. It also allows adding interface-related observation such as traveltimes of Ps and PmP in the joint inversion to eliminate the trade-off between interface depth and Vp/Vs ratio and therefore to reduce the uncertainties of results. Numerical tests show that the method is computationally efficient and the inversion results are robust and independent of the initial model. Application of the method to a dense linear array across the Wabash Valley Seismic Zone (WVSZ) produced a high-resolution crustal image in this seismically active region. The results show a 51–55-km-thick crust with a mid-crustal interface at 14–17 km. The crustal Vp/Vs ratio varies from 1.69 to 1.90. There are three pillow-like, ∼100 km apart high-velocity bodies sitting at the base of the crust and directly above each of them are a low-velocity anomaly in the middle crust and a high-velocity anomaly in the upper crust. They are interpreted to be produced by mantle magmatic intrusions and remelting during rifting events in the end of the Precambrian. The current diffuse seismicity in the WVSZ might be rooted in this ancient distributed rifting structure.

Late Holocene Deformation near the Southern Limits of the Wabash Valley Seismic Zone of Kentucky and Indiana, Central United States, with Seismic Implications

ABSTRACTThe Wabash Valley seismic zone (WVSZ) is a region of diffuse, modern intraplate seismicity in the central United States with a history of strong, late Quaternary and Holocene seismicity as determined through paleoliquefaction studies. Yet, there are no specific faults linked to these strong WVSZ paleoearthquakes, some of which were as large as Mw 7.2–7.5. A multidisciplinary investigation of a linear, 5-kilometer-long and ∼3-meter-high scarp on the Ohio River floodplain in the southernmost WVSZ in western Kentucky evaluated whether the scarp is a fluvial landform or a tectonic feature. Geomorphic mapping and optically stimulated luminescence geochronology show that the age and orientation of the scarp are inconsistent with surrounding fluvial landforms. Trenching, core drilling, seismic reflection, electrical resistivity profiling, and cross sections of petroleum well logs all indicate a blind fault directly underlies the scarp. The scarp is interpreted to be the fold axis of a down-to-the-west monocline formed in alluvium by slip on the underlying blind fault, herein named the Uniontown fault. The Uniontown fault connects the Hovey Lake fault, striking N20°E and having ∼0.5 km of documented strike-slip offset, with an unnamed fault complex to the south that strikes N40°E, suggesting the Uniontown fault is part of a larger, Paleozoic structure that has been reactivated with strike-slip deformation. Geomorphic mapping utilizing luminescence and radiocarbon geochronology indicates that folding and faulting occurred ∼3.5 ka. Paleoliquefaction was suppressed by a thick clay cap in the main Ohio Valley, but paleoliquefaction features are widespread on Ohio River tributaries. Gravel dikes at one site had a maximum age of 3.4±0.4 ka, confirming the region has experienced strong, late Holocene shaking. Estimates using vertical displacement and rupture length indicate that slip on the Uniontown scarp could produce an Mw 6.2–7.7 earthquake, which is comparable to other large paleoearthquakes in the WVSZ paleoseismic record.

Measuring PmP travel times using teleseismic S-wave waveform data*

Joint inversion for crustal velocity structure using only surface wave dispersion and receiver function data often suffers the non-unique solution problem due to lack of P wave velocity constraint in the data. Here we developed a method for measuring PmP travel time using teleseismic S wave waveform. The major improvement by this method over the previous one-layer-crust search method is its use of a more realistic multi-layer-crust (MLC) velocity model in the study region based on available information. Numerical tests show that compared with the previous search method the MLC search method is faster and more reliable and accurate. One limit of the MLC search method is the requirement of a multi-layer background model, thus this method might not be applicable in regions where the velocity structure is unknown. Nevertheless, our numerical tests show that the MLC search method is robust in the sense that small deviations of the background model from the true velocity model do not influence the results severely. We applied the MLC search method to data from a temporary linear array across the Wabash Valley Seismic Zone in the central USA. We obtained 157 PmP travel-time measurements that show lateral crustal structure variation in the region. The measurements provide additional constraints in joint inversion for crustal velocity structure in this seismically active region.

Crustal Structure Beneath the Wabash Valley Seismic Zone From the Joint Inversion of Receiver Functions and Surface‐Wave Dispersion: Implications for Continental Rifts and Intraplate Seismicity

AbstractSeismicity of several intraplate seismic zones in the North American midcontinent is believed to be related to reactivation of ancient faults in Precambrian continental rifts by the contemporary stress field. Existence of such a rift system beneath the Wabash Valley Seismic Zone (WVSZ) is not clear. Here we obtained a crustal structural image along a 300‐km‐long profile across WVSZ using a dense linear seismic array. We first calculated teleseismic receiver functions of stations and applied the Common‐Conversion‐Point stacking method to image crustal interfaces and the Moho. We then used ambient noise cross correlation to obtain phase and group velocities of Rayleigh and Love waves. Finally, we jointly inverted the receiver function and surface wave dispersion data to determine shear wave velocity structure along the profile. The results show a thick (50‐ to 60‐km) crust with a typical Proterozoic crustal layering: a 1‐ to 2‐km thick Phanerozoic sedimentary layer, an upper crust ∼15 km thick, and a 30‐ to 40‐km‐thick lower crust. The unprecedented high‐resolution image also reveals a 50‐km‐wide high‐velocity body above an uplifted Moho and several velocity anomalies in the upper and middle crust beneath the La Salle Deformation Belt. We interpreted them as features produced by magmatic intrusions in a failed, immature continental rift during the end of Precambrian. Current seismicity in WVSZ is likely due to reactivation of ancient faults of the rift system by a combination of stress fields from the far‐field plate motion and prominent crustal and upper mantle heterogeneities in the region.

Geophysical and Geological Evidence for Quaternary Displacement on the Caborn Fault, Wabash Valley Fault System, Southwestern Indiana

Geophysical and Geological Evidence for Quaternary Displacement on the Caborn Fault, Wabash Valley Fault System, Southwestern Indiana

Rupture Directivity of the 18 April 2008 Mt. Carmel, Illinois, Earthquake from Modeling of Local Seismic Waveforms

The 2008 M_w 5.2 Mt. Carmel earthquake is the largest earthquake in the last 50 yrs in southeastern Illinois, near the north termination of the north‐northeast‐trending Wabash Valley fault system (WVFS). The earthquake shows almost pure strike‐slip focal mechanism, but it is still uncertain which nodal plane (NP) is the ruptured fault plane. To resolve the fault plane, we determine rupture directivity of the earthquake via the relative centroid method. We begin with inverting the point‐source solution (strike 297°/dip 84°/rake 1° for NP1, strike 206°/dip 89°/rake 173° for NP2, and centroid depth 16 km) and then determine the relative location between the centroid and hypocenter via regional waveform fitting. Two M 4+ aftershocks are used as reference events, and the waveform time shifts of reference events with respect to the 1D velocity model are used to calibrate the path effects. The results show that the Illinois mainshock ruptured to east‐southeast along the 297° NP for about 2–3 km, consistent with relocated aftershock distribution, and we infer that the sinistral causative fault connects the north‐northwest‐trending La Salle anticlinal belt and the north‐northeast‐trending WVFS.

Spatially Varying Stress State in the Central U.S. From Bayesian Inversion of Focal Mechanism and In Situ Maximum Horizontal Stress Orientation Data

AbstractThe crustal stress state in the Midcontinent of the United States has been a focus of research for many years due to anomalously high rates of seismicity in the region. This interest is recently renewed because of ongoing wastewater injection and CO2 sequestration in the Illinois Basin and its potential to induce seismicity. When fluid is injected, pore pressure increases, decreasing effective normal stress and increasing the potential for faults optimally oriented with respect to the direction of maximum compressive stress to slip. In order to better manage this risk, we present a study on the spatial distribution of principal stress orientations in the Illinois Basin and surrounding region using focal mechanism solutions and in situ measured maximum horizontal stress orientation indicators from borehole breakout, core fracture, overcoring, hydraulic fracture, and strain gauge measurements. To invert these data for the deviatoric stress tensor, we extend the damped‐least stress inversion of Hardebeck and Michael (2006, https://doi:10.1029/2005JB004144) with the mixed linear/nonlinear Bayesian inversion scheme of Fukuda and Johnson (2010, https://doi:10.1111/j.1365‐246X.2010.04564.x). The stress inversion provides an objective means to estimate nonlinear parameters including the spatial smoothing parameter, unknown data uncertainties, and selection of focal mechanism nodal planes. Results indicate a systematic rotation of the maximum horizontal stress orientation (SHmax) across a 1,000‐km width of the Midcontinent. SHmax rotates from ~N60°E to ~E/W orientation across the southern Illinois Basin. The principal stress orientations are largely consistent with the observed pattern of strike‐slip faulting and reverse faulting near the New Madrid and Wabash Valley Seismic Zones.

Conjugate Faulting in the Wabash Valley Fault Zone Exhibited by the 20 November 2012mb 3.6 Earthquake, a Mt. Carmel Late Aftershock

ABSTRACT The 18 April 2008 Mt. Carmel earthquake ( M w 5.2) was the largest event in the central United States in 40 years. This Wabash Valley earthquake had hundreds of aftershocks delineating of a left‐lateral northwest–southeast‐trending strike‐slip fault in contrast to the north–south (N–S)‐trending faults dominating the Wabash Valley fault zone. The sequence suggests conjugate faulting as part of the associated intraplate tectonics. 20 November 2012 saw another earthquake ( m b 3.6) occurring near the same location. The EarthScope Transportable Array, active in Illinois from May 2011 to May 2014, provided an opportunity to obtain high‐quality seismic data to study this event in its regional context. Mt. Carmel and its largest aftershock were largely strike slip in nature; they both have N–S extension as secondary components of their moment tensors. Likewise, body‐wave polarities show that the focal mechanisms of the 2012 earthquakes are primarily strike slip with orientations similar to those of the Mt. Carmel earthquakes. The 2012 mainshock and its aftershock ( m b 2.1) do not lie on the east–west trend of the main Mt. Carmel aftershock sequence, but rather align with an N–S‐trending sequence made of the Mt. Carmel mainshock and a handful of early aftershocks. The two distinct sequences suggest continuing conjugate faulting, with the more southerly trending group denoting possible reactivation of one of the N–S‐trending faults making up the Wabash Valley fault zone.

Microseismicity of the Intraplate Northern Wabash Fault System and La Salle Anticlinorium, Illinois Basin, U.S.A.

ABSTRACT Within the Illinois basin, the northern Wabash Valley seismic zone (WVSZ) and adjoining La Salle anticlinal belt have produced three moderate‐sized earthquakes ranging from m b 4.7 to m b 5.2 in the past 50 years. However, few smaller events from this area are recorded in regional catalogs. The Wabash fault system extends from the junction of the Rough Creek and Cottage Grove faults in the south along the Wabash River into the La Salle anticlinal belt to the north. These faults extend through the Paleozoic and into the Precambrian basement. Two of these moderate‐sized earthquakes occurred north of the terminus of the Wabash Valley fault system where it meets the La Salle anticline. The La Salle anticline, a Precambrian basement feature, is oriented northwest to southeast. The La Salle anticline creates up to a 750 m uplift in the above Paleozoic strata. This uplift creates faults within the Paleozoic strata and within the La Salle anticline. This study uses a nine‐seismograph phased array near the southern terminus of the La Salle anticline to monitor the microseismicity of the region. Analyzing the seismicity of the region over a six‐month period found 834 events. The vast majority were determined to be mine blasts or otherwise human induced. Two small earthquakes ( M L ∼1.0) located near the La Salle anticline. These earthquakes likely occur on faults associated with the anticline. The lack of microearthquakes suggests the b ‐value of the La Salle region is lower than the typical value of 1. The b ‐value is consistent with other intraplate regions and previous studies of the WVSZ finding values nearer 0.7 or even potentially as low as 0.6.

Possible Earthquakes Recorded in Stalagmites from a Cave in South‐Central Indiana

Abstract The stalagmites collected from Donnehue’s Cave, south‐central Indiana, may record seismic events near the Wabash Valley fault system located in the midwestern United States. Results of uranium–thorium disequilibrium dating of four stalagmites showing pronounced shifts in their central growth axes, episodes of growth initiation/cessation, and results of optically stimulated luminescence (OSL) dating of flood deposits within the cave reveal both the possibilities and problems associated with using stalagmites as indicators of seismic events. The first of a set of twin stalagmites, initiated about 300,000 yr B.P., recorded a major flood event (correlated with an OSL dated flood deposit) at about 246,000 yr B.P. as a central growth‐axis shift and stopped growing at about 170,000 yr B.P. Other shifts present within this stalagmite contained embedded fine sediments associated with flooding. Its twin began growing at about 170,000 yr B.P., contained no shifts, and stopped growing at about 100,000 yr B.P. Both stalagmites resumed growing at about 6000 yr B.P. that overlaps in time, within sampling and analytical error, with an M w 7.1–7.3 event recorded by liquefaction features dated, by others, at 6100±200 yr B.P. Another stalagmite began growing at about 55,000 yr B.P. and recorded multiple central growth‐axis shifts showing dissolution and fine sediment layers associated with flood events within the cave. The third and youngest stalagmite contained no evidence of flooding and was initiated about 1800 yr B.P. (the same time, within error, as an M w 6.2 seismic event recorded in the Wabash Valley sediments). This stalagmite had shifts in its central growth axis that occurred at the same time (within statistical error) as New Madrid seismic zone events at A.D. 660 and 900, and contained a shifted regrowth layer that may be coincident with either the A.D. 1811–1812 series or the A.D. 1895 event. Online Material: Photographs of in situ stalagmites and of locations of flood deposit samples.

The Meadow Bank: A Nontectonic Linear Feature in the Wabash Valley Seismic Zone

The Meadow Bank (MB) is a 10-km-long and 5- to 7-m-high linear scarp in glacial sediments, trending north-northeast along the west edge of the Wabash River Valley in southeast Illinois. The MB is within the Wabash Valley seismic zone and closely parallels the trace of the Herald–Phillipstown fault zone, which lies 1 km west of the MB. Previous geophysical studies found no underlying faulting but did not provide a conclusive explanation for how such a long straight feature could have been formed. Recently available mine maps in the area demonstrate that there has been no post-Pennsylvanian faulting below the MB. We used closely spaced boreholes to develop evidence that the MB is the result of erosion, not tectonics. Most likely, the MB is an erosional scarp formed by a late-glacial outburst flood.

Strain accumulation in the New Madrid and Wabash Valley seismic zones from 14 years of continuous GPS observation

AbstractThe mechanical behavior—and hence earthquake potential—of faults in continental interiors is an issue of critical importance for the resultant seismic hazard, but no consensus has yet been reached on this controversial topic. The debate has focused on the central and eastern United States, in particular, the New Madrid Seismic Zone, struck by four magnitude 7 or greater earthquakes in 1811–1812, and to a lesser extent the Wabash Valley Seismic Zone just to the north. A key aspect of this issue is the rate at which strain is currently accruing on those plate interior faults, a quantity that remains debated. Here we address this issue with an analysis of up to 14.6 years of continuous GPS data from a network of 200 sites in the central United States centered on the New Madrid and Wabash Valley seismic zones. We find that the high‐quality sites in these regions show motions that are consistently within the 95% confidence limit of zero deformation. These results place an upper bound on strain accrual on faults of 0.2 mm/yr and 0.6 mm/yr in the New Madrid and Wabash Valley Seismic Zones, respectively. For the New Madrid region, where a paleoseismic record is available for the past ∼5000 years, we argue that strain accrual—if any—does not permit the 500–900 year repeat time of paleo‐earthquakes observed in the Upper Mississippi Embayment. These results, together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors, indicate that either tectonic loading rates or fault properties vary with time in the New Madrid Seismic Zone and possibly plate wide.

Paleoseismology in the New Madrid and Wabash Valley Seismic Zones, central United States

Paleoseismology in the New Madrid and Wabash Valley Seismic Zones, central United States

Aftershocks of the 2008 Mt. Carmel, Illinois, Earthquake: Evidence for Conjugate Faulting near the Termination of the Wabash Valley Fault System

On 18 April 2008, a moderate-sized earthquake ( Mw 5.2) occurred near the Indiana-Illinois state border within 3 km of the Mt. Carmel–New Harmony fault at a depth of 16 km near the northern termination of the Wabash Valley fault system (WVFS). A total of 257 aftershocks were recorded over the next month by a fourteen-station temporary network deployed by Indiana University and University of Memphis/Center for Earthquake Research and Information (CERI). The number of recorded aftershocks is greater than aftershocks recorded from previous earthquakes in the WVFS of similar magnitude within the last 50 years. The number and density of local stations allowed the generation of precise hypocentral relocations using the combination of waveform cross-correlation and joint hypocentral techniques. The relocated hypocenters indicate a well-defined near-vertical fault plane striking 295° and dipping 85° to the north. Analysis of the temporal distribution of aftershocks reveals an anomalously productive aftershock sequence relative to other earthquakes in the region, including four aftershocks with M > 4.0, the largest of which reached 0.6 units of the mainshock, an unusually low b value of 0.579, and Omori law decay parameters of a = –0.662 and p = 1.152. The inferred fault orientation is consistent with the focal mechanism of the mainshock and nearly orthogonal with respect to the trace of the neighboring Mt. Carmel–New Harmony fault. The interpreted fault orientation suggests that the aftershock sequence occurred on a transfer structure near the fault termination. The structure may be related to the change in deformation styles suggested by the transition from the northeast-trending WVFS to the northwest-trending La Salle anticlinorium.

Probabilistic Seismic-Hazard Assessment Including Site Effects for Evansville, Indiana, and the Surrounding Region

Abstract Evansville, Indiana, is one of the closest large urban areas to both the New Madrid Seismic Zone, where large earthquakes occurred in 1811–1812, and the Wabash Valley Seismic Zone, where there is evidence of several large prehistoric earthquakes in the last 14,000 yr. For this reason, Evansville has been targeted as a priority region for urban seismic-hazard assessment. The probabilistic seismic-hazard methodology used for the Evansville region incorporates new information from recent surficial geologic mapping efforts, as well as information on the depth and properties of near-surface soils and their associated uncertainties. The probabilistic seismic-hazard calculation applied here follows the method used for the 2008 United States Geological Survey (USGS) national seismic-hazard maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2% probability of exceedance in 50 yr, which are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. Spectral accelerations at 0.2-s period range from 0.6 to 1.5 g , values that are much greater than those of the USGS national seismic-hazard map, which assume B/C site conditions with an average shear-wave velocity of 760 m/s in the top 30 m. The presence of an ancient bedrock valley underlying the current Ohio River flood plain strongly affects the spatial pattern of accelerations. For 1.0-s spectral acceleration, ground motions are significantly amplified due to deeper soils within this structure, to a level comparable to that predicted by the national seismic-hazard maps with D site conditions assumed. For PGA and 0.2-s spectral acceleration, ground motions are significantly amplified outside this structure, above the levels predicted by the national seismic-hazard maps with uniform D site conditions assumed.

Earthquake Scenario Ground Motions for the Urban Area of Evansville, Indiana

The Wabash Valley seismic zone and the New Madrid seismic zone are the closest large earthquake source zones to Evansville, Indiana. The New Madrid earthquakes of 1811–1812, more than 180 km from Evansville, produced ground motions with Modified Mercalli Intensity VII near Evansville. Liquefaction evidence has been documented less than 40 km away from Evansville resulting from two large earthquakes that occurred within the past 12,000 years in the Wabash Valley. Two earthquake scenarios are described in this paper that demonstrate the expected ground motions within the 33 by 42 km2 region around Evansville based on a repeat earthquake from each of these source regions. The simulations take into account the 1-D amplification of ground motion in the unconsolidated soil layer using a new laterally varying model of seismic velocity and bedrock depth. The objective of this work is to demonstrate to what extent the local variability in ground motions due to local site effects impacts the assessment of seismic hazard. These scenario maps are a valuable complement to the probabilistic seismic hazard maps for the same area (Haase, Nowack, Choi, and Bowling 2011) because the scenario maps illustrate the direct relationship between an earthquake source and the potential hazard. Also, the scenario ground motions can be used to calculate liquefaction hazard (Haase, Nowack, and Choi 2011). These ground motion maps, along with those for the probabilistic seismic hazard and liquefaction hazard, will be useful for evaluating the potential losses from earthquakes and can be used for planning, development, hazard mitigation, and emergency response. Two earthquake source zones near Evansville, Indiana, that are considered capable of producing an M 6.5 or greater earthquake are the New Madrid seismic zone, near New Madrid, Missouri, and the Wabash Valley seismic zone along the southern border between Illinois and Indiana. The New …