Interpreting the Earthquake Source of the Wabash Valley Seismic Zone (Illinois, Indiana, and Kentucky) from Seismic-reflection, Gravity, and Magnetic-intensity Data

Abstract

Reprocessing of seismic-reflection data reveals new images of upper-to middle-crustal structures beneath the Wabash Valley seismic zone, located north of the New Madrid seismic zone within the seismically active southern Illinois basin. Four intersecting deep seismic profiles (243 km total) indicate an anomalous, 5-10-km-wide zone of dipping reflections and diffractions below the western flank of the Wabash Valley fault system (WVFS). The zone corresponds in places to gently arched regions of Paleozoic strata. The reflector zone can be interpreted as a result of either (or a combination of) magmatic intrusion or structural deformation. The area encompassing the reflection profiles has experienced several moderate magnitude (3.0 _< mbLg_~< 5.5) earthquakes during the past 50 years, defining the central part of the Wabash Valley seismic zone. The hypocenter of the largest 20th-century earthquake in the central USA midcontinent (9 November 1968, mbLg 5.5) corresponds to the most prominent zone of dipping middle-crustal reflections, just west of the WVFS. Both the focal mechanism (moderately dipping reverse fault) and the expected rupture zone size (-2.9 km fault length) of this earthquake are consistent with the orientation and size of observed reflectors. Dipping reflector patterns in the Precambrian crust are not collinear with fault surfaces updip in the Paleozoic sedimentary section. This indicates that shallow Paleozoic structures are effectively "decoupled" from deeper, possibly seismogenic structure, which suggests that understanding Paleozoic structure is not the key to understanding the earthquake source. The complex dipping crustal reflectivity beneath the WVFS is typical of Paleozoic continental convergent zones observed elsewhere (e.g., Appalachian orogen) and thus may suggest a preserved Proterozoic suture, possibly associated with the distal Grenville orogeny or an older event.Although magnetic intensity, Bouguer gravity, and seismic-reflection data present different means of understanding the deep geology of the area, their integration aids in limiting the number of admissible interpretations. The reflection profiles indicate a variable zone of anomalous crustal structure, including the dipping reflector zone, along a trend of northeast-trending gravity and magnetic highs locally defining the Commerce geophysical lineament (CGL), which is a suspected source of seismic hazard in the central USA midcontinent. Three-dimensional inverse modeling of the residual isostatic gravity anomaly values indicates that the upper part of the dipping reflector zone beneath the CGL lies near an important density boundary in the upper Precambrian crust. The results of our study suggest that the seismogenic source just north of the New Madrid seismic zone consists, in part, of a pre-existing fabric of blind thrusts localized along preexisting igneous intrusions, locally coincident with the CGL. This suggests that the CGL may be seismogenic in places and thus a potential seismic hazard. The variation in the expression of ...

Updated Seismic Hazard in the Southern Illinois Basin: Geological and Geophysical Foundations for Use in the 2002 USGS National Seismic-hazard Maps

“…Langenheim and Hildenbrand (1997) estimated that the CGL is 20-40 km wide; we use 30 km. McBride et al (2002) used deep seismicreflection profiles to estimate a width of 5-10 km for the inferred fault system that underlies the CGL southwest of the Vincennes bend. However, if the bend has been acting as a restraining bend for many earthquake cycles, the resulting network of reverse, right-lateral, and oblique-slip faults might have broadened as deformation proceeded.…”

Tectonic Framework of the Southwestern Margin of the Illinois Basin and Its Influence on Neotectonism and Seismicity

“…This parallelism has been suggested to be evidence for a structural relation between the two anomalies (Langenheim and Hildenbrand, 1997;Stephenson et al, 1999). Based on potential-field models and seismic-reflection data, McBride et al (2002) describe the source of the CGL in the southern Illinois basin as a strongly deformed Precambrian feature, approximately 5-10 km wide. modeled magnetic and gravity data for the Illinois basin and concluded that the source of the CGL follows the southeastern boundary of a dense and magnetic, northeasttrending igneous complex.…”

Fault-Plane Determination of the 18 April 2008 Mount Carmel, Illinois, Earthquake by Detecting and Relocating Aftershocks

“…1), which consists of a network of northeast-southwesttrending faults (Ault and Sullivan, 1982;Ault et al, 1985;Rene and Stanonis, 1995;Woolery, 2005). The faults were first discovered by petroleum exploration drilling in the early 1900s and were then widely studied by geological and geophysical investigations (Pratt et al, 1992;McBride et al, 2002;Duchek et al, 2004;McBride et al, 2007). Previous studies suggested that those faults were high-angle normal faults that formed in late Pennsylvanian or younger and pre-Pleistocene time (Rene and Stanonis, 1995;Woolery, 2005).…”