A TOMOGRAPHIC ANALYSIS OF THE WABASH VALLEY SEISMIC ZONE

The Wabash Valley Seismic Zone (WVSZ), or Wabash Valley Fault System (WVFS), is an intraplate region of higher-than-average seismicity located within the Illinois Basin. This region has produced three 5.0+ magnitude earthquakes within the last 50 years, and recent paleoseismological studies have also shown evidence for 7.0+ magnitude events occurring within the history of the fault system. The prominent faults within the system have very little surface expression, so available data of the orientation, trend, and extent of the fault system comes from seismic reflection data and are observed at depth within mine shafts throughout Illinois. The Wabash Valley faults are contained between the LaSalle Anticlinal Belt, a prominent Precambrian complex of en echelon folds to the north, and the junction of the east/west trending Cottage Grove and Rough Creek faults to the south. Most of the faults trend to the NE and can display vertical offsets into the hundreds of meters, penetrating the Precambrian basement rock at depth. Though there are some similarities in fault orientation and trend to the faults contained within the New Madrid Seismic Zone to the SE, the possible connection to and the tectonic setting of the Wabash Valley Seismic Zone is ambiguous. As part of the Earthscope project to map the crustal structure of the North American continent, data was passively collected along a 400-kilometer Flexible Array across Illinois and Indiana for the purpose of wide angle and deep seismic profiling in the form of tomography. Utilizing a new ray tracing methodology based on Dijkstra’s algorithm, tomograms of the study area were created by inverting for Vp data collected from this array. Our tomograms show no evidence for a rift pillow system under the Wabash Valley as predicted, but there is evidence for a dipping fast anomalous zone that has been previously hypothesized from reprocessed COCORP data. We consider this COCORP reflector as an upper layer of eclogite above old, subducted crust with a higher impedance contrast than the surrounding mantle material. Further investigation is warranted, particularly with respect to ray directionality as rays arriving from north of the array do not travel through the fault system itself, whereas southern rays do. It is our hope that further studying including only southern ray paths may illuminate a supposed rift pillow system