FLUVIAL ANOMALIES IN THE WABASH VALLEY SEISMIC ZONE AND THEIR IMPLICATIONS FOR NEOTECTONIC ACTIVITY

The Wabash Valley Seismic Zone (WVSZ) in the central United States experiences infrequent, moderate earthquakes. Evidence from paleoliquefaction studies, however, indicates the region has experienced numerous strong Quaternary and Holocene earthquakes ranging from M_w 6.0 to M_w 7.5. While numerous shallow faults have been identified in the WVSZ, primarily through petroleum exploration in the Illinois Basin, nearly all WVSZ faults are blind faults buried under thick glacial outwash and loess deposits, making paleoseismic fault studies challenging. Only one instrumentally recorded WVSZ earthquake occurred on a known fault, and none of the known paleoseismic events have been linked to a specific fault or fault zone. Lacking any detailed studies on seismogenic faults, the true seismic hazard posed by the WVSZ remains uncertain. Low gradient alluvial rivers like the Wabash River can be highly sensitive to small slope changes associated with subtle crustal deformation. Consequently, analyses using high-resolution LiDAR elevation data make the identification of neotectonically generated fluvial anomalies, and the locations of blind faults, relatively easy to detect once other potential causes are eliminated. This research analyzed LiDAR data from the lower WVSZ and identified several zones of fluvial anomalies that would be consistent with active deformation. These include:1) a change in a reach of the lower Wabash River from highly meandering to a straight channel impinged on the eastern valley wall; 2) two adjacent, 180 degree channel direction changes of the Wabash River that flow directly up valley for more than 3.5 km before shifting 180 degrees to flow back down-valley; 3) a 180 degree flow direction change of the Little Wabash River, around a suspected fault, that would be a 50% decrease in channel slope lacking surface deformation; and 4) compressed meanders on the Little Wabash River where a suspected fault crosses the channel. All these anomalies occur within the meisoseismal zone of a M_w 7.1 earthquake that occurred ~15.0 ka suggesting the seismogenic fault responsible for one of the largest earthquakes in the region could still be active. The next phase of this research is to complete a series of shallow geophysical measurements at the fluvial anomalies, along with core drilling, to see if vertically displaced or deformed sediments can be identified and age dated.