INSIGHTS INTO THE WABASH VALLEY SEISMIC ZONE AND SURROUNDING REGIONS REVEALED BY THE OIINK SEISMIC DEPLOYMENT

The New Madrid, Saint Genevieve, and Wabash Valley Seismic Zones (WVSZ) comprise the most seismically active portion of the North American continental interior. The WVSZ has drawn increasing attention in recent years due to moderate-sized earthquakes appearing to occur along a series of northeast trending high-angle transtensional faults in southeastern Illinois and southwestern Indiana. However, few events to date have been mapped to specific fault structures. The OIINK EarthScope FlexArray, deployed from 2012 to 2015, provides insight into the location and rate of seismicity in the WVSZ and its relationship to the neighboring seismic zones. Mining around the Saint Genevieve Seismic Zone and WVSZ complicates local earthquake identification. For roughly 8 hours of each day, frequent blasts obscure small earthquakes in the seismogram records. Distinguishing earthquakes from blasts is a time intensive process involving associating picks from an automated LTA-STA detector, followed by visually reviewing events based on characteristic differences between blasts and earthquakes. Comparing the high to low frequency content of the waveforms of detected events helps identify blasts, which generally have larger signals from low-frequency surface waves. Through this approach, our analysis identified an average of slightly less than one earthquake per month in the WVSZ during 2013 and the beginning of 2014. For comparison, the New Madrid network and NEIC identified four of these events. These events are located within a region slightly north and west of the surface trace of the Wabash Valley Fault Zone faults and occur at depths between ~20 and 25 km. In sampling the Saint Genevieve Seismic Zone, along the Illinois and Missouri border to the west of the WVSZ, the OIINK Array found an average of more than one earthquake per month at depths concentrated between 10 and 20 km. Seismicity in both these areas lies deeper than the more frequent events in the New Madrid Seismic Zone, which cluster at a depth of less than 10 km. The spatial extent of events in all three of these seismic zones correlates with low upper mantle velocities at 100 km depth. Better characterizing seismicity in the WVSZ and surrounding areas will help illuminate links deeper structures may have to localizing deformation in this portion of the midcontinent.