Date of Award
Master of Science
The Cottage Grove Fault System is an East-West trending system of strike slip faults within Southern Illinois that has been explored for mineral resources but never systematically examined for seismicity or seismic hazard. Due to its location between the seismically active Wabash Valley, Saint Genevieve, and the New Madrid Seismic Zones, and the prevalence of nearby structural features, this fault system merits its own systematic study. Using existing data from the EarthScope Transportable Array, seismic activity and implications for hazard are explored through microseismicity. Over a two-year period, the closest two seismometers to the CGFS were utilized to search for microseismicity along the fault. Analysis was done through visually assessing waveforms and frequency-amplitude plots, which can help differentiate mine blasts and earthquakes based on the frequency content of the waveform. During the 2-year deployment, a total of 94 seismic events were detected, with 5 previously unrecorded earthquakes located within the Cottage Grove Seismic Zone, although none were located on the main fault. The greatest magnitude of the Cottage Grove Fault System events found was an M_L 1.5 and the smallest an M_L 0.8. The methodology found all seismic events mb 2.3 or greater listed by the Center for Earthquake Research and Information (CERI) catalog, within a 150 km radius. Missed events from the CERI catalog were small and distant. Finding earthquakes near the Cottage Grove Fault System undetected by the CERI network demonstrates that the region has a degree of previously undetected seismic activity. Preliminary event detection is better explained by a b-value of 0.7 than a b-value of 1.0, suggesting current estimates of the hazard of the CGFS is underestimated.
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