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

Department of Earth and Atmospheric Sciences, Saint Louis University, St Louis, Missouri 63108

Risheng Chu

Seismological Lab, California Institute of Technology, Pasadena, California 91125

Online Material: Local magnitudes, and new and catalog locations of the 2008 Illinois earthquake sequence.

We developed a sliding-window cross-correlation (SCC) detection technique and applied the technique to continuous waveforms recorded by the Cooperative New Madrid Seismic Network stations following the 18 April 2008 Illinois earthquake. The technique detected more than 120 aftershocks down to M_L 1.0 in the 2 week time window following the mainshock, which is three times more than the number of aftershocks reported by the seismic network. Most aftershocks happened within 24 hrs of the mainshock. We then relocated all events by the double-difference relocation algorithm. Accurate P- and S-wave differential arrival times between events were obtained by waveform cross correlation. After relocation, we used the L1 norm to fit all located events by a plane to determine the mainshock fault plane. The best-fit plane has a strike of 292°±11° and dips 81°±7° to the northeast. This plane agrees well with the focal mechanism solutions of the mainshock and four largest aftershocks. By combining the aftershock locations and focal mechanism solutions, we conclude that the 18 April earthquake occurred on a nearly vertical left-lateral strike-slip fault orienting in the west-northwest–east-southeast direction. The fault coincides with the proposed left-stepping Divide accommodation zone in the La Salle deformation belt and indicates reactivation of old deformation zone by contemporary stresses in the Midcontinent.