Conjugate Faulting in the Wabash Valley Fault Zone Exhibited by the 20 November 2012mb 3.6 Earthquake, a Mt. Carmel Late Aftershock

Abstract

ABSTRACT The 18 April 2008 Mt. Carmel earthquake ( M w 5.2) was the largest event in the central United States in 40 years. This Wabash Valley earthquake had hundreds of aftershocks delineating of a left‐lateral northwest–southeast‐trending strike‐slip fault in contrast to the north–south (N–S)‐trending faults dominating the Wabash Valley fault zone. The sequence suggests conjugate faulting as part of the associated intraplate tectonics. 20 November 2012 saw another earthquake ( m b 3.6) occurring near the same location. The EarthScope Transportable Array, active in Illinois from May 2011 to May 2014, provided an opportunity to obtain high‐quality seismic data to study this event in its regional context. Mt. Carmel and its largest aftershock were largely strike slip in nature; they both have N–S extension as secondary components of their moment tensors. Likewise, body‐wave polarities show that the focal mechanisms of the 2012 earthquakes are primarily strike slip with orientations similar to those of the Mt. Carmel earthquakes. The 2012 mainshock and its aftershock ( m b 2.1) do not lie on the east–west trend of the main Mt. Carmel aftershock sequence, but rather align with an N–S‐trending sequence made of the Mt. Carmel mainshock and a handful of early aftershocks. The two distinct sequences suggest continuing conjugate faulting, with the more southerly trending group denoting possible reactivation of one of the N–S‐trending faults making up the Wabash Valley fault zone.