The 18 June 2002 Caborn, Indiana, Earthquake: Reactivation of Ancient Rift in the Wabash Valley Seismic Zone?

The 18 June 2002 Caborn, Indiana, earthquake (MW 4.6) occurred on a steeply dipping fault at a depth of about 18 km. Regional and teleseismic waveform data were used to obtain an accurate hypocenter with uncertainties better than about (plus-minus) 2 km in both vertical and horizontal directions. The source mechanism determined from regional waveform analysis is predominantly strike slip along near-vertical nodal planes (dips 82° and 84°) striking 28° and 297°. The nearly horizontal P axis trends 252° and plunges 10°. The close proximity of the epicenter to the trace of the compound Caborn fault, and good agreement between the strike and dip of that fault and the source mechanism of the 2002 event, suggests that the earthquake may have occurred on that fault. Most of the Wabash Valley faults as mapped from the shallow subsurface data extend into the Precambrian basement, to at least 7 km depth, without significantly changing their orientation. The June 2002 event at 18 km depth and the south-central Illinois earthquake on 9 November 1968 (mw 5.3), which occurred at 25 km depth, suggest that the seismogenic depth in the Wabash Valley seismic zone extends down to at least 18 km depth. This earthquake is unique because it is the largest instrumented event to date that is clearly located within the area spanned by the Wabash Valley fault system. Its focal mechanism-vertical strike-slip faulting at depth-may suggest that buried faults associated with a possible Precambrian rift system are being reactivated by the contemporary east-east-northeast-trending regional horizontal compressive stress.