The 18 June 2002 Caborn, Indiana, earthquake: Reactivation of ancient rift in the Wabash Valley seismic zone?

Publication Type  Journal Article
Year of Publication  2003
Authors  Kim, W. Y.
Journal Title  Bulletin of the Seismological Society of America
Volume  93
Issue  5
Pages  2201-2211
Journal Date  Oct
ISBN Number  0037-1106
Accession Number  ISI:000186096000023
Key Words  eastern north-america; proterozoic basement features; northeastern united-states; regional seismograms; intraplate seismicity; illinois earthquake; midcontinent; tectonism; holocene; stress
Abstract  

The 18 June 2002 Caborn, Indiana, earthquake (M-w 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 2 kin in both vertical and horizontal directions. The source mechanism determined from regional waveform analysis is predominantly strike slip along near-vertical nodal planes (dips 82degrees and 84degrees) striking 28degrees and 297degrees. The nearly horizontal P axis trends 252degrees and plunges 10degrees. 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 knit depth, without significantly changing their orientation. The June 2002 event at 18 km depth and the south-central Illinois earthquake on 9 November 1968 (m(w) 5.3), which occurred at 25 kin depth, suggest that the seismogenic depth in the Wabash Valley seismic zone extends down to at least 18 kin 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.

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