AFTERSHOCK SOURCE MECHANISMS FROM THE JUNE 9, 1994, DEEP BOLIVIAN EARTHQUAKE

The M(w) 8.3 Bolivia earthquake occurred on June 9, 1994, at a depth o f 636 km. This is the largest deep event in recorded history and ruptu red a portion of the down-going Nazca slab unknown to have ruptured pr eviously. We recorded the main shock and aftershocks on the BANJO and SEDA portable, broadband seismic arrays deployed in Bolivia during thi s event. Myers et al. (this issue) identified and located 36 aftershoc ks (M>2) for the 10-day period following the main shock. We use a grid search technique to determine focal mechanisms for 12 of these afters hocks ranging in magnitude from 2.7 to 5.3. We compare the observed P to SV and SH ratios to a series of synthetics that represent different fault plane orientations. We find consistent focal mechanisms with th e T-axis roughly horizontal and oriented approximately east-west, and the P-axis predominantly vertical. The aftershock focal mechanisms ind icate a rotation of the P-axis within the slab from down-dip compressi on prior to the main shock to a near-vertical direction afterwards. Th is observation is consistent with the release of shear stress on the n ear-horizontal rupture plane and the subsequent rotation of the maximu m compressive stress to a fault -normal orientation.