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 of 636 km. This is the largest deep event in recorded history and ruptured a portion of the down‐going Nazca slab unknown to have ruptured previously. We recorded the main shock and aftershocks on the BANJO and SEDA portable, broadband seismic arrays deployed in Bolivia during this event. Myers et al. (this issue) identified and located 36 aftershocks (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 aftershocks 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 the T‐axis roughly horizontal and oriented approximately east‐west, and the P‐axis predominantly vertical. The aftershock focal mechanisms indicate a rotation of the P‐axis within the slab from down‐dip compression prior to the main shock to a near‐vertical direction afterwards. This observation is consistent with the release of shear stress on the near‐horizontal rupture plane and the subsequent rotation of the maximum compressive stress to a fault ‐normal orientation.