IMPLICATIONS OF SPATIAL AND TEMPORAL DEVELOPMENT OF THE AFTERSHOCK SEQUENCE FOR THE M(W) 8.3 JUNE 9, 1994 DEEP BOLIVIAN EARTHQUAKE

On June 9, 1994 the M(w) 8.3 Bolivia earthquake (636 km depth) occurre d in a region which had not experienced significant, deep seismicity f or at least 30 years. The mainshock and aftershocks were recorded in B olivia on the BANJO and SEDA broadband seismic arrays and on the San C alixto Network. We used the joint hypocenter determination method to d etermine the relative location of the aftershocks. We have identified no foreshocks and 89 aftershocks (m > 2.2) for the 20-day period follo wing the mainshock. The frequency of aftershock occurrence decreased r apidly, with only one or two aftershocks per day occuring after day tw o. The temporal decay of aftershock activity is similar to shallow aft ershock sequences, but the number of aftershocks is two orders of magn itude less. Additionally, a m(b) similar to 6, apparently triggered ea rthquake occurred just 10 minutes after the mainshock about 330 km eas t-southeast of the mainshock at a depth of 671 km. The aftershock sequ ence occurred north and east of the mainshock and extends to a depth o f 665 km. The aftershocks define a slab striking N68 degrees W and dip ping 45 degrees NE. The strike, dip, and location of Be aftershock zon e are consistent with this seismicity being confined within the downwa rd extension of the subducted Nazca plate. The location and orientatio n of the aftershock sequence indicate that the subducted Nazca plate b ends between the NNW striking zone of deep seismicity in western Brazi l and the N-S striking zone of seismicity in central Bolivia. A tear i n the deep slab is not necessitated by the data. A subset of the after shock hypocenters cluster along a subhorizontal plane near the depth o f the mainshock, favoring a horizontal fault plane. The horizontal dim ensions of the mainshock [Beck et al., this issue; Silver et al., 1995 ] and slab defined by the aftershocks are approximately equal, indicat ing that the mainshock ruptured through the slab.