9521921 Stein This research will take advantage of an extraordinary opportunity to use space-based geodesy to study a deep earth process. On 9 June 1994, the largest deep earthquake ever seismologically recorded occurred 650 km beneath Northern Bolivia. Its extraordinary size is illustrated by the fact that ground motion was felt as far away as Seattle and Toronto, 9000 km from the epicenter. This earthquake has become the focus of intensive seismological study, because the physical process permitting seismic failure at these extreme depths is a long-standing unresolved question. Although geodetic studies have never been previously used to study a deep earthquake, they are practical in this case. The earthquake is so large that centimeter-level static displacements above the hypocenter appear likely, based on both seismological observations and theoretical calculations. Through good luck, the Principal Investigators established a geodetic network in Bolivia and measured it using the Global Positioning System only a month before the earthquake. Hence a resurvey of the network would yield the first direct observation of static displacements following a deep earthquake. The comparison of geodetic and seismological results would be especially valuable because the physical process causing deep earthquakes is still unknown, but is most likely very different from that for shallow earthquakes. In particular, because the leading but controversial model for the mechanism of deep earthquakes is that they result from solid state phase changes, there may be significant differences between coseismic and long term deformation. Time is of the essence, because the geodetic network was established to measure the expected steady-state deformations associated with shortening across the Andes. As a result, the more time that passes, the poorer the chances of separating the effects of the earthquake from the tectonic shortening. Moreover, the cost to measure the earthquake effects can be reduced significantly by cooperation with a German group who plan measurements in Bolivia in May, 1995. Although the German study, which was planned before the earthquake, would by itself be inadequate to resolve the earthquake displacement, a modest investment in additional measurements would permit joint achievement of the goal. This research is funded under the Small Grants for Exploratory Research program and is a component of the National Earthquake Hazard Reduction Program. ***
