THE 9 JUNE 94 BOLIVIAN DEEP EARTHQUAKE - AN EXCEPTIONAL EVENT IN AN EXTRAORDINARY SUBDUCTION ZONE

We investigate the physical setting of the Bolivian shock based on the history of the subducting Nazca plate, intraslab seismicity, deep sei smic moment release, and seismic tomography. South America has two bro ad regions of reverse are curvature. Subduction constrained to this un ique geometry produces slab kinking contortions that may cause unusual slab thickening as they sink to the bottom of the transition zone and encounter resistance to penetration into the lower mantle. Such conto rtions are observed at intermediate depths ''upstream'' from the slab source regions of both the great 1970 Colombian and 1994 Bolivian even ts. Thickening helps explain how the Nazca slab accommodates large sei smic source dimensions at depths of 625-650 km.