RUPTURE CHARACTERISTICS OF THE DEEP BOLIVIAN EARTHQUAKE OF 9 JUNE 1994 AND THE MECHANISM OF DEEP-FOCUS EARTHQUAKES

The M(w) = 8.3 deep (636 kilometers) Bolivian earthquake of 9 June 199 4 was the largest deep-focus earthquake ever recorded. Seismic data fr om permanent stations plus portable instruments in South America show that rupture occurred on a horizontal plane and extended at least 30 b y 50 kilometers. Rupture proceeded at 1 to 3 kilometers per second alo ng the down-dip azimuth of the slab and penetrated through more than a third of the slab thickness. This extent is more than three times tha t expected for a metastable wedge of olivine at the core of the slab, and thus appears to be incompatible with an origin by transformational faulting. These large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the Earth's su rface.