FAST RISE TIMES AND THE PHYSICAL-MECHANISM OF DEEP EARTHQUAKES

EARTHQUAKES at depths of > 300 km are similar to shallower events in that they are dominantly of double-couple character 1, implying that shearing motion has taken place at depth. But because increased friction at these high pressures inhibits brittle fracture 2,3, various other mechanisms, related to phase transformations, have been invoked to explain the occurrence of deep earthquakes 4-10. As yet, however, no consistent differences have been found between the source characteristics of deep (> 300 km) and intermediate-depth (< 300 km) earthquakes 2,11-13. Here we report a systematic global survey of the rise times and stress drops of deep and intermediate earthquakes. (The rise time is defined as the time from rupture initiation to peak moment release rate.) When the rise times are scaled to the seismic moment release of the events, their average is nearly twice as fast for events deeper than approximately 450 km as for shallower events. This difference may ultimately provide an experimental means of testing proposed mechanisms for the generation of deep seismicity.