DURATION OF DEEP EARTHQUAKES DETERMINED BY STACKING OF GLOBAL SEISMOGRAPH NETWORK SEISMOGRAMS

The duration of each subevent of 48 earthquakes with magnitude larger than 5.5 and depth greater than 100 km was determined from stacked tra ces of broadband records of Global Seismograph Network stations. We fi tted the source time function by one or more triangles convolved with attenuation. We found that global stacks of displacement seismograms y ield reliable estimates of the rupture duration. The durations, scaled to a moment of 10(19) N m, of both the subevents and the entire earth quake show a slight decrease with depth from 9 s for events at 100 km depth to about 7 s for events at 600 km depth. Assuming that the ruptu re velocity is a constant fraction of the shear wave speed, this decre ase can be completely explained by the increase in shear velocity of 2 0%. In this sense, deep earthquakes are comparable to intermediate one s. For some intermediate-depth events, Vidale and Houston [1993] found durations up to twice as long. We find that almost all of their slow events have been recorded at large epicentral distances. At these dist ances, we conjecture that the end of the P wave train may be extended by the arrival of reflections from the D'' layer.