Introduction

The mechanism of deep earthquakes in subducted oceanic lithosphere in the transition zone remains enigmatic, although there have been several studies about the mechanism for these events (e.g.Seilver et al., 1995; Raleigh and Patterson, 1965; Ogawa, 1987; Hobbs and Ord, 1988). It is known that first motions from deep earthquakes show four-lobed radiation patterns typical of shallow shear dislocation(Honda, 1932). At such depth, because of tremendous pressure, frictional sliding similar to that for shallow events is not possible, yet they must have a separate mechanism that produces earthquake motions much like their shallow counterparts. Interpretation of constitutive relations for the deep-focus events is the one of the approaches to finding the mechanism for these events. For the great Bolivia earthquake, which occurred on June 9, 1994 with a moment magnitude of 8.2, Kanamori et al.(1998) argued that the source process was dissipative based on calculation of seismic efficiency from the observed low rupture velocity. In their paper they show that once rupture initiated, melting could occur, further reducing friction and promoting fault slip. In this study, using several large deep-focus earthquakes, we compute the dynamic and static stress change inferred from kinematic source models of deep-focus earthquakes to see if they have a more dissipative process on average than those of the shallow events, and to also see the range of variability of these parameters among the set of deep earthquakes that have reasonable set of models.

Berkeley Seismological Laboratory
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