Seismic Body Wave Constraint on Mechanisms of Intermediate-Depth Earthquakes

Source parameters of six large earthquakes (7.1 <= M_w <= 7.7) in the depth range between 112 and 277 km from 5 different subduction zones are derived from teleseismic body waves recorded by global seismic networks. Except for the October 21, 1995 Mexican event which is consistent with the model of simple rupture on a planar surface, the earthquakes investigated show complex rupture processes that can be explained by irregularities in the focal region as is the case of shallow events. The inferred velocity of the moment release varies between 2.5 and 4 km/s, corrresponding to 40--90% of the shear velocity at the focus. Four of the six earthquakes show stress drops in the range observed for shallow events (1--10 MPa). Ruptures extend mainly parallel to the strike of subducting slabs, with dimensions reaching up to 50 km. The extent of rupture perpendicular to slab dip of generally less than ~15 km is comparatively small. This suggests that the rupture during large intermediate-depth earthquakes propagates predominantly parallel to the strike of the subducting plate and that the seismogenic zone perpendicular to the dip direction is limited to a narrow width. The relatively small rupture size perpendicular to slab dip suggests that the rupture process does not extend into regions of significantly different temperatures. The spatial extent of the earthquakes investigated is consistent with the orientation of trench-parallel faults that are generally observed in near-trench-outer-rise regions, suggesting that these events may result from reactivation of such pre-existing faults at intermediate-depths through dehydration of hydrous minerals concentrated in these zones of weakness. Depths of most of the earthquakes are consistent with the dehydration embrittlement of serpentine, the Bolivian earthquake of January 23, 1997 seems to be too deep (277 km) for such explanation.