R. Tibi, G. Bock and C. H. Estabrook (Potsdam)
Seismic Body Wave Constraint on Mechanisms of Intermediate-depth Earthquakes

Source parameters of six large earthquakes ($7.1 \le M_{w} \le 7.7$) 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. The inferred
velocity of the moment release varies between 2.5 and 4 km/s (40--90\% of the 
$S$-wave velocity at the focus). Static stress drops are mostly 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 60 km. The extent of rupture perpendicular to slab dip
is comparatively small and does not exceed about 13 km. 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. Although
depths of the 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. We suggest, however, that this
earthquake is also related to the serpentine dehydration reaction which in this 
case is kinetically delayed to the great depth due to the low nucleation/growth 
rates of the phase $A$ at $T=500$--$600^{\circ}$C.