Seismological constraints on the mechanism of deep earthquakes: temperature dependence of deep earthquake source properties

Seismological observations are of vital importance for understanding the me chanism of deep earthquakes. Most of the seismological observables for eart hquakes deeper than 300 km are similar to shallow earthquakes. Deep earthqu akes clearly represent shear failure on a planar surface as shown by their double couple mechanisms. Deep earthquake aftershock sequences show tempora l decay rates and magnitude-frequency relations (b-values) that are similar to shallow earthquakes, with the aftershocks occurring preferentially alon g the mainshock fault planes. In addition, deep earthquake rupture velociti es are similar to those of shallow earthquakes. However, there are also som e observations that are clearly distinctive relative to shallow earthquakes . Stress drops of deep earthquakes show a large variation but are larger, o n average, than shallow earthquakes. Different deep seismic zones show very different b-values, in contrast to shallow earthquakes, which show similar b-values worldwide. In addition, deep earthquakes show fewer aftershocks t han shallow earthquakes. A variety of observations suggest that deep earthquakes are highly sensitiv e to the temperature of the slab. Both deep earthquake b-values and the rat e of deep earthquake aftershock occurrence are inversely correlated with th e temperature of the deep stabs, suggesting that these factors are temperat ure-controlled to an extent much greater than with shallow earthquakes. Lar ge deep earthquakes in warm stabs show slower rupture velocities, larger st ress drops, and lower seismic efficiencies than similar earthquakes in cold slabs. The width of deep seismic zones is also probably temperature contro lled, but deep earthquake rupture can propagate outside the normal limits o f Benioff zone seismicity. Simple thermal models for the Tonga slab near th e 1994 deep earthquake suggest that the temperature at the rupture terminat ion point was at least 200 degreesC warmer than the temperature that limits smaller earthquakes in the slab. These observations can be used to evaluate physical models for deep earthqu akes, including brittle slip along fluid-weakened faults, transformational faulting, and thermal (and perhaps melt lubricated) shear instabilities. It is difficult to explain the large fault widths of some deep earthquakes us ing a fluid weakening model, since hydrated materials are expected in only a narrow depth zone at the top of the slab, and the fault planes do not hav e the expected orientations for reactivated faults. The lateral extent of t he largest deep earthquakes cast doubt on the transformational faulting mod el, in which events should be confined within a narrow metastable wedge. Se ismological studies have also failed to find evidence for the existence of metastable olivine in slabs. The temperature dependence of deep earthquakes argues in favor of a temperature-activated phenomenon, such as thermal she ar instabilities and perhaps fault zone melting. (C) 2001 Elsevier Science B.V. All rights reserved.