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

Abstract

Seismological observations are of vital importance for understanding the mechanism of deep earthquakes. Most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes. Deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms. Deep earthquake aftershock sequences show temporal decay rates and magnitude–frequency relations ( b-values) that are similar to shallow earthquakes, with the aftershocks occurring preferentially along the mainshock fault planes. In addition, deep earthquake rupture velocities are similar to those of shallow earthquakes. However, there are also some observations that are clearly distinctive relative to shallow earthquakes. Stress drops of deep earthquakes show a large variation but are larger, on 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 than shallow earthquakes. A variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab. Both deep earthquake b-values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs, suggesting that these factors are temperature-controlled to an extent much greater than with shallow earthquakes. Large deep earthquakes in warm slabs show slower rupture velocities, larger stress drops, and lower seismic efficiencies than similar earthquakes in cold slabs. The width of deep seismic zones is also probably temperature controlled, but deep earthquake rupture can propagate outside the normal limits of Benioff zone seismicity. Simple thermal models for the Tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200°C warmer than the temperature that limits smaller earthquakes in the slab. These observations can be used to evaluate physical models for deep earthquakes, 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 using 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 have the expected orientations for reactivated faults. The lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model, in which events should be confined within a narrow metastable wedge. Seismological 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 shear instabilities and perhaps fault zone melting.