Spatial Distribution of Deep Earthquakes Tied to Strain-rate Variations in Slabs

Deep earthquakes occur in subducting tectonic plates at depths of 300 to 680 km. While these earthquakes share many similarities with shallow earthquakes, they must occur by a different failure mechanism due to the high pressures at which deep earthquakes occur. Proposed mechanisms for deep earthquakes include transformational faulting due to metastable phases (olivine or pyroxene), thermal shear instability, and perhaps dehydration embrittlement. Among these, transformational faulting is the most commonly assumed mechanism, based on its ability to explain the global depth profile when the age-dependent temperature of the lithosphere is accounted for. However, such analysis fails to explain the variability in seismicity both within and among the world's subduction zones. I propose that these observed variations in deep seismicity are controlled by the variation in strain rate in the subducting slabs. Numerical simulations of subduction with non-linear rheology and compositionally-dependent phase transitions, exhibit strong variability in the strain rate magnitude both in space and time. High strain rates occur in bending regions of the slab and migrate as the slab buckles and folds at the base of the transition zone. However, in between these strongly-deforming regions the strain rate is low due to the strong temperature-dependence of viscosity and high yield strength of the slab. While not usually explicitly addressed, all deep earthquake mechanisms require high strain rate. Therefore, in addition to temperature, strain rate may limit where deep earthquakes occur, explaining why there are large gaps in seismicity (i.e., low strain-rate regions), variable maximum depth of seismicity (due to location of bending in slabs), and higher rates of seismicity where slabs exhibit a more complex shape (i.e., are undergoing more deformation). The results presented here cannot distinguish between possible failure mechanisms for deep earthquakes. However, they do suggest a new approach for testing these mechanisms that would combine the thermal and strain rate constraints with appropriate rheological models. Specifically, I will present preliminary results linking the conditions in deforming slabs (T, P, stress, strain-rate) to thermal shear instability, one of the proposed mechanisms for deep earthquakes.