Are intermediate depth earthquakes caused by plastic faulting?

- Semi-brittle failure data are consistent with failure via plastic faulting.
- Laboratory semi-brittle faulting data extrapolated to subducting oceanic crust.
- Semi-brittle failure via plastic faulting is possible to depths ∼300 km.
- Depths for plastic faulting correlate with breaks in depth-frequency of earthquakes.

The increase in confining pressure and temperature with depth precludes frictional sliding beyond a few tens of kilometers, yet earthquakes occur to depths greater than 600 km. Although rock failure is often conceptualized as either brittle under low pressure and temperature or ductile at higher pressure and temperature, an intermediate brittle-like failure mode, sometimes called “transitional” or “semi-brittle faulting”, is well known experimentally. In contrast to brittle and ductile failure, models for semi-brittle terminal failure strength are lacking. We show that semi-brittle failure is consistent with the dictates of adiabatic instabilities that result in failure via plastic faulting. Although adiabatic instabilities have been suggested as a possible cause of deep (>500 km) earthquakes, extrapolation of laboratory semi-brittle faulting data using the theory of plastic faulting with a rheology and geotherm typical of subducting oceanic crust indicates that adiabatic instabilities are more likely at intermediate depths to about 300 km.