Deep Earthquakes
Deep Earthquakes Edge-on view of deep seismicity in the Tonga subduction zone, with earthquake locations indicated by their 95% confidence ellipsoids; background seismicity is shown in blue. Most earthquakes occur within the seismically active cores of deep slabs, but aftershocks of a large 1994 earthquake (green ellipsoids) and the last subevent of the mainshock (red ellipsoid) are located outside the normal seismic zone, demonstrating that mantle material around the slab can shear during a large earthquake, likely due to transient high strain rates. (Image courtesy of D. Wiens.)
The mechanism responsible for generating earthquakes at great depth is still unknown. Sliding along a dry fault should be prohibited by the tremendous pressures at depths greater than 50 km, which would cause the frictional resistance to sliding to exceed the strength of rock. Yet, earthquakes are observed down to depths of 700 km within subducting slabs of cold lithospheric material. Proposed mechanisms include high pore pressures (and hence reduced normal stresses on faults) caused by water escaping from hydrous minerals (dehydration embrittlement), sudden loss of strength associated with metastable phase transitions along shear planes (transformational faulting), and runaway ductile shear instabilities, possibly including fault zone melting. These notions make predictions that can be seismologically tested. For example, seismic imaging should reveal the existence of a seismically low-velocity metastable wedge of olivine if deep earthquakes are caused by this phase transition, and there should be a lack of identically located repeating events for the mechanisms of dehydration embrittlement and metastable phase transitions. Specially designed instrument deployments are necessary for improving constraints on deep earthquake processes.