Deep Earthquakes: Old Faults or New?

Although deep‐focus earthquakes occur deep in the mantle, where high temperatures and pressures should make impossible brittle failure (the machanism of shallow quakes), seismically, deep quakes look very much like their shallow counterparts. For this reason, researchers have sought mechanisms that resemble shallow faulting, but that involve processes occurring in regions prone to deep earthquakes—that is, in slabs of oceanic plates descending into the mantle. The figure below depicts a cross section of such a slab (dark green and dark blue) as it descends into the mantle (light green and light blue) as it descends into the mantle (light green and ligh blue). According to one theory, the dehydration of magnisium/iron silicated, occurs from 100 to 125 km beneath the surface and liberates water the could weaken and reactivate a preexisting fault plane. This dehydration mechanism probably accounts for earthquaks occuring at depths below 75 km down to 300 km, and possibly down to 670 km.