When a deep earthquake happens, what happens mechanically? Is the rock failure process similar for deep and shallow earthquakes? Or are deep earthquakes an entirely different phenomenon? Since seismic rupture involves the rupture of a crystalline material, what happens at the molecular level?

For the most part scientists who tackle these questions work in rock mechanics laboratories, although seismological observations do place some constraints on the answers. Moreover, at present scientists don't all agree about the answers. This chapter will approach these questions from a historical perspective, focusing on the early proponents for various important ideas, and then summarizing our current state of knowledge.

In general, seismological evidence indicates that temperature and pressure are critical factors affecting where deep earthquakes do and don't occur. Deep earthquakes are almost nonexistent except beneath subduction zones, where cold lithosphere convects downward into the mantle. Moreover, deep-focus earthquakes occur in zones where the subducted lithosphere is coldest, i.e., where the thermal parameter Φ, the product of lithospheric age and vertical descent rate, is highest (see Section 2.4 and Figs. 2.8 and 4.7). Thus, another important question we should ask is: how do temperature and pressure affect the mechanical processes that allow rock to fracture or deform?