As an important part of the Earth's circulation system, subduction zone is characterized by active mountain building, high frequency earthquakes, and complex geological conditions. Earthquakes in subduction zones can be divided into inter and intra plate categories in terms of the hypocenter relative to the subducting slab, or shallow (0~60 km), intermediate (60~300 km), and deep (≥300 km) depth earthquakes according to focal depth. Shallow earthquakes include interplate earthquakes and shallow inland intraplate earthquakes while both intermediate and deep depth earthquakes are intraplate earthquakes. Shallow earthquakes are caused by cracking of intact rocks and frictional sliding along the preexisting faults because the shallow part of the Earth stays at low temperatures and low pressures where rocks are dominated by brittle behavior. At greater depths, however, both temperature and pressure are high, so rocks deform by ductile flow because brittle and frictional behavior is suppressed unless fluids are present to help push open the cracks. Thus the origin of intermediate and deep depth earthquakes should be different from that of shallow events. Two important mechanisms of intermediate depth earthquakes have been recognized: dehydration embrittlement and plastic shear instability. The origin of deep depth earthquakes is still enigmatic but most likely caused by phase transition induced faulting. However, not all of intermediate and deep depth earthquakes can be simply interpreted by a single mechanism as a combination of multiple mechanisms may act together in the initiation and propagation of the earthquakes. Intermediate depth earthquakes, for example, may be produced by fluid related embrittlement in brittle wall rocks and/or by stick—slip along dehydrating antigorite rich fault zones, depending on the magnitude of effective confining pressure. The deep depth earthquakes such as the 1994 Bolivia earthquake, whose seismogenic zones are much wider than the cold core of metastable olivine, as predicted by anticrack faulting model, could be initiated by phase transition induced faulting and then propagated by plastic shear instability. The deep depth earthquakes such as the 2013 Okhotsk earthquake, whose seismogenic zones are as wide as the cold core, may result totally from phase transition induced faulting alone. If water in nominally anhydrous minerals (e.g., olivine, wadesleyite, and ringwoodite) has released, dehydration embrittlement may also trigger deep depth earthquakes in the transition zone. In contrast, plastic shear instability, which can make its own contribution to the propagation of deep earthquakes, is often responsible for most of repeating deep earthquakes.