Patterns of deep seismicity reflect buoyancy stresses due to phase transitions

Thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies. Finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near ∼400 km depth, down-dip compression over ∼400-690 km (peaking at ∼550 km), and transition to rapidly fading tension below ∼690 km. Such features, even when complicated by olivine metastability, are consistent with observed patterns of deep seismicity. That such a simple model, neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations, successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs. It also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release.