2D stress rotation in the Tonga subduction region

Abstract

Despite their enigmatic origins, deep earthquakes provide unique information about the stress distribution in slabs. Down-dip compression is expected at depth, due to resistance from endothermic phase transition near 660 km and increasing viscosity. Focal mechanisms for ordinary deep (620-680 km) earthquakes in the Tonga subduction zone exhibit down-dip compressional stresses in agreement with this expectation, but unusually deep (≥ 680 km) earthquakes exhibit vertical tension and horizontal compression. Here we present a numerical modelling study of the Tonga slab focused on stress patterns in the transition zone. The subducting Pacific plate is one of the oldest and thus coldest plates in the world, so our parametric study tests the effects of rheology and phase transitions on stress evolution for an exceptionally cold subducting plate. Our results suggest that direct buoyancy effects from the endothermic phase transition at 660 km depth are overprinted by bending-related forces. A stress pattern best fitting seismogenic stresses is found for the coldest tested subducting plate (150 Myr old) and a viscosity interface decoupled from the 660-km phase transition and shifted to 1000-km depth. An abrupt change of stress orientations is observed when the slab, temporarily deflected by the phase transition, penetrates to the shallow lower mantle and the fold of the flat-lying part is tightening.