Dynamic conditions for large shallow intraslab earthquakes: four categories of subduction zones

Abstract Large shallow intraslab earthquakes (LSIEs) of over magnitude-7 rarely occur at shallower depths (≤ 60 km) in the subducting plates of the circum-Pacific and northern margin of the Australian Plate off Indonesia. Previous studies have suggested that most LSIEs occur under surface tectonic conditions with a lateral stress gradient across the back-arc to the fore-arc on the overriding plate based on seismological and geological evidence. In this study, dynamic conditions for the occurrence of LSIEs were studied using the intraslab stress state, stress state of the overriding plate, motion speed of overriding and subducting plates, and trench migration speed determined by plate motion models. LSIEs prefer tectonic conditions under (1) an extensional-to-neutral stress state in the back-arc, (2) a compressional stress state in the fore-arc, and (3) down-dip tension in the shallower part of the subducting plate. The results suggest that the Earth’s subduction zones can be classified into four categories. This categorization is informed by the behavior of underlying mantle flow, i.e., the magnitude of slab suction flow in the mantle under the subducting plates, which is related to the degree of plate-slab coupling, and the scale of return flow in the mantle wedge under the overriding plates, which varies the combination of stress states in the fore-arc and back-arc.