Repeating Earthquakes in the Continental Crust and Subducting Slabs, and Seismological and Tectonic Implications: Proposal for an Extended Fault-valve Model

It has become clear that repeating earthquakes (repeaters) prevalently occur not only at plate boundaries but also in the continental crust and subducting slabs, coexisting with regular earthquakes (Nakajima and Hasegawa, 2023). Based on this prevalent occurrence of repeaters, previous research is reviewed and the generation mechanism of inland crustal and intraslab earthquakes and its seismological and tectonic implications are discussed. Furthermore, based on this discussion, an extended fault-valve model for shallow large earthquakes is proposed. Repeaters in the continental crust and subducting slabs are interpreted to occur as repeated slips of isolated asperity patches caused by stress loading due to aseismic slips in the surrounding stable regime, similar to those at plate boundaries. Regular earthquakes are slips of asperity patches due to surrounding aseismic slips as well, which indicates that shallow inland earthquakes and intraslab earthquakes have the same generation mechanism as interplate earthquakes. This suggests that aseismic slips occur extensively and more universally within the continental crust and subducting slabs than was previously recognized, which further suggests that aseismic slips play an important role in tectonics in subduction zones such as deformation of the island-arc crust. High pore fluid pressure is assumed to be the main cause of aseismic slips. The conventional fault-valve model (Sibson, 1986, 1990) is modified for shallow large earthquakes as intermittent aseismic slips (slow slip events) are thought to occur frequently around asperity patches that cause large earthquakes. Thus, aseismic slips also play a fundamental and important role in the earthquake cycle, and are estimated to increase significantly in frequency immediately before large earthquakes. This process helps to enhance the stress applied to asperity patches that cause large earthquakes. The signals of the aseismic slips are thought to be weak, so they have rarely been detected to date. It is important to develop new methods that can detect such weak signals of aseismic slips.