Modifications of epidemic-type-aftershock-sequence models for characterizing diffusive shear slips of deep long-period earthquakes

SUMMARY Deep long-period earthquakes (DLPs) are the major seismic events in the lower crust and can provide important information on the evolution of the Earth. To capture any fluid-related diffusive phenomena, we attempted to translate the swarm-like nature of DLPs into a diffusive one by modifying the epidemic-type aftershock sequence (ETAS) model, which is a standard tool for evaluating seismicity. The modified ETAS model explained the seismicity of DLPs in eastern Shimane, Japan successfully. The diffusional migration rate may not be as high as the rupture velocity of ordinary earthquakes, but it is still sufficient to associate slow deformation with low-frequency seismic radiation. The principal mechanical process of DLPs is likely shear slip, since the observed diffusivity (103–105 m2/s) is much larger than estimates for shallow volcanic events (100–101 m2/s), but close to those of tectonic tremors (104–105 m2/s). While the areal extent of aseismic slips is greater on the plate interface than in a less-mature system at volcanic roots, tectonic low-frequency earthquakes tend to be smaller than volcanic DLPs. The concept that seismic patches cause these earthquakes via universal diffusive shear slips can independently explain this paradox.