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Nonvolcanic low-frequency earthquakes (LFEs) usually occur in young and warm subduction zones under condition of near-lithostatic pore fluid pressure. However, the relation between the LFEs and the subducting slab age has never been documented so far. Here we estimate the lithospheric age of the subducting Philippine Sea (PHS) slab beneath the Nankai arc by linking seismic tomography and a plate reconstruction model. Our results show that the LFEs in SW Japan take place in young parts (similar to 17-26 Myr) of the PHS slab. However, no LFE occurs beneath the Kii channel where the PHS slab is very young (similar to 15 Myr) and thin (similar to 29 km), forming an LFE gap there. According to the present results and previous works, we think that the LFE gap at the Kii channel is caused by joint effects of several factors, including the youngest slab age, high temperature, low fluid content, high permeability of the overlying plate, a slab tear, and hot upwelling flow below the PHS slab.

Plain Language Summary Nonvolcanic low-frequency earthquakes (LFEs) have been widely observed in recent years, which usually occur in young and warm subduction zones in the circum-Pacific regions and sometimes may become precursors of large megathrust earthquakes. However, the generating mechanism of the LFEs is still not very clear. In this study we estimate the lithosphere age of the subducting Philippine Sea slab and find a close relationship between the slab age and the LFE distribution in SW Japan. An LFE gap exists at the Kii channel. We think that the gap is caused by joint effects of several factors, including the youngest slab age, high temperature, low fluid content, high permeability of the overlying plate, a slab tear, and hot upwelling flow below the Philippine Sea slab. The present results shed new light on the generating mechanism of the LFEs and subduction dynamics.