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Simple relationship between seismic activity along Philippine Sea slab and geometry of oceanic Moho beneath southwest Japan



Many earthquakes with various types of focal mechanisms occur along the subducting Philippine Sea slab (PHS) beneath southwest Japan, and the hypocentre distribution is quite inhomogeneous. In this region, distinct non-volcanic deep low-frequency tremors are often observed. To clarify the cause of these apparently complicated and peculiar seismic activities, we first construct a detailed configuration of the oceanic Moho (slab Moho) within the PHS based on a receiver function (RF) analysis. To convert the time-domain RFs into their depth-domain equivalents, we adopt a fine-scaled velocity model based on the traveltime tomography. The depth contours of the oceanic Moho exhibit complexities, but the active region of the intraslab earthquakes corresponds well to the oceanic Moho geometry. In the western part of Shikoku, the intraslab seismicity is very high and the focal mechanisms mainly exhibit normal faults with east–west extension axes. Since the PHS is subducting to the west and the dip angle of the slab becomes steep in the western part of this region, the stress field in this region can be attributed to the slab pull induced by the westwards slab subduction. Similar types of focal mechanisms are observed in the Tokai district with a focal depth greater than 35 km. Since the oceanic Moho exhibits a ridge shape in this region, its stress field can be attributed to the slab pull and the effect of slab bending. Shallow earthquakes in the Tokai and eastern Shikoku regions have strike-slip focal mechanisms and their extension axes are parallel to the strike of the oceanic Moho. These focal mechanisms may be influenced by both the oblique subduction of the slab and low slab dip angle. Earthquakes beneath the Kii Peninsula exhibit strike-slip or downdip compression focal mechanisms with east–west compression axes. Since the Moho beneath this peninsula shows a valley shape, compressional stress occurs around the Moho. Moreover, the slab convergence compresses this valley. This also induces the east–west compression field in this region. We have newly confirmed the existence of an aseismic slab beneath the northern part of the Kinki district, central part of Japan. In this aseismic region, the dip direction of the slab is parallel to that of the slab convergence direction. This tendency is observed in the central part of Shikoku, where the slab seismicity is relatively low. These features imply that the intraslab seismicity is constrained by the interaction between the slab geometry and slab motion. We also revealed the possibility that the oceanic Moho beneath not only Ise Bay but also the Kii Channel has a ridge shape. In both these regions, the non-volcanic low-frequency tremor activity becomes low. More than 80 per cent of the tremor epicentres are distributed within contour depths of 38 ± 3 km of the oceanic Moho. We find that the interaction between the Moho geometry and the plate motion of the subducting PHS plays an important role in determining the local stress field and the location of both seismicity and low-frequency tremors.