Three‐dimensional seismic velocity structure in and around the Philippine Sea plate subducting beneath southwestern (SW) Japan is determined by applying double‐difference tomography method to arrival time data for earthquakes obtained by a dense nationwide seismic network in Japan. A region of low S wave velocity and high Vp/Vs of several kilometers in thickness is recognized immediately above the region of intraslab seismicity in a wide area from Tokai to Kyushu. This characteristic layer dips shallowly in the direction of slab subduction. Compared with the upper surface of the Philippine Sea slab based on seismic reflection and refraction surveys on seven survey lines, we interpret that the low‐Vs and high‐Vp/Vs layer corresponds to the oceanic crust of the Philippine Sea slab. On the basis of the position of the low‐Vs and high‐Vp/Vs layer and the precisely relocated hypocenter distribution of intraslab earthquakes, the upper surface of the Philippine Sea slab is reliably determined for the entire area of SW Japan. Nonvolcanic deep low‐frequency earthquakes that occurred associated with the subduction of the Philippine Sea slab are distributed along the isodepth contour of 30 km in SW Japan, except for the Tokai district where the depth of deep low‐frequency earthquakes becomes gradually deeper toward northeast.

[1] We first determine the configuration of the upper surface of the Pacific (PAC) slab beneath Kanto, Japan, from the distribution of interplate earthquakes relocated by an appropriate 1-D velocity model. Then, traveltime tomography is carried out to estimate three-dimensional seismic velocity structures around Kanto using 735,520 P wave and 444,049 S wave arrival times from 6508 local earthquakes. The obtained results suggest that the Philippine Sea (PHS) slab is subducting to depths of 130-140 km without a gap, even to the northwest of the Izu collision zone. We subsequently define the lateral extent of the contact zone between the bottom of the PHS slab and the upper surface of the PAC slab (PHS-PAC interface) and reveal that the slab contact zone underlies a wider area beneath Kanto in harmony with the Kanto plain. The downdip limit of interplate (thrust-type) earthquakes on the PAC slab is deepened by $30 km locally under the slab contact zone. This deepening is probably caused by a lower-temperature environment in the PAC slab, resulting from the overlap with the PHS slab subducting above and consequent thermal shielding by the PHS slab from the hot mantle wedge. We detect an extremely low-velocity anomaly in the easternmost portion of the PHS slab, which is probably attributable to serpentinization of mantle peridotite. Interplate earthquakes are almost absent along the PHS-PAC interface overlain by the serpentinized mantle in the PHS slab, suggesting that ductile deformation takes place along the interface because of low viscosity of the serpentine.Citation: Nakajima, J., F. Hirose, and A. Hasegawa (2009), Seismotectonics beneath the Tokyo metropolitan area, Japan: Effect of slab-slab contact and overlap on seismicity,

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A massive earthquake of a magnitude (M) of 9.0 occurred on March 11, 2011, off the Pacific coast of the northeastern part of Honshu, Japan. Centroid Moment Tensor analysis of the mainshock indicates that it was the reverse fault type, with a WNW-ESE compressional axis. The earthquake occurred on the plate boundary between the island arc and the Pacific plates. Three aftershocks exceeding M 7 occurred within 40-min after the mainshock, and the aftershock area covered a wide range of 500-km × 200-km. Seismicity became active one month before the mainshock, and it continued for two weeks in an adjacent area northeast of the mainshock. Furthermore, foreshock activity with maximum M 7.3 started in the same area two days before the mainshock. Seismic activities increased in almost the entire area of the Japanese Islands after the mainshock. We infer that these earthquakes were induced by the mainshock. JMA displacement-amplitude magnitude of the mainshock was determined to be 8.4, which was smaller than the moment magnitude of 9.0. Key words: Foreshock and aftershock, massive earthquake on the plate boundary, induced seismicity, northeastern Japan.

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