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The geometry of the Philippine Sea slab (PHS) subducting beneath the Japanese islands has been imaged to 400 km depth beneath the Kyushu and Chugoku regions, whereas the PHS slab geometry beneath the Hokuriku region has only been determined to ~ 140 km depth, thereby indicating a large east–west asymmetry in the slab subduction. However, geologic evidence suggests that there was symmetrical east–west seafloor spreading along the axis of the Kinan seamount chain when the Shikoku basin was an active spreading center in the PHS plate. This inconsistency suggests that the PHS slab should be present beneath the Hokuriku region. Here we perform P-wave travel-time tomography across central Japan and conduct a two-dimensional plate subduction numerical simulation that reproduces the dual subduction of the PHS and Pacific (PAC) plates to elucidate the PHS slab geometry beneath central Japan. The tomography results reveal a high-velocity anomaly at ~ 150–250 km depth that extends from Wakasa bay to Noto peninsula and a slab window beneath the Hokuriku region. The numerical simulation results suggest that the PHS slab may have torn when it collided with the PAC slab, with the once leading edge of the PHS slab now present along the upper surface of the PAC slab beneath Noto peninsula. These results indicate that the PHS slab exists at ~ 250 km depth beneath the Hokuriku region, although it has been torn owing to its collision with the PAC slab, with this tear propagating westward to form a triangular slab window beneath the Hokuriku region. [ABSTRACT FROM AUTHOR]

The 1891 Nobi earthquake was the largest historic intraplate earthquake in Japan. The rupture origin corresponds to a zone of high strain rate inferred from geodetic data. To understand the geologic setting of this event, we deployed temporary seismic stations in the area. Shear-wave splitting analysis was performed using data from temporary and permanent seismic stations, revealing significant lateral variations in polarization directions. Polarization directions of NE-SW, ESE-WNW, and ENE-WSW were observed in the northeastern, central, and southwestern parts of the study area, respectively. The NE-SW- and ENE-WSW-aligned polarizations are consistent with the subduction directions of the Philippine Sea plate and Pacific plate, respectively; thus, shear-wave splitting in the northeastern and southwestern regions of the study area is likely caused by mantle wedge anisotropy, a consequence of mantle flow caused by the subducting oceanic slabs. However, the ESE-WNW orientations observed in the central Chubu Region are inconsistent with the subduction direction of either slab. Regions of low seismic velocity and low resistivity have been reported in the inferred position of the mantle wedge; these heterogeneities are thought to be caused by fluid rising from the dehydrated oceanic slabs. Thus, the ESE-WNW polarization in central Chubu could be a consequence of structural heterogeneities created by fluid to the crust from the mantle. The presence of crustal fluid is closely related to weakening, and the faults responsible for the 1891 Nobi earthquake are located just above the anisotropic region. Because fluids in the crust weaken the surrounding rock, this could explain the occurrence of the 1891 Nobi earthquake. [ABSTRACT FROM AUTHOR]