首都圏直下とその付近でのフィリピン海プレートスラブの形態

We present a new view of the morphology of slab (s) subducted just beneath the Tokyo Metropolitan Area in Japan. Previously, several different models of the surface geometry of the subducted Philippine sea plate slab (PH slab) have been published mainly using seismicity data (e.g., Nakamura and Shimazaki, 1981; Maki, 1984; Kasahara, 1985; Ishida, 1992; Noguchi, 1998; Hori, 2006).<BR>In this study, first we discriminate a previously unknown seismic slab (called slab SG, or seismic slab SG) above the downgoing Pacific plate slab (PC slab), second identify the possible internal structure of slab SG, and third demonstrate tectonic evolution models. It is clear that the currently known surface contours of PH slab indicate the shallowest part of slab SG as well.<BR>Most previous studies assumed a PH slab with a constant thickness, and paid little attention to the tectonic characteristics of the vertical extent and/or the bottom geometry of slab SG with variable thickness. The bottom extent of the seismic slab SG beneath the Metropolitan area reaches 36.5N at least. The horizontal extent of seismic slab SG covers most of the lowland Kanto Plain. The bottom depth of slab SG is approximately 120 km near 36.5N and 139.0E, being the same as the surface depth of the PC slab there. Below the Sagami Trough axis near 34.5N and 140.0E, the lowest portion of slab SG is located at a depth of 80 to 90 km. The western bottom end zone of slab SG generally strikes in the NNW-SSE direction, being approximately parallel to the volcanic front.<BR>We suggest four basic morphology models of slab SG as follows. (1) Slab SG consists of both the PH slab at a shallower depth and a deeper underlain slab (slab SL). (2) A bookshelf-like configuration of northwardly inclined multi-slabs on the PC slab due to the intermittent southward shift of accumulation sites of short slab tips with episodic subduction at just south of the previously active paleo-Sagami Trough (s). We emphasize that the above evolutional bookshelf model is, to some extent, similar to the sediment layer accretion process near the deep trench system, but the dynamic situation is not the same. (3) A structure combining models (1) and (2). (4) Slab SG is merely the eastern part, having been cooled by the downgoing PC slab, of 65-70 km thick lithosphere of the Izu Outer Block (JOB) without any other slab components.<BR>To clarify the structure of the SG slab in more detail, we should incorporate high-gain seismic data from the on-line operating dense seismic observation network deployed at both marine and land areas covering the metropolitan area. In addition, we must study the effects of 3D mantle wedge circulation with dehydration process due to the subduction of both PC and PH slabs from different directions, as well as the space-time evolution history of accretion tectonics at the northern end zone of the Philippine sea plate.