HR: 0800h
AN: T51B-0556    [Abstracts]
TI: 3-D Geometry of the Philippine Sea Plate Slab just beneath the Tokyo Metropolitan Area and its Vicinity, Japan
AU: * Eguchi, T
EM: eguchi@nda.ac.jp
AF: National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa, 239-8686, Japan
AU: Hori, S
EM: sadaki@bosai.go.jp
AF: National Res. Inst. for Earth Sci. and Disaster Prevent., 3-1,Tennodai, Tsukuba, Ibaraki, 305-0006, Japan
AB: We present a new view of three-dimensional geometry of slab(s) subducted just beneath the Tokyo metropolitan area, central Japan. Previously, several different models of the surface geometry of the subducted Philippine sea plate slab (PH slab) have been published using seismological data. First, we discriminate previously unknown seismic slab (called slab SG, or seismic slab SG) above the downgoing Pacific plate slab (PC slab), second propose possible models of the slab SG internal structure, and third discuss and demonstrate the tectonic background of slab SG. It is clear that the recent surface models of PH slab corresponds to the shallowest part of slab SG. Most of previous studies 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 slab SG beneath the Metropolitan area reaches 36.5N at least. The horizontal extent of seismic slab SG covers most of the Kanto Plain Lowland. The bottom depth of slab SG is approximately 120km near 36.5N and 139.0E, being the same as of the surface depth of PC slab there. Below the Sagami trough axis near 34.5N and 140.0E, slab SG locates at a depth of 80 to 90km. We confirmed that southernmost part of slab SG extends to areas around 34.3N and 140.3E at least, where seismic observation network density is not sufficient to locate correctly seismic events. The deepest portion of slab SG generally strikes the NNW-SSE direction, being approximately parallel to the volcanic front. The thickness of slab SG decreases gradually at areas west of the NNW-SSE striking deepest corner. We propose four basic internal models of slab SG as follows. (1) Slab SG consists of both PH slab at shallower depth and a deeper underlain slab (slab SL). (2) Bookshelf-like configuration of northwardly inclined multi-slabs on 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). The evolutional bookshelf model is partly similar to accretion process near the deep trench system, but 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 to 70km thick lithosphere of IOB (Izu Outer Block). IOB is the northeastern margin of the Philippine sea plate, ranging from the volcanic front along the Izu-Bonin Island arc to the Izu-Bonin trench.
DE: 7218 Lithosphere (1236)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8123 Dynamics: seismotectonics
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting