HR: 16:15h
AN: S44A-02    [Abstracts]
TI: The Kanto Subduction Zone: Seismogenic Layers, Slab Deformations and Volcanism Associated With the two Subducting Philippine Sea Plate and Pacific Plate
AU: * Noguchi, S
EM: shin@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tennodai, Tsukuba, 305-0006 Japan
AB: Understanding the details of complex seismicity and tectonic processes in the Kanto subduction zone, which is located at the junction of northeastern Japan arc and Izu-Ogasawara arc, is important in various fields. The most important factor characterizing plate convergence in the Kanto area is that the young, hot and laterally heterogeneous forearc portion of the Philippine Sea plate (PHS) descends into the mantle wedge and collides with the old and cold Pacific plate (PAC). The interaction among overriding, subducting and mutually colliding plates, each having different plate movement, is responsible for complex seismic activity and various subduction modes as have been observed by recent seismic and geodetic networks. Among several noticeable features of seismicity, we focus on the intermediate-depth seismicity of the double Wadati-Benioff seismic zones (WBZ) of the Pacific slab subducting beneath the Izu-Ogasawara arc, and its relation with the overlapping boundary of two slabs, i.e., the contact interface between the two slabs (PHS-PAC interface). Along the upper seismic plane of WBZ, we recognize a remarkable band of intermediate-depth seismicity, which distributes in southeast-northwest direction with a systematic increase of depth northwest from about 90 to 150 km. Focal mechanism solutions of this band are characterized by normal faultings. Associated with this intense seismicity band, we examine the intersection of the bottom of PHS and the upper boundary of PAC (the loci of the deeper limit of PHS-PAC interface), by taking cross-sections of hypocenter distributions. We find the deeper limit of interface also locates southeast-northwest in the depth range between 70 and 100 km and almost parallel with the intense seismicity band with distance 30-40 km apart. To explain reasonably the northwest deepening of the intermediate-depth seismic band, we use the model of dehydration embrittlement of serpentine and temperature structure in the slab proposed by Yamasaki and Seno (2003, JGR, 108). Because PHS-PAC interface deepen northwest, the colder temperature structure of the Pacific slab underneath may remain deeper northwest. As a result the dehydration loci and the corresponding seismogenic layer in the slab may deepen northwest. Further in the overlying asthenospheric mantle the ascending path of aqueous fluid and magma formation point may also be deepened northwest. This inference accords with the location of the volcanic front in western Kanto, which runs northwest above the upper seismic plane of WBZ across isodepth lines with depth range bewteen about 110 and 160 km along the Izu-Ogasawara arc.
DE: 7230 Seismicity and seismotectonics
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting