HR: 16:15h
AN: T14A-02    [Abstracts]
TI: Fragmentation on a grand scale: A slab fragment dislodged from the descending Pacific plate slab wedged Beneath Tokyo
AU: * Stein, R S
EM: rstein@usgs.gov
AF: U.S. Geological Survey, MS 977, Menlo Park, CA 94025, United States
AU: Toda, S
EM: s-toda@aist.go.jp
AF: Active Fault Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, 1-1, Tsukuba, 305-8567, Japan
AU: Bozkurt, S B
EM: sbozkurt@geomatrix.com
AF: Geomatrix Consulting, 2101 Webster Street, Oakland, CA 94612, United States
AU: Kirby, S H
EM: skirby@usgs.gov
AF: U.S. Geological Survey, MS 977, Menlo Park, CA 94025, United States
AU: Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, 980-8578, Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Tohoku University, Sendai, 980-8578, Japan
AB: The Eurasian-Philippine Sea-Pacific plate triple junction located 250 km southeast of Tokyo not only produces megathrust subduction earthquakes along the Sagami trough (including the 1703 M=8.3 Genroku and 1923 M=7.9 Kanto shocks) but is also associated with enigmatic M~6-7 earthquakes directly beneath Tokyo (such as the 1855 M~7.1 Ansei-Edo earthquake and 2006 M=6.0 Chiba shocks). In the past, the Tokyo quakes have been interpreted as resulting from a highly warped portion of the Philippine Sea slab scraping against the Pacific slab below. However, on the basis of microearthquake distributions, seismic tomography, bathymetric analysis, paleomagnetic data on past plate motions, magnetic seafloor anomalies, and seismic stress inversion, we argue instead that a 100-km-wide by 25-km-thick fragment of the Pacific plate lithosphere detached from the descending slab several million years ago and is now jammed between the Pacific, Philippine Sea and Eurasian plates beneath Tokyo. We explore and visualize the complex three-dimensional nature of this problem through a series of seismicity and plate animations. We identify a pronounced ‘snow-plow' shape to the double seismic zone of the descending Pacific plate slab, which closely parallels the sharp curvature of the overlying volcanic front. We argue that the fragment was probably dislodged when two buoyant seamount chains on the Pacific plate collided with the Japan-Izu trench east of Tokyo. Since breakup, the fragment has been pushed 100 km to the northwest by the leading edge of the Philippine Sea plate slab. Whether the plate-scale fragmentation that we find is common or rare is difficult to assess, because our analysis is possible only because of the extraordinarily rich data of southwest Japan. Nevertheless, subducting seamount chains appear to deform trenches and bend slabs in many circum-Pacific settings, modifying seismic behavior and perhaps causing slab fracture and fragmentation.
DE: 8138 Lithospheric flexure
DE: 8166 Stresses: deep-seated
DE: 8168 Stresses: general
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: 2007 Fall Meeting