HR: 17:45h
AN: S22F-08    [PDF]
TI: High-Resolution Subducting Slab Structure Beneath Northern Honshu, Japan, Revealed by Double-Difference Tomography
AU: * Zhang, H
EM: hjzhang@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Thurber, C
EM: thurber@geology.wisc.edu
AF: University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, WI 53706 United States
AU: Shelly, D
EM: dshelly@pangea.Stanford.EDU
AF: Stanford University, Department of Geophysics, 397 Panama Mall, Stanford, CA 94305 United States
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Department of Earth and Planetary Science, Tokyo, 113-0033 Japan
AU: Beroza, G C
EM: beroza@pangea.Stanford.EDU
AF: Stanford University, Department of Geophysics, 397 Panama Mall, Stanford, CA 94305 United States
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Tohoku University, Research Center for Prediction of Earthquakes & Volcanic Eruption, Graduate School of Science, Sendai, 980-8578 Japan
AB: Double seismic zones separated by ~30 km exist at intermediate depths of ~50-~160 km in northeast Japan. The upper and lower plane of earthquakes occur mainly in oceanic crust and mantle, respectively, and are assumed to be induced by dehydration reactions of various hydrous minerals. We obtain the seismic structure of the subducting slab with unprecedented resolution beneath Northern Honshu, Japan by applying newly developed double-difference tomography to the two planes of seismicity of the double seismic zone, which provides evidence supporting the above hypothesis. The upper plane consists of two distinct regions: one with relatively high Vp/Vs ratio (1.77-1.8) at 60-90 km depth and one with low Vp/Vs ratio (1.7-1.77) at 90-110 km depth. These two regions may correspond to the transformations of metabasalt/metagabbro to blueschist and blueschist to eclogite, respectively. There is a conspicuous anomaly of very low Vp/Vs ratio (1.6-1.7) associated with the lower plane, in sharp contrast with high Vp/Vs ratio (~1.8-1.85) in the region between the two planes. The forsterite-enstatite-H$_{2}$O system from serpentine dehydration for the lower plane and possible partial hydration of the region between the two planes of earthquakes may explain the observed features.
DE: 7218 Lithosphere and upper mantle
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting