HR: 16:45h
AN: T24B-04    [Abstracts]
TI: Complex upper mantle anisotropy beneath Japan: shear wave splitting observations and geodynamical modeling
AU: * Long, M D
EM: mlong@mit.edu
AF: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: van der Hilst, R D
EM: hilst@mit.edu
AF: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Hager, B H
EM: bhhager@mit.edu
AF: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: de Hoop, M V
EM: mdehoop@mines.edu
AF: Center for Wave Phenomena, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401 United States
AB: The characterization of seismic anisotropy in the upper mantle has the potential to elucidate patterns of mantle deformation. In the context of a subduction zone, this line of inquiry is both especially intriguing and especially challenging, because the presence of volatiles or melt in the mantle wedge or three-dimensional flow induced by complex slab morphology can complicate the relationships between deformation and anisotropy. In this paper we report the results of an intensive effort to fully characterize the shear wave splitting behavior at broadband seismic stations in Japan due to deformation-induced anisotropy in the upper mantle. Japan is an excellent area to study complex anisotropy associated with subduction zone processes; it overlies two different subducting plates with different ages, thermal histories, plate velocities, and slab morphologies. Therefore, Japan can serve as a natural laboratory to test hypotheses relating to the effects of melt, stress variations, and volatile content on the relationships among tectonic processes, deformation, anisotropy, and splitting observations. We have assembled a database of teleseismic shear wave splitting measurements for F-net, a network of more than 65 broadband stations throughout Japan. We have measured splitting parameters for over 1900 individual S, SKS, and SKKS phases. At many F-net stations, the observed splitting patterns are complex and cannot be explained with simple, single-layer anisotropic models. Complexity in splitting pattern appears to be spatially correlated with complexity in slab morphology. We observe trench-parallel fast directions in the Ryukyu arc, which rotate to trench-perpendicular moving northward along the array into Kyushu and along the downdip direction of the Phillippine slab. This rotation from trench-parallel fast directions close to the trench to trench-perpendicular farther away from the trench is also observed in the Kanto-Shikoku regions, moving downdip above the Pacific slab, and on Hokkaido. We seek to explain this persistent feature, along with other features of our shear wave splitting dataset, by comparing splitting observations with a suite of geodynamical models that take into account the unique slab geometries beneath Japan.
DE: 9320 Asia
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting