HR: 14:25h
AN: S32C-04    [PDF]
TI: Shear wave splitting and upper mantle anisotropy beneath Japan
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
AB: Shear wave birefringence is a consequence of seismic anisotropy and is often used, with constraints from mineral physics, to characterize the pattern of upper mantle deformation. In the context of a subduction zone, however, the relationship between measured shear wave splitting parameters ($\phi$, $\delta$t) and geodynamical processes is not straightforward. The three-dimensional pattern of anisotropy in a subduction zone may reflect processes such as corner flow in the mantle wedge, flow around the slab edge, back-arc extension, and motion of the overriding plate. This relationship may be further complicated by complex slab morphology, by the presence of frozen anisotropy in the slab itself, and by the presence of volatiles such as water. In this study, we take advantage of dense station coverage in Japan and use seismic phases covering a wide range of incidence angles, incoming polarization angles, and backazimuths. We take advantage of the good data coverage needed to consider complexities in structure such as multiple anisotropic layers, dipping symmetry axes, and small-scale lateral variations in anisotropic properties. We utilize data from the Japanese F-net network, which comprises 65 broadband seismic stations. We have compiled a database of approximately 1500 splitting measurements of S, SKS, and SKKS phases at F-net stations, and investigate the variations of measured splitting parameters with incoming polarization angle and incidence angle. In the southern part of the array, along the Ryukyu arc, we find that fast directions are consistently trench-parallel, with splitting times of 1 second or more. Moving northward along the array, the measured splitting patterns become more complicated, with significant variations in apparent splitting parameters that indicate complex anisotropic structure. Additionally, measured fast directions vary significantly over short length scales, and stations separated by less than 100 km often exhibit very different splitting behavior. This increase in complexity of anisotropic structure coincides geographically with the complicated slab morphology of the subducting Pacific plate. At stations on Hokkaido, to the north, and Kyushu, to the south, we see some evidence that the fast direction of anisotropy may rotate from trench-parallel close to the trench to subduction-parallel further away from the trench, which may correspond to a change in stress conditions and/or volatile content.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting