HR: 0800h
AN: T21B-0519    [Abstracts]
TI: Shear Wave Splitting and Seismic Anisotropy in the Mariana Mantle Wedge from a Combined Land-Sea Deployment
AU: * Pozgay, S H
EM: spozgay@seismo.wustl.edu
AF: Washington University in St Louis, Department of Earth and Planetary Sciences, St Louis, MO 63130 United States
AU: Wiens, D A
EM: doug@seismo.wustl.edu
AF: Washington University in St Louis, Department of Earth and Planetary Sciences, St Louis, MO 63130 United States
AU: Shiobara, H
EM: shio@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Tokyo, 113-0032 Japan
AU: Sugioka, H
EM: hikari@jamstec.go.jp
AF: Institute for Frontier Research on Earth Evolution (IFREE), JAMSTEC, Yokosuka, 237 Japan
AB: We investigate anisotropy and mantle flow in the Mariana mantle wedge using data from the Mariana Subduction Factory Imaging Experiment. The passive component of this experiment consisted of 20 broadband PASSCAL seismographs deployed on the island chain and 58 ocean-bottom seismographs across the arc and the backarc. This dense seismic network recorded from May-June, 2003 until April-May, 2004 and 66 instruments returned useful data. Analysis of shear wave splitting from S waves propagating from intermediate and deep earthquakes in the Mariana slab to this array provides an unparalleled opportunity to study the pattern of anisotropy and mantle flow in the arc and backarc region. We analyze local S wave anisotropy using the shear wave splitting method of Silver and Chan (1991). To check the robustness of our results, we also utilize the cross-correlation method (Bowman and Ando, 1987) and visually inspect particle motions. Preliminary results mostly based on the 20 land seismic stations indicate a complex pattern of anisotropy in the region. Fast splitting directions and magnitudes are highly dependent on source location and depth. Along-arc delay times generally range from small for southern hypocenters near Guam ($<$ $\sim$0.4 s) to larger ($>$ $\sim$0.6 s) for sources in the northern part of the arc. Fast directions generally range from N-S to NW-SE at southern stations, and N-S to NE-SW at northernmost stations. We interpret these results in terms of mantle flow using the conventional assumption that the fast anisotropic direction results from lattice preferred orientation and denotes the direction of maximum extension. There is little evidence of a large-scale convergence-parallel mantle circulation patterns previously reported for this region based on observations from a single station at Guam. Rather, these results indicate that along-strike flow patterns predominate in most regions of the arc. Complete analysis of splitting directions and magnitudes in the region will provide a rare opportunity to observe seismic anisotropy across a subduction zone forearc, arc, and backarc spreading center.
DE: 9355 Pacific Ocean
DE: 8100 TECTONOPHYSICS
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting