HR: 09:45h
AN: T31A-07    [PDF]
TI: Mantle Seismic Anisotropy in the Izu-Bonin Subduction System
AU: * Anglin, K
EM: Karen.Anglin@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287 140 United States
AU: Fouch, M J
EM: fouch@asu.edu
AF: Arizona State University, Department of Geological Sciences Box 871404, Tempe, AZ 85287 140 United States
AB: The objective of this study is to determine the location and extent of seismic anisotropy within the Izu-Bonin (IB) subduction system to constrain mantle flow dynamics in the region. Of particular importance is an improved understanding of the role of water in strain development and seismic anisotropy in hydrated mantle. The IB system is a key location in which to evaluate this problem as it presumably contains a hydrated mantle wedge and is a viable candidate for relatively simple 2D mantle flow. However, the dearth of seismic stations in IB presents significant challenges in measuring shear wave splitting in the region. We therefore apply a unique approach for imaging mantle seismic anisotropy to provide new constraints on patterns of strain and mantle flow in the IB system. We apply an analysis technique that determines residual splitting between teleseismic S and sS for intermediate- and deep-focus events. In this method, we estimate receiver-side anisotropy by applying particle motion analysis to teleseismic S. To limit the receiver-side anisotropic effect on the sS waveform, we use the resulting splitting parameters (fast polarization direction and relative delay time) from the S phase to apply particle motion corrections to the sS phase. We then evaluate shear wave splitting in sS to provide an estimate of seismic anisotropy recorded by sS within the upper mantle on the source side of the raypath. In our current dataset, we have examined residual sS splitting in waveforms in which no discernible splitting is evident on teleseismic S, thus limiting the potential effect of receiver-side upper mantle anisotropy on sS. Preliminary results from analysis of several source-receiver pairs exhibit fast polarization directions oriented ~E-W (i.e., convergence-parallel) and splitting times ranging from 1.75 s to 2.75 s. These results are consistent with the limited body of existing shear wave splitting analyses of local S phases for northernmost regions of the IB mantle wedge; however, splitting times from the current study are roughly double those estimated by previous work, as expected given the extended path length of sS relative to local S within the mantle wedge. While we emphasize that these results are preliminary, analysis of our dataset will continue to provide new constraints on the lateral and depth distribution of seismic anisotropy beneath the Izu-Bonin region.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting