HR: 14:10h
AN: S23D-03    [Abstracts]
TI: Imaging the Andaman and Sunda Subduction Zones Using Regional Double-Difference Tomography
AU: * DeShon, H R
EM: hdeshon@memphis.edu
AF: Center for Earthquake Research and Information, University of Memphis, 3890 Central Ave., Memphis, TN 38152, United States
AU: Zhang, H
EM: hjzhang@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139, United States
AU: Thurber, C H
EM: clifft@geology.wisc.edu
AF: University of Wisconsin-Madison, 215 W Dayton St., Madison, WI 53706, United States
AU: Engdahl, E
EM: engdahl@iaspei.org
AF: University of Colorado-Boulder, 2200 Colorado Ave., Boulder, CO 80309, United States
AB: We present an extension of the double-difference (DD) local earthquake tomography algorithm to teleseismic scales and show initial results for the Andaman and Sunda subduction systems. The 2004 and 2005 great Sumatra earthquakes and the resulting aftershock sequences generated thousands of globally recorded events and illuminated the shallow seismogenic zone. We focus analysis on earthquakes that meet the stringent Engdahl-van der Hilst-Buland (EHB) location quality criteria, as these events provide the most reliable information for imaging the complexities of the seismogenic zone. Where broadband waveforms exist, we use a frequency- based automatic picking technique to identify additional first arrivals and depth phase onset times for inclusion in the dataset. Waveform cross-correlation is used to reduce relative picking errors between earthquakes with similar waveforms, leading to higher precision differential times. To extend the DD tomography code for use with teleseismic raypaths, we integrated a hybrid ray tracer that combines a finite-difference (FD) travel time calculator and pseudo-bending algorithm to solve for P and S first arrivals. This advancement allows us to use teleseismic data to more precisely determine the relative location of subduction zone earthquakes, especially their focal depths, and solve for 3D velocity heterogeneity. The FD travel time calculator is computationally stable and capable of computing travel times to all points in the model and can locate diffractions in ray shadow zones. It can find the correct solution even in strongly heterogeneous medium. However, its accuracy depends on grid spacing size and computation time may be unacceptable if the grid size is too small for the desired accuracy. Therefore, the FD travel time calculator is used to provide an approximate initial raypath. We then apply the spherical pseudo-bending algorithm of Koketsu and Sekine (1998) to further improve raypath accuracy. Because of non-linearity of the two-point problem, this algorithm may fail to find the true two-point ray path in a heterogeneous medium if the initial ray path to start with is far from the true solution. By combining the two algorithms, we can efficiently and accurately find the ray path solution even in strongly heterogeneous medium.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: 2007 Fall Meeting