HR: 08:15h
AN: S31E-02    [Abstracts]
TI: Non-linear 3D Born Shear Wave Tomography in Southeastern Asia
AU: * Cao, A
EM: acao@seismo.berkeley.edu
AF: UC Berkeley, 215 McCone Hall, Berkeley, CA 94720, United States
AU: Panning, M
EM: mpanning@princeton.edu
AF: Princeton University, Guyot Hall, Princeton, 08544, United States
AU: Kim, A
EM: ahyi@seismo.berkeley.edu
AF: UC Berkeley, 215 McCone Hall, Berkeley, CA 94720, United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: UC Berkeley, 215 McCone Hall, Berkeley, CA 94720, United States
AB: We have developed a 3D radially anisotropic shear velocity model of the upper mantle in southeastern Asia from the inversion of long period seismic multimode waveforms. Our approach is based on normal mode perturbation theory, specifically, on a recent modification of the Born approximation, which we call "N-Born", and which includes a non-linear term that allows the accurate inclusion of accumulated phase shifts which arise when the wavepath traverses a spatially extended region with a smooth velocity anomaly of constant sign. We apply the N-Born approximation in the forward modeling part and calculate linear 3D Born kernels in the inverse part. Our starting model is a 3D radially anisotropic model which we derived from a large dataset of teleseismic multimode long period waveforms in the period range 60 to 400 s, using a finite-frequency 2D approximation (NACT, Li and Romanowicz, 1995). This model covered a larger region of East Asia (longitude 30 to 150 degrees and latitude -10 to 60 degrees), while our N-Born model is restricted to a smaller subregion (longitude 75 to 150 degrees and latitude 0 to 45 degrees) for computational efficiency. In this subregion, our N-Born isotropic and anisotropic models are both parameterized at relatively short wavelengths corresponding to a spherical spline level 6 (~200km). Our N-Born model can fit waveforms as well as the NACT model, with up to ~ 83% variance reduction. While the models agree in general, the N-Born isotropic model shows a stronger fast velocity anomaly beneath the Tibetan plateau in the depth range of 150 km to 250 km, which disappears at greater depth, consistent with other studies. More importantly, the N-Born anisotropic model can recover well the downwelling structure associated with subducted slabs. Beneath the Tibet plateau, radial anisotropy shows V_SH>V_SV, which is indicative of horizontal rather than vertical flow and may help distinguish between end member models of the tectonics of Tibet.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: 2007 Fall Meeting