HR: 0830h
AN: S31D-0787    [PDF]
TI: Waveform Modeling of 3D Structure of D" Region Using A Coupled SEM/Normal Mode Approach
AU: * To, A
EM: toh@seismo.berkeley.edu
AF: U.C. Berkeley Seismological Laboratory, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760 United States
AU: Gung, Y
EM: gung@seismo.berkeley.edu
AF: U.C. Berkeley Seismological Laboratory, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760 United States
AU: Capadeville, Y
EM: yann@seismo.berkeley.edu
AF: Institut de Physique du globe de Paris, 4, place jussieu, Paris, 75005 France
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: U.C. Berkeley Seismological Laboratory, 215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760 United States
AB: The presence of strong lateral heterogeneity in D" is now well documented and presents challenges for seismic modeling. The main challenges are the limited global sampling of D" and the theoretical limits of validity of the present modeling tools, such as standard ray theory and mode approaches. We use coupled normal mode/Spectral Element Method (SEM) to compute synthetic seismograms of Sdiff in the D" part of a tomographic model(SAW24b16, M\'egnin and Romanowicz, 2000) down to corner frequency 1/12s. SEM allows to take into account strong heterogeneity in a rigorous manner. The coupled method is much faster than standard SEM, when the numerical part of the computation is restricted to the D" region. In the rest of the mantle, the wave field is computed using efficient normal mode summation. As a first step, we consider a radially symmetric model outside of the D" region, and compare Sdiff synthetics with observed waveforms for a collection of deep earthquakes, for which the effect of strong heterogeneity in the crust and upper mantle is avoided. Observed and synthetic travel time trends are very consistent and in many cases the observed residuals are significantly larger. This indicates that the tomographic model only represents the smooth features of the real structure. Observed waveform amplitudes and SEM synthetics are somewhat less consistent. We compare the predictions for 800 Sdiff phases using SEM with those obtained by more approximate methods : ray theory and NACT (Non-linear asymptotic coupling theory, a normal mode perturbation approach). We discuss systematic trends in the travel times predicted by the different methods, compared to observations. Starting with the tomographic model, and correcting for mantle structure outside of D" using approximate NACT predictions, we next invert for perturbations to the tomographic model, using the coupled SEM/mode computation for the forward part of the modeling, in several regions of D" under the Pacific, which are well sampled by available Sdiff data. We discuss the resulting changes in the D" model.
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Seismology [S]
MN: 2003 Fall Meeting