HR: 13:55h
AN: S33D-02    [Abstracts]
TI: Constraints on Lateral S Wave Velocity Gradients and the Shape of the Pacific Superplume
AU: * To, A
EM: toh@seismo.berkeley.edu
AF: UC Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720 United States
AU: Romanowicz, B
EM: barbara@seismo.berkeley.edu
AF: UC Berkeley Seismological Laboratory, 215 McCone Hall, Berkeley, CA 94720 United States
AU: Capdeville, Y
EM: capdevil@ipgp.jussieu.fr
AF: Institut de Physique du globe de Paris, 4 place jussieu, Paris, 75005 France
AB: We have recently documented that a sharp lateral boundary exists at the southern edge of the Pacific superplume (To et al., 2005). The set of SHdiff wave forms, which graze the South Pacific superplume, have similar features to those observed previously at the southeastern edge of the African superplume. They both show a rapid shift of the arrival time with respect to azimuth and are followed by postcursors. The similarity of the two observed SHdiff waveform sets at relatively high frequencies indicates that the low velocity regions in the lower mantle under Pacific and Africa, observed as the strong degree-2 pattern in shear velocity tomographic models, have a similar nature also at finer scales. We used the coupled mode/spectral element method (CSEM, Capdeville et al., 2003), which can handle strong lateral variations of the velocity in the D", to construct synthetic waveforms. The results show that, by increasing the gradient of the fast and slow anomaly based on an existing global tomographic model, those features of waveforms can be produced. The postcursors are explained as refractions from the lateral boundary in D" region. The result suggests that it is important to take into account the heterogeneity outside of the great circle path. Modeling these pulses can help constrain the shape and velocity contrast at the superplume boundaries, at the base of the mantle. We have assembled a large dataset of Sdiff waveforms and travel time throughout the Pacific region. We here report that also in other regions bordering the South Pacific superplume, observed Sdiff travel times vary rapidly over small ranges of azimuth and/or distance, and can be better fit by increasing the amplitude and lateral gradients of the large scale velocity anomalies in a tomographic S velocity model. We present preliminary results of a new multi-step approach, in which we combine our finite frequency tomographic inversion methodology (NACT, Li and Romanowicz, 1996), in order to obtain a good starting 3D model, with forward modelling of travel times (taking into account finite frequency effects), and finally waveforms, using CSEM. We show that this approach holds great promise to better constrain the 3D structure within and around the Pacific superplume.
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
SC: Seismology [S]
MN: Fall Meeting 2005