HR: 0830h
AN: S31C-0781    [PDF]
TI: Rayleigh wave phase velocities, shear wave structure and azimuthal anisotropy beneath southern California
AU: * Yang, Y
EM: Yingjie_Yang@brown.edu
AF: Yingjie Yang, 324 Brook street Brown University Gelogy Department, Providence, RI 02912 United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Donald W. Forsyth, 324 Brook street Brown University Gelogy Department, Providence, RI 02912 United States
AB: We use normal mode Rayleigh wave phase and amplitude data recorded at the TriNet network in southern California to invert for phase velocities at periods from 25 to 143 s. These phase velocities were used to obtain 3-D S-wave velocity structure in the upper mantle. Phase velocities on the Pacific plate side of the plate boundary are systematically higher than on the North American side, suggesting that seismic velocity contrast between these two plates extends to the upper mantle. In the upper mantle, there is a pronounced low velocity anomaly beneath the Long Valley/Mono Lake region, which has not been observed by previous tomographic studies. This low velocity anomaly is consistent with melting extending to the base of the crust beneath this part of the western Basin and Range province, as suggested based on the composition of late Cenozoic basalts (Wang et al., JGT, 2002). There is a high velocity anomaly under the Transverse Range and a slightly slow velocity anomaly under the Salton Trough, both of which have been observed in previous body and/or surface wave tomographic studies. Assuming uniform anisotropic structure in the whole study area, the strength of anisotropy is about 2.5% at all periods. However, the fast direction varies with period. The fast direction of apparent anisotropy is nearly W-E at periods less than 50 s, consistent with the fast polarization axis of SKS splitting measurements in Southern California. At periods larger than 67s, the fast direction changes to NW-SE, subparallel to the plate boundary. This two-layer azimuthal anisotropy structure is in contrast to the one-layer SKS splitting model for southern California, implying that lateral heterogeneity may affect the apparent anisotropy of long-period surface waves. If anisotropy is allowed to vary laterally in our models, we find a minimum in azimuthal anisotropy in the vicinity of the Transverse Range, suggesting possible more vertical alignment of the olivine a-axis in a region of downwelling.
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2003 Fall Meeting