HR: 0800h
AN: S41A-0251    [Abstracts]
TI: The ZH ratio Analysis of Global Seismic Data
AU: * Yano, T
EM: tomoko@crustal.ucsb.edu
AF: UCSB, Department of Earth Science University of California, Santa Barbara, CA 93106, United States
AU: Shikato, S
EM: shikato@geol.ucsb.edu
AF: UCSB, Department of Earth Science University of California, Santa Barbara, CA 93106, United States
AU: Rivera, L
EM: luis@sismo.u-strasbg.fr
AF: EOST-IPGS, 5 rue Rene Descartes, Strasbourg, F67084, France
AU: Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: UCSB, Department of Earth Science University of California, Santa Barbara, CA 93106, United States
AB: The ZH ratio, the ratio of vertical to horizontal component of the fundamental Rayleigh wave as a function of frequency, is an alternative approach to phase/group velocity analysis for constructing the S-wave velocity structure. In this study, teleseismic Rayleigh wave data for the frequency range between 0.004Hz to 0.04Hz is used to investigate the interior structure. We have analyzed most of the GEOSCOPE network data and some IRIS GSN stations using a technique developed by Tanimoto and Rivera (2007). Stable estimates of the ZH ratios were obtained for the frequency range for most stations. We have performed the inversion of the measured ZH ratios for the structure in the crust and mantle by using nonlinear iterative scheme. The depth sensitivity kernels for inversion are numerically calculated. Depth sensitivity of the lowest frequency extends to depths beyond 500 km but the sensitivity of the overall data for the frequency band extends down to about 300km. We found that an appropriate selection of an initial model, particularly the depth of Mohorovicic discontinuity, is important for this inversion. The inversion result depends on the initial model and turned out to be non-unique. We have constructed the initial model from the CRUST 2.0. Inversion with equal weighting to each data point tends to reduce variance of certain frequency range only. Therefore, we have developed a scheme to increase weighting to data points that do not fit well after the fifth iteration. This occurs more often for low frequency range, 0.004-0.007Hz. After fitting the lower frequency region, the low velocity zone around a depth of 100km is observed under some stations such as KIP (Kipapa, Hawaii) and ATD (Arta Cave, Djibouti). We have also carried out an analysis on the resolving power of data by examining the eigenvalues-eigenvectors of the least-squares problem. Unfortunately, the normal matrix usually has 1-2 very large eigenvalues, followed by much smaller eigenvalues. The third one is often an order of magnitude smaller. The largest eigenvalue is always dominated by an eigenfunction that has the peak at the surface. It indicates that the ZH ratio is sensitive to shallow structure but it has limited form in resolving power for underlying structure. We will report on the details on the resolving capabilities of the ZH ratios.
DE: 7200 SEISMOLOGY
DE: 7205 Continental crust (1219)
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: 2007 Fall Meeting