HR: 11:20h
AN: S42A-05    [Abstracts]
TI: The Measurement and Interpretation of Surface Wave Group Arrival Times
AU: * Masters, G
EM: gmasters@ucsd.edu
AF: UCSD, MC0225 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Kane, D
EM: dlkane@ucsd.edu
AF: UCSD, MC0225 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Morrow, J
EM: jmorrow@ucsd.edu
AF: UCSD, MC0225 9500 Gilman Drive, La Jolla, CA 92093-0225 United States
AU: Zhou, Y
EM: yingz@gps.caltech.edu
AF: Cal Tech, Seismological laboratory, Pasadena, CA 91125 United States
AU: Tromp, J
EM: jtromp@gps.caltech.edu
AF: Cal Tech, Seismological laboratory, Pasadena, CA 91125 United States
AB: We have recently developed an efficient technique for measuring the relative group arrival times of surface waves by using cross-correlation and cluster analysis of waveform envelope functions. Applying the analysis to minor arc Love and Rayleigh waves in the frequency band 7 to 35 mHz for all events over magnitude 5.5 results in a dataset of over 200,000 measurements at each frequency for long period Rayleigh waves (frequency less than 25 mHz) and about 100,000 measurements at the shorter periods. Analysis of transverse components results in about half as many Love wave measurements. Simple ray theory inversions of the relative arrival times for apparent group velocity produce maps which are accurate representations of the data (often over 90% variance reduction of the relative arrival times) and which show features strongly correlated with tectonics and crustal thickness. The apparent group velocity variations can be extremely large: 30% velocity variations for 20 mHz Rayleigh waves and 40% variations for 30 mHz Rayleigh waves and can have abrupt lateral changes. This raises the concern that non-ray theory effects could be important. Indeed, a recent analysis by Dahlen and Zhou (personal communication) suggests that the group arrival times should be a functions of both the group velocity AND the phase velocity. The simplest way to test the interpretation of the measurements is to perform the analysis on synthetic seismograms computed for a realistic model of the Earth. Here, we use the SEM with a model which incorporates realistic crust and mantle structure. We are currently computing synthetics for a suite of roughly 1000 events recorded globally that extend to a period of 18 seconds. We shall present the results of applying both ray-based and finite frequency inversions to the synthetic data as well as evaluating the effects of off path propagation at short periods using surface wave ray tracing.
DE: 7205 Continental crust (1219)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 7290 Computational seismology
SC: Seismology [S]
MN: Fall Meeting 2005