HR: 16:45h
AN: S34B-04    [Abstracts]
TI: Probing the crust by Microseisms
AU: * Tanimoto, T
EM: toshiro@geol.ucsb.edu
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AU: Alvizuri, C
EM: rodricels@hotmail.com
AF: Institute for Crustal Studies, University of California, Santa Barbara, CA 93106 United States
AB: Study of microseisms has a long history, and a part of its effort has been targeted to using the signals to probe the crustal structure (e.g. Aki, 1957; Toksoz, 1964). With new developments in seismic instrumentation and networks, there are clearly new opportunities for improving our understanding of these signals and utilizing them for constraining shallow crustal S-wave velocity. Enigmas on the source of excitation, especially the source locations of double-frequency microseisms, may also be resolved in the near future. We report our analysis of microseisms by using networks based in Southern California. Preliminary analysis in other parts of the world indicates that there are distinct differences in the characteristics of microseisms from place to place, but in this paper we will mainly focus on the Southern California Data using three-component seismograms. Main characteristics that we have found are (1) microseisms mostly consist of Rayleigh waves, which have been known for over 70 years now, but microseisms do contain some other signals such as S waves. But there is a way to remove such non-Rayleigh wave signals by using the 90-degree phase shift between the horizontal and vertical components. (2) The directions of source can be inferred approximately from Rayleigh wave horizontal particle motions and are shown to be fairly constant throughout the year, pointing toward the Pacific coast in general. (3) In basin structures, there are some indications of reflection of such energy, complicating the phase in data. The ellipticity of Rayleigh waves can be measured from such data as a function of frequency from 0.1 to 0.2 Hz. This frequency range is specific to the double-frequency microseisms in Southern California and can change from region to region, generally towards wider frequency range such as 0.1-0.4 Hz. Ellipticity data contain seasonal variations in our results at the moment whose cause we do not exactly understand. But the averaged ellipticity data over a year can be solved for shallow S-wave velocity structure. Feasibility of such an approach was shown by Boore and Toksoz (1969) for a four-layered model before (for longer periods at 10-50 seconds), but the microseism data can now be inverted using almost continuously varying kernels in depth. Our results indicate that ellipticity data (frequency range 0.1-0.2 Hz) have sensitivities to the upper crust (0-10 km) and our results seem to require modifications to the standard model in the region (SCEC CVM3.0).
DE: 3285 Wave propagation (0689, 2487, 4275, 4455, 6934)
DE: 7205 Continental crust (1219)
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: Fall Meeting 2005