HR: 1340h
AN: S33A-0298    [Abstracts]
TI: A Centerless Circular Array Method: Extracting Maximal Information on Phase Velocities of Rayleigh Waves From Microtremor Records From a Simple Seismic Array
AU: * Cho, I
EM: ikuo-chou@aist.go.jp
AF: Geological Survey of Japan, AIST, Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Tada, T
EM: kogutek@rs.kagu.tus.ac.jp
AF: Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601 Japan
AU: Shinozaki, Y
EM: sinozaki@rs.kagu.tus.ac.jp
AF: Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601 Japan
AB: We have developed a Centerless Circular Array (CCA) method of microtremor exploration, an algorithm that enables to estimate phase velocities of Rayleigh waves by analyzing vertical-component records of microtremors that are obtained with an array of three or five seismic sensors placed around a circumference. Our CCA method shows a remarkably high performance in long-wavelength ranges because, unlike the frequency-wavenumber spectral method, our method does not resolve individual plane-wave components in the process of identifying phase velocities. Theoretical considerations predict that the resolving power of our CCA method in long-wavelength ranges depends upon the SN ratio, or the ratio of power of the propagating components to that of the non-propagating components (incoherent noise) contained in the records from the seismic array. The applicability of our CCA method to small-sized arrays on the order of several meters in radius has already been confirmed in our earlier work (Cho et al., 2004). We have deployed circular seismic arrays of different sizes at test sites in Japan where the underground structure is well documented through geophysical exploration, and have applied our CCA method to microtremor records to estimate phase velocities of Rayleigh waves. The estimates were then checked against "model" phase velocities that are derived from theoretical calculations. For arrays of 5, 25, 300 and 600 meters in radii, the estimated and model phase velocities demonstrated fine agreement within a broad wavelength range extending from a little larger than 3r (r: the array radius) up to at least 40r, 14r, 42r and 9r, respectively. This demonstrates the applicability of our CCA method to arrays on the order of several to several hundreds of meters in radii, and also illustrates, in a typical way, the markedly high performance of our CCA method in long-wavelength ranges. We have also invented a mathematical model that enables to evaluate the SN ratio in a given microtremor field, and have applied it to real data. Theory predicts that our CCA method underestimates the phase velocities when noise is present. Using the evaluated SN ratio and the phase velocity dispersion curve model, we have calculated the apparent values of phase velocities which theory expects should be obtained by our CCA method in long-wavelength ranges, and have confirmed that the outcome agreed very well with the phase velocities estimated from real data. This demonstrates that the mathematical assumptions, on which our CCA method relies, remains valid over a wide range of wavelengths which we are examining, and also implies that, even in the absence of a priori knowledge of the phase velocity dispersion curve, the SN ratio evaluated with our mathematical model could be used to identify the resolution limit of our CCA method in long-wavelength ranges. We have thus been able to demonstrate, on the basis of theoretical considerations and real data analysis, both the capabilities and limitations of our CCA method.
DE: 0935 Seismic methods (3025, 7294)
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7255 Surface waves and free oscillations
SC: Seismology [S]
MN: Fall Meeting 2005