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New Circular-Array Microtremor Techniques to Infer Love-Wave Phase Velocities

Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan kogutek{at}ni.aist.go.jp

Yuzo Shinozaki

Department of Architecture, Faculty of Engineering, Tokyo University of Science, 1-14-6 Kudan Kita, Chiyoda-ku, 102-0073 Tokyo, Japan

We present simple and novel formulas that help to directly infer phase velocities of Love waves (c^L) using two-component, horizontal-motion, circular-array records of microtremors. Our formulas are analogous to those of the spatial autocorrelation (SPAC) method, a technique of microtremor exploration that is popularly used to infer phase velocities of Rayleigh waves (c^R). Although the existing theory of the SPAC method does provide the possibility to estimate c^L, this can only be done by solving a nonlinear system of equations, wherein the unknowns to be solved for also include c^R and the Rayleigh-to-Love power partition ratios. By contrast, c^L is the only unknown to appear in our formulas.

The field applicability of the c^L estimation methods based on the proposed formulas—which we name the SPAC+L, SPAC-L, and CCA-L (where CCA stands for centerless circular array) methods—is demonstrated by analysis results for two test sites. Among the three techniques proposed, the SPAC+L method distinguished itself as the best-performing. The valid wavelength ranges of the three methods had lower limits in the area of 2–5 times the array radius r and upper limits in the area of 10–25r.

Similar circular-array microtremor techniques that involve c^L alone were proposed in recent years, but the methods we present here are either logistically less expensive or mathematically simpler than any of them.