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

Abstract

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 10–25 r . 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.