Determination of focal mechanisms of nonvolcanic tremor using S wave polarization data corrected for the effects of anisotropy

Abstract

We propose a method to determine the focal mechanisms of nonvolcanic tremors using anisotropy-corrected S polarizations measured directly from noisy tremor signals. The successful retrieval of polarization angles in noisy signals owes much to the observation that, in general, tremors propagate slowly and therefore do not change their location immediately. This feature of tremors enables us to use longer time windows to compute polarization angles (e.g., 1 min or longer), resulting in a stack of particle motions. We applied the method to a tremor episode that occurred beneath the Kii Peninsula in April 2013. Although the majority of inferred focal mechanisms suggested that tremors manifest shear slip on the plate interface, some events with strike-slip components were also found to have occurred. The proposed method provides us with a tool for investigating smaller-scale spatial distributions and temporal variations of focal mechanisms throughout tremor zones.