Estimating Focal Mechanism of Small Earthquakes Using S/P Amplitude Ratios of Distributed Acoustic Sensing Records

Abstract

We proposed a method for estimating focal mechanisms based on S/P amplitude ratios from distributed acoustic sensing (DAS) records for single earthquakes and successfully obtained the mechanisms from four events. This method involves measuring the maximum amplitude ratio between the S- and P-waves of the DAS records, followed by searching for the parameters of the focal mechanism. We applied this method to the DAS records of four shallow earthquakes from central Japan with magnitudes ranging from M2.2 to 3.4. The estimated mechanisms were consistent with the distribution of P-wave polarities from conventional seismometers despite recording only one component along the cable. This straightforward approach may be effective because of the wave scattering caused by near-surface heterogeneities and the varying orientations of the cable sections corresponding to the channels. Consequently, the observed scattered waves preserved the overall radiation patterns, allowing us to obtain the radiation patterns from a single-component record.