2.2 Estimation of focal mechanisms
We estimated the focal mechanisms based on the amplitude ratios of the
waveforms using the method of Yoshida et al. (2019b), which is similar
to that of Dahm (1996). We used six focal mechanisms determined by the
JMA (Fig. 1b) to represent effects of the path and site on the waveform.
We determined the focal mechanisms of 161 earthquakes with
ML ≥ 2. We used displacement waveforms obtained by
integrating the velocity waveforms recorded at the 20 stations (green
triangles in Fig. 1a) surrounding the hypocenters. The vertical
component was used for the analysis of the P-wave, whereas radial and
transverse components were used for that of the S-wave. We applied a
2–5 Hz bandpass filter to the waveforms, cutting them out with time
windows of 2.8 s for P-waves and 4.3 s for S-waves starting 0.3 s before
their arrival.
We used waveform cross-correlations to measure the amplitude ratios
between target and reference events. The amplitude ratios were obtained
for pairs with absolute correlation coefficients above 0.75. We used
principal component analysis (PCA) to measure the amplitude ratios.
We only estimated the mechanism solutions when amplitude ratios were
obtained for more than 20 channels. We eliminated the results when the
Variance Reduction (VR) was below 80:
\(\text{VR}=\ \left(1-\frac{\sum_{k=1}^{n}\left(d_{k}-s_{k}\right)^{2}}{\sum_{k=1}^{n}d_{k}^{2}}\right)\ \bullet 100,\)(1)
where \(d_{k}\) and \(s_{k}\) are the observed and calculated
displacement amplitude ratios, respectively, at channel \(k\).