Detection of Seismic Anisotropy From Low-Frequency Earthquakes: Eastern Shikoku

Abstract:

In this study, we detected seismic anisotropy from low-frequency earthquakes (LFEs) by the JMA (Nishide et al., 2000) during a tremor episodes at the plate interface of the PHS slab beneath eastern Shikoku, southwest Japan by S-wave splitting analysis (e.g., Ando et al., 1983). We applied this method for the isolated phases of LFEs, which are regarded as S-waves and used by hypocenter determination. In this method seismic anisotropy is represented by polarization direction of fast S-wave (direction of anisotropy) and delay time between two S-waves. In order to examine the difference between anisotropy within the subducting plate and around the plate interface, we also investigated anisotropy of slab earthquakes, which occurred within the slab. Furthermore, we measured anisotropy of crustal earthquakes in order to evaluate anisotropy near receiver, because anisotropy obtained by S-wave splitting analysis is strongly affected by anisotropy in shallow crust and often masked by it.

We used filtered seismograms (2-8 Hz) at stations of Hi-net deployed by NIED in all the analyses. Results of S-wave splitting analyses of LFEs showed that the directions of anisotropy are tend to be orientated in the East−West-trending direction and the delay times are 0.2-0.4 sec. The delay times of LFEs are larger than those of slab earthquakes and crustal earthquakes (~ 0.2 s), suggesting a possibility that there exists a unique anisotropy in the source regions of LFEs. However, the direction of anisotropy is similar to crust anisotropy in the study area, implying that the anisotropy of LFE might be masked by crustal anisotropy. In order to rigorously discuss anisotropy around the source region, we have to remove the effect of the anisotropy in the crust.