Azimuthal Anisotropy Variations in Japan from Shear Wave Splitting Analysis of Foreshocks

Precursory anisotropy change before a major earthquake remains a controversial topic in earthquake seismology. Japan is an ideal place to study this topic because it experiences numerous large earthquakes and has a dense seismic network. We analyze shear-wave splitting (SWS) data from foreshocks recorded at four Hi-net seismic stations (JUOH, ASGH, SKGH, and TYNH) for four major earthquakes in Fukushima, Shizuoka, Tottori, and Kumamoto. SWS parameters (fast polarization direction and delay time) are calculated using a semi-automatic algorithm. The average fast directions at the four stations are either parallel to the local maximum horizontal stress orientation or the fault strike, consistent with previous studies. Both SWS parameters do not show noticeable temporal variations before large earthquakes at station JUOH, ASGH, and TYNH. At station SKGH in Tottori, the delay time varies randomly within a small range for most of the foreshocks from 25 days to 1 day before the main earthquake. However, a sharp jump of delay time (0.03 s) is observed within two hours between two groups of events before the mainshock in Tottori, evidencing a precursory anisotropy change. The later foreshocks are also closer to the mainshock, indicating that the dilatancy process is strictly limited in time and space. Our results suggest that it is difficult to observe the change of anisotropy before large earthquakes due to the lack of ideal foreshocks or close stations. Precursory anisotropy might be observed when there are favorable foreshock datasets. More SWS measurements from foreshocks need to be conducted to corroborate the findings in this study.