15:30 〜 16:30
[J02-P-12] Tidal triggering of earthquakes after the 2011 Tohoku earthquake
I investigated correlations between tides and earthquakes off the Pacific coast of eastern Japan for about five years after the 2011 Tohoku earthquake (Mw 9.1). A previous study has shown a high correlation in the northern part of the focal area of the Tohoku earthquake, where the mainshock rupture initiated, in about ten years prior to the Tohoku earthquake (Tanaka, 2012). The data I used are the Global Centroid Moment Tensor (CMT) solutions of shallow earthquakes (H < 70 km) with Mw 5.0 or larger for the period from 1976 to 2015. For each event, I calculated tidal shear stresses on the fault plane (Tanaka et al., 2002), and assigned a tidal phase angle at the time of occurrence. Based on the distribution of tidal phase angles, I tested whether they concentrate near some particular angle or not by using the Schuster's test (Schuster, 1897). In this test, the result is evaluated by p-value, which represents the significance level to reject the null hypothesis that the earthquakes occur randomly irrespective of tidal phase angle. After the Tohoku earthquake, no significant correlation was found in the area where a high correlation was found before the Tohoku earthquake. The p-values in this area are larger than 10% in the period of about five years after the Tohoku earthquake. On the other hand, small p-values were observed on the northwest side of the large slip area of the Tohoku mainshock. A small p-value of 2.8% was obtained in the area near the coast, where large postseismic afterslip has been identified by geodetic measurements (Sun et al., 2014; Iinuma et al., 2016). In this region, no significant correlation was found for about 35 years prior to the Tohoku earthquake. The p-value was smallest (1.6%) just after the Tohoku earthquake, and gradually increased with time. This behavior seems to be correlated with time evolution of afterslip which shows rapid decay over time (Ozawa et al., 2012).