Delayed Triggering Process for the Mj6.4 Eastern Shizuoka Earthquake of 15 March 2011

We investigated the triggering process for the Mj6.4 Eastern Shizuoka earthquake of 15 March 2011, which occurred 4 days after the 2011 Mw9.0 Tohoku-Oki earthquake and about 4 minutes after the Mj6.2 Fukushima-Oki earthquake. The static Coulomb failure stress change on the fault of the Eastern Shizuoka earthquake from the Tohoku-Oki earthquake, was about 20kPa, and the largest dynamic stress change by the passing surface waves was about 200kPa. The largest dynamic stress change from the Fukushima-Oki earthquake and the largest tidal stress change after the Tohoku-Oki and before the Eastern Shizuoka earthquake were about 0.3kPa and 1.4kPa, respectively, while those at the onset of the Eastern Shizuoka earthquake were at most 0.01kPa and − 0.5kPa, respectively. We also analyzed seismicity by detecting earthquakes immediately preceding the Eastern Shizuoka earthquake, which is done using a matched filter technique. A single M1.0 event that occurred about 17 hours before the Eastern Shizuoka earthquake and located about 3km NNE from the hypocenter was found, however, this event may not be classified as a foreshock if we consider the background seismicity in this region before 2011. We propose that the seismic cycle was possibly advanced by about 10 1 to 10 2 years and delayed triggering might have occurred for the Eastern Shizuoka earthquake. The eventual earthquake was ready to occur to some extent, when the Tohoku-Oki earthquake occurred. The fault strength had significantly decreased due to imposed large dynamic and static stress changes from the Tohoku-Oki earthquake and probably its large aftershocks, in terms of a rate and state dependent friction law.