Recurrence behavior of short‐term slow slip and correlated nonvolcanic tremor episodes in western Shikoku, southwest Japan

Abstract

From the National Research Institute for Earth Science and Disaster Prevention Hi‐net tiltmeter data, we determine time‐dependent source processes of seven repeating short‐term slow slip events (SSE) in the western Shikoku region, southwest Japan, over the period from 2002 to 2007. The tiltmeters record clear signals of SSEs characterized by a slow rise, a week‐long duration, and a magnitude on the order of 10−2−10−1μrad. We apply a time‐dependent slip inversion method to the tilt records for the seven SSEs. For each SSE, a slip area migrates in the strike direction of the subducting slab at a rate of ∼10 km d−1. Comparing the slow slip area with epicenters of accompanying nonvolcanic tremors in subday time resolution, we find that the two phenomena are located on a smaller area than previously reported and migrate as a group. A moment rate function for each SSE exhibits good correlation with the temporal change in tremor activity. A comparison of the slip distributions of all of the analyzed SSEs shows that there is a patch of slow slip area (∼30 km in diameter) that is shared by all of the episodes. In addition, the location of the slip patch is in good agreement with the epicenters of tremors and very low frequency earthquakes. These lines of evidence suggest that the identified slip patch is one of the main slow slip patches in the region and a recurrence of SSEs may be controlled by the patch and that these “slow earthquakes” might be different manifestations of a single slip process on the deep plate interface.