Slip Distributions of Short‐Term Slow Slip Events in Shikoku, Southwest Japan, From 2001 to 2019 Based on Tilt Change Measurements

In the southwest Japan subduction zone, short‐term slow slip events (SSEs) accompanying nonvolcanic tremor (ETS: episodic tremor and slip) occur repeatedly with recurrence intervals of several months. It is important to know the detailed slip areas of the SSEs because short‐term SSEs occur at the downdip extension of a megathrust earthquake rupture zone, and hence, the SSEs play a key role in the stress buildup processes of the megathrust earthquakes. Although most of the previous studies model a SSE fault motion with uniform slip on a rectangular fault, this may lead to a bias in estimating the slip area. In this study we estimate a spatial slip distribution for each of the short‐term SSEs from tilt offset measurements. In total, the slip distributions of 61 SSEs in Shikoku from January 2001 to March 2019 are obtained. The ETS zone in Shikoku is divided into three segments in terms of slip history of the SSEs. Before 2011, the slip areas of most of the SSEs are limited in any one of the three segments. After 2012, however, the number of SSEs having a slip area that extends to multiple segments increases. The slip histories at each position in the ETS zone suggest that the short‐term SSE activity is affected by the occurrence of nearby long‐term SSEs. The average slip rate over 18 years suggests that an estimated slip deficit in the ETS zone in western Shikoku could be released by long‐term SSEs, or coseismic slip or afterslip of the megathrust earthquakes.