Relation between the coupling and tremor rates in the transition zone around the Shikoku region

We examine the spatiotemporal distributions of the interplate couplings and the tremor count rate in the Shikoku region, which is located in the western part of the Nankai subduction zone in Japan, from July 2007 to June 2015 by comparing the results of static geodetic inversion using onshore global navigation satellite system data and the published tremor catalog. We observe that the temporal fluctuations in these two rates correlated well at a few spot-like points in the tremor-occurrence zone at a deeper location next to the strongly coupled area. The well-correlated points are densely distributed around the long-term slow slip area in the Bungo Channel and are randomly dispersed in the central and eastern portions. The correlation coefficients of the well-correlated points differ from point to point; however, at some points, zero tremor counts can be expected when the coupling rates reach the plate convergence rate. This indicates that tremors occur on the plate interface and that the tremor count rates can be used as proxies for monitoring the small fluctuations in interplate coupling.