Detection of inland SSE based on GNSS baseline change around earthquake swarm area

Interplate slow slip events (SSEs) around the subduction zone are important phenomena to understand the mechanism of the large interplate earthquakes. On the other hand, SSEs in inland Japan have been also reported using geodetic data, although the number of cases are less. It is thought that these are caused by aseismic slips on the deeper extention of inland fault or on the shallow detachment of a geological structure border. According to previous studies, sevelal inland SSEs have been occurred around shallow seismic swarm area. Therefore, there might be some relationship between those seismic activity and inland SSE. In order to detect more such cases of inland SSEs, we analyze seismicity and baseline change of GEONET GNSS sites in Hokkaido, Japan, since October 1997 to December 2016. To pick up SSE candidate regions based on seismicity, we define the region with a threshold using the JMA unified earthquake catalogue. When the number of earthquakes lager than M 4.0 and located in shallow depth (less than 30 km) is more than three in one year, we regard the region as a candidate of SSE potential region. Then, we try to detect transients from GNSS baseline changes crossing those region. As a result, we found four SSE candidate regions from seismicity data. Among of them, two areas show the obvious GNSS baseline changes which might related to the candidate of SSE. One is in northern Hokkaido, which has been already detected by our previous study. The other one is detected around Hidaka region, southern Hokkaido, started on September 2011. In this area, there is a M5.1 earthquake at 5 km depth on September 7, 2011 without coseismic surface displacement. This moment corresponds to the beginning of the transient. Then, the gradual baseline change has been lasted for 4 months. There is no active fault around this region, but existence of an active structure and its aseismic activity has been pointed out from geological and topographic studies. Therefore, this seismic swarm after M5.1 main shock might activated it. We confirm that this method is effective to detect inland SSEs in Hokkaido region. We are going to find more cases in Japan in near future.