[S-CG45] Science of slow-to-fast earthquakes

[SCG45-04] Long-term slow slip events with and without tremor activation in the Bungo Channel and Hyuganada, southwest Japan

*Hitoshi Hirose¹, Takeshi Matsushima², Takao Tabei³, Takuya NISHIMURA⁴ (1.Research Center for Urban Safety and Security, Kobe University, 2.Institute of Seismology and Volcanology, Kyushu University, 3.Faculty of Science and Technology, Kochi University, 4.Disaster Prevention Research Institute, Kyoto University)

Keywords:GNSS, aseismic slip, slow earthquakes, Nankai subduction zone, plate interface

Slow slip events (SSEs) lasting for approximately one year occur every 6-8 years around the Bungo Channel in the southwest Japan subduction zone. The slip time evolution of the latest Bungo Channel SSE that occurred in 2018-2019 has been studied; however, the detailed spatial and temporal relationship between the slip process and other nearby phenomena, such as tectonic tremors and SSEs, is not well understood. Moreover, the migration of such long-term SSEs from Hyuganada to Shikoku through the Bungo Channel has been suggested, but a slip process connecting the SSEs has not been observed.

In this study, we utilized 21 continuous global navigation satellite system (GNSS) stations around the Bungo Channel and Hyuganada that have been installed by us since 2014 in addition to GNSS Earth Observation System (GEONET) stations to improve the spatial resolution of such interplate slip. Based on these data, we estimate the spatial and temporal slip evolutions of the major SSE in 2018-2019, which was accompanied by tremor activity in the deep episodic tremor and slip (ETS) zone, and a smaller SSE in 2015-2016 without tremor activity.

We show that the slip area of the major SSE overlaps the ETS zone, whereas that of the smaller SSE does not. This strongly suggests that synchronized tremor activity with an SSE requires a slip close to or overlapping the ETS zone. We also show two distinct slip propagation paths from the Oita area during the 2018-2019 sequence: one is a southward propagation to the Miyazaki area, leading to an SSE around the Miyazaki Plain, and the other is an eastward propagation to an area close to Cape Ashizuri, where "invading slip" is proposed to propagate from the ETS zone to a shallower megathrust source area. These slip propagations may be two of fundamental slip modes that connect slow-slip patch-like areas around the Bungo Channel and Hyuganada.

Acknowledgments
The GEONET GNSS data were provided by Geospatial Authority of Japan. The tremor catalog was provided by National Research Institute for Earth Science and Disaster Resilience. This study was supported by JSPS KAKENHI Grant Numbers JP16H06474, JP21K03702, and JP21H05206, the research grant from the Tokio Marine Kagami Memorial Foundation, the MEXT Earthquake and Volcano Hazards Observation and Research Program, and ERI JURP 2020-A-03 in Earthquake Research Institute, the University of Tokyo.