[M-TT37] New Developments in Earth Science Explored by Dense GNSS Observation Networks

[MTT37-03] Modeling the Crustal Deformation Field in Kyushu with post-seismic deformation of the 2016 Kumamoto earthquake using dense GNSS observation data

*Yushi Nagayama¹, Satoshi Matsumoto², Yuhei Yuasa³, Takeshi Matsushima², Kentaro Emoto² (1.Department of Earth and Planetary Sciences, Graduate School of Science, Kyushu University, 2.Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, 3.Japan Meteorological Agency)

Keywords:Lower crust, Anelastic strain rate, Kumamoto earthquake, Post-seismic deformation

In Kyushu Island, southwest Japan, some characteristic tectonic structures, such as the Beppu-Shimabara graben and the sear zones in the southern part of Kyushu, might contribute to seismic activity in the crust. The deformation rate field in Kyushu is also complex. The deformation field is influenced not only by these tectonic structures but also by large earthquakes occurring within the crust. Over the past several decades, several large earthquakes (Mj>6) have occurred in Kyushu. In particular, the 2016 Kumamoto earthquake sequence accompanied with the largest foreshock Mj6.5, mainshock Mj7.3 caused a large rupture along the Hinagu and Futagawa fault zones, resulting in the large co-seismic crustal deformation and stress change observed. Additionally, remarkable post-seismic deformation continues after the major earthquakes. According to previous studies, the post-seismic deformation is mainly attributed to after-slip at the extended part of the earthquake fault zone and viscoelastic relaxation in the upper mantle and lower crust (e.g., Pollitz et al., 2017; Moore et al., 2017; Liu et al., 2024). On the other hand, Yuasa and Matsumoto (2023) investigated the deformation field of the Kyushu crust during a relatively steady-state period prior to the Kumamoto earthquake occurrence. They modeled the crustal deformation by four components: (1) deformation of the lower crust, (2) interplate coupling, (3) volcanic activity, and (4) rigid body motion of the crust. They estimated the distribution of anelastic strain rates in the lower crust and slip deficit at the plate boundary.
As mentioned above, modelings of the post-seismic deformation following the Kumamoto earthquake and the steady-state deformation field of the Kyushu crust have been conducted in the previous studies. However, there are no studies that comprehensively model both of these aspects. Therefore, in this study, we model the deformation rate field in Kyushu Island including the response of the 2016 Kumamoto earthquake based on the dense GNSS observation data.
We used GNSS data from GEONET installed by the Geospatial Information Authority of Japan (GSI), and the GNSS station of SoftBank Corp. After removing seasonal variations and seismic or artificial offset from these data, displacement velocity vectors were calculated for each GNSS station. The deformation rate field of the target area during the post-Kumamoto earthquake period was estimated using model parameters adopted in Yuasa and Matsumoto (2023) from the displacement velocity vector data.
The results show that the area with larger anelastic strain rates in the lower crust exist around the focal area of the Kumamoto earthquake compared with before the earthquake. In addition, large residuals between observed and expected data around the focal area, suggest the presence of post-seismic deformation that may be modeled by other specific models. Based on these results, we can model the deformation field in Kyushu Island including the post-seismic deformation of the 2016 Kumamoto earthquake, such as after-slip and viscoelastic relaxation.

Acknowledgments
The SoftBank's GNSS observation data used in this study was provided by SoftBank Corp. and ALES Corp. through the framework of the "Consortium to utilize the SoftBank original reference sites for Earth and Space Science".