[J02-1] Recent large earthquakes I

[J02-1-05] Postseismic deformation of 2016 Kumamoto earthquake by the dense GNSS continuous observation

^○Shigeru Nakao¹, Takeshi Matsushima², Takao Tabei³, Makoto Okubo³, Tadashi Yamashina³, Takahiro Ohkura⁴, Takuya Nishimura⁴, Takuo Shibutani⁴, Masahiro Teraishi⁴, Takeo Ito⁵, Takeshi Sagiya⁵, Kenjiro Matsuhiro⁵, Teruyuki Kato⁶, Jun'ichi Fukuda⁶, Atsushi Watanabe⁶, Yusaku Ohta⁷, Satoshi Miura⁷, Tomotsugu Demachi⁷, Hiroaki Takahashi⁸, Mako Ohzono⁸, Teruhiro Yamaguchi⁸, Kazumi Okada¹ (1.Kagoshima University, Kagoshima, Japan, 2.Kyushu University, Fukuoka, Japan, 3.Kochi University, Kochi, Japan, 4.Kyoto University, kyoto, Japan, 5.Nagoya University, Nagoya, Japan, 6.The University of Tokyo, Tokyo, Japan, 7.Tohoku University, Sendai, Japan, 8.Hokkaido Universiy, Sapporo, Japan)

The 2016 Kumamoto earthquake (M7.1) occurred on April 16 following the M6.5 earthquake on April 14, 2016. The maximum intensity 7 was observed in Mashiki-machi two times. These earthquakes occurred in the two active faults zone, Hinagu fault zone and Futagawa fault zone. After the occurrence earthquake activity enlarged to east from the aftershock area of the 2016 Kumamoto earthquake. Earthquake activity become high in northern region of Aso volcano and central part of Ohita prefecture. Our group started settlement of the continuous GNSS observation site in aftershock area of main shock, around Aso volcano and central Ohita prefecture in order to observe postseismic deformation and relationship between seismic activity and crustal deformation.
Twenty-one GNSS sites have been set up by April 28. There are nine sites in aftershock area, four sites in east area to Aso volcano, four sites in the central Ohita prefecture and three sites in southern part of Hinagu fault zone. Bernese GNSS Software Ver. 5.2 is used for GNSS data analysis of our newly sites together with GEONET and JMA GNSS sites in volcanoes (Aso, Kujyu and Garan-dake) in Kyushu for the period from April 15. We used CODE precise ephemerides and CODE Earth rotation parameters. The coordinates of the GNSS sites are estimated with respect to ITRF2008.
North-east displacement at the sites of western side of Hinagu fault and south-west displacement at the site of eastern side of Hinagu fault are observed. Postseismic deformation continues in January, 2017. After slip is assumed the cause of observed postseismic deformation. Fault parameters of two faults (Futagawa fault and Hinagu fault) are estimated. Fault slip on Hinagu fault is larger than that Futagawa fault. Width of two faults is 40 km and dip is 89 and 57 degree. It seems that after slip fault extend to the mantle. Long time observation of postseismic deformation is needed to separate deformation caused by after slip and response of viscoelasticity.