[M-IS34] The 2016 Kumamoto Earthquake and related crustal activities

[MIS34-02] Urgent joint seismic observation of the 2016 Kumamoto earthquake - Seismic activities and their background -

*Hiroshi Shimizu¹, Yoshihisa Iio⁸, Shin'ichi Sakai⁶, Tomomi Okada⁵, Hiroaki Takahashi³, Toshiki Watanabe⁷, Kazuhiko Goto², Takahiro Ohkura⁹, Youichi Asano¹⁰, Takeshi Matsushima¹, Yusuke Yamashita⁸, Manami Nakamoto¹, Masahiro Miyazaki⁸, Satoshi Matsumoto¹, Koki Aizawa¹, Azusa Shito¹, Kazunari Uchida¹, Rintaro Miyamachi¹, Megumi Kamizono¹, Yoshiko Teguri¹, Hiroki Miyamachi², Shigeru Nakao², Hiroshi Yakiwara², Shuichiro Hirano², Mako Ohzono³, Takahiro Shiina³, Takada Masamitsu³, Masayoshi Ichiyanagi³, Teruhiro Yamaguchi³, Okada Kazumi³, Masahiro Kosuga⁴, Ryosuke Azuma⁵, Naoki Uchida⁵, Kentaro Emoto⁵, Yusaku Ohta⁵, Toshiki Kaida⁵, Tomofumi Kozono⁵, Syuichi Suzuki⁵, Ryota Takagi⁵, Tomotsugu Demachi⁵, Hisashi Nakahara⁵, Takashi NAKAYAMA⁵, Satoshi Hirahara⁵, Toru Matsuzawa⁵, Satoshi Miura⁵, Mare Yamamoto⁵, Takaya Iwasaki⁶, Naoshi Hirata⁶, Takashi Iidaka⁶, Eiji Kurashimo⁶ (1.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 2.Graduate School of Science and Engineering,Kagoshima University, 3.Graduate School of Science, Hokkaido University, 4.Graduate School of Science and Technology, Hirosaki University, 5.Graduate School of Science, Tohoku University, 6.Earthquake Research Institute, The University of Tokyo, 7.Graduate School of Environmental Studies, Nagoya University, 8.Disaster Prevention Research Institute, Kyoto University, 9.Graduate School of Science, Kyoto University, 10.National Research Institute for Earth Science and Disaster Prevention)

Aitaro Kato⁶, Shigeki Nakagawa⁶, Yoshiko Yamanaka⁷, Takeo Ito⁷, Toshiko Terakawa⁷, Yuta Maeda⁷, Shinichiro Horikawa⁷, Kenjiro Matsuhiro⁷, Takashi Okuda⁷, Hiroshi Katao⁸, Yasuyuki Kano⁸, Tsutomu Miura⁸, Hiroo Tsuda⁸, Tomoya Muramoto⁸, Makoto Okubo¹¹, Tadashi Yamashina¹¹, Tomotake Ueno¹⁰
(11.Research and Education Faculty, Natural Sciences Cluster, Sciences Unit, Kochi University)

Keywords:the 2016 Kumamoto Earthquake, urgent joint seismic observation, hypocenter distribution, seismic activity, Futagawa-Hinagu fault

Large earthquakes of M6.5 and M7.3 occurred in April, 2016, in the Kumamoto prefecture, Kyushu, Japan. We are carrying out an urgent joint seismic observation by several universities and institutes in Japan in order to investigate the detailed feature of seismic activity of the Kumamoto erathquakes. In this observation, we installed more than sixty temporary seismic stations including eleven online telemetered stations in the inland area of middle Kyushu, which enable us to determine the space-time distribution of hypocenters and focal mechanism solutions.
The hypocenter of the M6.5 earthquake of April 14 locates beneath the northeastern end of the Hinagu fault zone. The aftershocks occurring before the M7.3 of April 16 were mainly aligned along an approximately 20 km long NE-SW trend, which roughly corresponds with the trace of the Futagawa-Hinagu fault zone. The hypocenters of the aftershock region were distributed on a nearly vertical plane at depths of 5 - 15 km, deeper at the central part and shallower at both NE and SW sides. The M6.5 was located near the central part of the aftershock region at a depth of approximately 13 km. The large aftershock of M6.4 occurred at the southwestern part of the aftershock region. The focal mechanism solution of the M6.5 is strike-slip fault type with N-S tension. These suggest the M6.5 earthquake was generated by a right-lateral strike slip of the nearly vertical Hinagu fault. However, both detailed hypocenter distribution and a nodal plane of the focal mechanism solution indicate the strike of the M6.5 fault is oblique to the trace of Hinagu fault.
The hypocenter of the M7.3 earthquake of April 16 locates about 5km WNW of the M6.5, and beneath the Futagawa fault zone. The aftershocks were roughly along the Futagawa-Hinagu fault zone, and induced earthquakes were activated along the Beppu-Shimabara graben. The hypocenters of the aftershock region were distributed at depths of 3 - 17 km dipping NW direction. The hypocenters at both NE and SW sides of aftershock region are shallower, however, the NE and SW extention of aftershock region become relatively deeper again. The focal mechanism solution of the M7.3 is strike-slip fault type with NW-SE tension, and its nodal planes are not consistent with the trace of both Futagawa and Hinagu fault. These suggest the initial rupture of M7.3 earthquake occurred on the different plane from the main rupture. In the period between M6.5 and M7.3, the migration of seismicity was recognized from the hypocenter of M6.5 to that of M7.3, which may be related with a trigger mechanism of M7.3 earthquake.
The Futagawa-Hinagu fault zone was a seismically active region since the seismic network was established in this area. In June 2000, the M4.8 earthquake occurred at almost the same place of M6.5 of April 14, and the focal mechanism was very similar to that of M6.5. These suggest that the stress level on the fault in this area has been high until the outbreak of the 2016 Kumamoto earthquake.
Acknowledgements:
This work is partly supported by MEXT KAKENHI Grant Number 16H06298, the Ministry of Education, Culture, Sports, Science and Technology of Japan under its Earthquake and Volcano Hazards Observation and Research Program, and Earthquake Research Institute, The University of Tokyo under Joint Usage Program.