9:20 AM - 9:35 AM
[MIS34-02] Urgent joint seismic observation of the 2016 Kumamoto earthquake - Seismic activities and their background -
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
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.
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.