09:20 〜 09:35
[MIS34-02] 2016年熊本地震の緊急合同地震観測 - 地震活動の特徴と背景 -
加藤 愛太郎6、中川 茂樹6、山中 佳子7、伊藤 武男7、寺川 寿子7、前田 裕太7、堀川 信一郎7、松廣 健二郎7、奥田 隆7、片尾 浩8、加納 靖之8、三浦 勉8、津田 寛大8、村本 智也8、大久保 慎人11、山品 匡史11、上野 友岳10
(11.高知大学教育研究部自然科学系理学部門)
キーワード：2016年熊本地震、緊急合同地震観測、震源分布、地震活動、布田川−日奈久断層帯
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.
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.