08:30 〜 09:00
[J02-1-01] Complex seismicity and hypocenter distribution of the 2016 Kumamoto earthquakes, Kyushu, Japan, and their relation to the stress field and crustal structure
invited
Large earthquakes of M6.5 and M7.3 occurred in April, 2016, in the Kumamoto prefecture, Kyushu, Japan. An urgent joint seismic observation by several universities and institutes in Japan was conducted in order to investigate the detailed feature of the seismic activity.
The hypocenter of the M6.5 earthquake of April 14 locates beneath the Hinagu fault zone. While 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 seismic activity was distributed along both the Futagawa and Hinagu fault zones, and the induced earthquakes were activated along the Beppu-Shimabara graben. However, detailed hypocenter distribution and the focal mechanism solutions indicate that the strike of the M6.5 fault is oblique to the trace of Hinagu fault, and that the initial rupture of the M7.3 occurred at off-fault of the Futagawa fault. In addition, the almost aftershocks did not occur at the fault plane on which the main rupture of M7.3 took place.
In this region, the high background seismicity had been observed, and M5.0 earthquake occurred in 2000. Inelastic strain distribution estimated by seismic moment tensor data implies that the source region corresponds to the relatively large strain area, which suggests the large inelastic strain created stress concentration. Although the N-S minimum principal stress is dominated in this region, the maximum and moderate principal stresses are spatially alternated. The complex fault system and seismicity could be attributed to such a uniaxial extension of deviatoric stress field.
Seismic tomography and magnetotelluric analysis show that the low velocity, moderate to high Vp/Vs ratio and conductive region exists below the large earthquakes. In addition to the stress concentration, the fluid supply from the lower crust probably trigger the large earthquakes and high-level seismicity.
The hypocenter of the M6.5 earthquake of April 14 locates beneath the Hinagu fault zone. While 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 seismic activity was distributed along both the Futagawa and Hinagu fault zones, and the induced earthquakes were activated along the Beppu-Shimabara graben. However, detailed hypocenter distribution and the focal mechanism solutions indicate that the strike of the M6.5 fault is oblique to the trace of Hinagu fault, and that the initial rupture of the M7.3 occurred at off-fault of the Futagawa fault. In addition, the almost aftershocks did not occur at the fault plane on which the main rupture of M7.3 took place.
In this region, the high background seismicity had been observed, and M5.0 earthquake occurred in 2000. Inelastic strain distribution estimated by seismic moment tensor data implies that the source region corresponds to the relatively large strain area, which suggests the large inelastic strain created stress concentration. Although the N-S minimum principal stress is dominated in this region, the maximum and moderate principal stresses are spatially alternated. The complex fault system and seismicity could be attributed to such a uniaxial extension of deviatoric stress field.
Seismic tomography and magnetotelluric analysis show that the low velocity, moderate to high Vp/Vs ratio and conductive region exists below the large earthquakes. In addition to the stress concentration, the fluid supply from the lower crust probably trigger the large earthquakes and high-level seismicity.