3-D Gravity Basement Structure Around Mashiki, Kumamoto, Japan
Shun Araki1,*, Tatsuya Noguchi2, Masao Komazawa3, Shoya Arimura2, Mitsuhiro Tamura2,*, Kei Nakayama2,*, Hitoshi Morikawa1,†, Takashi Miyamoto4, Kahori Iiyama1, Yoshiya Hata5, Masayuki Yoshimi6, Takao Kagawa2, and Hiroyuki Goto7
1Department of Civil and Environmental Engineering, Tokyo Institute of Technology
4259-G3-7, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
2Tottori University, Tottori, Japan
3Oyo Corporation, Tsukuba, Japan
4Yamanashi University, Yamanashi, Japan
5Osaka University, Osaka, Japan
6Geological Survey of Japan/AIST, Tsukuba, Japan
7Kyoto University, Kyoto, Japan
*Former student, current affiliation has been changed
Gravity survey has been carried out around central part of Mashiki, Kumamoto, Japan, where was severely damaged by 2016 Kumamoto earthquake. Dense observations were performed at more than 450 sites around the central part of Mashiki. The observation sites satisfy enough density to discuss density structure shallower than 500-meter depth around the target area. After applying some corrections to the observed data, Bouguer anomaly is obtained and three dimensional model of gravity basement is estimated. As a result, a graben runs parallel to the Akitsukawa River and some isolated small basins are found inside of the graben. The central part of Mashiki is located immediately above of the one of such the small basins. We also analyze focusing and defocusing effects of ray of seismic wave under very simple assumptions and it is found that the central part of Mashiki locates close to a focusing area.
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