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.

1. [1] M. Yamada, J. Ohmura, and H. Goto, “Wooden building damage analysis in Mashiki town for the 2016 Kumamoto earthquakes on April 14 and 16,” Earthquake Spectra, 2017 (in press).
2. [2] Y. Bai, B. Adriano, E. Mas, and S. Koshimura, “Machine learning based building damage mapping from the ALOS-2/PALSAR-2 SAR imagery: Case study of 2016 Kumamoto earthquake,” Journal of Disaster Research, Vol.12, No.sp, pp. 626–655, 2017.
3. [3] Y. Hata, F. Minato, M. Yamauchi, Y. Okawa, Y. Ueda, S. Atoji, H. Goto, M. Yoshimi, A. Furukawa, H. Morikawa, T. Ikeda, and T. Kagawa, “Evaluation of local site effects with very high density in central Mashiki Town, Kumamoto Prefecture, Japan, Based on temporary earthquake observations and microtremor measurements, and its applications,” Proceedings of the International Workshop on the 2016 Kumamoto Earthquake, Fukuoka, March 6, 2017.
4. [4] Y. Hata, A. Furukawa, H. Goto, M. Yoshimi, H. Morikawa, T. Ikeda, and T. Kagawa, “Investigation of tombstone condition at graveyard sites in central Mashiki Town, Kumamoto Prefecture, Japan, and its applications,” Proceedings of the International Workshop on the 2016 Kumamoto Earthquake, Fukuoka, March 6, 2017.
5. [5] J. Akamatsu, K. Nishimura, H. Morikawa, S. Sawada, K. Onoue, H. Saito, M. Jido, T. Kagawa, K. Kamura, K. Sato, K. Furuno, and M. Komazawa, “Bedrock Structure around Faults and Its Relation to Earthquake Disaster,” Earthquake Proof Design and Active Faults, (Edited by Y. Kanaori), Elsevier Science, pp. 199–216, 1997.
6. [6] M. Komazawa, H. Morikawa, K. Nakamura, J. Akamatsu, K. Nishimura, S. Sawada, A. Erken, and A. Onalp, “Bedrock Structure in Adapazari, Turkey – A Possible Cause of Severe Damage by the 1999 Kociaeli Earthquake –,” Soil Dynamics and Earthquake Engineering, Vol.22, Nos.9–12, pp. 829–836, 2002.
7. [7] H. Goto, C. Takahashi, Y. Ishii, S.-Q. Ling, K. Miyakoshi, H. Morikawa, Y. Sato, S. Sawada, Y. Shingaki, Y. Suzuki, D. Takabatake, M. Joshima, “Deep subsurface structure estimateed by microtremors array observations and gravity surveys in Kashiwazaki area, Japan,” Soils and Foundations, Vol.49, No.4, pp. 651–659, 2009.
8. [8] Web page on GEONET, http://terras.gsi.go.jp/geo_info/geonet_top.html(inJapanese) [accessed July 27, 2017]
9. [9] M. Komazawa, “Gravimetric analysis of Aso Volcano and its interpretation,” Journal of Geodetic Society of Japan, Vol.41, pp. 17–45, 1995.
10. [10] Geological Survey of Japan, “Gravity CD-ROM of Japan, Ver.2,” National Institute of Advanced Industrial Science and Technology (AIST), 2004.
11. [11] V. K. Gupta and N. Ramani, “Some aspect of regional-residual separation of gravity anomalies in a Precambrian terrain,” Geophysics, Vol.45, pp. 1412–26, 1980.
12. [12] Strong Motion Seismograph Networks (K-NET, KiK-net), http://www.kyoshin.bosai.go.jp/NationalResearchInstituteforEarthScienceandDisasterResilience(NIED) [accessed July. 28, 2017]
13. [13] K. Nisimura, J. Akamatsu, and M. Komazawa, “3D bedrock structure of Okayama plain, wset Japan, as inferred from gravity anomalies and its relation to damage distribution during the 1946, M8.0, Nankai earthquake.” Proceedings of 15^th World Conference on Earthquake Engineering, Paper #1334, 2012.
14. [14] A. Nozu, “Characterized source model for foreshock (M6.5) of 2016 Kumamoto earthquake,” http://www.pari.go.jp/bsh/jbn-kzo/jbn-bsi/taisin/sourcemodel/somodel_2016kumamoto_z.html,PortandAirportResearchInstitute(PARI)(inJapanese) [accessed July 28, 2017]
15. [15] A. Nozu, “Characterized source model for mains hock (M7.3) of 2016 Kumamoto earthquake,” http://www.pari.go.jp/bsh/jbn-kzo/jbn-bsi/taisin/sourcemodel/somodel_2016kumamoto.htmlPortandAirportResearchInstitute(PARI)(inJapanese) [accessed July 28, 2017]