岩盤工学（Rock Engineering）

[3305-12-02] Prevention of Large Earthquakes beneath Big Cities

○Yoshiaki Fujii¹, Jun-ichi Kodama¹, Daisuke Fukuda¹ (1. Hokkaido University)

司会：船津貴弘（九州大学）
Chairman: Takahiro Funatsu (Kyushu university)

キーワード：Large earthquakes beneath big cities, Prevention, Induced seismicity, Silica carbide composite, Water injection

Twenty two M ≥ 6 earthquakes occurred beneath big cities between 2000 and 2016, for example, in Japan excluding aftershocks. They induced severe human and property damages. By Kumamoto 2016 (M6.5), for example, 110 people died, 184,643 buildings including the Kumamoto Castle collapsed and the economic loss was 2.4-4.6 trillion JPY. Gradual release of seismic energy by injecting water through a drill hole to the seismic fault is proposed. An M6 is occurring at the average interval of 100 years in Kumamoto for example. One thousand M4 earthquakes (an M4 causes almost no damage in Kumamoto) in 100 years will release the seismic energy for an M6. Namely, 10 M4 for one year or ca. an M4 for a month will prevent an M6 earthquake. The necessary water amount is 9.4 x 10⁵ m³ for an injection based on a study on mostly cases of enhanced geothermal system (EGS). The scheduled injections would be carried out very carefully under a dense microseismic and seismic monitoring to ensure safety. Focal depths of typical large earthquakes under big cities are deeper than the deepest records of drilling. The problems of deep drilling are high temperature and corrosion and would be solved by the silica carbide composite and electro pulse drilling. The total cost of ca. 56 billion JPY is much cheaper than the economic damage. The risk of unexpected induction of an M6 can be minimized by gradual injection under careful monitoring. The injection can be cancelled when the released energy exceeds an M4.