Lessons for active fault assessment learned from the destructive 2016 Kumamoto earthquakes in Japan

The 2016 Kumamoto earthquake composed of magnitude-6.5 foreshock on April 14th and 7.3 main shock struck on 16th was generated by a well-known the Futagawa-Hinagu active fault causing severe damage by strong ground motions as severe as JMA intensity 7 (equivalent to MMI 10-11) twice in the central part of Kyushu, Japan. Long-term earthquake prediction for this region from the Japanese government is more or less consistent with what happened, several problems were left for discussion on 1) segmentation of faults; 2) mechanism of strong ground motion; and 3) unexpected surface fault ruptures along unmapped active fault traces. The Futagawa-Hinagu fault was assessed in 2002 and 2013 by Japanese Government. In the 2002 evaluation, the fault was identified as a 100km-long fault, and divided into three segments based on geometry of active fault traces. On the other hand, the 2013 evaluation separated this fault system into two independent faults namely Futagawa fault and Hinagu fault based on additional date such as gravity and sub-surface geology. However, tectonic geomorphology emphasizes continuity of tectonic landforms along the faults, and does not support the evaluation done in 2013. The main shock attacked wider area including the strongly shaken area of the foreshock, and prominent surface fault ruptures only appeared with the main shock, while minor fractures recognized mainly on metaled roads in foreshock area, were reactivated by the main shock. From these data, we consider that foreshock and main shock were generated by activities of the same fault, suggesting that the 2002 evaluation is appropriate. According to this evaluation, the foreshock should be regarded as "one size smaller earthquake than that expected from the fault", and we should have noticed the possible successive occurrence of larger earthquake after the foreshock. It is often stated that strong ground motion occurs in any location; however, this understanding may be misleading. Narrow severe damage zone along the fault in Mashiki downtown implies that strong seismic waves were generated in the shallow portion, probably near surface, of the crust. Therefore conventional anti-earthquake measures in Japan are inadequate for JMA seismic intensity 7, and it is necessary to specify regions in which the seismic intensity could reach such level.