2019 Volume 2019 Issue 50 Pages 33-44
During the 2016 Kumamoto earthquake (Mw7.0), the Futagawa segment of the Futagawa fault zone and a part of Takano-Shirahata segment of the Hinagu fault zone ruptured and a 30-km-long surface rupture with right-lateral strike slip appeared. Although surface rupture distribution, co-seismic displacement and paleoseisimicity have been revealed after the 2016 event, the slip rate in the central part of the Futagawa segment is not estimated. In order to reveal vertical and horizontal slip rates, we conducted geomorphic analysis and field survey in and around Futa, Nishihara Village, Kumamoto prefecture. Co-seismic vertical displacement was estimated by elevation difference between pre- and post-earthquake digital elevation models (DEMs) created from LiDAR survey data and accumulated vertical and horizontal displacements of geomorphic features were measured by using those DEMs. Additionally, we investigated an outcrop to reveal the shallow fault structure and to estimate an age of geomorphic surface. From accumulated displacement and ages of geomorphic surface and Takayubaru lava, vertical and horizontal slip rates of the Futagawa fault were estimated to be 1 mm/yr or larger. Moreover, recurrence interval was estimated to be a thousand to two thousand years by using estimated slip rates and co-seismic displacements of the 2016 event, consistent with results of recent paleoseisimic surveys on the Futagawa segment. These results, slip rate and recurrence interval of the Futagawa fault, are higher than previous estimation and suggests that the Futagawa fault is one of the most active faults in Kyushu region.
