2017 Volume 2017 Issue 46 Pages 27-32
The 16 April 2016 Mw＝7.0 Kumamoto earthquake accompanied ～ 31-km-long surface rupture along the NE part of the Hinagu fault and the Futagawa fault (Kumahara et al, 2016). The surface rupture zone along the Futagawa fault mostly exposed right-lateral strike slip up to 2.2 m (Shirahama et al., 2016), whereas a ～ 10-km-long normal faulting surface rupture with a maximum of 2-m vertical separation mostly along the previously mapped Idenokuchi fault located 1-2 km south of and sub-paralleled to the Futagawa fault. Here we report an outcrop at the oblique-normal faulting surface rupture at the riverbed of the Kanayama River which runs through Shimojin, Mashiki Town. The site is located 300 m south east of the Futagawa fault and a 50 cm vertical slip occurred at the 2016 Kumamoto earthquake. Although we only had a brief time to observe the outcrop due to levee wall construction, we observed a normal fault (f1) responsible for the 16 April earthquake and cuts recent gravel units. Along the f1 strand, we measured 1.5 m of cumulative vertical displacement of the gravel unit including the 2016 slip. A detrital charcoal recovered from the gravel unit yields an age of 1,990-2,300 cal BP. It enables us to estimate a vertical slip rate of 0.65～0.75mm/yr. Thus it is highly likely that the coseismic simultaneous rupture of the both fault might have occurred repeatedly during the recent events.
