Out-of-sequence thrust faults developed in the coseismic slip zone of the 1946 Nankai earthquake (Mw=8.2) off Shikoku, southwest Japan

A multi-channel seismic (MCS) reflection survey was conducted to study the subsurface structure of the coseismic slip zone of the 1946 Nankai earthqua ke (Mw=8.2) off Shikoku Island in 1997. We could identify a splay fault sys tem consisting of several sigmoid out-of-sequence thrust (OST) faults dippi ng landward with slope 10 similar to 25 degrees on the poststack depth migr ated MCS profiles. Most of the OSTs are apparently developed from the subdu cting oceanic basement to the seafloor in the forearc region, cutting both underthrust sediments and the overriding accretionary prism. In addition, m ost of the OSTs are within the coseismic zone of the 1946 Nankai earthquake and the interseismic locked zone. The OSTs are considered to be related to large interplate earthquakes including the 1946 Nankai earthquake. These O STs may be responsible for tsunami generation following deformation of fore arc accretionary wedge.