Detailed structural image around splay‐fault branching in the Nankai subduction seismogenic zone: Results from a high‐density ocean bottom seismic survey

To investigate megathrust earthquake and tsunami generation in a subduction seismogenic zone, it is important to know the detailed structure around the plate boundary and active splay‐fault system. The Nankai Trough, southwestern Japan, is among the best studied subduction zones with splay faults in the world. This paper presents a detailed structural image around the splay fault in the coseismic rupture zone of the 1944 Tonankai earthquake in the central Nankai Trough, based on results from a wide‐angle, high‐density ocean bottom seismograph (OBS) survey. Our seismic image clearly shows for the first time the subduction structure along with the splay‐fault reflections imaged on a previously obtained seismic reflection profile. The correspondence between the splay‐fault reflections imaged on the seismic reflection profile and our reflectivity image was confirmed by converting the time‐migrated section to a depth section by using a velocity model estimated from the wide‐angle OBS data in this study. The high‐resolution seismic image obtained by the high‐density OBS survey demonstrates a zone of low‐velocity gradient just above and along the splay fault. We focused specifically on the detailed structure around the splay fault. A simple amplitude analysis of the wide‐angle reflections showed that it is likely that a thin low‐velocity layer with velocities 0.5–1.5 km/s slower than those in the surrounding rock exists above the splay fault. The presence of this layer suggests elevated fluid pressure in the fault zone, which cut through the relatively homogeneous rock of the accretionary prism and generated past megathrust earthquakes.