3:45 PM - 4:00 PM
[SSS02-19] Plate geometry, splay fault and tsunamigenic earthquake in the southernmost Ryukyu trench
In order to improve our understanding seismic potentials and structure controlling the seismogenic process in the Ryukyu subduction zone, we started a new 8-year project that consists of four two-dimensional active-source seismic experiments and extensive passive-source seismic observations covering the entire Ryukyu arc. In 2013, active-source seismic data were collected in the southernmost Ryukyu trench that crosses the potential source region of the 1771 Yaeyama earthquake (Nakamura, 2009). For refraction/wide-angle reflection analyses, seismic wave from air-gun shots were recorded at a total of 60 ocean bottom seismographs with approximately 6-km spacing on a ~390-km-long profile. On the same line, multichannel seismic (MCS) reflection profiling using the ~6-km-long, 444-channel streamer cable was also carried out.
Using this data set, we succeeded in imaging the plate boundary down to ~30 km depth. The dip angle of the slab increases from ~5 degree closer to the seafloor to ~20 degree at greater depths. In the fore-arc region, we found a fault branches from the plate boundary to the seafloor and they form a low-velocity accretionary wedge in between. This splay fault and accretionary wedge almost overlap the source region of the 1771 Yaeyama earthquake proposed by Nakamura (2009) and thus may have played a role in tsunami generation. The slab contacts with the overriding wedge mantle at depths greater than ~25 km. This region shows greater reflectivity at the plate boundary and is also coincident with repeating slow-slip events (Heki and Kataoka, 2008). These results probably suggest a changing frictional property along the plate interface with increasing depths.