Seismic structure of the off Fukushima, southern Japan Trench region

The Japan Trench (JT) is one of the most active seismogenic zones in the world. Large destructive interplate earthquakes, such as the 2011 Tohoku earthquake, have occurred repeatedly around the JT. Because they occurred along the boundary where the Pacific plate is subducting beneath the North American plate, it is very important to understand the geometry of the plate boundary to elucidate the mechanisms of these interplate earthquakes. Therefore, many seismic experiments using active sources were conducted to study the seismic structure and the plate boundary geometry. However, the detailed deep structure of the off Fukushima region, which is in southern part of the JT, is poorly known because few seismic refraction experiments have been conducted. We therefore conducted a seismic refraction and reflection experiment in the off Fukushima region in order to obtain a crustal structure and plate boundary geometry.

From December 2012 to January 2013, a seismic refraction and reflection experiment was conducted in the off Fukushima region using the R/V Kairei of the Japan Marine Science and Technology Center. An airgun array of 7800 cubic inches total capacity was shot along a seismic line. The line runs perpendicular to the trench axis and its length is approximately 180 km. Thirty-one ocean bottom seismometers were deployed on the line. R/V Kairei also towed a 444-channel hydrophone streamer for the multichannel reflection study (MCS).

First-arrival tomography method (Fujie et al., 2006) was used to obtain the seismic structure model. The initial model was constructed by the result of MCS data analysis and result of previous study (Miura et al., 2003). From the obtained preliminary structure model, the top of the upper island-arc crust exists in shallow part beneath the seafloor. Similar structure was also seen in the previous study (Miura et al., 2003). We will continue to further analysis and obtain a deeper part velocity structure.

This study was supported by "Geophysical and geological studies of earthquakes and tsunamis for off-Tohoku district, Japan" conducted by the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.