But as the depth of the subduction gets deeper, the property of the rocks near the plate boudary change and the strain energy which is released when the great earthquake occurs can be accumulated in this area. It is difficult, however, to locate the upper limit of the seismogenic zone from the land-based observations. We have carried out the seismological observations for the detection of the local seismicity using ocean bottom seismometers in this area several times in the past to locate the upper limit of the seismogenic zone. The main objective theme of this 3-D survey is to study how the physical properties of rocks at the boundary of plates change when they subduct from the trough axis to the seismogenic zone. Surveys in the Nankai Trough Rectangle painted in red is the 3-D box of this survey.
Thin lines drawn in green is previous survey lines.
Orange circles denote the location of the ocean bottom seismometers. The contents of survey The Maurice Ewing, the vessel used in this survey, is outfitted with equipment of high quality for the reflection seismic survey. The streamer is 6 km long and the receiver has 240 channels. 7 air guns were towed from each side of the stern, or 14 air guns in total with a 70 litters capacity, and these guns were fired at every 50 meters. Also a gun depth sensor was attached to the air gun to record its depth at the time of the firing.