ABSTRACT To accurately estimate the coseismic rupture area in a Nankai megathrust earthquake and predict seismic and tsunami hazards, various three-dimensional models of the subducting plate geometry and simple seismic velocity models of the subduction zone in SW Japan have been proposed. However, to ensure consistency among studies, more realistic and reliable standard models must be developed. Here, we use wide-angle ocean-bottom seismographic survey results to develop models of the three-dimensional geometry of the subducting plate and of the three-dimensional P-wave velocity structure around the Nankai Trough. We confirmed the reliability of the proposed models by comparing theoretical first arrivals, calculated from two-dimensional structure models sampled from the three-dimensional model along seismic profile lines, with observed traveltime data. The proposed models are the first to be visible and to attempt to represent the actual seismic velocity structures around the entire Nankai Trough in SW Japan. Although the spatial validity of the three-dimensional velocity structure model could not be strictly evaluated, we confirmed that the differences between hypocenter parameters determined from previously published seismic tomography results and those obtained by using our three-dimensional structure model were sufficiently small (latitude and longitude within ±0.1° and depth within approximately ±5 km). Therefore, our three-dimensional structure model is suitable for use as an initial model for hypocenter determination.

Abstract The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the Izu Peninsula to Guam Island (Fig. 2f.1) and provides an excellent example of an intra-oceanic convergent margin where the effects of crustal anatexis and assimilation are considered to be minimal (Stern et al. 2003; Tatsumi Stern 2006). The current IBM activity is caused by subduction of the Pacific Plate beneath the Philippine Sea Plate, which dips at 35° at the northern tip of this system and ia nearly vertical at Mariana. Evolution of the IBM Arc since 50 Ma has strongly in?uenced the present architecture of the Japanese archipelago. One important event during this arc system’s evolution is back-arc rifting from 15-25 Ma that separated the IBM Arc system from the remnant Kyushu-Palau Ridge by c. 500 km and created the young oceanic lithosphere of the Shikoku Basin that is now being subducted beneath the SW Japan Arc. It should be further stressed that recent geophysical and geological survey results suggest this intra-oceanic arc is an active site of both creation and growth of the continental crust. This chapter will highlight the tectonic and structural evolution of the IBM arc system.