Geological structures and fault distributions in the Sagami Trough offshore Boso Peninsula

Around the Boso Peninsula, central Japan, the Philippine Sea Plate (PHS) is subducting beneath the Honshu Island along the Sagami Trough and the Pacific Plate is subducting beneath the PHS plate and the Honshu Island along the Japan Trench. The area offshore Boso Peninsula has very complicated geological histories by the influence of highly oblique convergence of the PHS plate and collision of the Izu-Bonin Arc since 15Ma. The geologic body of this region is composed of the accretionary prism, some part of which is exposed in the Miura and the southern Boso Peninsulas. The geologic body of the offshore Boso Peninsula is also considered as the accretionary prism. Moreover, this area is accompanied with the seismogenic zone in which the large-scale earthquakes such as the 1703 Genroku and 1923 Taisho Kanto earthquakes repeatedly occurred. Additionally, the tsunami and crustal movements also occurred together with earthquakes in this area. Although tsunami generation suggested seafloor crustal movements, the distributions of active faults as well as deep structure near the plate boundary have not been yet well understood by previous studies (ex. Nakamura et al., 1987, EPSL; Kimura et al., 2009, Tectonophysics). The objective of this study is to elucidate the submarine structure and distribution of active faults offshore Boso Peninsula in the Sagami Trough using various kinds of data sets as the bathymetric map, IZANAGI side-scan imagery, and Multi-channel seismic (MCS) reflection profiles. These data sets were acquired by JAMSTEC, Japan Coast Guard and Ocean Research Institute (now Atmosphere and Ocean Research Institute), Univ. Tokyo, respectively. In the area offshore Boso Peninsula, geomorhological lineaments were recognized in high accuracy bathymetric map and side-scan sonar imagery. These lineaments are interpreted to be continuous fault scarp morphologies. Along the Katsuura canyon, an approximately 125 km-length lineament was identified. This lineament is continuously traced from the Kamogawa Graben in the Boso Peninsula. Around the Katsuura Basin, an increase in the width of the canyon to the westward was observed across the lineament. In addition, changes in water depth were also found in the same location. Therefore, the fault system of this region suggests dextral and reverse components of displacements. Seismic profiles of the area offshore Boso Peninsula provided very clear images of the upper boundary of PHS plate, trough filled sediments of the Sagami Trough, and the forearc area of the Honshu Island arc composed of the accretionary prism. Landward dipping faults were recognized in the accretionary prism between the Boso and Katsuura canyons. These faults are distributed in the NW-SE direction along the Sagami Trough and interpreted as splay faults branched from upper boundary of the PHS plate (Kimura et al., 2009, Tectonophysics). The positions of the splay faults coincide with the Boso and Katsuura Canyons suggesting that the activity of the splay fault contributes to the development of the both canyons.