[S-CG48] Ocean Floor Geoscience

[SCG48-P13] Active deformation structures of the Nankai forearc basin off Muroto-Ashizuri using seismic reflection data

*Konosuke TERUI^1,2, Juichiro Ashi^1,2 (1.Course of Natural Environmental Studies, Graduate School of Frontier Sciences, The University of Tokyo, 2.Atmosphere and Ocean Research Institute, The University of Tokyo)

Keywords:Nankai Trough, Strike Slip Fault, Sub-bottom profiler, Multi channel seismic reflection survey, lineament

In the Nankai Trough, the inter-plate type earthquakes occur repeatedly and affect the topography of the coastal areas significantly. For example, Cape Muroto was uplifted 1.2 m by the 1946 Nankai Earthquake and was estimated to have been uplifted at an average rate of 2 mm/y by periodic earthquakes (Yoshikawa et al, 1964, Geogr. Rev. Japan). Later studies, however, reported that the influence of uplift by inter-plate type earthquakes was small in the Holocene uplift of Cape Muroto, with a dominant intermittent uplift of 2-4 m every 1000-2000 years (Maemoku, 2001, Jour. Tokyo Geogr. Soc.). The proposed mechanisms for this intermittent uplift include splay faults from the plate boundary (Maemoku, 2006, Jour. Tokyo Geogr. Soc.) and/or asymmetric folds caused by north-south trending and westward dipping faults around Cape Muroto (Matsu'ura, 2015, Geomorphology). As for the large-scale structure of the sea area, an inverted L-shaped uplift leading from Cape Muroto to the outer ridge zone was formed by oblique plate subduction (Awata and Sugiyama, 1989, Jour. Seismol. Soc. Japan), and the distribution of submarine active faults has been reported based on bathymetry and seismic reflection data (Okamura, 1990, Jour. Geol. Soc. Japan).
This study aims to map faults with high precision and to assess their activity in the offshore area between Cape Ashizuri and Cape Muroto in the Nankai Trough. Three types of data were used: sub-bottom profiler (SBP) data, which explores the shallow structure of the seabed, multi-channel seismic reflection survey (MCS) data and seabed topography data. The SBP records were obtained by carrying the SBP onboard the towed remotely operated vehicle NSS during the KH-15-2 and KH-16-5 cruises. MCS data were provided by JAMSTEC. Seafloor topographic data was sourced from GEBCO 2023.
Two clear lineaments with good continuity are recognized in the seafloor topography. Lineament L1 extends approximately 50 km southwestwards from the Tosa-Bae (knoll south of Cape Muroto), with small hights approximately 20 m higher than the surrounding area and depressions area approximately 50 m lower than the surrounding area. Lineament L2 stretches from off Cape Ashizuri to the south of Tosa-Bae, curving northwards and continuing eastwards for about 300 km to offshore Cape Shionomisaki. Along the lineament, there are small hights about 20 m higher than the surrounding area and depressions up to about 800 m lower than the surrounding area. In addition, there are two elongated continuous depressions of less than 10 m between L1 and L2, which, together with L1 and L2, form an echelon arrangement.
In the MCS data, a negative-flower structure develops in the section intersecting L2 off Ashizuri. In the SBP data, a sedimentary layer dipping to the south is observed in the surface layer near L2 south of Tosa-Bae. Furthermore, sedimentary layers dipping to the north develop to the south of L2, both of which suggest repetitive tilting due to the feature of increasing dip with depth. Lineaments L1 and L2 both have good continuity, with convex and concave areas, and flower structures are observed in some areas. No distinct structure showing a sense of displacement is currently available, but it is presumed to be a fault with lateral displacement. In addition, since the SBP record shows repeated deformation through to the younger sedimentary layers at the surface, these lineaments can be interpreted as active faults, and they are presumed to have had a significant influence on the topographic development of the area around Cape Muroto.