Abstract The Meiwa tsunami of AD 1771 is regarded as an extremely strong tsunami event causing devastating damage in Japan in historical times. Earlier studies explored the possibility that a submarine landslide enhanced the Meiwa tsunami waves. We collected detailed seafloor bathymetry data, sub-bottom structure data and surface sediments in a putative Meiwa tsunami source region to ascertain any signature related to a submarine landslide in the forearc region, which is located south of Ishigaki-jima. The forearc-region seafloor is characterized by its surface submarine landslide morphology. However, the investigated magnetic fabric of surface sediment revealed that there was no landslide mass deposit during historical times. The described landslide morphology in the basin is unrelated to the generation or enhancement of the AD 1771 Meiwa tsunami, although the disturbed relief in the topography of the study area indicates that the forearc region is susceptible to slope failure because of its tectonic setting.

Abstract The progressive development of microfabrics in sediments was investigated in an active subduction zone using cores recovered during the Integrated Ocean Drilling Program Expedition 362. A sequence of Paleocene–Holocene rock was recorded at Site U1480, which is located in the Sunda Trench, south of Sumatra. Hemipelagic clayey sediments from the drilled succession were recovered from a depth of 1327.23 m. Cardhouse fabrics were observed initially, consisting of clay flakes with edge-to-edge (E–E) or edge-to-face (E–F) contacts. These initial microfabrics developed into compacted bookhouse fabrics (which are random and consolidated), and consist of clay flakes with E–F or face-to-face (F–F) contacts as the domain. These microfabrics developed due to individual particle sliding. Bookhouse fabric developed into a shape-preferred orientation of clay minerals with F–F contacts as a shaly fabric due to domain sliding. Finally, clay flakes with F–F contacts deformed into an undulated shape due to further increased overburden pressure. These pervasive fabrics comprising clay flakes stacked with F–F contacts resisted shear deformation due to their strong cohesion. This strong cohesion in the undulated shaly fabric might be an important factor in the locking of shallow tsunami-generating slip in subduction zones.

Abstract Submarine landslides triggered by earthquakes can generate turbidity currents. Recently, several studies have reported that the remobilization of surface sediment triggered by earthquakes can also generate turbidity currents. Such sedimentary processes may be influenced by sediment characteristics, seafloor morphology and seismic motions. Here, we verify surface sediment remobilization using sedimentary records from the Nankai forearc region, SW Japan. We collected multi-core and piston core samples from a small confined basin, mainly composed of silty clay or very fine sand. Radiocaesium measurements of the multi-core show consistently high values in the upper 17 cm and low values below this depth. Rapid sediment deposition after 1950 is assumed, and the most likely cause is the 2004 off the Kii Peninsula earthquake. Based on calculations using bathymetric maps and palaeocurrent data, settlement of the upper 17 cm can be explained by redeposition of the surface ( c. 1 cm) slope sediment around the basin. Muddy turbidites are also identified in the piston core. The gap in radiocarbon age observed around 2.0 m bsf (metres below seafloor) implies similar sedimentary processes. Our study represents the first examination of surficial remobilization from sedimentary cores in the Nankai forearc region.

Abstract The National Gas Hydrate Program Expedition 02 was conducted in early 2015 using the Drilling Vessel Chikyu in the western part of the Bay of Bengal, India. During drilling off Vishakhapatnam, NE India, some bottom-simulating reflectors were penetrated, and numerous mass-transport deposits (MTDs) were identified. The recovered cores were composed of post-late Miocene muddy slope deposits containing the late Miocene–Pliocene hiatus that is widespread in that region. Based on detailed visual core descriptions and calcareous nannofossil biostratigraphy, two major MTD-rich intervals were identified: the Pleistocene interval above the hiatus, and the middle–late Miocene interval below it. Although the MTDs in both intervals are composed of variously coloured clay–silt blocks in an olive-black or olive-grey silty clay matrix (muddy MTDs), the Pleistocene MTDs consist of larger-sized blocks (mostly less than a few metres but with some >10 m) without clear shear fabrics, whereas the Miocene MTDs contain smaller blocks (<0.1 m) with asymmetrical shear fabrics. The muddy blocks are composed of older components (Pliocene–Cretaceous) compared with the depositional ages of the MTDs. The high abundance of MTDs above the hiatus and the depositional ages of the interbedded coherent layers indicate that large-scale MTDs occurred repeatedly during the Pleistocene. Such repeated MTDs contributed to maintaining the high sedimentation rate in this area and potentially provided stable pressure and temperature conditions for the formation of gas hydrates.

Abstract Large earthquakes and related tsunamis serve as triggering mechanisms that generate turbidity currents which form turbidites. The event deposits from the recent 2011 Tohoku-oki earthquake and tsunami are observed throughout a wide area along the Pacific coast of Tohoku, northern Japan, extending from the coast through the shelf and slope, to the trench floor. Spatio-temporal correlation of turbidites and other tsunamigenic deposits, such as those generated in the 2011 event, can be used to reconstruct the recurrence history of large earthquakes and tsunamis. Here we use sediment cores and sub-bottom profiles to analyse the depositional setting along the Japan Trench, and show that the environment is ideal for preserving turbidites. The subducting Pacific Plate creates graben or basins along the trench floor that accommodate the episodic deposition of fine-grained turbidites; and interseismic hemipelagic deposits that form with high sedimentation rates along the Japan Trench effectively cover earthquake-induced turbidites and preserve the deposits as a geological record of large earthquakes. Therefore, small deep-sea basins with high sedimentation rates, such as in and around the Japan Trench floor, are favourable environments for studies of turbidite palaeoseismology.

Abstract We conducted a palaeomagnetic study on the Cenozoic sedimentary sequences of the Nankai Trough, recovered by the Integrated Ocean Drilling Program Expedition 322 in SE Japan. Sedimentary sections of Late Miocene age from the two subduction input sites (sites C0011 and C0012) recorded a pattern of magnetic polarity reversals that correlates well with the known magnetic polarity time scale. The polarity of characteristic remanent magnetization could be identified throughout the majority of the recovered cores of the two sites, following removal of a low-stability drilling-induced remanence. Most of the observed magnetostratigraphy from the characteristic directions is in good agreement with that to be expected from the stratigraphic position of the sequence deduced from the biostratigraphic data. Palaeomagnetic data from both shipboard and shore-based studies indicate changes in the rate of sedimentation from 9.5 to 2.7 cm/kyr at about 11 Ma, suggesting that some fundamental palaeoenvironmental change in the Shikoku Basin and/or significant tectonic event may have occurred in Late Miocene.