Kazuko Usami

Spatial variability in sediment lithology and sedimentary processes along the Japan Trench: use of deep-sea turbidite records to reconstruct past large earthquakes

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.

High-resolution chronology of sediment below CCD based on Holocene paleomagnetic secular variations in the Tohoku-oki earthquake rupture zone

Using high-resolution paleomagnetic data, we examined the potential for obtaining precise ages from sediment core samples recovered from deep-sea basins close to rupture zones of the 2011 and earlier earthquakes off Tohoku, Japan. Obtaining detailed stratigraphic ages from deep-sea sediments below the calcium compensation depth (CCD) is difficult, but we found that the samples contain excellent paleomagnetic secular variation records to constrain age models. Variations in paleomagnetic directions obtained from the sediments reveal systematic changes in the cores. A stacked paleomagnetic profile closely matches the Lake Biwa data sets in southwest Japan for the past 7,000 years, one can establish age models based on secular variations of the geomagnetic field on sediments recovered uniquely below the CCD. Comparison of paleomagnetic directions near a tephra and a paleomagnetic direction of contemporaneous pyroclastic flow deposits acquired by different magnetization processes shows precise depositional ages reflecting the magnetization delay of the marine sediment record.

Morpho-sedimentary features and sediment dispersal systems of the southwest end of the Ryukyu Trench: a source-to-sink approach

The southwestern Ryukyu Trench near Taiwan is an ideal place for source-to-sink studies because of the short sediment transport route between the terrestrial sediment source in Taiwan and the marine sink in the Ryukyu Trench. Bathymetric and seismic reflection data and core samples from the area around the southwestern Ryukyu Trench were used to identify features of the trench–arc system, including submarine canyons, the trench wedge, bathymetric ridges, and forearc basins, which together form two distinct sediment dispersal systems: a longitudinal (trench-parallel) system and a transverse (trench-normal) system. The longitudinal sediment dispersal system carries sediments eroded from the Taiwan orogenic belt eastward, primarily along the Hualien Canyon and a channel–terminal fan system at its mouth, and deposits them in the southwestern end of the Ryukyu Trench. The transverse sediment dispersal system carries sediments eroded from the Ryukyu Islands downslope and deposits them in the Hoping, Nanao, East Nanao, and Hateruma forearc basins, behind the barrier formed by the E–W-trending Yaeyama Ridge on the trench-slope break. The presence of pyrrhotite, a characteristic component of sediments sourced from Taiwan, in a seafloor sample from the Ryukyu Trench and its absence in a sample from the East Nanao forearc basin support the view that the southwestern Ryukyu Trench is longitudinally fed by sediment derived from Taiwan, whereas the trench-slope forearc basins receive sediment transported transversely downslope from the Ryukyu Islands.