Small‐scale spatial variation in near‐surface turbidites around the JFAST site near the Japan Trench

Abstract

AbstractThis paper aims to improve our understanding of the depositional processes associated with turbidites related to recent earthquake events. A series of short sediment cores (ca. 20–30 cm long) were recovered from the landward slope of the Japan Trench around JFAST (Japan Trench Fast Drilling Project) site C0019 by a remotely operated vehicle, KAIKO 7000 II, and the sample sites were accurately located using an LBL (long base line) acoustic navigation system. The properties of the cores were analyzed using visual observations, soft X‐ray radiographs, smear slides, measurement of anisotropy of magnetic susceptibility, and analysis of radioactive elements (134Cs, 137Cs, and excess 210Pb). For the first time, small‐scale (ca. 200–1000 m) spatial variations in recent earthquake‐triggered deep‐sea turbidites, the formation of which was probably linked to the 2011 Tohoku‐oki earthquake, are described. We also examine the submarine landslide that probably generated the sediment unit below the turbidites, which is thought to be an important process in the study area. The spatial distribution and characteristics of the near‐surface seismoturbidite obtained immediately after the earthquake, presented here, will enable precise calibration of offshore evidence of recent earthquakes, and thus facilitate the use of the sedimentary archive for paleoseismic interpretations. Furthermore, although sampling for turbidite seismology on steep slopes has not been widely performed previously, our results suggest that the recent event deposits may be continuously tracked from the slope to the basin using a combination of the present sampling method and conventional large‐scale investigation techniques.