9:15 AM - 9:30 AM
[SCG67-02] Heterogeneous distribution of pelagic input sediments in the Japan Trench and its impact on seismic slip propagation
All of coring sites are located in off-Sanriku area of the Japan Trench, north of the 2011 rupture area. Seven piston cores (PC01-07) were retrieved from seaward trench slope (PC05 and PC06), horst (PC03 and PC04), graben (PC01), and graben edge (PC02 and PC07) during the R/V Shinsei Maru KS-15-3 cruise. Sediment thickness estimated from seismic profiles are ~30-90 m at horst and seaward trench slope sites, and ~130-340 m at graben/graben edge sites, respectively. Visual core descriptions and successive density and magnetic susceptibility measurement by multi-sensor core logger (MSCL) on split core surfaces as well as X-ray CT imaging of whole-round cores have been performed at Kochi Core Center. Ages of tephra layers were estimated by comparing mineral assemblages and refractive indices of volcanic glasses to those of catalog values, and averaged sedimentation rates of each core were estimated.
Core lithologies are mainly diatomaceous clay/silty clay, with including tephra layers. Sedimentation rates of seaward trench slope, horst, graben, and graben edge are estimated to be ~20-40, ~5-20, ~45, ~1 cm/kyr, respectively. According to these sedimentation rates, sediments on seaward trench slope and horst sites have been deposited within the last 160-660 kyr. Our results suggest that entire pelagic sediments, including smectite-rich pelagic clay, have been removed by some reasons in the last 1 million years, where the thickness of incoming sediment is thin. The lack of smectite-rich pelagic clay may contribute to stop rupture propagation of 2011 Tohoku Earthquake at off-Sanriku Japan Trench. More understanding on sediment dynamics of deposition and erosion at trench outer rise is needed to link subduction input and megathrust earthquakes.