EXTRACTING THE DEEP MARINE RECORD OF EARTHQUAKES FROM THE JAPAN TRENCH MARGIN SEDIMENTS

Shallow water coastal settings are dynamic with frequent discontinuities from sediment reworking. The focus of this study is deep marine sediment that preserves the most continuous record of deposits related to tectonic events, specifically, large earthquakes and tsunamis. They can devastate densely populated coasts along subduction margins, as the 2011 Mw9.0 Tohoku earthquake and tsunami did in Japan. High-resolution multibeam bathymetry, subbottom profiling and sediment cores were acquired during several expeditions from Japanese (Natsushima) and European (Sonne) research vessels. We studied the sedimentation record of the Japan Trench margin in water depths of 4000m-8000m. Textural (core descriptions, photos, grain size) and physical properties (magnetic susceptibility) of the upper 1m of 10 piston and trigger cores were interpreted based on previously obtained short-lived radioisotope ages and acoustic images.

Sand turbidites interpreted as earthquake event deposits were documented in two depositional environments: 1) In steep (6°) regions of the forearc slope between 4000-6000 m of water depth we found multiple indurated sand turbidites that lack excess Pb-210 radioisotopes. They are interpreted as earthquake deposits older than 1850 years. Earthquake induced slope failures are likely to account for the incomplete record there and for sedimentation in catchments downslope. 2) The sedimentary record in the southern, central and northern segments of the trench is distinct. The 2011 Tohoku earthquake signature was preserved near the area of maximum deformation and southern segment. In the central segment, six sand turbidites are older than 1960’s. In the northern part of the trench, four turbidites were documented in the upper 70 cm. The youngest can be linked with the Mw 7.7 1994 Sanriku earthquake.

These results indicate that event deposits in deep-water catchments such as the Japan Trench can record earthquakes and thus constrain location and size of large prehistoric ones. The upcoming International Ocean Discovery Program Expedition 386 will recover 40m long piston cores from the Japan Trench. They will provide a long record of event deposits for a better understanding of the time-space distribution of earthquake ruptures, possible interactions between them, and segmentation of the boundary.