Sedimentation Triggered by the 2011 Tohoku Megathrust Earthquake along the Japan Trench
Abstract:Recent developments in the field of subaqueous paleoseismology have provided information about the sedimentation record of earthquakes and about the long-term seismicity of fault systems. In 2013, the Japan Agency for Marine-Earth Science and Technology conducted expeditions NT13-02 and NT13-19 to the 2011 Tohoku Mw 9.0 megathrust earthquake and tsunami source, with R/V Natushima in 800-5,900 m water depth. The goal was identifying earthquake-triggered deposits and mapping their spatial and temporal distribution, as a strategy to recognize the sedimentary signature of Tohoku-like events and measure recurrence intervals for seismic hazard assessment. Twenty-four piston cores, 3 to 6 m long, were recovered during the NT13-19 expedition along a 300 km-long portion of the mid-slope terrace. This elongated structure is parallel to the strike of the Japan Trench, and located landward of the frontal prism where deformation is most intense. Faults, sometimes forming steep scarps, define small (5km long) confined basins that were targeted for coring.
Radioisotopes 137Cs and xs210Pb measured in the cores mark the 2011 Tohoku earthquake-related sedimentation. Detection of 134Cs and enrichment of 137Cs provided a 2011 Fukushima reactor signature, which was found in the upper 5cm of several cores and was buried 10-15cm in others. Very high activities of xs210Pb were measured in the upper half-meter of the majority of the cores providing evidence of very recent depositional events that we are linking to the 2011 earthquake. We envision the shaking by the earthquake fluidized a layer of surface sediment, which then moved downslope and was deposited where surface slope decreased. These sediments also incorporated 137Cs derived from global fallout over the past half century. These deposits can be recognized in the cores because they are homogeneous and lack bioturbation. The thickest ones (~1m) have soft sediment deformation features at their base. Along the mid-slope terrace, they are thicker in the region where maximum 2011 rupture displacement was documented. Several older event horizons separated by bioturbated sediments are recognized in the cores. Usami et al. (2014) used tephra chronology to estimate an average recurrence interval of 100-500 yrs in most cores, although some cores suggest recurrence of 1500-2000 yrs.