Expedition | 503

The 2011 – Mw 9.0 – Tohoku-oki earthquake and tsunami was a catastrophic geological event with major societal consequences. Unexpected shallow and large coseismic slip contributed to the large tsunami. Short historical and even shorter instrumental records limit our perspective of earthquake maximum magnitude and recurrence, and thus are insufficient for fully characterise Earth’s complex and multiscale seismic behaviour and its consequences. The geological record is a reliable tool for reconstructing the history of giant earthquakes with long recurrence intervals and to help reduce epistemic uncertainties in seismic-hazard assessment. Results of previous research based on up to 40-m long piston cores suggested that megathrust earthquakes have been recorded as thick turbidites in the central Japan Trench. This research has also documented that the Japan Trench trench basins, which are part of the hadal ocean and thus among the deepest places on our planet, act as terminal sinks for sediment and carbon, stimulating highly active subseafloor microbial communities and potentially efficiently sequester carbon.

Therefore, the ultra deep-sea trench-fill sediments in the central Japan Trench are the best archive of past giant earthquakes to expand turbidite paleoseismology toward a much longer time and to study the hadal carbon cycle. Furthermore, the seismic profiles indicated that a deformed structure created during large coseismic slip was detected in the trench-fill sequence of central Japan Trench. Such deformation is likely another geological evidence of the extreme slip which propagated to the trench (“slip-to-the-trench”) and is considered important for causing outstanding large tsunami. Establishing a chronology for such deformation horizons in seismic profiles may improve our understanding of the recurrence of tsunamigenic slip-to-the-trench vs deep megathrust rupture modes.

International Ocean Drilling Programme (IODP³) Expedition 503 plans to drill a trench basin in the central Japan Trench to recover the whole trench-fill sequence for dating and establishing event-stratigraphy for paleoseismologic interpretations and further investigations of earthquake related element cycles in hadal trench environment. Combining stratigraphy and chronology of thick event deposits, interstitial-water geochemistry proxy-data for past fluid flow pulses and core-to-seismic correlation to paleo-slip-to-trench events, we would like to clarify how often the slip-to-trench events have occurred. Unravelling the complete trench-fill sedimentary record and pore water profile will significantly advance our understanding of the nature and recurrence of hadal trench tsunamigenic slip, the underlying megathrust earthquakes and related geohazards, as well as the effects on enhancing carbon accumulation in the hadal trench that may stimulate carbon transformation and eventual export into the subduction zone.