Unexpected shallow and large slip on the plate boundary fault during the 2011 Tōhoku-oki earthquake contributed to a large tsunami. However, our knowledge on the past history of such slips is limited due to the long recurrence interval of the events. IODP³ Expedition 503 recovers the whole trench-fill sequence in the central Japan Trench to understand the nature and recurrence of hadal trench tsunamigenic slips and their earthquake-related carbon accumulation and element cycles.

Freshened water exists in continental shelf aquifers worldwide. One remarkable example is the US Atlantic shelf, where groundwater in shallow sand aquifers >100 km offshore has salinity that is 10 % of seawater salinity. Expedition 501 will drill sites off Massachusetts, US, to estimate how much freshened water there is, processes that emplace the water, and rates of emplacement. The expedition will also assess whether salinity patterns have important implications for microbial processes.