The 2011 Tohoku earthquake (Mw 9.0) and accompanying tsunami devastated much of the northeast coast of Honshu, Japan, and highlighted many of the poorly understood aspects of how great earthquakes occur. An important aspect of the event was the very large amount of slip (~50 m) on the shallow portion of the megathrust, which is the largest displacement ever recorded in an earthquake (Fujiwara et al., 2011; Ito et al., 2011; Sato et al., 2011). Based on current ideas of the rheology and behavior of shallow faults, the large amount of displacement and its location, which reached the trench, was not anticipated (Ammon et al., 2011; Avouac, 2011; Ide et al., 2011). The huge amount of movement on the shallow portion of the megathrust and associated displacement of the ocean floor was the main cause of the destructive tsunami (Ito et al., 2011; Ide et al., 2011; Fujii et al., 2011).

The scientific community needs to learn as much as possible from this extreme event. Specifically, we want to clarify the physical conditions and state of stress that enabled the large slip to occur. We also hope to identify the effects of the earthquake on the fault zone so that we can identify the recent event and other events in the geological record. With understanding of the physical conditions on the fault zone and knowledge of past earthquakes, we will be able to improve evaluations of future occurrences of large tsunamis in the Tohoku region, as well as for other subduction zones around the world (e.g., Ozawa et al., 2011; Ammon et al., 2011).

An important aspect of Integrated Ocean Drilling Program (IODP) Expedition 343 (Japan Trench Fast Drilling Project [JFAST]) is the priority on collecting time-dependent observations of fault properties following a large earthquake. Measurements of the decaying temperature of the fault zone, as well as analyses of the changing stress and chemical properties of the fault rocks, are important observations that need to be made soon after the earthquake (Brodsky et al., 2009; Fulton et al., 2010). The scheduling of this expedition within 13 months following the earthquake reflects the rapid response taken by IODP to provide scientific information about large-scale geohazards that severely impact our societies.