The 2011 Tohoku earthquake (Mw 9.0) off the Pacific coast of Japan produced huge slip (~50 m) on the shallow portion of the fault close to the toe of the megathrust. The large displacement on this portion of the subduction zone and the magnitude of the devastating tsunami that took more than 19,000 lives and inflicted massive damage along the northeast coast of Honshu was unexpected by earthquake scientists. The main science goal of the Japan Trench Fast Earthquake Drilling Project (JFAST) is to understand the physical mechanisms and dynamics of large slip earthquakes, which is a fundamental issue that is currently poorly understood. Specifically, the level of frictional stress during the earthquake rupture and the physical characteristics of the fault zone will be investigated. This topic has obvious social consequences globally for evaluating severe shaking and large tsunamis from future earthquakes at subduction zones. The scientific objectives of JFAST include characterizing the fault and wall-rock composition, fault architecture, and the nature of heat and pressure within and around the fault zone, located approximately 1000 m below seafloor. Two riserless holes will be drilled during Integrated Ocean Drilling Program (IODP) Expedition 343: one logging-while-drilling hole to determine in situ stress and locate the fault zone and one hole to acquire core samples from the fault zone. Each hole will be completed with an observatory deployment comprising a suite of temperature and pressure sensors.