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The main science goal of the Japan Trench Fast Drilling Project (JFAST) is to understand the physical mechanisms and dynamics of large slip earthquakes, which is fundamental to understanding the huge tsunami that caused extensive damage during the 2011 Tohoku earthquake. Specifically, the level of frictional stress during the earthquake rupture and the physical characteristics of the fault zone are investigated through drilling. The objectives of JFAST include locating the fault that ruptured during the Tohoku event using logging while drilling (LWD); characterizing the composition, architecture, and fundamental mechanisms of dynamic frictional slip and healing processes along the fault by taking core samples; and estimating the frictional heat and stress within and around the fault zone by placing a temperature measurement observatory across the fault. During the main JFAST expedition (Integrated Ocean Drilling Program [IODP] Expedition 343 in April and May 2012), LWD was completed in a borehole drilled to 850.5 meters below seafloor (mbsf) (total depth [TD] = 7740 meters below sea level [mbsl]), and a coring hole was drilled to 844.5 mbsf (TD = 7734 mbsl) to acquire 21 cores that spanned the two main fault targets. Because of delays associated with severe weather and technical challenges of operating in great water depths, installation of the observatories for temperature and pressure measurements was not completed during the main expedition; however, temperature sensors were successfully deployed during a short technical extension of JFAST, IODP Expedition 343T (JFAST II), in July 2012. Principal results of Expedition 343/343T include the following: 1. The overall structure at the drill site consists of a prism of faulted and folded clayey to silty mudstones above, and in fault contact (at ~820 mbsf) with, a largely undeformed, relatively thin sequence of hemipelagic and pelagic sediments that were deposited on top of the incoming Pacific plate. The primary constituents of the mudstones that make up the prism are terrigenous silt and clay, vitric ash, and biogenic silica. The fault contact, interpreted as the plate boundary decollement, is defined by a subhorizontal