DescriptionThe 2011 Tohoku-oki, Japan earthquake (Mw9.0) produced the largest fault slip ever recorded for an earthquake, up to 50 meters on the shallow portion of the subduction megathrust. This slip produced the very large tsunami (up to 40 meters) that devastated much of the northeast coast of Honshu, Japan. Before the earthquake, this region of the plate boundary was thought to be an area of mainly aseismic slip by many researchers, so the huge co-seismic displacements and resultant tsunami were a shocking surprise to the seismological community. In response to the earthquake, the Japan Trench Fast Drilling Project (JFAST) was launched by the Integrated Ocean Drilling Program (IODP). This research program was designed to investigate the physical conditions and rupture mechanisms that produced the large slip, by drilling of seafloor boreholes to the plate boundary fault. During April/May and July 2012, three boreholes located at a site close to the Japan Trench about 90 km east of earthquake epicenter, successfully reached the plate boundary fault at depths of about 820 meters below seafloor. These boreholes enabled geophysical logging, core sampling and installation of a temperature observatory in the vicinity of the fault zone. These data are currently being analyzed to understand the physical properties and stress conditions that enabled the very large fault slip. For more information on JFAST see http://www.jamstec.go.jp/chikyu/exp343/e/ .
Kyoto University, Earthquake Hazards Division.
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