Integrated Ocean Drilling Program (IODP) Expedition 343, named the Japan Trench Fast Drilling Project (JFAST), drilled ocean floor boreholes through the fault zone of the 2011 Tōhoku-Oki earthquake (M9.0) to enhance understanding of the rupture process and tsunami generation. This project investigated the very large fault slip that caused the devastating tsunami by making borehole stress measurements, sampling the plate boundary fault zone, and taking temperature measurements across the fault zone. The results show that the earthquake rupture occurred in a narrow fault zone (< 5 m). Based on both laboratory experiments on fault zone material and temperature monitoring of the fault zone, we show that the fault had very low friction during the earthquake. The low friction properties of the fault zone might be attributed to high smectite clay content of the sediment. These physical and frictional characteristics contributed to the unusually large fault slip that occurred during the earthquake and generated the tsunami.
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