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.

Mori, J., F. Chester, E.E. Brodsky, and S. Kodaira. 2014. Investigation of the huge tsunami from the 2011 Tōhoku-Oki, Japan, earthquake using ocean floor boreholes to the fault zone. Oceanography 27(2):132–137, https://doi.org/10.5670/oceanog.2014.48.

Byerlee, J.D. 1978. Friction of rocks. Pure and Applied Geophysics 116:615–626, https://doi.org/10.1007/BF00876528.

Chester, F.M., C. Rowe, K. Ujiie, J. Kirkpatrick, C. Regalla, F. Remitti, J.C. Moore, V. Toy, M. Wolfson-Schwehr, S. Bose, and others. 2013. Structure and composition of the plate-boundary slip-zone for the 2011 Tohoku-Oki earthquake. Science 342:1,208–1,211, https://doi.org/10.1126/science.1243719.

Fujiwara, T., S. Kodaira, T. No, Y. Kaiho, N. Takahashi, and Y. Kaneda, 2011. The 2011 Tohoku-Oki earthquake: Displacement reaching the trench axis. Science 334:1,240, https://doi.org/10.1126/science.1211554.

Fulton, P.M., E.E. Brodsky, Y. Kano, J. Mori, F. Chester, T. Ishikawa, R.N. Harris, W. Lin, N. Eguchi, S. Toczko, and Exp. 343/343T and KR13-08 Scientists. 2013. Low coseismic friction on the Tohoku-Oki fault determined from temperature measurements. Science 342:1,215–1,217, https://doi.org/10.1126/science.1243641.

Hasegawa, A., K. Yoshida, and T. Okada. 2011. Nearly complete stress drop in the 2011 MW 9.0 off the Pacific coast of Tohoku earthquake. Earth, Planets, and Space 63:703–708, https://doi.org/10.5047/eps.2011.06.007.

Ida, Y. 1972. Cohesive force across the tip of a longitudinal shear crack and Griffith’s specific surface energy. Journal of Geophysical Research 77:3,796–3,805, https://doi.org/10.1029/JB077i020p03796.

Lin, W., M. Conin, J.C. Moore, F.M. Chester, Y. Nakamura, J.J. Mori, L. Anderson, E.E. Brodsky, N. Eguchi, and Expedition 343 Scientists, 2013. Stress state in the largest displacement area of the 2011 Tohoku-Oki earthquake. Science 339:687–690, https://doi.org/10.1126/science.1229379.

Mori, N., T. Takahashi, and the 2011 Tohoku Earthquake Tsunami Joint Survey Group. 2011. Nationwide post-event survey and analysis of the 2011 Tohoku earthquake tsunami. Coastal Engineering Journal 54:1–27, https://doi.org/10.1142/S0578563412500015.

Tanioka, Y., and K. Satake. 1996. Fault parameters of the 1896 Sanriku tsunami earthquake estimated from tsunami numerical modeling. Geophysical Research Letters 23:1,549–1,552, https://doi.org/10.1029/96GL01479.

Ujiie, K., H. Tanaka, T. Saito, A. Tsutsumi, J.J. Mori, J. Kameda, E.E. Brodsky, F.M. Chester, N. Eguchi, S. Toczko, and Expedition 343 and 343T Scientists. 2013. Low coseismic shear stress on the Tohoku 1 megathrust determined from laboratory experiments. Science 342:1,211–1,214, https://doi.org/10.1126/science.1243485.

This is an open access article made available under the terms of the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution, and reproduction in any medium or format as long as users cite the materials appropriately, provide a link to the Creative Commons license, and indicate the changes that were made to the original content. Images, animations, videos, or other third-party material used in articles are included in the Creative Commons license unless indicated otherwise in a credit line to the material. If the material is not included in the article’s Creative Commons license, users will need to obtain permission directly from the license holder to reproduce the material.