Achievements of the rapid response drilling (IODP Expedition 343: J-FAST) after the 2011 off the Pacific coast of Tohoku Earthquake

Abstract

The first offshore ‘rapid response drilling,’ a drilling immediately after an earthquake for a scientific purpose, was conducted at the Japan Trench after the 2011 off the Pacific coast of Tohoku Earthquake as IODP Expedition 343. It brought various insights into the mechanism of huge tsunami induced by the mega earthquake. Major findings include 1) the fault zone is thin (5 meters) and weak (0.2-0.26 and 0.08-0.1 as its friction coefficient under low and high velocity slip conditions respectively), 2) low friction of the fault zone at seismic slip rates is due to abundance of smectite (a clay mineral) and thermal pressurization effects that can explain the large co-seismic slip, 3) the stress field dramatically changed from compressional (pre-earthquake) to extensional (post-earthquake). Remaining issues are to understand 1) the complete profile of physical property from the seafloor to the basement, 2) the factors that determine the slip heterogeneity along strike. Another scientific drilling is expected to solve these in near future.