Stress State in the Largest Displacement Area of the 2011 Tohoku-Oki Earthquake

Weiren Lin, Marianne Conin, J. Casey Moore, Frederick M. Chester, Yasuyuki Nakamura, James J. Mori, Louise Anderson, Emily E. Brodsky, Nobuhisa Eguchi, Expedition 343 Scientists, Becky Cook, Tamara Jeppson, Monica Wolfson-Schwehr, Yoshinori Sanada, Saneatsu Saito, Yukari Kido, Takehiro Hirose, Jan H. Behrmann, Matt Ikari, Kohtaro Ujiie, Christie Rowe, James Kirkpatrick, Santanu Bose, Christine Regalla, Francesca Remitti, Virginia Toy, Patrick Fulton, Toshiaki Mishima, Tao Yang, Tianhaozhe Sun, Tsuyoshi Ishikawa, James Sample, Ken Takai, Jun Kameda, Sean Toczko, Lena Maeda, Shuichi Kodaira, Ryota Hino, Demian Saffer

The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced a maximum coseismic slip of more than 50 meters near the Japan trench, which could result in a completely reduced stress state in the region. We tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the Integrated Ocean Drilling Program approximately 1 year after the earthquake and by inferring the pre-earthquake stress state. On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis, in situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate interface, which is consistent with coseismic fault weakening and a nearly total stress drop.