[S-CG46] スラブ内地震とその発生メカニズム

[SCG46-02] Absolute slab stress at intermediate depths below northeast Japan

★Invited Papers

*Brent Delbridge¹、Heidi Houston²、Saeko Kita³、Youichi Asano⁴、Roland Burgmann⁵ (1.Los Alamos National Laboratory、2.University of Southern California、3.Building Research Institute、4.National Institute of Earth Science and Disaster Resilience、5.University of California, Berkeley)

キーワード：subduction, earthquake, stress

Knowledge of the state of stress in subducting slabs is essential for understanding their mechanical behavior and provides constraints on the proposed physical processes that generate intermediate-depth earthquakes, a long-standing puzzle in seismology. Here we develop a novel framework and dataset to determine for the first time the absolute deviatoric stress tensor within the subducing slab at intermediate depths. We report rotations of the principal stress axes by 20 degrees at the time of the M9 Tohoku-oki earthquake, which are observed to recover on a time-scale of only a few years. The large stress rotations observed imply that deviatoric stresses within the deep slab (>70 km) are only a few times larger than the stress changes due to the M9 (0.5 MPa). Thus, only a small differential stress on the scale of a few MPa is available to drive intermediate-depth earthquakes. This is substantially lower than expected from conventional fault mechanics. We show that combining stress fields calculated from coseismic slip distributions with the stress orientations before and after the mainshock allows us to invert for the full deviatoric stress tensor. Our inverted stress tensors show that low effective-friction controls earthquake failure in the subducting slab at intermediate depths, and rules out mechanisms which require large background driving stresses. The results and analysis presented here suggest a mechanically-weak, fluid-rich subducting slab.