A weak subducting slab at intermediate depths below northeast Japan

Knowledge of the state of stress in subducting slabs is essential for understanding their mechanical behavior and the physical processes that generate earthquakes. Here we develop a novel framework that uses a high-resolution focal mechanism catalog to determine, for the first time, the full deviatoric stress tensor within the subducting slab at intermediate depths. We show that by combining the static stress calculated from coseismic slip distributions with the stress orientations before and after the mainshock, that deviatoric stress within the slab at intermediate depths must be very low ( ∼ 1 MPa). These results preclude earthquake source mechanisms that require large background driving stresses, favoring a mechanically weak subducting slab, thus providing quantitative constraints on the physical processes that generate intermediate-depth earthquakes.

Methods
The focal mechanism data, and their spatial relationship with respect to the plate interface of the subducting slab (Kita et. al., 2010). The p-wave polarity focal mechanisms were obtained from NIED using the online MOWLAS system. The hypocenters were obtained from the JMA. The F-Net moment tensors were obtained from the NIED.