Subduction faults accumulate stress during long periods of time and release this stress suddenly,during earthquakes, when it reaches a threshold. This threshold, and the shear strength,controls the occurrence and magnitude of earthquakes. We consider a 3-D model to derive an analytical expression for how the shear strength depends on the fault geometry, the convergence obliquity, and frictional properties,and the stress field orientation. We then consume estimates of these different parameters in Japan to infer the distribution of shear strength along a subduction fault. We show that the 2011 Mw9.0 Tohoku earthquake ruptured a fault portion characterized by unusually small variations in static shear strength. This observation is consistent with the hypothesis that large earthquakes preferentially rupture regions with relatively homogeneous shear strength. With increasing constraints on the different parameters at play, our approach could, and in the future,succor identify favorable locations for large earthquakes.
Source: usgs.gov