STRUCTURAL CONTROLS ON THE MW 9.0 2011 OFFSHORE-TOHOKU EARTHQUAKE

What controls the way in which a great earthquake slips? Minor variations on the fault plane can have profound influence on the behaviour in the event. Joint seismic tomography exploiting P and S wave arrivals conducted before the 2011 Offshore Tohoku earthquake reveals an area comparable to the faulting surface for the 2011 March 11 event with different properties from other areas along the shallow part of the subduction zone. Within this area there are patches on the subduction zone with slightly lowered shear wavespeed that appear to act as barriers to slip in the great earthquake. Segmentation of the slip process can be imaged by back projection of seismograms from the US Array; the areas of greatest energy release avoid these barrier zones. The structural variations are likely to associated with the effects of fluids acting on structures present in the subducting plate as the oceanic lithosphere bends at the trench hinge, since fluids have a strong effect on the shear modulus.