A water wall in the Tohoku forearc causing large crustal earthquakes

We present high-resolution tomographic images in source areas of 26 large crustal earthquakes (M 6.0-7.2) which occurred in Northeast Japan (Tohoku) during the past 120 yr from 1894 to 2014. Prominent low-velocity (low-V) and high Poisson's ratio (high-sigma) anomalies are revealed in the crust and mantle wedge under the source areas. Beneath the volcanic front and backarc areas, the low-V and high-sigma zones reflect arc-magma related high-temperature anomalies which are produced by joint effects of corner flow in the mantle wedge and fluids from dehydration of the subducting Pacific slab. The hot anomalies cause locally thinning and weakening of the brittle seismogenic layer above them. Low-frequency micro-earthquakes are observed in the lower crust and uppermost mantle in or around the low-V zones, which reflect ascending of arc magma and fluids from the mantle wedge to the crust. No volcano and magma exist in the forearc area due to low temperature there, hence the low-V zones in the forearc reflect fluids from the slab dehydration. The ascending fluids may have produced a 'water wall' in the mantle wedge and crust beneath the forearc area. When the water enters active faults in the crust, the fault-zone friction is reduced and so large earthquakes can be induced. These results indicate that the nucleation of a large earthquake is not entirely a mechanical process, but is closely associated with subduction dynamics and physical and chemical properties of rocks in the crust and upper mantle. In particular, arc magma and fluids play an important role in the seismogenesis.