Abstract
I introduce a model for generation of interplate, intraslab and shallow inland earthquakes in the NE Japan subduction zone and for magma genesis and ascent within the mantle wedge beneath the arc. Waveform inversion studies of recurrent earthquakes have revealed that large slip areas(asperities) of successive ruptures are in the same place on the plate boundary. Back slip inversions of GPS data show that asperities once ruptured by past large earthquakes are locked at present. These observations strongly suggest that the asperity model is applicable to the process of seismic and aseismic slip on the plate boundary in this subduction zone. Studies on spatial distribution of intermediate-depth earthquakes and seismic velocity structure within the slab provide the evidence which supports the dehydration embrittlement hypothesis for the generation of intraslab earthquakes beneath this arc. Seismic tomography studies have shown an inclined sheet-like seismic low-velocity zone in the mantle wedge at depths < 150km, which probably corresponds to the upwelling flow of the subduction-induced convection system. This upwelling flow reaches the Moho immediately beneath the volcanic front, suggesting that the volcanic front is formed by this upwelling flow. Aqueous fluids dehydrated from the slab are probably transported upward through this upwelling flow to reach the crust, where the surrounding crustal rocks might be weaken resulting in local contractive deformation under the current compressional stress field of this arc. GPS data have revealed a notable concentration of contraction deformation there. Shallow inland earthquakes are also concentrated in the upper crust of this locally large contraction deformation zone, suggesting that their occurrence is closely related with local weakening of the crust by aqueous fluids originated from the slab dehydration.
