The 2007 Niigata earthquake: Effect of arc magma and fluids

To clarify the generation mechanism of large crustal earthquakes in the Amur-Okhotsk plate boundary, we determined high-resolution 3D seismic velocity (V_p, V_s) and Poisson's ratio structures in Niigata, Northeast Japan using a large number of P- and S-wave arrival times from the aftershocks of the 2004 and 2007 Niigata earthquakes as well as many intermediate-depth earthquakes in this area. Our results show that the Niigata mainshocks occurred around zones of low-velocity and high-Poisson's ratio, which suggest that fluids may have influenced the nucleation and rupture processes of the 2004 and 2007 Niigata earthquakes. A zone with pronounced low-velocity and high Poisson's ratio is revealed under the source areas of the 2004 and 2007 Niigata earthquakes in the lower crust and uppermost mantle, which may reflect the arc magma and fluids ascending from the upper-mantle wedge. These results indicate that the generation of the 2004 and 2007 Niigata earthquakes was affected by the ascending magma and fluids released by the dehydration process of the subducting Pacific slab.