Crustal heterogeneity in the 2007 Noto-Hanto earthquake area and its geodynamical implications

To understand the genesis of the 25 March 2007 Noto-Hanto earthquake (Mw 6.6), we determined high-resolution 3-D seismic velocity (Vp and Vs) and Poisson's ratio (σ) images in the epicenter area using 24,698 P-wave and 22,927 S-wave arrival times from 809 earthquakes and 265 sP depth-phase data collected from seismograms of 162 earthquakes beneath the Japan Sea. The sub-oceanic events are relocated accurately by using P-, S- and sP depth-phase arrival time data jointly. Our results demonstrate that the seismic velocity (Vp and Vs) and Poisson's ratio (σ) vary markedly in the source area. In and around the main shock hypocenter, low-velocity and high-σ anomalies are revealed, which are apt for accumulation of differential strain to bring the brittle failure. We infer that the zone with pronounced low-velocity and high Poisson's ratio beneath the source area reflects fluid-related anomalies derived either from the dehydration of the subducting Pacific slab or from permeation of sea-water through deep-seated active faults or both to the main shock hypocenter, which in turn may have facilitated the rupture initiation of the main shock and its aftershock sequence by weakening the overlying seismogenic layer beneath the region. Our study reinforces the concept of fluid-driven earthquakes in the peninsular regions, elsewhere in the world.