Configuration of the Moho discontinuity beneath the Japanese Islands derived from three-dimensional seismic tomography

Abstract

The Mohorovičić discontinuity (Moho) is defined on the basis of an abrupt increase in seismic velocity in the lithosphere which has been observed using seismic refraction and receiver function analysis methods worldwide. Moho depth varies regionally and remains a fundamental parameter of crustal structure. We present a new method of mapping the Moho using a 3D seismic tomography model. Since the tomographic method cannot locate discontinuities, we treat the Moho as a zone of high velocity gradient. Maximum lower crust/minimum upper mantle P-wave velocities in Japan are known to be 7.0km/s and 7.5km/s, respectively. We map the residual between isovelocity surfaces of 7.0km/s and 7.5km/s to find areas where the residual is small, the separation between the surfaces is narrow, and the velocity gradient is high. The Moho is best constrained where the isovelocity surfaces are close together, and under much of Japan, they are <6km and rarely >10km apart. We chose an isovelocity surface of 7.2km/s as a representative Moho ‘proxy’ in these areas. Our resulting ‘Moho’ map under Japan compares favorably with existing regional Moho models that were obtained from controlled-source seismic investigations. The ‘Moho’ varies from shallow (25–30km) to deep (>30km), and this variability relates to the structural evolution of the Japanese islands: the opening of the Sea of Japan back-arc, ongoing arc-arc collisions at the Hidaka and Izu collision zones, ongoing back-arc extension in Kyushu, and a possible failed back-arc extensional event of Mesozoic age. It is apparent that the Moho is less well-constrained in areas where the crustal structure has been modified by magmatic activity or thickened due to arc-arc collision.