3-D Seismic velocity structure and stress regime under Japan Islands

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We have used seismic tomography to study the 3-D crust and mantle structure and seismotectonics of the Japan Islands. A close relationship between large crustal earthquakes and structural heterogeneities is revealed. Most of the large crustal earthquakes occurred in or around the areas of low velocity that represent weak sections of the seismogenic crust. In volcanic areas the crustal weakening is caused by active volcanoes and arc magma. In forearc areas fluids are suggested in the large earthquake source zones, which have contributed to the rupture nucleation. The fluids originate from the dehydration of the subducting Philippine Sea slab. These results indicate that large earthquakes do not strike anywhere, but only anomalous areas which can be detected with geophysical methods.A new approach was used to conduct global tomographic inversions to study the deep structure of subducting slabs and mantle plumes. Strong and wide fast anomalies are visible in the transition zone depths under subduction regions, which suggests that most of the slab materials are stagnant in the transition zone before finally collapsing down to the lower mantle. Plume- like slow anomalies are clearly visible under the major hotspot regions in most parts of the mantle, in particular, under Hawaii, Iceland, South Pacific and Africa. The slow anomalies under hotspots usually do not show a straight pillar shape, but exhibit winding images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. As a consequence, hotspots are not really fixed but can wander on the surface.