Evidence for the location and cause of large crustal earthquakes in Japan

Large crustal earthquakes (magnitude 5.7-8.0, depth 0-20 km) during a period of 115 years from 1885 to 1999 in Japan occurred in or around zones of low seismic velocity revealed by seismic tomography. The low-velocity zones may represent weak sections of the seismogenic crust. The crustal weakening is considered to be closely related to the subduction of the oceanic Pacific and Philippine Sea plates in this region. Along the volcanic front and in back-arc areas the crustal weakening may be caused by active volcanoes and magma chambers. In the forearc areas, fluids are detected in the earthquake source zones, which may have contributed to the crustal weakening and the rupture nucleation. The fluids may be related to the dehydration of the subducting Philippine Sea slab. These results indicate that large crustal earthquakes do not strike anywhere, but only anomalous areas that may be detected with geophysical methods.