Lateral variations in the seismogenic layer beneath the Japan Islands

To better understand the earthquake dynamics of the Japan subduction zone, we investigated the lateral variations of the seismogenic layer in the crust beneath the Japan Islands using about 30,000 well-located crustal earthquakes (M 1-5). These shallow events (0-20 km depths) were selected from the Japan University Network Earthquake Catalog (1985-1993) and the aftershocks of the 1995 Kobe earthquake (M 7.2). The hypocenters were relocated precisely using a 3-D ray tracing method [Zhao et al, 1992] and a 3-D velocity model [Ochi, Asamori and Zhao, 1999]. From the hypocentral distribution of these events we estimated the depth distributions of the upper and lower boundaries and the thickness of the seismogenic layer in Japan. Then we examined the relationships among the thickness of the seismogenic layer, active faults, volcanoes, tomography, and large crustal earthquakes (M>5.7) from 1885 to 1999. We obtained the following results. (1) There are large variations in depth range and thickness of the seismogenic layer. There is a trend that large crustal earthquakes occur in areas where the seismogenic layer changes abruptly. Those areas may represent weak sections of the crust and so are subject to tectonic stress and are prone to large earthquakes. (2) The upper and lower boundaries of the seismogenic layer become shallow toward volcanoes, indicating that the seismogenic layer is controlled by the geothermal regime. (3) The seismogenic layer exhibits large variations across and along the fault zones. (4) A correlation is visible between the thickness of the seismogenic layer and seismic tomography, suggesting that tomographic images of the crust contain the effects of lateral material heterogeneities, as well as thickness changes of the seismogenic layer. These results may improve our understanding of the earthquake dynamics and contribute to the assessment and the mitigation of seismic hazards of the Japan Islands