Seismic structure of the source region of the 2007 Chuetsu-oki earthquake revealed by offshore-onshore seismic survey: Asperity zone of intraplate earthquake delimited by crustal inhomogeneity

The 2007 Niigata-ken Chuetsu-oki earthquake occurred on July 16, 2007 with a magnitude of 6.8. Immediately after the mainshock, a dense network of ocean-bottom seismometers and temporary land seismic stations were deployed to obtain the accurate aftershock location. A seismic survey using ocean-bottom seismometers, land stations and controlled sources at sea and on land was also conducted along three profiles to estimate the detailed velocity structure of the source region. A thick sedimentary layer covers the crust, and this layer is thickest near the coast. The upper crust has a P-wave velocity of 6. km/s and a large lateral heterogeneity with respect to thickness and velocity. The lower crust, with P-wave velocity of 7. km/s, has a thickness of approximately 10. km. The thickness of the crust is estimated to be approximately 24. km. The precise aftershock distribution was obtained by using the high-resolution velocity structure. The aftershocks in the upper crust form a plane dipping to the southeast. Most of the aftershocks are located in the upper crust; in addition, a small number of aftershocks in the lower crust seem to be positioned on the same plane formed by the aftershocks in the upper crust. The mainshock fault estimated from the aftershock distribution is positioned in the high-velocity body of the upper crust. A large deformation of the sediments above the epicentral region is interpreted to be due to the repeated occurrence of large earthquakes. The lower crust immediately below the fault has low velocity. The source region of the 2007 event in the upper crust is sandwiched by a weakening structure. We suggest that the ductile deformation of both the sediments and the lower crust causes stress accumulation in the source region.