Crustal structure associated with the Atotsugawa fault system in Central Honshu, Japan

Well-located earthquake hypocenters along the Atotsugawa fault system have been obtained as a result of recent seismic observations by a dense network. The fault system is one of the well-known active faults in Japan and the only one that has a creeping section. Seismicity is clearly high along the three faults of the fault system. A large earthquake (M7.0) occurred on the fault system in 1858. The cutoff depth of seismicity is relatively deep on the middle part of the fault and shallows towards both ends, where there are active volcanoes. In order to study the relationship between the distribution of earthquakes and the heterogeneous structure, we conducted seismic explosion surveys along the fault and in the surrounding areas.

Four explosions were shot in the northeast part of the Atotsugawa fault to study its structure in 2000. We recorded seismic arrivals from these explosions along the Mozumi-Sukenobe fault and the Atotsugawa fault. In 2001, a large-scale seismic explosion survey was carried out from the Tokai district to the Noto Peninsula, crossing the Atotsugawa fault system. We used these data to analyze the crustal structure with relation to the seismicity. From simple analyses, we obtained the thickness of the surface layer, which is estimated to be about 1 km along the fault profile, and 3 to 8 km along the profile crossing the fault. Normal move-out (NMO) corrections with static corrections using the surface layer structure, are applied to the waves to obtain a rough image of reflectors in the crust. Crustal velocity structures are derived by a ray tracing method, in which the surface and basement layer structures, mentioned above, are being used as an initial model. Furthermore, we study the spatial extension of these reflectors by use of the NMO corrections on the seismic explosion data recorded by the routine seismic network.

Comparing these structures with the earthquake focal depth distributions along and across the Atotsugawa fault, we obtain the following conclusions: 1) Most earthquakes occur only in the layer with a velocity of 6.0-6.2 km/s and events are very scarce in the surface layer. 2) There are two distinct reflectors at the depths of about 12-15 km and 20-23 km. 3) The lower reflector seems to be located at the base of the seismogenic layer, regardless of whether the seismic activity is high or low. 4) The lower reflector is situated in the middle to lower crust, which is usually reflective. 5) The reflectors seem to be distributed not only beneath the fault system but also over the whole central northeast Chubu district, although the strength of the reflectors varies from place to place.