Seismic Reflection Survey Across the Fukaya-Ayasegawa Fault System in the Northwestern Margin of Tokyo Metropolitan Area, Japan.

Several active faults, such as the Fukaya and Ayasegawa faults, have been traced fragmentarily from northwest to southeast in the northwestern margin of the Tokyo Metropolitan area, Kanto district. Because of the thick sediment cover in the Kanto plain, a considerable part of the fault system may be located as a blind fault. In 2005 the Earthquake Research Committee published a long-term evaluation for the Kanto-Heiya-Hokuseien (i.e. the northwestern margin of the Kanto plain) Fault Zone including these faults. According to the evaluation, the total length is about 82km, the average vertical displacement rate is 0.2-0.4m/ky, the displacement for one faulting event is 5-6m, and magnitude of a supposed event is M8.0. To clarify the detailed structure downto the pre- Neogene basement at the gapped area between the Fukaya and Ayasegawa faults in Saitama prefecture, we conducted a 10km-long seismic reflection survey which covers from the lowland in the northern Kounosu city to the eastern foot of the Yoshimi hills. The processed section shows clear images of the basement, some flexures and the two unconformities: the Kurotaki unconformity at about 3Ma and the Niwaya unconformity at about 15Ma. The basement, which crops out in the Yoshimi hills, is steeply dipping to the northeast in the western side of the section, whereas that in the eastern side is nearly horizontal and is situated at about 4km in depth. The flexural zone in the shallower part is totally 3km wide and shows cumulative deformation of reflectors since 3Ma at least. These flexures show no topographic expression, except the easternmost one which corresponds to the northern extension of the Ayasegawa fault. The long-term vertical displacement rate since 3Ma is estimated as about 0.1m/ky in the easternmost one. It is very interesting that the rate is equal to that estimated by using the 70ka terrace deposits. The displacement rate of the whole flexural zone since 3Ma is about 0.2m/ky which is comparable to or somewhat smaller than that of the Fukaya fault in the north.