推定される京都市内「横大路断層」における反射法地震探査

抄録

Seismic reflection survey was conducted in Yokooji, the middle of the Kyoto basin in 1992. The reason why this site was chosen is that this is just the place where UEMURA and NAKATSUKA (1992) inferred a buried fault (named Yokooji fault) from wells' data. According to them, there is a systematic sudden change in depth of the lower sand and gravel layer which is widely spread at the depth of about 10m in the middle of the Kyoto basin. They have suggested that its change was caused by a buried fault running from east to west, which is likely a sub-fault of the Arima-Takatsuki tectonic line.
Though no topographical feature is found in this area, such a buried fault can have a high activity when the movement has begun in the recent past. From the disaster prevention point of view, it is hence important to investigate the subsurface structure of the possible fault to determine its activity even in the middle of a basin. It also can provide some information of the basin's growth history which leads to a better understanding of the Quaternary tectonics.
An air gun was used as a sourse apparatus. Through the data-processing for the CDP profiling, geological information of the site was obtained as follows:
(1) A velocity model up to the basement is obtained. The depth of the basement is estimated as 400-500m.
(2) A few reflections from interfaces in the Osaka group (unconsolidated sediments above the basement) are discerned. The key layer (Ma3) at 200m-300m below the ground surface is found to be consistent with the result of existing boring surveys. Considering Ma3 is 0.9 million years old, it is estimated that the middle of the Kyoto basin has been subsiding at the rate of 0.2m-0.3m/1000 years.
(3) From the revealed shape of the interfaces in the Osaka group, neither a large fault nor a large fold is found at the very position where UEMURA and NAKATSUKA (1992) suggested a buried fault.