Abstract
The Ujigawa fault with about 9km length has been discovered by a seismic reflection (P waves) survey in the central part of the Kyoto basin and has been identified as a quite large active concealed fault revealing a flexure within the thick sediments.
To elucidate such characteristics as distribution, activity and faulting history of this fault, we have carried out several survey lines for seismic reflections and deep drilling surveys across this flexure zone, and have also performed volcanic ash analysis, carbon-14 dating, pollen and diatom analysis by using core samples obtained. In this paper, we report the results of these surveys, especially on the vertical displacement during the late Quaternary. Because of technical restriction in the survey methods, we can not discuss about the strike-slip movement.
The Ujigawa fault extends with ENE-WSW direction from the confluence of the Katsura and the Uji rivers through the course along the Uji to the south rim of the Momoyama hills, where greatly changes its trace from the southeast edge to continue the reverse fault trending north to south. The southwestward extension of this fault, on the other hand, may trace along the south side of the Takatsuki-Tennouzan fault, part of the Arima-Takatsuki fault zone, which main zone is situated in the further west-southwest along the northern fringe of Osaka basin.
From the identification of the wide-spread volcanic ash layers and the measurement of radiocarbon ages in the core samples obtained, the latest activity of the Ujigawa fault is presumed to have taken place after about 3,000 years ago. The amount of vertical displacement per single earthquake is estimated to be 1.2-2.1m and the average recurrence interval of earthquakes is roughly 16,500± 6,500 years. The average slip rate of vertical displacement for this fault was 0.09-0.12m per thousand years during, at least, the last 520,000 years. This value indicates that the activity for the Ujigawa fault belongs between the lowest rank of the class B and the highest rank of the class C in Japanese classification.
The Ujigawa fault appears to have shared an important role in the separation of the Kyoto sedimentary basin, where the northern part has relatively thin layers of the Quaternary sediments with 200-300m thickness and the southern part accompanies relatively thick layers of 500-600m.
