Abstract
The writers describe the location and active mode of a fault (Funai fault) lying concealed under the town area of Oita City, by analyzing lots of the drilling survey, the reflection method and the Geo-slicer investigation.
The Funai fault is the eastern part of“Beppu bay south coast fault group”, which com p osed of the Horita-Asamigawaa fault, Beppu bay south coast fault and Funai fault, extended from west to east with extension about 20km.
The very position in Oita plain is considered from Kasugaura to the south of Maizuru bridge located at the left bank of Oita river.
It was c o nfirmed in the drilling survey, and the amount of vertical displacement of the K-Ah volcanic ash layer in a main fault is 16m at the Funai castle traverse line. According to existing bore hole data, the maximum amount of displacement of the Funai fault is 18m to the west of Funai castle. The average rate of vertical displacement after the K-Ah volcanic ash (6,300 years BP) is 2.2-2.5m/1000 year, and the Funai fault is estimated one of the A class active faults in Kyushu.
The latest activity of the Funaifault is thought to be time between before 1,540yBP dated at the lower part of the uppermost mud layer which does not show the displacement and after 2,350yBP dated in the peat layer in upper sand and gravel bed which shows displacement.
The problem in the future is the clarification of the fault continuation to the further east, and it is necessary to elucidate accurate distribution and the activity history of the concealed faults lying under whole the Oita plain.
