Abstract
In the eastern part of Oita Plain, there are several concealed active faults with ENE-WSW direction, and they have been active in Holocene epoch. To presume the position and the activity of the faults which lay beneath the plain, the reflection method, the drilling survey and Geo-slicer investigation were done.
The northern fault named Misa Fault passes the vicinity of an former coastline and the fault in the south named Shimura fault can be confirmed even in the vicinity of Otozu river. The Oita Group and equivalent layers are covered directly with Holocene deposit in the south of the Shimura fault. The deposit of Late Glacial-Late Interglacial Stage is thick covered with Holocene deposit on the north side. In addition, marine deposit of Late Interglacial Stage exists in the subordinate position and the Late Glacial deposit is considerably thicker than that of the south from the Misa fault.
On the other hand, the fault confirmed in the Hioka district, thick deposit of Late Interglacial Stage distributes at the south of the fault, and is different from the Shimura fault. Judging from the geological features, the fault confirmed in the Hioka district has a similar character as the Funai fault. '
Thus, the Misa and the Shimura faults which are concealed by Late Quaternary deposit in this region, greatly restrict the distribution of the stratum since Late Pleistocene. The base of Holocene marine deposit(ca.10,000 years ago) is about 48m in amount of displacement and the K-Ah volcanic ash layer is 26.5-30m by the Misa fault, and the vertical slip rates are about 4.8m and 4.2-4.8m/1000 year each other. The activity of the Misa fault since Holocene is judged to be A class.
