The Oita district is a standard area for the Cenozoic stratigraphy in West Japan. Above all the Tsurusaki hills is regarded as the most important area including the type sections of three formations. Main parts of the Takio, the Higashiwasada and the Tsurusaki formations are distributed respectively in the central to north area, in the south and north extremity. They are overlain by the higher terrace sediments. In the south of the hill, the gravels of the Higashiwasada formation, the Takio formation and the higher terrace sediments are developed in intricate pattern and their stratigraphic relation is misleading suggesting that some of these are possibly contemporaneous. While the known index fossil from the formation is only Stegodon orientalis from the basal part of the Tsurusaki formation. Accordingly the supposed ages of the Takio and Higashiwasada formation based on an indirect correlation is not accurate. Therefore it is important to clarify their ages. For settling these problems, the fission track age determination of the accessory zircon from pyroclastic rocks from above mentioned formations were done after a systematic field survey. In norder to observe fission tracks easily and correctly, the etching procedure was examined and the condition was finally settled as following; the etchant was 2:1 mixture of 48%HF and 98% H2SO4 and etching is carried out under pressure at 250°C The Shikido pyroclastic rock member of the Higashiwasada formation and the Hada pyroclastic rock member of the Takio formation were selected for examination. The age of the Shikido and the Hada pyroclastic rock members are (6.52±1.34)×106 year and (1.44±0.28)×106 year, respectively. The age of the Shikido pyroclastic rock member is comparable with some volcanic rocks in Central Kyushu, for example biotite andesite in Kitatakaki District, Nagasaki Prefecture (5.7m.y.B.P.), biotite hornblende rhyolite in Hirado City (5.8m.y.B.P.), etc. Considering sporadical distribution, in Central Kyushu of volcanic rocks with similar lithology to those of the Higashiwasada formation, it must be concluded that the volcanic activity of this age is more extensive than has been supposed. The Hada pyroclastic rock member is correlative with the Meisei tuff bed at MaO horizon of the Osaka group. Comparing the relative position of the Hada pyroclastic rock member and the Stegodon orientalis horizon in the Oita group with the relative position of the Meisei tuff bed and the Stegodon orientalis horizon in the Osaka group, the writer conclude that the Takio formation correspons to the lower and middle parts of the Osaka group.
The Noto Peninsula, which projects northeastwards from central Japan is the largest peninsula in the area along the Sea of Japan. This peninsula mostly consists of low relief erosion surfaces and marine terraces truncating the Neogene rocks. Many active faults which displace these geomorphic surfaces as well as alluvial fans are observed as shown in Fig. 1. Figures 2 to 12 represent the detailed topographies and profiles near and across the active faults. All the active faults are expressed as clear fault scarps or scarplets, and most of them are reverse faults with upwarping of the terrace surfaces on the upthrown side. Active faults in this peninsula are classified into three types according to their bearing on geomorphic development. Type I is the first order active fault which resulted in the differentiation of mountain blocks as indicated in Fig. 1. Bijosan I, II and Sekidosan Faults belong to this type. Ochi Depression delineated by these faults at both margins is a kind of ramp valley in a restricted sense rather than graben, as shown in Fig. 13. Fault scarplets at younger uplifted fans (L1) indicate the faulting has still continued until recently. Type II is the second order fault, represented by large scale height difference of marine terraces, and caused subdivision of each mountain block. Togigawa and Sakami Faults belong to this type. All the other active faults except those mentioned above belong to type III, which has resulted in local deformation of marine terrace surfaces. Faults of this type are usually less than 2km in length and less than 20m in vertical displacement. It is especially interesting that the seaward portion of terrace surfaces generally upthrust against their inland parts. Therefore, active faults of type III can be easily recognized by such an abnormal inland-facing scarplet on terrace surface. Active faults in this area are listed in Table 3. It is noticed that the rate of faulting is always more than 10cm/1, 000 years in types I and II, while it is usually less than that in type III. The amount of vertical displacement even in type III is, however, thought too large to be caused by a single earthquake, so that repeated faultings must be considered. Direction of principal axis of maximum compressive stress is N40-60°W, which is inferred from the frequency distribution of trend of active reverse faults shown in Fig. 14. Fault mechanism of a destructive earthquake of 1933 shows also a maximum pressure direction of approximately E-W, probably with a reverse faulting. The direction above mentioned is almost the same as that in the inland areas of central Japan. It is noteworthy, however, that there is a clear difference in fault type between the Noto Peninsula and the other areas of central Japan where strike-slip active faults predominate.