The term “active fault (Katsu-danso)” appeared in the 1920's in papers of some geomorphologists in Japan. It has been used for faults active in Quaternary or late Quaternary, although the usage of the word “active” and its age-limit are different by authors. At any rate, the active fault is significant in geology in that the evidence of fault movements is recorded in the topography on the fault trace which enables detail analysis of the faulting, and in that the faulting might be detectable by means of geophysical methods, and it might be closely related to the occurrence of an earthquake. Recent studies in Japan showed that an active fault moves under a regionally-stressed condition of the crust, which has persisted during recent geologic time. If so, an “active fault” could be defined as a fault which is active under the present-day stress system. Then, the origin, orientation, and variation in time and space, of the “present” stress system are to be an important and fruitful research subject on the Tertiary to Quaternary tectonic history of the Japanese Islands. Some of the recent investigation on the active fault began to focus on this line.
Studies of active faults in Japan have started from two points: the geological studies of the earthquake faults by geologists and the studies of the fault topography by geomorphologists.
The geological studies of the earthquake fault was commenced by Koto, B. (1894) who had observed the surface break of the Mino-Owari earthquake of 1891 along the Neo Valley fault. Since then, geologists have investigated and described more than ten earthquake faults from Japan and Formosa. Particularly through the experiences of 1927- and 1930-earthquakes, it became clear that the mode of displacement along the earthquake fault during the earthquake is the same in the sense of displacement as the mode of the long-term displacement through the geologic periods. For example, Kuno, H. (1936) found that the Tanna fault which moved a few meters left-laterally during 1930 earthquake, has accumulated left-lateral displacement ca. 1000m since the middle Pleistocene.
Almost independently of the geologists'works, geomorphologists had investigated the fault topography of Japan. By these studies, it had become clear before the War II that there are many fault scarps and fault valleys in this country, which have been produced probably by Quaternary faulting. Tsujimura, T. (1932) published a distribution map of topographically-recognized faults, in which 413 fault systems were registered.
Recently, the geological and geomorphological studies have been jointly performed and the movement-history of some active faults in Quaternary time are clarified quantitatively (Table 1). It is also shown that many active faults hitherto considered to have only vertical displacement are strike-slip faults accompanied by lesser dip-slip components. The Atera fault (Sugimura & Matsuda, 1965), the Atotsugawa fault (Matsuda, 1966), and the Median Tectonic line (Kaneko, S., 1966; Okada, A., 1968), which was described recently, are examples.
Some features of the active strike-slip faults described from Japan may be summerized as follows:
A number of active faults or fault zones are present particularly in central Japan (the Chubu, Kinki, and Shikoku districts). They are, however, less than one hundred kilometers in length and a few kilometers in total displacement of the basement rocks, except for the Median Tectonic line (and probably the Itoigawa-Shizuoka Tectonic line).
There is a marked regularity in the fault systems of the central Japan between the trend of faults and the sense of the displacement: the NW-trending faults are left-lateral, whereas the NE-trending faults are right-lateral. This implies that the earth's crust of the region is under the same stress system having the maximum (compressional) principal axis of approximate east-west.
