The Senya fault system is one of the most distinguished active reverse faults in Japan. It strs eches along the eastern fringe of northern part of the Yokote Basin, Akita Prefecture. Four main earthquake faults (surface brakes) appeared along the Senya fault system, when the Riku-u Earthquake (M=7.2) occurred in the Mahiru Mountains on 31 August 1396 (Fig. 1). Dip angle of the fault shows about 20 degrees at the fault outcrop at Jyurokusawa (Fig. 3). Prehistoric fault activities along the fault system have been revealed by observing the stratigraphy at fault outcrops of Jyurokusawa and Komorizawa (Figs. 3 and 5) and by examining fault topographic references at Sankichi-zawa (Figs. 4 and 7). Seismic events of two different periods are inferred from fault activities shown by the geological and topographical evidences along the fault system. One is 1896 A. D. that is, Riku-u Earth-quake. The other event is estimated to have occurred after ca. 4, 400 yr B. P., but no destructive prehistoric earthquake have been occurred during the period between 1894 A. D. and ca. 2, 700 yr B. P.. It will be safe to say that the pre-1896 earthquake occurred between 2, 700 and 4, 400 yr B. P. (Fig. 8).
The purpose of this symposium was to discuss some problems on regional systems of coral islands located in the Ryukyu Islands, or the South-West Islands of Japan. Especially on some regional characteristics of physical environments related with social and cultural problems were expected to discuss, to construct the new methodology of regional geography. The participants in this symposium were made up of fifteen reporters and five commentators. The papers presented by reporters are as follow: Takahashi, T., Kawana, T. and Koba, M.: Geomorphological characteristics of coral islands in the Ryukyu Islands. Urushibara, K.: Calcarious soils and their genetic environments in the Ryukyu Arc. Furukawa, H.: Water behavior in the Calcarious Islands. Toguchi, K., Yoshikawa, H. and Mezaki, S.: Physical characteristics and human activities in the Islands of Okinawa Prefecture: land use pattern by multi-variate analysis. Nakamura, K., Kobayashi, S., Moriwaki, H. and Nakai, T.: Seasonal pattern of traditional agriculture in the Ryukyu Arc. Domae, R.: Traditional co-operative shops and structure of village community in the Ryukyu Islands. Ogawa, T.: Okinawa Islands and Mainland Japan from view point of cultural geography. Nakamatsu, Y. Physical environments and traditional religion in the Ryukyu Islands. On these reports, the commentators posed challenging questions, made constructive comments and led the discussions with attendants fruitfully. The discussions were focused on the following points: 1. How to treat some gaps of conception on physical environments between recent geography and that of inhabitants, to construct the regional geography of these islands. 2. Some methodological problems on regional geography of the Ryukyu Islands as another KULTURKREIS in Japan located in the coral islands world.