The α track etch method, which is one of the geochemical survey methods for the mapping and detection of active faults and the evaluation of their activities, has been applied to the Kannawa and Kozu-Matsuda fault areas for the purpose of the study of active faults. The method conventionally measures relative radon concentration in the soil gas by counting the track density (tracks/cm2·day) recorded on a small piece of plastic film (cellulose nitrate) which is sensitive to α-ray radiation. As the result of the track measurement on survey lines crossing the Kannawa fault and the younger fault which cuts the former at Yumotodaira, the town of Yamakita, Kanagawa Prefecture, it was clarified that the peak in the track density appeared just above the points on the both fault lines which were shown by HOSHINO and HASE (1977). As the results of the track measurement on the survey lines K at the vicinity of the Kamonomiya tunnel on Shinkansen, Japanese National Railways and S1, S2, and S3 at Sogodai, the town of Ohi, Kanagawa Prefecture, crossing the Kozu-Matsuda fault system, the following was clarified: 1. A group of several peaks in the track density at the western segment of survey lines S1 and S2 corresponds, to“a”fault of the Kozu-Matsuda fault system which was shown by Research Group for Active Faults (1980). 2. A group of several peaks in the track density at the eastern segment of the survey line S2 corresponds to the eastern fault in parallel with“a”fault of the Kozu-Matsuda fault system. 3. Because there are many peaks in the track density on the survey lines S1 and S2, it is supposed that there exists a wide fault fracture zone under the Sogodai hill. 4. Some peaks in the track density on the survey line K correspond to“d”fault of the Kozu-Matsuda fault system. 5. Considerable difference in the track density between“a”and“d”faults may suggest the difference in the activity of both faults.
Depositional environments of the Makinohara Upland area, Shizuoka Prefecture, are determined from detailed sedimentological, paleontological and stratigraphical data. The results are summarized as follows: 1) The Furuya Formation (maximum thickness 30m) is composed mainly of fine-grained sediments that were deposited in shallow embayments during transgressions of the sea up narrow valleys in the early stage of a so-called Last Interglacial Age. 2) The Furuya Formation can be divided into five superposed mappable lithologic cycles (coded I-V), each of which shows a nearly complete, fining-upward, sedimentary sequence consisting of, in ascending order, a basal gravel, a sand, sandy clay, and finally clay (Figs. 5, 6). 3) The Makinohara Formation (maximum thickness 50m) comformably overlies the Furuya Formation. The sedimentary surface of the Makinohara Formation forms the terrace known as the Makinohara Upland. The Makinohara Formation is laterally divided into two lithofacies; poorly sorted gravel proximally, and well sorted sands in the distal parts of the buried valleys. The sands that were deposited near the river mouth and near the bay mouth and coastal areas of the“Paleo-Sagara-Bay” are considered deltaic and coastal sand bank deposits, respectively. 4) The history of sedimentation in the study region is interpreted as follows (Figs. 10, 11): (a) Before the Last Interglacial Age two major valleys had been cut to Neogene basement. (b) During the early stage of the interglacial transgression, fluvial sediments consisting of coarse sands and gravel were deposited along the distal parts of the buried valleys (Sedimentary cycles I-II). (c) As the transgression continued, marine water soon inundated the buried valleys and the initial“Paleo-Sagara-Bay”was formed (Sedimentary cycles III-IV). (d) During a standstill of the sea at the time of maximum transgression, a “Paleo-Sagara-Bay”became filled with fine sediments, eventually forming a marsh (Sedimentary cycle V). (e) After the maximum phase of the transgression, the“Paleo-Sagara-Bay”was totally covered by coarse fluvial deposits and/or coastal and deltaic sand. 5) Because the last (marsh) phase of“Paleo-Sagara-Bay”was at sea level, the amount of differential crustal movement within the study area can be closely estimated. It has been determined that in the uplands (Makinohara) of the northern part of the study area the contact between the Furuya Formation and the Makinohara Formation has been uplifted at least 150m (Figs. 12, 13).