中部日本に発達する阿寺断層は第四紀において最も活動的な断層の一つである.断層露頭の詳しい観察や変位した河岸段丘の調査により,この断層に沿って発生する有史以前の地震活動が明らかにされた. 有史以前の地震の時期を決定するための14C年代測定用の数多くの腐植土壌が加子母村小郷の阿寺断層の露頭から得られた.また,地震活動に伴う断層変位量を測定するために,木曾川右岸の坂下地域において河岸段丘の変位量を計測した.地震は断層に沿ってそれぞれ約10,300y. B. P.以前, 7,900y. B. P., 5,500y. B. P.そして1,850y. B. P.に繰り返し発生している.地震の発生間隔は決して等しくなく,先行する地震に伴う変位量にそれぞれの間隔が対応している.断層変位を受けた河岸段丘から得られる平均垂直変位速度は,地震活動に対応する変位量とそれに引き続く地震の発生間隔から0.71m/103年と算出される.また,垂直変位量と左横ずれ変位量は1:5の割合になっている (Sugimura and Matsuda, 1965) ことより,平均左横ずれ変位速度は3.55m/103年と算出される.将来の阿寺断層に沿っての大地震の発生は, Time-predactable recurrence model (Shimazaki and Nakata, 1980) の考えをもとにすれば,約1,000年後に推定される.
The objective of the present paper is to make clear soil water characteristics of Kanto Loam by presenting data obtained in laboratory. The experimental apparatus originally developed by Richards was used to measure the pressure head and the hydraulic conductivity for two undisturbed test samples collected from Tachikawa and Musashino loam formations at Yatabe near Tsukuba Academic New Town. The results obtained are summarized as follows: 1. The hysteresis effect between the pressure head and the water content is clearly shown for Kanto Loam. 2. Maximum water content for the sorption curve is smaller than that of the desorption curve because a part of pore space is filled with entrapped air. The values of critical saturation for two test samples are 0.93 and 0.95. 3. The water-entry value is nearly equal to a half of the air-entry value. 4. The unsaturated hydraulic conductivity depends on the pressure head and the water content.
Uphill-facing scarplets are often distributed on the high mountain slopes of Japan. They have been called “double ridges”, or “linear depression”, the origin of which has been disputed among geomorphologists. In this paper the author investigated their topographical characteristics such as mode of occuerrence, height, distribution and cross sections of rides on which they appear. He also refers to their origin applying a two-dimensional finite-element calculation of the self gravitational stress to a model mountain, on the slopes of which the scarplets are often distributed. The results can be summarized as follows. 1. The scarplets are mostly recognized in high mountain areas with high relief caused by the rapid uplift during the Quarternary (Fig. 2). 2. Most of them appear on the mountain slopes higher than 2, 000m a. s. l., but some are recognized on the slopes lower than 1, 000m a. s. l. in case of the slopes with high relief (figs. 3, 5 and 6). 3. They are mainly formed on ridges deeply dissected by V-shaped valleys (e. g. the Kurobe) (Fig. 7), cross sections of which consist of two parts; very steep lower slopes formd by deepening and gentle upper slopes or flat surfaces around ridges. Therefore, the sections generally show convex outlines as a whole (Figs. 8 and 9). Scarplets are usually located just above breaks between steep and gentle slopes. 4. Brittle fractures caused by the self load of mountain will be expected to occur around ridges of mountain (Figs. 11 and 12) by the analysis applying the finite-element method. Therefore, uphill-facing scarplets may be originated from the subsidence of ridges caused by local faulting.