稗田山大崩壊の崩積土と崩壊の要因  [in Japanese] Cause of the Hieda-yama Large Landslide  [in Japanese]

In 1911-1912 a large landslide had occurred at the north rfot of Hieda-yama in the upper reaches of the Ura-kawa river, a tributary of the Hime-kawa river. It gave rise to the great debris now which dammed up the Hime-kawa river. Eventually the dam eollapsed and brought out much disasters in the lower reaches of the Hime-kawa river. The present paper discusses the cause (internal factors) of this large landslide. For elucidating the cause, the authors have analyzed also the landform of the old slide area east of Itakura-zawa in the north-eastern part of Hieda-yama. The conclusion reached by the authors is itemized below. 1. One of the internal factors of the large landslide in the above-mentioned area is the existence of decperosional valleys of the Ura-kawa, Kanayama-zawa and old Kanayama-zawa rivers which show almost straight courses. These erosional valleys must have been produced by the two events: 1-A. Formation of the margin of the collapse basin created by the high-angle faults that took place along the rivers. 1-B. Effusion of the Hieda-yama andesite which widely covered the area after the basin's collapse, immediately followed by the gush of hot spring accompanied by mineralization, along the above-mentioned marginal fault. Thus pyrite and other minerals were exuded, resulting in what is called an alteration zone. The ground water in the alteration zone comes to be of sulfuric acid type and intensely weathers the surrounding rocks, so that the severed ownward erosion as mentioned above would progress. 2. Another internal factor of the Hieda-yama large landslide is the occurrence of tension cracks due to gravity, running parallel to the surfacc of the Hieda-yama side cliff of the valleys produced by the severed ownward erosion. Shear cracks also were developed along the base of the cliff. The two types of cracks, in the process of their development, grew into mutually connected clefts. These cracks could have worked as a cause of repeated landslides. 3. A lowland formed in the 1911-1912 large landslide is extending in front of Hieda-yama at present. It is considered that old sliding blocks wered eveloped there previously, in the manner similar to that of the old sliding blocks east of Itakura-zawa today. Accordingly, a large scale landslides like the 1911-1912 event may take place in this area of old landslide if earthquake of heavy rainrall should tigger.