The large-scale landslide landforms, wish source areas wider than 1 km were mapped by air-photograph interpretation in the Tohoku district between latitude 38°N and latitude 42°N. On the basis of the mapping, characteristics of the distribution of those large-scale landslide landforms are discussed from geomorphological and geological points of view.
Six regions are recognized as those of concentration in the central part of the Ou Mountains, the Shirakami Mountains, the Hinotodake-Kamura Mountains and the northern part of the Asahi Mountains. Those regions mostly coincide with the areas of the maximum uplift during the Quaternary time in the Tohoku district. Those are also underlain by the zones of the Lower and Middle Miocene sedimentary rocks and submarine volcanic rocks, commonly called “Green Tuff”, and the Quaternary volcanic rocks. The topography in those zones are characterized by cap rock structures.
The large-scale landslide landforms of the Quaternary volcanic areas are dominantly found in the old and/or dormant volcanoes, and are roughly divided into three types from the main contributing factors of landsliding, the volume in source area, morphology and so on.
(1) Type 1 is a catastrophic landslide or collapse of a volcanic cone associated with volcanic activity. Its tophographic and structural feature is composed of the so-called the collapse caldera of the source area and hummocky relief of the deposition area. The latter is a typical topographic feature of volcanic dry avala nche deposits composed of megablocks and interstitial fine material. The volume of the source area is the largest of the three types. Examples can be seen in Iwate volcano, Iwaki volcano, Gassan volcano, Cheikai volcano, Zao volcano and Shirataka volcano.
(2) Type 2 shows a typical landslide landform associated with a gentle arc-shaped main scarp and a moving mass. Main scarp of this type is found on the slope of a thick lava flow near the top of the volcano.
The activity of the volcanoes, where the landslides of this type occurred, ceased in the late Pleistocene and seems to be almost dormant at present.
The contributing factor to the landsliding may be a large quantity of water supply related to the late Quaternary climatic change.
Water are prepared in the form of the perennial snow patches and are supplied by the rapid melting of the perennial snow patches with the rising air temperature. The formative age of the perennial snow patches ranges from the late Pleistocene to the eariest Holocene (period about 18, 000y. B. P. to 8, 000y. B. P.).
Because the sea level of this period is lower than that of the present, mountains were vigorously dissected. Then the stability of the mountain slopes rapidly deteriorated and a number of large-scale landslides occured.
Those landslides are found in Hachimantai volcano, Yakeishi volcano, Kurikoma volcano, Funakata volcano, Gassan volcano and Zao volcano.
(3) Type 3 is recognized along the dissected vally wall in the pyroclastic deposits and pumice fall deposits. Type of movement is mostly rotational. The volume in source area is smaller than that of the type 2. However, the same contributing factor may be important for the landslides of type 3 as the type 2.