Abstract
The Kanto Earthquake in 1923 triggered a widespread debris-avalanching on the slopes of the Tanzawa Mountains (Fig. 2), located about 50 km southwest of Tokyo, and since then small debris avalanches have frequently taken place there due to heavy rainfalls. For two small sample areas, A and B (Fig. 1), the scars of debris avalanche on the aerial photographs taken in 1946 and 1971 were plotted on maps of a scale of 1:5000 by a photo-grammetric method with Autograph A8 and A7 (Fig. 3 and Table 1).
The scars are classified into the following three types according to their origin. 1) “New scars” formed after 1946 can be recognized only on the photographs taken in 1971. 2) Scars recognized both on the photographs taken in 1946 and 1971 are called “continuously naked scars”. 3) Naked scars only on the photographs taken in 1946 are called “vege-tated scars”.
It is considered that the mountain slopes are eroded by such processes as catastrophic debris avalanches, continuous debris fall in the naked and vegetated scars, and soil erosion on vegetated slopes. If each temporal change in area of these scars and vegetated areas (Sai, Sb, Sc and Sd) is known, the volume (DV) eroded during the period from t1 to t2 can be calculated from the formula (1). But it is assumed that each erosion rate of various processes is constant with time as shown in Table 2.
1) The maximum and minimum volumes of erosion for one year (1970_??_1971) were estimated by formulas (2) and (3) on the assumption that all of new scars were formed by debris-avalanching in the year of and before 1970.
2) Many scars observed on 1946's photographs can not be recognized on 1971's, being covered by vegetation grown during 25 years. The decreasing rate (R) of the area of these scars is expressed by formula (4). If new scars formed after 1946 have also been covered with vegetation at the same rate as R, area of new scars in 1971 may be ex-pressed by the formula (5), provided that new debris avalanches occurred with the same area (Sai) at every accident. The eroded volumes during the period from 1946 to 1971 were obtained as shown Table 3, assuming that the debris-avalanching happened every year, every 3 years, every 5 years, every 10 years and once in 1959. If it happened every 5 years, the eroded volume in the area A is expressed as the volume of solid in Fig. 5. Judging from the record of daily rainfalls in Yokohama about 40km east of the surveyed area, it is considered to be the most probable among the above cases that torrential rainfalls have occurred at an interval of about five years and triggered the debris-avalanching during the period in the Tanzawa Mountains (Fig. 4).
The most preferable estimate of erosion rate per unit area in the surveyed areas is in the order of 103m3/km2 year. This rate seems to be consistent with the maximum value of the various estimates of denudation rates from the sedimentation rate in many reservoirs of Japan. Most of the scars in 1946, which are inferred to have occurred triggered by the earthquake in 1923, are extraordinarily larger in area than the recent scars. These large debris avalanches may have occurred at an interval of 100 years or more, but small debris avalanches have occurred every several years and, therefore, are considered to be one of the most continuous erosion processes together with soil erosion.
From the consideration mentioned above it is concluded that the amount of erosion by continuous erosion processes during long period are comparable in order with that by cata-strophic erosion processes such as debris avalanches of extraordinarily large scale. If the above-estimated erosion rate is applicable to erosion processes in geologic times, the moun-tains would be dissected several hundred meters in relative height during several hundred thousand years.
