Restored Image of Volcanic Sector Collapse at Old Yotei Volcano, Southwestern Hokkaido, Japan, from a Geomorphological Interpretation of the Distribution of Hummocks

Abstract

Hummocks are conically shaped mounds, which occur following catastrophic sector collapses, and are mainly observed around volcanoes. Empirical relationships of hummocks proposed by Yoshida et al. (2012) are basically applicable if hummocks now distributed on a debris avalanche depositional area do not have remarkable disturbances due to topographical barriers or modifications after deposition. Using these empirical relationships, the Old Yotei volcano, whose megacollapse in the Late Pleistocene has not been clarified yet, is examined. The results show that the size-distance relationship of hummocks (N ＝ 297) , distributed west of the volcano can be expressed by an exponential function as, A ＝ exp (-D) , where A is the average area of a hummock within a certain span and D is the average distance from the source. For the Old Yotei volcano, the values of coefficients and of the fitted curve are 170286.4 and 0.000429, respectively. Based on a geomorphological interpretation of both and values, it can be concluded that the collapse volume of the Old Yotei volcano is ca.1.3 km 3 , and the equivalent coefficient of friction of the debris avalanche is 0.129. The latter also reveals a high probability that the pre-collapse summit of the Old Yotei volcano reached 1700 m a.s.l. Thus, the image of the flank collapse of the Old Yotei volcano has been restored more decisively by the distribution of hummocks.

Magnitude-Frequency Distribution of Hummocks on Rockslide-Debris Avalanche Deposits and Its Geomorphological Significance

“…Hummocks were identified by using a stereoscope to manually interpret aerial photographs, with occasional reference to a geomorphic map, such as the Land Condition Map of Volcanoes published by the Geographical Survey Institute (GSI) of Japan ( Figure 2). The hummock size-distance relationship and morphological processes of some of these terrains have been investigated previously [14,[18][19][20][21][22]. For two avalanche deposits on Bandai volcano, a few small hummocks were previously excluded from the regression analysis of the size-distance relationship, as likely derived from secondary and much smaller collapses following (possibly just after) the huge main collapse event [18].…”

“…Therefore, in this research, the equivalent coefficient of friction (H/L) was used as a non-dimensional mobility index to characterize rockslide-debris avalanche events ( Table 1). Some of the values in the table are based on published sources [14,[18][19][20][21][22], and others have been re-calculated or newly estimated. In general, H/L values of volcanic rockslide-debris avalanches range from 0.05 (more mobile) to 0.18 (less mobile) [24], and all of the avalanches included in this study have H/L values in this range.…”