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 mega-collapse 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
=α exp (−β D
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 α and β of the fitted curve are 170286.4 and 0.000429, respectively. Based on a geomorphological interpretation of both α and β values, it can be concluded that the collapse volume of the Old Yotei volcano is ca.1.3 km3
, 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.
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