Decrease of size of hummocks with downstream distance in the rockslide-debris avalanche deposit at Iriga volcano, Philippines: similarities with Japanese avalanches

A morphometric investigation of the longitudinal distribution of hummocks at the southeastern foot of Iriga volcano in the Philippines showed that hummock size decreases away from the volcano. Aerial photographs and GIS analysis revealed that the size–distance relationship can be expressed as the exponential function A?=?α exp (?β D), where A is the area of a hummock and D is its distance from the source. This relationship is the same as that observed previously for freely spreading debris avalanches in Japan, including two avalanches at Bandai volcano. This size–distance relationship provides information about the physical characteristics of the event: the α value shows a strong correlation with the volume of the collapsed mass of the volcanic edifice, and the β value shows a strong correlation with the coefficient of friction of the debris avalanche. Thus, morphometric analysis of hummocks created by a volcanic avalanche illuminates both the physical properties of the volcanic body and the mobility of the avalanche. For the Iriga debris avalanche, the observed longitudinal hummock distribution is clearly a function of the volume of the collapsed mass and the coefficient of friction of the avalanche. The relationships so defined appear to be a geometric effect related to the areal extent of freely spreading hummocky avalanche deposits, especially their longitudinal dimensions.