Abstract
A magnitude-frequency analysis of rockslide-debris avalanche deposits was performed. Hummocks are conical mounds formed in debris avalanche deposits from the catastrophic sector collapse of a mountain (often volcanic) that represent relatively cohesive fragments of the mountain edifice. Examination of 17 debris avalanche deposits in Japan and the Philippines showed that, in general, the larger the magnitude of the hummocks, the smaller their frequency. Hummocks followed an exponential distribution: log10N(x) = a – bx, where N(x) is the cumulative number of hummocks with magnitude ≥ x and a and b are constants; x is equal to log10A, where A is the area of a hummock. The constants a and b were positively correlated. The value of b, which differs among avalanches and in this analysis ranged between 1 and 3, may be controlled by the mobility of the debris avalanche. Avalanches with higher mobility (relatively longer runout) have higher b and potentially produce more numerous fragments forming hummocks (i.e., higher a). From the above correlation, the magnitude-frequency relationship can be used to roughly estimate the original height of the collapsed volcanic body, if the runout distance of the rockslide–debris avalanche can be estimated with sufficient accuracy.
Highlights
Many studies have investigated the magnitude-frequency relationship of earth surface processes and landforms [1,2]
Occasionally called molards, are conical mound-like landforms that protrude from the surface of debris avalanche deposits associated with the catastrophic sector collapse of a volcanic edifice or other mountain [6,7,8,9]
Such hummocks generally decrease in size downstream [6,10,13], and this phenomenon has been quantitatively investigated in Japan [14]
Summary
Many studies have investigated the magnitude-frequency relationship of earth surface processes and landforms [1,2]. Failures of volcanic masses consisting of alternating lava and tephra deposits are reported to produce hummocks when these brittle domains separate from each other during the collapse [11,12] Such hummocks generally decrease in size downstream [6,10,13], and this phenomenon has been quantitatively investigated in Japan [14]. Simple models that describe the progressive breaking up of volcanic masses or rockslide-debris avalanche blocks (hummocks) during avalanche spreading [15,16,17] have been partly proposed to explain this decrease In these models, the volcanic edifice is initially broken into a few mega-blocks, and these mega-blocks subsequently break up along pre-existing weak surfaces before their emplacement as hummocks. The magnitude-frequency distribution of hummocks may provide new geomorphological insight into the processes of rockslide-debris avalanches
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