Distribución de tamaños de bloques en avalanchas de rocasentendiendo el significado no fractal de un proceso sismo-activo

Resum

We have investigated the block size distribution (BSD) and their respective roughness and eccentricities within four rock avalanche deposits triggered seismically onto the western and southern steep limestone crags of Sierra de la Sobia (Cordi- llera Cantábrica, NW Iberia). The BSD has been determined by the digital analysis of ortho-images with enough resolution for the hand-outlined block size (ri) > 1dm and it is characterized by the slope, D of its log(frequency)-logri histogram. The BSD is not fractal; we observe two slope for all BSDs. The larger block size fractions (ri > 1m) have slope D-values ranging between 2.70 and 2.24, whereas the block size fractions ri < 1m have very lower D-values. Therefore, the block fragmenta- tion is developed by a similar breakage dynamic process within all the rock avalanches analysed, and it is characterized by two-stage: (i) dynamic and ballistic fragmentation from the aftershock rupture scarp; (ii) the further fragmentation by wear and attrition from the scar slope to the accumulation body, caused the BSD-homogenization within the block size fractions r i < 1m. The block size at the slope change (rk) is the same for all the accumulation bodies analysed and such slope change probably represents a change in the dominant fragmentation mechanism from (i) to (ii). rk is a lithology-dependent parameter (limestone in this study) that might be controlled by the fragmentation law of Kick (1985).