Age and recurrence of coseismic rock avalanches in Sierra de la Sobia (Cantabrian Mountains, Spain)

Abstract

Dating of rock avalanche clusters along faults may provide paleo-earthquake data for locations where direct fault-rupture evidence is rare. This study focuses on the rock avalanche cluster preserved in the western slope of Sierra de la Sobia (Cantabrian Mountains, Spain), where evidence of Quaternary faulting was previously reported at the Marabio Fault segment, a splay of the long-lived León Fault. In a previous study we interpret this cluster as seismically induced based on the kinematic analysis of local fault structures, the block size distribution analysis of several rock avalanche accumulation bodies, and the stability analysis of current slopes. In this new study we date these deposits to assess the timing of the recurrent events and evaluate a paleo earthquake record. We determine the ages of 20 boulders from the coarse carapace of two calcareous rock avalanche deposits using the 36Cl Cosmic Ray Exposure dating technique. Our results indicate that the youngest instability event occurred 8.5 ± 0.6 ka ago and was simultaneously recorded by the recent lobes of the two rock avalanches and could correspond to the latest severe earthquake striking this area. The analysis of boulder age clusters in the middle and distal lobes of both rock avalanches reflects up to four additional coseismic instability episodes between ∼22.0 and ∼10.9 ka, with a recurrence interval of ∼3.7 ka on average. Extending this chronological analysis to other rock avalanche clusters could improve our understanding on landscape evolution and its potential for developing regional paleo-earthquake catalogs for the Cantabrian Mountains and other similar contexts.