Gravitational slope processes triggered by past earthquakes on the western Cantabrian mountains (sierra de la Sobia, northern Spain)

Abstract

Sierra de la Sobia is a linear mountain range of arcuate shape, located west of the Cantabrian Mountains. The longlived crustal León Fault runs along its western hillslope, and its trace is partially covered by multiple rockavalanches. Here we discuss if recurrent paleo-earthquakes in the M6+ range could have acted as the triggering factor for rock-avalanches in a region where the historical and instrumental seismic records only include seismic events up to Ms 4.6. Evidence concurring with a coseismic origin includes: (i) kinematic of current minor transverse and parallel faults to Sierra de la Sobia being consistent with a horizontal North-South compression of the Cantabrian Arc; (ii) slopes of the depletion zones that would become unstable if horizontal seismic ground rose 0.10–0.15 g; and (iii) block size-reduction by dynamic fragmentation creating block size distribution with fractal dimension, ranging between 2.24 and 2.70. U/Th dating of seven calcite precipitates coating the blocks of cemented rock-avalanche deposits suggesting that cementation has taken place in multiple episodes since MIS-9a. Neotectonics of Sierra de la Sobia are expressed in subtle changes in relief after extremely rapid coseismic events, with average recurrence intervals much longer than the time covered by the regional historical and instrumental period.