LAUSR

Abstract Sierra de la Sobia is a linear mountain range of arcuate shape, located west of the Cantabrian Mountains. The long-lived crustal Leon Fault runs along its western hillslope, and its trace is partially covered by multiple rock-avalanches. 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 co-seismic events, with average recurrence intervals much longer than the time covered by the regional historical and instrumental period.