Mechanisms of co- and post-seismic landslides in the Colca valley, Peru, following M5+ earthquakes

During earthquakes, landslides are triggered both co-sesmically and post-seismically over several weeks or years. The triggering mechanisms of these two phases encompass a combination of dynamic loading during the shaking, fluid migration from sediment contraction, bulk damage inside the landslide mass, fluidization of clay layers, and subtle interplays between landslide units. These different mechanisms are still poorly quantified, due in particular to the paucity of dynamic parameters acquired on landslides during earthquakes. Slow-moving landslides, with their persistent motion through time, provide a unique opportunity for monitoring different physical parameters, including displacements and material mechanical properties. As a consequence, they provide a strong interest for studying the mechanisms of landslides during seismic forcings.

The Maca and Madrigal landslides, located in southern Peru at an altitude of 3,400 m, are two nearby slow-moving landslides (~1m/year) located in a highly seismic environment (Colca valley). For this reason, the two sites have been instrumented since 2012 and 2017 respectively, with permanent GNSSs and broadband seismometers. In October 2021, March 2022 and June 2023, the instruments recorded the response of the landslides to 3 major shallow earthquakes (M_l5.2, 5.5, 5.2 at distance between 3-15 km). Both landslides displayed a co- and a post-seismic motion of different magnitudes and characteristics. In particular, the post-seismic motion is systematically delayed by 2 days on the Madrigal landslide, and the October 2021 earthquake reactivated the whole landslide mass, inactive for at least 6 years. We analyze this dataset, together with seismic, InSAR, Pléiades satellite images and weather data to decipher the mechanisms at play during the co- and post-seismic phases.