Landslides are the most deadly secondary effect of earthquakes in mountainous regions. Dr Noelle Bontemps (University Grenoble Alpes) has uncovered the secrets of a slow landside in Peru by monitoring post-seismic landslide dynamics in both the dry and wet seasons. GPS and seismic data confirm that in combination, earthquakes and rainfall cause greater landslide motion than either force alone. There is a strong decrease in soil rigidity following earthquakes. Rigidity recovers rapidly during the dry season and slowly in the wet season, with soil rigidity a key factor in controlling landslide movement.

Globally, landslides are the most deadly secondary effect of earthquakes in mountainous areas. Despite this, the mechanisms behind landslide triggering remain poorly understood; nowhere is this truer than in regions where landslide kinematics are intricately linked to seismic activity and rainfall. Improved understanding of landslide triggering is vital for effective hazard management and mitigation.

To tackle this gap in knowledge, a group of researchers has spent 10 years building an international collaboration focused on a remote corner of South America. Scientists from the Institute de Recherché pour le Development (IRD) in France, and those from the Institute Geological Mineral y Metallurgical (INGEMMET) in Peru have worked together in the high Andes to uncover the secrets of a persistent, slow landside on the southern wall of Peru’s Colca Valley.

Landslides are widely perceived as catastrophic events; to the casual observer, landslides conjure an image of soil, rocks, and other debris hurtling downslope at speeds on the order of meters per second. However, slow landslides move at a fraction of this speed, creeping downslope at just millimetres to meters per year, with some persisting for decades or longer. Unless transitioning to rapid flow through catastrophic failure, such events tend not to pose a threat to life; however, they are major drivers of morphological change, sometimes on a landscape-scale, and can cause extensive damage to property
and infrastructure.