Deep-seated gravitational slope deformations triggered by extreme rainfall and agricultural practices (eastern Michoacan, Mexico)

"The study of deep-seated gravitational slope deformations (DSGSD) in Mexico is scarce; therefore, their localization and causes are highly overlooked. The present paper examines the characterization of the DSGSD of Jungapeo and Las Pilas in eastern Michoacan state, currently active and endangering their inhabitants. An integrated study, including detailed lithology, morpho-structural inventories, analysis of land use, and pluviometric regime, was performed and complemented with differential global positioning system monitoring networks. Both landslides developed over highly weathered volcano-sedimentary rocks. On the one hand, the Jungapeo landslide has an estimated volume of 990,455 m3 with steady decreasing velocity rates from 41 to 15 cm/month in the first monitoring period to 13–3 cm/month in the last one. On the other hand, the Las Pilas landslide estimated volume is about 1,082,467 m3 with a stable velocity rate of 1.3 to 0.1 cm/month. Despite the multi-storeyed style of activity, two behaviors of instability were distinguished: slow deformation and secondary landslide stages. The conditioning factors for slow deformation in both DSGSD are the combination of weathered lithology with clay- and sand-rich content, and the shift toward intensive monoculture. The triggering factor is related to excess water produced by an inefficient flood-irrigation system that also generates an atypical acceleration behavior in both landslides during the dry season. The DSGSD activity thus predisposes the generation of tension cracks and secondary scarps from which the collateral landslides are triggered by atypical rainfall, such as that of 2010.