SLOPE STABILITY OF THE LA PAZ BASIN, BOLIVIA

The La Paz basin, Bolivia, has been a landslide hotspot throughout the Holocene. It was the site of numerous large (10⁷ to 10⁹ Mm³) failures during the early and middle Holocene. The city of La Paz, which covers most of the basin floor, experiences damaging (10² to 10⁶ Mm³) landslides nearly every year. However, historic failures and the present activity of the large paleolandslides are poorly documented. We use remote sensing and field investigation to characterize landslide activity in the La Paz basin since the early twentieth century. Interpretation of air photos (1935 to 2006) and high-resolution optical satellite imagery (2005 to 2011), along with geologic maps of La Paz, provides data on the location, type, source material, and general timing of historic landslides. Advanced interferometric synthetic aperture RADAR (InSAR) analysis of RADARSAT-2 scenes (2008 to 2011) identifies and quantifies slow slope deformations. InSAR results show recent motion at several ancient landslides of decimeters per year, which is probably indicative of long-term activity. More than 100 landslides have occurred in the city since ca. 1935, although some small events undoubtedly have escaped notice because of resettlement soon after failure. Over the past two decades, during which the record is nearly complete, 25 of 42 events occurred during the four-month rainy season. Historic landslides are concentrated near the limits of still-active paleolandslides, where differential motion reduces stability. Additionally, lateral margins of numerous small recent landslides correspond to gullies in-filled during urban expansion since ca. 1975. The Pampahausi paleolandslide was moving slowly before 2011, when a catastrophic reactivation destroyed ~2 km² of the city and displaced ~6000 residents. It has continued to move since then. Other large paleolandslides could similarly accelerate to produce more rapid mass movements. Our insights into landslide processes and controls in La Paz can aid urban risk reduction by guiding land-use-planning and mitigation measures.