Changes in ground deformation prior to and following a large urban landslide in La Paz, Bolivia revealed by advanced InSAR

Abstract

We characterize and compare creep preceding and following the 2011 Pampahasi landslide (~40 Mm 3 ± 50%) in the city of La Paz, Bolivia, using spaceborne RADAR interferometry (InSAR) that combines displacement records from both distributed and point scatterers. The failure remobilised deposits of an ancient landslide in weakly cemented, predominantly 15 fine-grained sediments and affected ~1.5 km 2 of suburban development. During the 30 months preceding failure, about half of the toe area was creeping at 3-8 cm/a and localized parts of the scarp area showed displacements of up to 14 cm/a. Changes in deformation in the 10 months following the landslide are contrary to the common assumption that stress released during a discrete failure increases stability. During that period, most of the landslide toe and areas near the headscarp accelerated, respectively, to 4-14 and 14 cm/a. The extent of deformation increased to cover most, or probably all, of the 20 2011 landslide as well as adjacent parts of the slope and plateau above. The InSAR-measured displacement patternssupplemented by field observations and by optical satellite images -indicate that kinematically complex, steady-state creep along pre-existing sliding surfaces temporarily accelerated in response to heavy rainfall, after which the slope quickly achieved a slightly faster and expanded steadily creeping state. This case study demonstrates that high-quality groundsurface motion fields derived using spaceborne InSAR can help to characterize creep mechanisms, quantify spatial and 25 temporal patterns of slope activity, and identify isolated small-scale instabilities. Characterizing slope instability before, during, and after the 2011 Pampahasi landslide is particularly important for understanding landslide hazard in La Paz, half of which is underlain by similar, large paleolandslides.Nat.