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Seismically induced rock slope failures resulting from topographic amplification of strong ground motions: The case of Pacoima Canyon, California |
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| Authors: | Sergio A Sepúlveda William Murphy Randall W Jibson David N Petley |
| Institution: | aSchool of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK bDepartamento de Geología, Universidad de Chile, Casilla 13518- Correo 21, Santiago, Chile cU.S. Geological Survey, Box 25046, MS966, Denver Federal Center, Denver CO 80225, USA dDepartment of Geography, University of Durham, Durham DH1 3LE, UK |
| Abstract: | The 1994 Northridge earthquake (Mw = 6.7) triggered extensive rock slope failures in Pacoima Canyon, immediately north of Los Angeles, California. Pacoima Canyon is a narrow and steep canyon incised in gneissic and granitic rocks. Peak accelerations of nearly 1.6 g were recorded at a ridge that forms the left abutment of Pacoima Dam; peak accelerations at the bottom of the canyon were less than 0.5 g, suggesting the occurrence of topographic amplification. Topographic effects have been previously suggested to explain similarly high ground motions at the site during the 1971 (Mw = 6.7) San Fernando earthquake. Furthermore, high landslide concentrations observed in the area have been attributed to unusually strong ground motions rather than higher susceptibility to sliding compared with nearby zones. We conducted field investigations and slope stability back-analyses to confirm the impact of topographic amplification on the triggering of landslides during the 1994 earthquake. Our results suggest that the observed extensive rock sliding and falling would have not been possible under unamplified seismic conditions, which would have generated a significantly lower number of areas affected by landslides. In contrast, modelling slope stability using amplified ground shaking predicts slope failure distributions matching what occurred in 1994. This observation confirms a significant role for topographic amplification on the triggering of landslides at the site, and emphasises the need to select carefully the inputs for seismic slope stability analyses. |
| Keywords: | Landslides Earthquakes Site effects Slope stability |
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