Dynamic analysis of a slope failure from the 6 August 1979 Coyote Lake, California, earthquake

Abstract

The Coyote Lake, California, earthquake of 6 August 1979 (M_L = 5.7) provided a rare opportunity to perform a dynamic numerical analysis of a seismically induced slope failure using actual strong-motion records. The earthquake displaced a preexisting slump on the northeast shore of Lake Anderson and formed a fissure 20 m long; offsets of 9 mm vertically and 18 mm horizontally were measured across the fissure. The slump is 9 km northwest of the located epicenter but within 100 m of the mapped trace of the Calaveras fault. Two strong-motion records were written in the vicinity of the slump: Coyote Creek, 5 km to the southeast, and Gilroy #6, 15 km to the southeast. These records combined with field measurements of the slope, estimates of the material properties, and a dynamic numerical model based on the Newmark analysis of seismic slope stability to calculate a predicted displacement for the landslide. A displacement of 27 mm, calculated using the N50°E component of the Gilroy #6 record, is in excellent agreement with the displacement measured in the field (21 mm). However, displacements of only 0.12 mm, calculated using the 250° component of the Coyote Creek strong-motion record, reflect significant local variations in seismic shaking intensity. Because strong-motion records and slope failures rarely coincide, this landslide provides an important test case for our understanding of the dynamics of seismically induced landslides, as well as a warning that local seismologic variations need to be considered in any design applications.