ABSTRACT
The Santa Clara Valley Water District monitors creep rates of the 80-hectare Penitencia Creek Landslide by measuring deflection of inclinometer casings and calculating shortening of terrestrial survey lines. Two significant earthquakes (Morgan Hill and Loma Prieta) caused discrete, almost immediate, creep displacements of 5 mm to 18 mm. Four ENSO events (El Niño Southern Oscillation) (1986–1987, 1993, 1994, and 1997–1998) caused landslide creep rates to accelerate to 10 mm to 39 mm per year, for periods of 5 to 11 months. Most of the active landslide creep occurs along a well developed shear surface in claystone of the Plio-Pleistocene Santa Clara Formation. The residual friction angle of the clay at the shear surface could be as low as 4 to 5 degrees. Creep displacements near the toe of the landslide have damaged roads, houses, and pipelines.
