Instrumental shaking thresholds for seismically induced landslides and preliminary report on landslides triggered by the October 17, 1989, Loma Prieta, California earthquake

Abstract

The generation of seismically induced landslide depends on the characteristics of shaking as well as mechanical properties of geologic materials. A very important parameter in the study of seismically induced landslide is the intensity based on a strong-motion accelerogram: it is defined as Arias intensity and is proportional to the duration of the shaking record as well as the amplitude. Having a theoretical relationship between Arias intensity, magnitude and distance it is possible to predict how for away from the seismic source landslides are likely to occur far a given magnitude earthquake. Field investigations have established that the threshold level of Arias intensity depends also on site effects, particulally the fracture characteristics of the outcrops present. in the case of the M= 7.1, October 17, 1989, earthquake that struck the San Francisco Bay region, thousands of landslides were triggered over an area of 14,000 km2. The most numerous were rock falls, rock slides and soil slides. Landslides produced a complex pattern of surface fractures, in addition to the ones of tectonic origin related to regional structures. The most common deformational features related to earthquake triggered landslides are extension fractures and scarp; the least common are welldefined folds or fractures along the toes.