Shaking intensity thresholds for rock falls and slides: Evidence from 1987 Whittier Narrows and superstition hills earthquake strong-motion records

Due to inconsistencies in using descriptive intensity scales to describe earthquake shaking necessary to trigger landslides, Arias intensity, a quantity calculated from seismic strong-motion records, has been employed as a measure of the seismic energy required to initiate landslides. Because Arias intensity is defined as the sum of all the squared acceleration values from a strong-motion record, it is a measure of the energy dissipated at a site by shaking. It incorporates both amplitude and duration information, making it more useful to compare with the failure of slopes than parameters, such as peak acceleration. Arias intensities calculated from the 24 October 1987 Superstition Hills and the 1 October 1987 Whittier Narrows earthquakes in southern California allow comparisons with the limits of landslides, specifically, falls and slides from rock and soil in these two earthquakes. These comparisons establish two shaking thresholds for the limits of falls and slides. A range of 0.08 to 0.6 m/sec is observed for Tertiary and younger deposits, while the range of 0.01 to 0.07 m/sec is observed for Mesozoic and older rocks. These two ranges of shaking levels are controlled by the relative degree of fracturing and aperture of fracture systems within the two general age groups of rocks and soils in the Los Angeles area and the Imperial Valley.