LANDSLIDES TRIGGERED BY THE 1994 NORTHRIDGE, CALIFORNIA EARTHQUAKE

The 17 January 1994 Northridge, California, earthquake (M(W) = 6.7) tr iggered more than 11,000 landslides over an area of about 10,000 km(2) . Most of the landslides were concentrated in a 1000-km(2) area that i ncluded the Santa Susana Mountains and the mountains north of the Sant a Clara River valley. We mapped landslides triggered by the earthquake in the field and from 1:60,000-nominal-scale aerial photography provi ded by the U.S. Air Force and taken the morning of the earthquake; the se mapped landslides were subsequently digitized and plotted in a GIS- based format. Most of the triggered landslides were shallow (1- to 5-m thick), highly disrupted falls and slides within weakly cemented Tert iary to Pleistocene elastic sediment. Average volumes of these types o f landslides were less than 1000 m(3), but many had volumes exceeding 100,000 m(3). The larger disrupted slides commonly had runout paths of more than 50 m, and a few traveled as far as 200 m from the bases of steep parent slopes. Deeper (>5-m thick) rotational slumps and block s lides numbered in the tens to perhaps hundreds, a few of which exceede d 100,000 m(3) in volume; Most of these were reactivations of previous ly existing landslides. The largest single landslide triggered by the earthquake was a rotational slump/block slide having a volume of 8 x 1 0(6) m(3). Analysis of the mapped landslide distribution with respect to variations in (1) landslide susceptibility and (2) strong shaking r ecorded by hundreds of instruments will form the basis of a seismic la ndslide hazard analysis of the Los Angeles area.