Special San Fernando Earthquake Edition

Seismically Triggered Landslides

Above San Fernando Valley

By Douglas M. Morton

The interpretation of aerial photographs has indicated that more than 1,000 landslides, distributed over a 100-square-mile (250 sq. km) portion of the hilly and mountainous terrain above San Fernando Valley were triggered by the earthquake and its aftershocks. Overall distribution of the landslides was determined primarily by the intensity of ground shaking; the local density of landslides reflected differing local geology. Where ridge tops have been severely fractured, abundant landslides may develop later when saturated with water.

The photos used in this preliminary study were a set of U. S. Geological Survey black-and-white aerial photographs taken on February 9 by American Aerial Surveys, Inc. (scale 1:10,000) and small scale color photographs and infrared photographs taken on February 9 by aircraft assigned to the National Aeronautics and Space Administration, and color photographs taken February 18 by the U.S. Air Force, scale 1:20,000. Photo coverage was adequate for the area (see figure, page 80), with the exception of the northeast part, where clouds largely obscured the ground.

Landsliding took place in response to the shaking of the ground during the primary earthquake shock, although some rockfalls that continued intermittently for several days along steep canyon walls (e.g., Pacoima and Little Tujunga Canyons) may have been, in part, triggered by aftershocks. Landslides were concentrated in a 100-square-mile (250 sq. km) area of foothills and mountains above San Fernando Valley south-southwest of the epicenter of the main shock, adjacent to the zone of surface faulting and within the area of principal aftershocks.

Approximately 1,000 landslides, ranging in length from about 50 feet to more than 1,000 feet (15 to 300 m) were mapped from the photographs and a few were checked in the field. One of the larger landslides, 600 feet (200 m) across is shown in the photo on page 84. Rockfalls, soil falls, debris slides, debris avalanches, and slumps were the principal types of landslides triggered by the earthquake. Surficial debris slides (6 inches to 3 feet 0.2 to 1 m-thick) and avalanches are probably the most widespread and common type of failure and were especially pronounced in areas underlain by sedimentary mlrs. Rockfalls were common on steep canyon walls cut in well fractured basement rock, but also occurred in particularly resistant sedimentary beds which formed a cliff. Soil falls occurred mainly on the steeper faces of recent stream terrace deposits along major drainages. Slumps were larguly limited to reactivation of older, pre-earthquake, slumps.

Landslide distribution was controlled primarily by the intensity of ground shaking. Within the area of intense shaking, variation in landslide density reflects local geologic conditions, such as type of rock units and degree of fracturing, including pre-existing landslide deposits and fault zones. As might be expected, very steep areas generally produced more landslides than more gentle topography.

The relation of the amount of landsliding to the intensity of shaking is particularly clear in the area underlain by eastward-striking Pico, Modelo, Towsley, and Saugus Formations west of San Fernando Valley. There, the number of landslides decreases markedly along strike westward from the valley away from the areas of intense shaking. The control of landslide distribution by rock units is best seen in the vicinity of Lopez and Bartholomaus Canyons (see figure, page 82).

Landslides were also common in man-made excavations. Numerous roads were blocked by landsliding in cut slopes, and innumerable rock falls occurred in road cuts throughout the San Gabriel Mountains.

No new large rotational or complex landslides, such as are characteristic of the San Gabriel and Santa Susana Mountains, were triggered in this earthquake. Apparently different circumstances have been responsible for slides of these kinds in the past. There are however, features along the valley margin near the San Fernando Ranger Station that may be indicative of incipient landsliding, perhaps related to the numerous fractures in the hills above the ranger station. If this is, or becomes a slide, it will be a large one.

The ground was fractured over a wide area, as shown in the figure on page 80. Ridge tops were shattered, as discussed on page 79; it is possible that, when saturated with water, these fractured ridge tops may give rise to more sliding than took place during and immediately after the earthquake.