Ground Failure Table of Contents Geologic Hazards

USGS Response to an Urban Earthquake -- Northridge '94

Landslides -- Common and Unexpected Effects

Documenting the Landslides and Their Effects

The earthquake caused tens of thousands of landslides over an area of 10,000 square kilometers. The landslides destroyed dozens of homes, blocked roads, disrupted pipe and powerlines, and blocked streams. They also generated massive clouds of dust, precipitating an epidemic of valley fever that caused three fatalities. While widespread and abundant throughout the region, landslides were most concentrated in the Santa Susana Mountains immediately north of the San Fernando Valley, and in the mountains farther north in the vicinity of Piru.

Scientists from the USGS arrived in southern California on the day of the earthquake and worked for several weeks documenting the extent and effects of landslides, and evaluating the continuing hazards they might pose. The USGS mapped the precise locations and shapes of more than 11,000 individual landslides using extensive fieldwork and high-resolution aerial photography taken by the U.S. Air Force on the day of the earthquake.

Location base map

This map shows landslide locations and shapes for a part of the earthquake-affected area. This inventory of landslides, digitized from high-resolution aerial photography, is the basis for making a predictive model of future seismic landslide hazards.
High detail Encapsulated PostScript files of the landslide inventory.
(UNIX Z compressed 6MB size)
(GZIP compressed 4MB size)
(PKZIP compressed 4MB size)

The areas of greatest landslide concentration occurred in weak, poorly cemented, geologically young rock and soil. These materials have been rapidly uplifted through the eons by events like the Northridge earthquake, and because of their weakness are rapidly eroded by rainfall, earthquake shaking, and other processes. The balance of rapid uplift and erosion in these weak materials has produced steep-walled canyons with high topographic relief, and slopes that fail readily during earthquakes and rainstorms.

The most common types of landslides were shallow rock falls and rock slides, numbering in the thousands, and up to tens of thousands if very small failures are counted. These landslides were highly disrupted, chaotic jumbles of soil, rock, and vegetal debris. In some areas, more than 75 percent of slope areas were denuded by landslides.

Santa Susana Mountains

The light-colored areas in this photograph are slopes laid bare by landslides in the Santa Susana Mountains caused by the earthquake.

Santa Susana Mountains

These failures ranged in volume from a fraction of a cubic meter to a few hundred thousand cubic meters. Some of the larger slides traveled several hundred meters from their sources. In some cases, landslide debris completely filled canyon bottoms, or traveled as far as 200 meters onto flatter areas beyond the bases of slopes. The debris in canyon bottoms poses a secondary threat because it can be mobilized during rainstorms to form debris flows. Such flows are common, problematical, and often disastrous throughout southern California, and their potential may now be enhanced in the earthquake-affected areas.

Deeper, more coherent landslides called slumps and block slides numbered in the tens to hundreds, and occurred primarily in the Santa Susana and Santa Monica Mountains. Although these were far less numerous than the shallow slides and falls, they contributed significantly to the total volume of landslide material because they tended to be much larger. A few of these features had volumes of several million cubic meters. Most of these occurred along a single ridge between Piru and Castaic Junction where a weak clay formation is exposed in the mountains.
Landslide damage to buildings, roads, pipelines, and other structures was not as severe as it might have been in a more extensively developed area. However, landslides blocked many roads in the Santa Susana and western San Gabriel Mountains, hampering relief efforts and exacerbating the overall transportation problems caused by the earthquake. Landslides did extensive damage to roads, pipelines, and well machinery in oil fields in the Santa Susana Mountains. Dozens of homes in the central and eastern Santa Monica Mountains were moderately or severely damaged by movements of large block slides.

Home in Palisades

This home in Pacific Palisades near Santa Monica was damaged when the coastal bluff on which it was built failed during the earthquake. The landslide caused half the house to be torn loose and cascade down the steep slope.

Landslide Susceptibility -- Where do Future Dangers Lie?

USGS scientists are using various properties of slopes and earthquake shaking to forecast the stability of slopes during future earthquakes in the region. The forecast is in the form of a susceptibility map of the Los Angeles 1:100,000 topographic map quadrangle. An example of a part of the susceptibility map shown here indicates 6 degrees of stability of slopes during earthquakes, ranging from extremely stable to unstable. Principal characteristics of the map are its great detail and its digital format. Additionally, the map was constructed in collaboration with county geologists, the California Division of Mines and Geology (CDMG), private consulting firms who are the people and entities most likely to need and use it in the future.

susceptibility map

This is a forecast of the stability of slopes during earthquake shaking in a mountainous area north of Los Angeles. This map can be used with earthquake-shaking information to produce a variety of scenarios of slope failures during postulated earthquakes of different magnitudes and locations. The map can be used for planning development in sloping areas, emergency-preparedness planning, and predicting lifeline disruptions in future earthquakes.

The susceptibility map is constructed using data for each 10-meter-square cell in the area covered by the map. These data include slope steepness and measures of the strength of geologic materials. The data are necessary inputs to a mathematical slope-stability model that yields a single numerical value for each cell. This value is called the critical acceleration, which is a measure of the level of earthquake shaking that must be exceeded to cause the slope in that cell to fail. The numerical values of critical acceleration can be grouped into colors for displays like the map sample shown on this page. Scientists can test and adjust the susceptibility map using the landslide inventory data derived from the Northridge earthquake.

Lessons Learned

The Northridge earthquake provided an unprecedented amount of high-quality data useful for analyzing instabilities in earth slopes subjected to seismic shaking. The highly detailed landslide inventories and hundreds of strong-motion records of variations in earthquake shaking allowed scientists to examine landsliding in great detail. These data, combined with high-resolution topographic and geologic data, resulted in the susceptibility map—a forecasting tool of exceptional quality and flexibility.

Scientists also learned, in the case of the valley fever outbreak, that earthquakes typically provide surprises beyond the realm of efforts to forecast hazards.

Valley Fever

A major outbreak of "valley fever" occurred in Ventura County in the weeks following the Northridge earthquake. In just 8 weeks, more than 200 valley fever cases were diagnosed, which is 16 times the normal infection rate. The cases were heavily concentrated in the Simi Valley rather than in the San Fernando Valley where the earthquake was centered. Both the timing and location of the valley fever cases suggests that the outbreak was caused by dust generated by landslides caused by the earthquake. Canyons in the Santa Susana Mountains, northeast of the densest cluster of valley fever cases, produced many dust-generating landslides during the shaking. Prevailing winds during and after the earthquake blew from the northeast to the southwest, and carried the dust from the canyons into Simi Valley and beyond. The valley fever outbreak led to hospitalization of more than 50 people and ultimately to three fatalities.

Valley fever thumbnail

Dust from landsliding blows out of the Santa Susana Mountains into the Simi Valley where an outbreak of valley fever occurred following the earthquake.

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