U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005–3107

September 2005

Prepared in Cooperation with the California Geological Survey

Southern California Landslides—An Overview

Printed on recycled paper

Southern California’s Complex Landscapes 

Contribute to Landslides

Southern California is tectonically 

active and geologically complex, and 

this complexity strongly controls 

when and where landslides occur 

throughout the region. (See the USGS 

Web site that describes the geological setting and history of southern 

California, http://scamp.wr.usgs.

gov/scamp/html/scgeo.html for more 

information.) In general, landslides 

are more common where slopes are 

steep and rocks are weak, and these 

conditions are all too common in 

southern California. Steep slopes 

are abundant because ongoing tectonic deformation pushes up rock 

that is too weak to support the steep 

slopes. Some of the uplifted material 

is geologically very young sediment 

that has not consolidated into more 

resistant rock. However, even older 

and harder rocks have been sheared 

and fractured by tectonic movement 

along the plate boundary, and thus 

they can be quite weak too. The weak rock weathers 

and erodes rapidly to form steep-sided gullies and 

larger valleys. It is this steep, rugged terrain carved 

into relatively weak material that sets the stage for 

widespread landsliding. Southern California experiences many kinds of landslides: shallow, rapid debris 

flows triggered by heavy rainfall, rock falls from 

steep slopes as a result of earthquakes, and slower 

moving slumps and earth flows on slopes that can 

fail under many different conditions.

Winter Rainstorms and Debris Flows

Almost all of southern California’s rain falls during 

the winter rainy season, which typically lasts from 

November through April. The most common landslides triggered by winter storms are debris flows 

(popularly called “mudslides”), which are shallow 

View of the La Conchita landslide taken March 16, 2005. The lightest-colored, exposed 

rock to the left of the center of the photo is the main scarp of the 1995 slide. The 

southeast part of the 1995 deposit (central part of photo; it extends into a housing 

development) remobilized in 2005. Photograph by Jonathan Godt, USGS. A report on 

the La Conchita landslide is available online: http://pubs.usgs.gov/of/2005/1067

Introduction

Southern California lies astride a major tectonic 

plate boundary defined by the San Andreas Fault 

and numerous related faults that are spread across 

a broad region. This dynamic tectonic environment has created a spectacular landscape of rugged 

mountains and steep-walled valleys that compose 

much of the region’s scenic beauty. Unfortunately, 

this extraordinary landscape also presents serious 

geologic hazards. Just as tectonic forces are steadily 

pushing the landscape upward, gravity is relentlessly tugging it downward. When gravity prevails, 

landslides can occur.
landslides of water-saturated soil and rock fragments 

that travel downslope rapidly as muddy slurries. The 

flowing mud carries rocks, vegetation, and other 

natural and man-made debris as it rushes down the 

slopes. Debris flows can reach speeds up to 56 kilometers/hour (35 miles/hour). Hillsides left denuded 

by wildfires, and drainages filled by debris washed 

off these denuded slopes, are also susceptible to 

flooding and debris flows during and immediately 

following heavy rainstorms. Debris flows most commonly form after heavy rainfall onto steep slopes 

underlain by weak rock units. Although many steep 

slopes appear stable when dry, they can, without 

warning, produce damaging debris flows when saturated by intense rainfall, water from broken pipes, 

or misdirected drainage from roads, roofs, or large 

paved areas.

As a general rule, at least 250 millimeters (10 

inches) of seasonal rainfall are needed to make 

southern California hillsides susceptible to debris 

flows. Once the seasonal rainfall exceeds 250 millimeters, intense rainfall—more than 50 millimeters 

(2 inches) in 6 hours in the lowlands or more than 

100 millimeters (4 inches) in 6 hours in the mountains—can trigger debris flows. Although the likelihood of debris flows begins to decline after a day 

or more of dry weather, deeper, 

generally slow-moving landslides 

can occur days, weeks, or months 

after a period of prolonged rainfall 

has ended.

Deep-Seated Landslides

Although much of the water 

from prolonged rainfall runs off 

or evaporates, some soaks into 

the ground. It gradually moves 

downward through soil and bedrock, particularly during years of 

above-average rainfall. In many 

places, fractures or impermeable 

layers of rock or soil deflect this 

downward-moving water laterally 

toward a slope face. There, the 

water can weaken the slope and 

cause landslides long after the rain 

has stopped. Movement is usually 

triggered by cumulative rainfall 

during long periods (weeks to 

years). Most landslides that result from this process 

are relatively deep earth flows and translational or 

rotational earth slides and rock slides. Translational 

landslides are typically a few meters to tens of meters 

deep, and rotational slides range in depth from 

several meters to tens of meters. Deep-seated translational and rotational landslides, including rock slides, 

tend to fail a little at a time and move more slowly 

than debris flows, but a few do accelerate to rapid 

movement. Consequently, these deep slides rarely 

cause loss of life, but they commonly destroy homes 

and infrastructure in hillside areas.

Earthquake-Induced Landslides

Ground shaking from earthquakes is another important cause of landslides in southern California. The 

most common types of earthquake-induced landslides are relatively shallow falls and slides, in which 

highly disrupted masses of rock and soil travel down 

slopes at high speed. For example, the January 17, 

1994, Northridge, California, earthquake (M = 6.7), 

triggered more than 11,000 landslides in an area of 

about 10,000 square kilometers (6,200 square miles), 

and almost all of the triggered slides were shallow, 

brittle failures of surficial rock and soil. The majority of the landslides were concentrated in the Santa 

Susana Mountains and the mountains north of the 

Denudation of slopes in the Santa Susana Mountains. All of the light-colored areas are 

landslides that failed during the Northridge earthquake; many of these existed before the 

earthquake and were reactivated. Harp, E.L., and Jibson, R.W., 1995, Inventory of landslides triggered by the 1994 Northridge, California earthquake: U.S. Geological Survey 

Open-File Report 95–213, 17 p. http://pubs.usgs.gov/of/1995/ofr-95-0213/
For more information

1. The California Geological Survey (CGS) 

phone: (213) 239-0878, http://www.consrv.ca.gov/cgs/

2. U.S. Geological Survey, http://www.usgs.gov, 

http://landslides.usgs.gov

3. Information on southern California geology and the

Southern California Areal Mapping Project (SCAMP): 

http://scamp.wr.usgs.gov/

4. Campbell, R.H., 1975, Soil slips, debris flows, and rainstorms 

in the Santa Monica Mountains and vicinity, southern California: U.S. Geological Survey Professional Paper 851, 51 p.

5. Many cities and counties also employ geologists or contract with consulting companies that are knowledgeable about 

landslides. For lists of registered professionals please see the 

Association of Engineering Geologists (AEG):

http://www.aegweb.org, and the American Society of Civil Engineers (ASCE): http://www.asce.org. 

Lynn Highland

National Landslide Information Center

U.S. Geological Survey

Federal Center, Box 25046, MS 966

Denver, CO 80225 USA

1-800-654-4966

http://landslides.usgs.gov

This fact sheet is available on-line at 

http://pubs.usgs.gov/fs/2005/3107

Publication layout by Margo L. Johnson

Landslide scars above US 101 west of Ventura. Photograph by 

Jonathan Godt, March 2005.

Santa Clara River valley, where most 

of the earthquake energy was directed. 

These areas are underlain by weak, 

geologically young rock that is being 

uplifted and severely deformed by compressional forces generated along the 

tectonic plate boundary. Analysis of the 

distribution of landslides induced by the 

Northridge earthquake forms the basis 

for seismic landslide hazard analysis of 

southern California. It was later discovered that dust generated by these same 

landslides caused an outbreak of valley 

fever (coccidioidomycosis) that hospitalized dozens of people and led to three 

fatalities. (See http://landslides.usgs.

gov/recent/archives/1997northridge.php

for more information on the valley fever 

outbreak.)

Man-Made Landslides

Although several types of humaninduced landslides are common in southern California, most can be avoided or mitigated. 

They are most commonly a result of building roads 

and structures without adequate grading of slopes, of 

poorly planned alteration of drainage patterns, and of 

disturbing old landslides. Most of these situations can 

be avoided through careful planning and design by 

qualified professionals and knowledgeable oversight 

by appropriate regulatory agencies.

Bluebird Canyon landslide, June 1, 2005, near Laguna Beach, California. This area 

experienced a landslide in 1978, about 50–100 meters (150–300 feet) to the west of 

the 2005 event. The 2005 landslide caused 350 homes to be evacuated, of which 

15 were damaged or destroyed. The movement was most likely caused by heavy 

rains in January and February 2005. This type of deep-seated landslide movement is common in southern California in the months following a season of heavy 

rainfall. Photograph by Jim Bowers, USGS.
Before a Storm:

1. Watch the patterns of storm-water drainage on 

slopes near your home, and note especially the 

places where runoff water converges, increasing 

flow over soil-covered slopes. Watch the hillsides 

around your home for any signs of land movement, such as small landslides or progressively 

tilting trees.

2. Become familiar with the land around you. Learn 

whether landslides have occurred in your area by 

contacting local officials, state geological surveys 

or departments of natural resources, and university departments of geology. Slopes that contain 

known landslides are likely to move again in the 

future.

3. Support your local government in efforts to 

develop and enforce land-use and building ordinances that regulate construction in areas susceptible to landslides and debris flows. Buildings 

should be located away from steep slopes, streams 

and rivers, intermittent-stream channels, and the 

mouths of mountain channels.

4. Contact your local authorities to learn about the 

emergency-response and evacuation plans for 

your area, and develop your own emergency plans 

for your family and business.

During Heavy Rainfall:

1. Stay alert and stay awake! Many debris-flow fatalities occur when people are sleeping. Listen to a 

radio for warnings of intense rainfall. Be aware 

that intense short bursts of rain may be particularly dangerous, especially after longer periods of 

heavy rainfall and damp weather.

2. If you are in areas susceptible to landslides and 

debris flows, consider leaving if it is safe to do so. 

Remember that driving during an intense storm is 

hazardous.

3. Listen for any unusual sounds that might indicate 

moving debris, such as trees cracking or boulders 

knocking together. A trickle of flowing or falling 

mud or debris may precede larger flows. If you 

are near a stream or channel, be alert for any sudden increase or decrease in water flow and for a 

change from clear to muddy water. Such changes 

may indicate debris-flow activity upstream, so 

be prepared to move quickly. Don’t delay! Save 

yourself, not your belongings.

4. Be especially alert when driving. Embankments 

along roadsides are particularly susceptible to 

landslides. Watch the road for collapsed pavement, 

mud, fallen rocks, and other indications of possible debris flows.

After Heavy Rains—When the Danger Is SlowMoving Landslides:

Areas prone to landslides:

• On or immediately above old landslides

• On or at the base of slopes

• In or at the base of minor drainage hollows

• At the base or top of an old fill slope

• At the base or top of a steep cut slope

• Developed hillsides where leach field septic systems are used

Features that might indicate potential landslides:

• Springs, seeps, or saturated ground in areas that 

have not typically been wet

• New cracks or unusual bulges in the ground, street 

pavements or sidewalks

• Soil moving away from foundations

• Ancillary structures such as decks and patios tilting or moving relative to the main house

• Tilting or cracking of concrete floors and foundations

• Broken water lines and other underground utilities

• Leaning telephone poles, trees, retaining walls, or 

fences

• Offset fence lines

• Sunken or down-dropped road beds

• Rapid increase in creek water levels; water may be 

darker or look muddier than usual

• Sudden decrease in creek water levels although 

rain is still falling or has just recently stopped

• Sticking doors and windows, and open spaces 

around jambs and frames that indicate they are out 

of plumb

What to do if you suspect imminent landslide danger:

1. Get out now!

2. Contact your local fire, police, or public works 

department.

3. Inform affected neighbors.

What Can You Do If You Live Near Steep Hills?