Frequently Asked Questions - Southern California

What can you tell me about the La Conchita landslide area?
When does the U.S. Geological Survey issue a landslide advisory, and under what conditions?
What is the difference between an advisory, a watch, and a warning?
How soon does the danger of landslides end after the rain stops?
Can major landslides and debris flows like those recently experienced in southern California happen in other parts of the U.S.?
Do human activities cause landslides?
What areas are most vulnerable to landslides in southern California?
When do U.S. Geological Survey landslide personnel conduct fieldwork in southern California?
What can I do to be prepared?
Where can I find maps that show landslide hazards for southern California?
Who can I talk to for more information?
How can I find a person knowledgeable about landslides?

1. Q: What can you tell me about the La Conchita landslide area?

Images
- January 2005 La Conchita Landslide Photo
- Video of La Conchita slope failure of 2005
- USGS Open-File Report: Landslide Hazards at La Conchita, California (OFR-2005-1067)
Published information about the 1995 movement of the landslide
Published articles
- O'Tousa, James, 1995, La Conchita landslide, Ventura county, California: AEG News, Association of Engineering Geologists, v. 38, no. 4, p. 22-24.
  
  The following excerpts from O'Tousa's article outline the occurrence of the 1995 movement:
  
  "The La Conchita landslide occurred catastrophically on March 4, 1995 at 2:03 p.m. pst. The landslide movement rate was very rapid with the majority of the movement occurring by 2:30 p.m. pst. The community of La Conchita had been notified of the potential for failure and the County of Ventura had an emergency response plan implemented. The landslide destroyed (buried) or damaged seven residences, some of which were mobile homes, but no human lives were lost. Following the main failure, the weather forecast predicted an additional several inches of rain during the following week, and on March 10, 1995, a debris flow occurred in the canyon west of the slide, damaging an additional four to five residences and an additional four to five residences and a banana plantation. The community was recommended to be evacuated following the initial failure..."
  
  "The La Conchita Community is located near the base of a 1.5:1 (horizontal to vertical ascending slope with approximately 600 feet of relief... Geomorphically, the March 4, 1995, landslide is a portion of a much larger, ancient landslide... Based on eyewitness accounts from the residents, the top of the landslide dropped vertically, then the bottom flowed out. These accounts also indicate that large amounts of dust were produced as the failure occurred."
  
  "The failure area is approximately 400 feet wide, 1100 feet long, and has relief of 500 feet. The slide is approximately 10 acres in size with a rough estimated volume of 600,000 cu. yds."
  
  "Movement of the upper portion of the landslide was first reported during the summer of 1994 as evidenced by surface cracks. Smaller failures had occurred between the summer of 1994 and the main failure (March 4, 1995). These failures occurred towards a previous canyon located along the left flank of the landslide..."
- Schuster, R.L., and Highland, L.M., 2001, Socioeconomic and Environmental impacts of landslides in the western Hemisphere: U.S. Geological Survey Open-File Report 01-0276, on-line at http://pubs.usgs.gov/of/2001/ofr-01-0276/
- Wilshire, H.G., Howard, K.A., Wentworth, C.M., and Gibbons, Helen, 1996, Geologic Processes at the Land Surface: U.S. Geological Survey Bulletin 2149, 41p. (contains a brief article by Andrei M. Sarna-Wojicki p. 2-4; 482 KB PDF)

2. Q: When does the U.S. Geological Survey issue a landslide advisory, and under what conditions?

A. The U.S. Geological Survey derives its leadership role in landslide hazard-related work from the Disaster Relief Act of 1974 (the Stafford Act). The Director of the USGS has been delegated the responsibility to issue warnings for an earthquake, volcanic eruption, landslide, or other geologic catastrophe (1974 Disaster Relief Act 41 U.S.C. 5201 et seq). When we have sufficient data or knowledge of a particular area, such as southern California, we can provide information on potential hazards. Specifically, if rainfall intensity-duration thresholds for landslide activity have been developed for an area, or if landslide-hazard maps have been produced, we can issue an advisory. We work in conjunction with the National Weather Service (http://www.nws.noaa.gov/) to issue advisories and press releases regarding the potential for landslide activity. These advisories are provided relatively infrequently.

3. Q: What is the difference between an advisory, a watch, and a warning?

A: An advisory is a general statement about the potential of landslide activity in a given region relative to developing rainfall predictions. An advisory may include general statements about rainfall conditions that can lead to debris-flow activity, and list precautions to be taken in the event of heavy rainfall. A watch means that landslide activity will be possible, but is not imminent. People in, or planning to travel through, a watch area should check preparedness and stay informed about developing weather patterns. Warnings indicate that landslide activity is presently occurring and extreme caution should be taken. Watches and warnings may be issued for discrete areas, and include advice about contacting an area’s local emergency centers. Watches and warnings for rainfall-induced debris flows are weather dependent and will closely track National Weather Service watches and warnings for flash flooding.

4. Q: How soon does the danger of landslides end after the rain stops?

A: Residents near mountain slopes, canyons, and landslide prone areas should stay alert even after heavy rain subsides. While the likelihood of debris flows begins to decline after a day or more of dry weather, some deep-seated landslides may occur days—even weeks to months—after long periods of intense rainfall.

Q: Can major landslides and debris flows like those recently experienced in southern California happen in other parts of the U.S.?

A: Landslides can and do occur in every state and territory of the U.S.; however, the type, severity and frequency of landslide activity varies from place to place, depending on the terrain, geology, and climate. Major storms have caused landslides in Washington state, Oregon, California, Colorado, Idaho, Hawaii, Virginia, Ohio, Pennsylvania, Puerto Rico, and elsewhere. Rapid snowmelt has caused landslides in Utah and Washington State.

6. Q: Do human activities cause landslides?

A: The causes of landslides are complex. The main causes of recent landslides in southern California were increased soil moisture and elevated ground-water levels that resulted from recent abnormally high rainfall. Detailed on-site investigation is required to determine the importance of human factors in causing any particular landslide.

7. Q: What areas are most vulnerable to landslides in southern California?

A: Detailed locations of areas susceptible to landslides are given on “Preliminary Soil-Slip Susceptibility Maps, Southwestern California: published as a U.S. Geological Survey Open-file Report (available on-line at http://geopubs.wr.usgs.gov/open-file/of03-17). In addition, areas that have been burned by recent wildfires are highly susceptible to debris-flow activity that can be triggered by significantly less rainfall than that which triggers debris flows from unburned hillslopes. These areas are delineated in the following USGS Open-file Reports:

Areas of potential danger from debris flow include areas downslope and downstream from the susceptible areas shown on the USGS maps. Fatalities, injuries, and property damage from debris flows most commonly occur in low-lying areas such as canyon floors and near the mouths of canyons.

8. Q: When do U.S. Geological Survey landslide personnel conduct field work in Southern California?

A. USGS landslide researchers have ongoing field projects in many areas of the U.S., including southern California. USGS scientists also respond to significant landslide events, including those that result in federally-declared disasters.

9. Q: What can I do to be prepared?

A. During the drier periods between storms, watch for fresh cracks or fissures on hillsides, tilting of trees or utility poles, or other signs of recent earth movement.

Watch for changes in the patterns of storm-water drainage on nearby hillslopes and streams. Obstructed drainage may increase the potential for landslides
Monitor local National Weather Service Flash-Flood announcements, which will contain more detailed information on specific times and places of concern for debris-flow activity.

Stay awake and alert because many debris-flow fatalities occur when people are sleeping. Listen for unusual sounds such as rushing water, cracking trees, or rolling boulders.
Be prepared to move quickly. Be especially alert when driving because road cuts and embankments are often susceptible to debris flows and rock falls. Watch for fallen trees and power lines.

10. Q: Where can I find maps that show landslide hazards for southern California?

A: Detailed locations of areas susceptible to debris flow are given on “Preliminary Soil-Slip Susceptibility Maps, Southwestern California: published as a U.S.Geological Survey Open-file Report 03-017 available on-line at http://geopubs.wr.usgs.gov/open-file/of03-17 , or call 1-888-USA-MAPS to purchase a hard copy. Maps that show debris-flow hazards from basins burned by the Old, Grand Prix, Piru, Verdale, Simi, Paradise, and Cedar Fires of 2003 are available on-line at: http://pubs.usgs.gov/of/2004/1072; http://pubs.usgs.gov/of/2003/ofr-03-481/; and http://pubs.usgs.gov/of/2003/ofr-03-475

11. Q: Who can I talk to for more information?

A: The U.S. Geological Survey has a toll-free number at 1-800-654-4966, and can be reached by email at NLIC@usgs.gov. The California Geological Survey has many publications and maps of interest on landslides: http://www.consrv.ca.gov/cgs/, or phone: 916-445-1825.

Call your local emergency management office (city and/or county) to find out what kind of emergency preparedness, maps, or advice is available. Information about contacting local offices can be found in the government pages of the telephone book, or through calling directory assistance.

12. Q: How can I find a person knowledgeable about landslides?

A. The California Geological Survey has several landslide specialists on staff in its southern California office (phone (213) 239-0878, e-mail cgsla@conserv.ca.gov). Many cities and counties of southern California also employ geologists who are knowledgeable about landslides. Geotechnical consultants have listings in the yellow pages and professional societies (http://www.aegweb.org/indexf.html; http://www.asce.org/) can help you contact geotechnical professionals in your area. Consultants can contract with you for detailed investigations of your property. USGS landslide researchers can be contacted using information listed on the Landslide Program website, “People and Projects”