Landsliding in Southern California, 2005
Jonathan Godt and Mark Reid
The winter storms of January and February 2005 triggered widespread landslide activity in the Southern California region. On March 16 and 17, 2005, Mark Reid and Jonathan Godt of the U.S. Geological Survey's Landslide Hazards Program and Pam Irvine of the California Geological Survey performed an aerial reconnaissance to assess the regional distribution of recent landslides and identify areas with exceptional concentrations of landsliding. Our aerial reconnaissance extended from the Ventura-Santa Barbara County line to northern San Diego County. The map and photographs shown on these web pages provide a preliminary overview of landsliding in the region, but are not a comprehensive inventory of either landslides that occurred this winter season or of damage caused by landslides. Each photograph shown is stamped with the position of the aircraft and the time (Pacific Standard Time) when the photograph was taken.
Throughout the area there is recent evidence of isolated, shallow landsliding in much of the steep terrain. In addition, there are local areas heavily impacted by abundant slope failures. Our initial assessment indicates that the areas with the greatest concentrations of recent landslides are located in Ventura County and in the mountainous areas north of Los Angeles. Most of the recent landslides were shallow failures triggered by storm rainfall. The ongoing hazard from most of these failures appears negligible. A notable exception is the landslide above the town of La Conchita (Photo 2), described in Open File Report 2005-1067.
We observed high concentrations of recent shallow landslides and debris flows in Ventura County along the Pacific coast west from the City of Ventura to the Santa Barbara County line (Photos 1, and 2) and in a band that runs east near Ventura along the north side of the Santa Clara River Valley (Photos 4, 5, and 25). The area from Fillmore east through Castaic (Photos 5 and 6) and north of Santa Clarita (Photos 7 and 8) appears particularly hard hit. In less populated areas, road and pipeline corridors were affected (Photo 3 and 24). Additionally, there are abundant recent shallow landslides on the north side of the Santa Susana Mountains southwest of Santa Clarita (Photo 9).
Along the range front of the San Gabriel and the San Bernardino Mountains from around Newhall in the west to Redlands in the east, there are scattered and locally moderate concentrations of shallow landslides and debris flows (Photos 10, 11, and 13). In many of the stream channels and debris basins along the range front there appears to be fresh debris and sediment (Photos 12, 21, 22, and 23). There are pockets of moderately abundant shallow slides along the west side of the Santa Anna Mountains from Corona to the Pacific Coast in Orange County (Photo 14). We also identified areas with scattered landslides in the Hollywood Hills (Photo 15), and on the south and north flanks of the Santa Monica Mountains near Malibu (Photos 16) and Thousand Oaks (Photo 17), respectively.
In the areas of northern San Diego County burned in 2003 by wildfire we saw little evidence of landsliding or debris flow activity. In the Paradise Fire area we observed some fresh sediment in stream channels and on a few fans (Photo 18); in the Cedar Fire we saw little evidence that a significant amount of sediment was mobilized by this winter season’s rains (Photos 19 and 20).