NEW LANDSLIDE MAPPING OF THE PITAS POINT AND VENTURA QUADRANGLES, VENTURA COUNTY, SOUTHERN CALIFORNIA

The California Geological Survey (CGS) recently completed new landslide mapping of the Pitas Point and Ventura quadrangles as part of the Seismic Hazards Zonation Program. These 1:24,000-scale maps will be published online in the CGS Landslide Inventory Map Series. The inventory maps were prepared by geomorphic analysis and interpretation of stereo-paired aerial photographs, LiDAR and NAIP imagery, on-line oblique photo-mosaics on Bing^TM, and topographic maps, as well as limited field observations and review of previous mapping. Landslide deposit and source (scarp) areas were digitally compiled and key attributes were assigned to each landslide.

Landslides are a significant geologic hazard in the Pitas Point and Ventura area, having caused the disruption of the road and railway along the coast as far back as 1865, damage to oil wells along the Ventura Avenue-Rincon anticline trend, and the destruction of homes in the town of La Conchita in 1995 and again in 2005, when 10 lives were lost. Landslides are common throughout the area because it is underlain primarily by weak, fine-grained, sedimentary strata that have undergone rapid, late Quaternary tectonic uplift, folding, faulting and subsequent erosion, resulting in the development of steep-sided canyons and coastal bluffs. Landslides, including local earth flows, are most concentrated in the “Mud Pit shale.” The distribution and failure mechanism of bedrock landslides are controlled primarily by the underlying geologic structure and its relationship to slope gradient and orientation. Translational and compound failures are dominant near fold axes and on dip slopes. Several large, dormant old landslides occur in the Sespe Formation on the flanks of the Red Mountain anticline. Rotational and wedge-type failures are the dominant mechanisms where neutral or antidip bedding is exposed in high steep slopes. Shallow debris slides and flows commonly develop in colluvium, weathered rock, and existing slide debris after periods of heavy rainfall, particularly in the Pico and Rincon Formations. Extensive Late Pleistocene debris deposits on the southwest side of Rincon Mountain are largely a combination of debris flow sheets and long-runout landslides that are unique in the area and have been designated as debris fans on the inventory map.