Abstract: Landslides and Groundwater Related to the 1992-1993 Winter Storms Along the Liberty Canyon Fault Zone in the Central Santa Monica Mountains, City of Agoura Hills, California

Frank E. Denison

The Liberty Canyon fault is a northwest-trending fault about 15 km long. The fault is bifurcated to the south or even two separate parallel faults. The north end of the fault appears to be braided. The fault is concealed by alluvium over most of its trace. New data from landslide investigation suggests that the fault turns westward at the north end of Liberty Canyon at the intersection of the Ventura Freeway. The fault either becomes a low-angle thrust fault or several parallel normal faults to the west. In addition to the fault zone, there are numerous folds in the Miocene marine sediments with axial traces that are parallel to and bend around the north end of the fault.

The Liberty Canyon fault zone and its related folds are only now being recognized as a significant groundwater barrier that localizes the development of pore-water pressures that activate landslides. The intense rains of 1993 triggered these conditions and reactivated landslides threatening numerous residences at the top and toe of the slides. In the study area, two recent landslides can be attributed to an accumulation of groundwater along faults, folds, and broken bedrock thus increasing the amount of water available for infiltration. Several residences were threatened and only one was lost in 1993 as a result of poor placement/construction of subdrainage and also to the lack of recognition and understanding of the significance of the folds and associated fault zone. Additional slop failures also occurred along this fault zone in 1969, 1978, and 1990 during periods of intense rainstorms.

The study area has been mapped by several researchers over the last 40 to 50 yr, however, there is no consensus as to the location of the Liberty Canyon fault, since it has not been observed at the surface. Subsurface data suggest that several hundred vertical feet of offset are on the east side of the fault.

The land use has changed from ranching to housing. Portions of the western Santa Monica Mountains along the Ventura Freeway corridor have now become an urbanized community within the cities of Agoura Hills and Calabasas. Initial construction of these residences began in the early 1960s and 1970s. The lack of a clear understanding of the geologic structure of this fault has allowed placement of some residences within an area with a previously unrecognized potential for storm damage during intense rainstorms.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994