Seismic Potential of the San Clemete Island Region

Saenz, Joseph M.¹, Frank E. Denison², James E. Slosson³ (1) Naval Facilities Engineering Service Center, Port Hueneme, CA (2) Frank Denison Consultant, Westlake Village, CA (3) Slosson and Associates, Van Nuys, CA

A large syncline trends north-south through Northwest Harbor. Synclines can focus and amplify earthquake energy considerably in the geologic strata. Therefore, Northwest Harbor has the potential to experience significant site amplification of seismic energy released nearby. Wilson Cove has a much smaller north-south tending, fault-bounded syncline. The potential of refraction of earthquake energy off a faults surface may cause site amplification.

Both Northwest Harbor and Wilson Cove are only about three miles from the San Clemente Fault. Historically, faults in the area have produced one large earthquake, a 5.74 magnitude event in 1951. More importantly, movement along the San Clemente Fault could theoretically generate an earthquake of magnitude 7.2. Such a large earthquake could cause bedrock accelerations to exceed gravity.

Distant earthquakes may cause site amplification to increase locally to damaging levels. The San Clemente Fault had been previously mapped in the study area, along with other fault-bounded synclines, and Miocene marine and volcanic rocks over Catalina Schist basement rocks. The vulnerability of San Clemente Island is compounded by the distribution of landslides that are encountered at the surface when ground shaking is amplified from the existing geologic structure.