SEISMIC AMPLIFICATION AND REFRACTION FROM ONSHORE AND OFFSHORE FAULT-BOUNDED SYNCLINES FROM THE GREAT SAND DUNE AND SYCAMORE CANYON AREA TO POINT MUGU, WESTERN SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

The Boney Mountain Fault zone has a mapped length of about 11 miles. It extends northeast from Sycamore Canyon and Great Sand Dune coastal area towards the city of Thousand Oaks, California. Within this coastal setting, the fault zone contains onshore exposures of several fault-bounded blocks, associated with two larger anticlines and one small syncline. The offshore structural complex shows a series of large fault-bounded anticlines and synclines that were identified by seismic surveys and linked to the Malibu Coast Fault, 1.5 miles south of the site. Theoretically, the presence of the fault-bounded synclines can focus and amplify earthquake energy. Therefore, the site area has the potential to experience significant site amplification of seismic energy released by nearby earthquakes. At the Great Sand Dune, the Boney Mountain Fault zone with left-reverse motion dissects large outcrop exposures. Examination of this area shows the faulted-rock out-crops of middle Miocene Topanga Formation intruded by the Conejo Volcanics. Observations at the site show that four faults have a high-angle dip with reverse separation, and four faults exhibit normal separation that appears to displace surface-cliff outcrops associated with two large slumps. Within these faults, the much older Miocene-aged(?), northern slump shows a back-rotation perpendicular to the cliff, and the more recent Quaternary southern slump/rockfall has a rotation parallel to the cliff.

An examination of past seismic activity found 580 earthquakes (M0.5 to 4.34) spanning 1973 to 2021 located within a 125 miles² area. Historically, faults just outside the area have produced one large earthquake, a 5.3 magnitude Point Mugu event in 1973. Movement along the Boney Mountain Fault zone could generate larger earthquake magnitudes ranging from M6.0 to M7.0. Such a large earthquake might cause bedrock peak ground accelerations ranging from 0.43g to 0.62g, accompanied by very strong to extreme shaking.

We believe that new evidence supported by faulted and fractured cliff outcrops, falling rock, with blocks of boulder-sized rock rubble over 5 feet in shape found at the base of the cliffs along Highway 1, coupled with greater earthquake magnitudes, and with the potential for seismic shaking amplified by fault-bounded synclines to exhibit greater site vulnerability.