New Investigations on the Hollywood-Raymond Fault Zone, Los Angeles, California

Janis Hernandez, Rufus D. Catchings, Robert R. Sickler, & Mark R. Goldman

Submitted August 4, 2016, SCEC Contribution #6496, 2016 SCEC Annual Meeting Poster #081

South of the Transverse Ranges in southern California, the transition zone between the Hollywood and Raymond faults has been mapped as a series of left-stepping, E-NE-trending faults (Weber et al., 1980). Geomorphic expression of faulting in this area has been obscured by urbanization, and limited geotechnical or fault investigations are available for review. As part of our evaluation of this area for Alquist-Priolo (AP) earthquake fault-zone mapping, we performed a review of existing studies, aerial photos, and new trench studies required by the City of Los Angeles. A new fault study conducted along the western end of the Raymond Fault revealed a steeply north-dipping, continuous fault strand that extends to within a few feet of the ground surface. This fault exhibits down-to-the-north displacement of Holocene marsh deposits and coincides with a marshy wetland that existed until the early 1920’s. Borehole cores reveal a parallel set of fault strands, constrained by radiocarbon methods to be > 11.7 ka. Because active shallow faulting exists at this site, a guided-wave seismic study was conducted to test whether the Raymond fault extended along a linear valley southwestward toward the Hollywood Fault. The data revealed discrete zones of high peak ground velocity amplitudes that coincide with 4 separate fault strands separated over a distance of about 350 meters. These studies support the hypothesis that the Raymond fault zone extends southwestward toward the Hollywood fault, and the trenching provides positive evidence of Holocene faulting further west along the Raymond fault than previously known. This new information can be used in preparation of AP earthquake fault zone maps, as well as in regional seismic hazards assessments that consider whether the Hollywood and Raymond faults are capable of rupturing together.

Key Words
Hollywood fault, Raymond fault, Transverse Ranges, guided wave

Citation
Hernandez, J., Catchings, R. D., Sickler, R. R., & Goldman, M. R. (2016, 08). New Investigations on the Hollywood-Raymond Fault Zone, Los Angeles, California. Poster Presentation at 2016 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology