Characterizing the Geometry, Kinematics, and Potential Connectivity at the Southern Termination of the Palos Verdes Fault

Abstract

The Palos Verdes fault (PVF) is an active component of the Inner California Borderland offshore strike-slip fault system that collectively accommodates ~6-8 mm/yr of Pacific-North American plate boundary shear. The PVF extends ~100 km from Santa Monica Bay in the north, across the Palos Verdes Peninsula, through the San Pedro Shelf and Slope to Lasuen Knoll in the Gulf of Santa Catalina. Slip rate estimates for the PVF range from ~1.5 to 3 mm/yr, and magnitude-length scaling relationships indicate the fault could generate a M 7.3 event. Despite posing a significant hazard for coastal communities in Southern California, the geometry at the northern and southern extents of the PVF remains poorly constrained. Several models have been proposed to explain the southern fault termination and the potential for connectivity with neighboring faults, the uncertainty of which has implications for seismic hazard analysis. These models include: 1) a throughgoing linkage between the PVF and Coronado Bank fault, 2) slip transfer between the PVF and the Newport-Inglewood-Rose Canyon fault along a distributed system of faults on the northeast side of Lasuen Knoll, and through faults in the San Mateo Trend, 3) PVF termination in a horsetail-splay southwest of Lasuen Knoll, and 4) thrust fault termination of the PVF at Lasuen Knoll.

Observations are presented from focused high-resolution 2D multichannel seismic reflection, and multibeam bathymetry surveys to reduce uncertainty in fault geometry and distinguish between the proposed fault termination and connectivity models. This study also incorporates, deep-penetration, low-resolution legacy industry seismic, and high-resolution legacy USGS seismic data to provide a higher density and distribution of datasets at different resolutions. A stratigraphic framework is constructed by extending late Quaternary stratigraphic markers throughout the study area. A sequence stratigraphic approach is used to interpret relative timing of fault-related deformation. This framework enables us to create a fault classification map and make temporal correlations between faults inferred to have mutual connectivity. The proposed models for kinematic-linkage or termination at the southern PVF are evaluated based on the analysis and interpretation of observations as compared to the expected style and location of deformation predicted by established models.

This study interprets a southward growth of east-west trending compressional folds northeast of the southern PVF that correspond with high relief at Lasuen Knoll. The folding is bound to the south by a deformation front that extends eastward from a ~1 km restraining bend in the PVF. North-dipping thrust faults are interpreted along the deformation front trending parallel to the folds. The compressional structures match the predicted style of deformation found in the contractional quadrant around right-lateral strike-slip fault-tip damage zones. Similarly, a potentially active extensional horsetail-splay damage zone is observed south of the termination, in the dilational quadrant. No evidence is found for kinematic or temporal linkage between the PVF and Coronado Bank fault, supporting more recent mapping by other Borderland researchers. Eliminating this connection model from future hazard forecasting models is recommended. Furthermore, no evidence for linkage between the PVF and Newport-Inglewood-Rose Canyon fault system or the faults of the San Mateo Trend is observed in the data. Lastly, a hybrid model of strike-slip fault termination is presented that is consistent with the extensional and compressional deformational patterns observed at the southern termination of the PVF at Lasuen Knoll. This work serves to reduce uncertainty in seismic hazard analysis and future rupture forecasting models, and provide updated structural, stratigraphic, and fault activity mapping for this portion of the Gulf of Santa Catalina.