Seismic-reflection profiles, 3D volumes and time slices were used to map detailed structural geometry within a 6-degree restraining bend in the fault zone. Geometric complexities of primary strike-slip faults, as well as numerous subsidiary faults provide insights into fault system evolution through time. High-resolution 3D seismic imaging of buried marine paleochannels allowed identification of piercing points for estimating Quaternary slip rates on the HFZ.
Seven paleochannels (channels A-G) were identified in the 3D volume. Discrete offsets of paleochannel thalweg piercing points across the two major strands of the HFZ were measured. Channel complex F, which is well imaged, crosses both the east and west strands of the HFZ, providing the best piercing points for estimating fault slip rates. Channel F is interpreted as an upper slope paleochannel incised during a low sea level stand in the Late Quaternary. Cumulative measured offset of the Channel F thalweg across the HFZ is ~600–650 m, within a range of 550–700 m.
Ages used in estimating temporal aspects of piercing point offsets are based on the evaluation of current global paleo–sea level curves, offshore well data, and sequence stratigraphy. The preferred age for channel abandonment is MIS 10 (~340ka), but age control is poor and both younger and older ages were considered. Based on the results of this study, the estimated slip rate for the HFZ at Point Sal is approximately 1–2 mm/yr., within a range of 0.4–5 mm/yr. Better geochronologic control through a targeted sediment age dating program would improve our understanding of Quaternary sequence stratigraphy and reduce uncertainty in slip rate estimates offshore the Central California Coast.