High-resolution three-dimensional seismic imaging reveals structure and slip rate of the Hosgri fault zone offshore Point Sal, California, United States

Abstract A 3-D low-energy seismic survey of the Hosgri fault zone (HFZ) west of Point Sal, California, USA, provides new insights into the structural complexity of the HFZ and a new slip rate estimate based on offset mid-Pleistocene (ca. 250–440 ka) paleochannels. The HFZ is mapped as a paired fault bend with 20°–30° restraining and releasing bends. The leading restraining bend parallels the Lions Head fault and is spatially coincident with a major erosional unconformity and the absence of reflections associated with the Marine Iso- tope Stage (MIS) 2 (ca. 20 ka) unconformity. Subsidence within the trailing releasing bend is indicated by vertical offset of the MIS 2 unconformity and possibly shallower (younger) horizons. Seven paleochannel complexes were identified within the 3-D volume as piercing points. One channel complex, designated F, was mapped across the entire HFZ. The youngest channel complex F thalwegs are thought to have been incised during the MIS 8a (ca. 250 ka) or MIS 10a (ca. 345 ka) sea-level low stands and exhibit a total dextral offset of 640 ± 80 m for a fault slip rate of 2.56 ± 0.37 mm/yr (95% confidence interval) to 1.86 ± 0.26 mm/yr. An older MIS 12a (ca. 440 ka) age of incision results in a correspondingly slower slip rate of 1.45 ± 0.21 mm/yr. Comparison of Point Sal slip rates with those of other HFZ piercing points indicates that northward-increasing right-lateral displacement along the HFZ could be accommodated by slip transfer from northwest-trending faults within the Los Osos domain.