Complex patterns of lateral strain transfer between the Ventura-Pitas Point and blind Southern San Cayetano faults, Ventura, Southern California

Chris Anthonissen, Lee J. McAuliffe, Jessica R. Grenader, James F. Dolan, John H. Shaw, Judith A. Hubbard, Ed J. Rhodes, & Thomas L. Pratt

Submitted September 11, 2022, SCEC Contribution #12472, 2022 SCEC Annual Meeting Poster #107

Continuously cored borehole data, cone-penetrometer tests (CPT) and seismic reflection data from several sites along the Ventura-Southern San Cayetano fault system allow us to document lateral changes in slip rate and strain transfer between these large reverse faults in the Western Transverse Ranges of Southern California. Near the eastern end of the Ventura fault at a site on Brookshire Avenue in Ventura, we document a latest Pleistocene (~15 ka) slip rate of 1 to 1.5 mm/yr. This slip rate is much slower than the 4.4 to 6.9 mm/yr Ventura fault slip rate documented farther west (Hubbard et al. 2014), indicating that slip is transferred eastward off the Ventura-Pitas Point fault and on to other structures. This conclusion is bolstered by the fact that whereas four large-magnitude earthquakes were documented by Rockwell et al. (2016) at Pitas Point ~15 km west of Ventura during the last ~7000 years, McAuliffe et al. (2015) showed that only two of these four events generated an uplift signal on the eastern part of the Ventura fault. Lateral variations in the structural geomorphic expression of this thrust fault may reflect different degrees of the blind thrust’s “breakthrough” towards the surface. Directly east of the Ventura fault, ~10 km to the east of Ventura, at a site on Briggs Road along the northern edge of the Ventura basin, high-resolution seismic reflection and borehole-CPT data reveal folding of young strata at the mountain front across an active synclinal axial surface, such that basinal strata are pulled up onto the mountain front during slip on the underlying thrust fault. These kinematics could be explained by either: (1) slip along a south-dipping back-thrust, which may come to the surface as the south-dipping Lion fault set, and/or (2) slip on a north-dipping thrust fault that is generating a fault-propagation fold at the northern margin of the Ventura basin. We consider both possibilities and discuss these in light of their implications for the structural evolution and seismic hazard of this large thrust fault system.

Key Words
Paleoseismology, Ventura, Structure

Anthonissen, C., McAuliffe, L. J., Grenader, J. R., Dolan, J. F., Shaw, J. H., Hubbard, J. A., Rhodes, E. J., & Pratt, T. L. (2022, 09). Complex patterns of lateral strain transfer between the Ventura-Pitas Point and blind Southern San Cayetano faults, Ventura, Southern California. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology