SCEC2021 is online September 12-17!  Registered participants click here.

Structure and seismic hazard of the Ventura Avenue anticline and Ventura fault, California: Prospect for large, multisegment ruptures in the Western Transverse Ranges

Judith A. Hubbard, John H. Shaw, James F. Dolan, Thomas L. Pratt, Lee J. McAuliffe, & Thomas K. Rockwell

Published June 2014, SCEC Contribution #1736

The Ventura Avenue anticline is one of the fastest uplifting structures in southern California, rising at !5 mm/yr (Rockwell et al., 1988). We use well data and seismic reflection profiles to show that the anticline is underlain by the Ventura fault, which extends to seismogenic depth. Fault displacement increases with depth, implying that the Ventura Avenue anticline is a fault-propagation fold. A decrease in the uplift rate since ~30±10 ka is consistent with the Ventura fault breaking through to the surface at that time, and implies that the fault has a recent dip-slip rate of ~4.1-8.1 mm/yr.
To the west, the Ventura fault and fold trend continues offshore as the Pitas Point fault and its associated hanging wall anticline (e.g., Sarna-Wojcicki et al., 1976; Yerkes and Lee, 1987; Kamerling and Nicholson, 1995). The Ventura/Pitas Point fault appears to flatten to a detachment, called the Sisar Decollement (e.g., Huftile and Yeats, 1995), then step down on a blind thrust fault to the north. Other regional faults, including the San Cayetano and Red Mountain faults, link with this system at depth. We suggest that below 7.5 km, these faults may form a nearly continuous surface.
Holocene terraces on the Ventura Avenue anticline suggest that it grows in discrete events with 5-10 m of uplift, with the latest event having occurred ~800 years ago (Rockwell, 2011). Uplift this large would require large earthquakes (M7.7-8.1) involving the entire Ventura/Pitas Point system, and possibly more structures along strike such as the San Cayetano fault. Due to the local geography and geology, such events would be associated with significant groundshaking amplification and regional tsunamis.

Citation
Hubbard, J. A., Shaw, J. H., Dolan, J. F., Pratt, T. L., McAuliffe, L. J., & Rockwell, T. K. (2014). Structure and seismic hazard of the Ventura Avenue anticline and Ventura fault, California: Prospect for large, multisegment ruptures in the Western Transverse Ranges. Bulletin of the Seismological Society of America, 104(3), 1070-1087. doi: 10.1785/0120130125.