Structure and Seismic Hazard of the Ventura Avenue Anticline and Ventura Fault, California: Prospect for Large, Multisegment Ruptures in the Western Transverse Ranges

The Ventura Avenue anticline is one of the fastest uplifting structures in southern California, rising at similar to 5 mm/yr. 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 offset increases with depth, implying that the Ventura Avenue anticline is a fault-propagation fold. A decrease in the uplift rate since similar to 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 similar to 4.4-6.9 mm/yr. To the west, the Ventura fault and fold trend continues offshore as the Pitas Point fault and its associated hanging wall anticline. The Ventura-Pitas Point fault appears to flatten at about 7.5 km depth to a detachment, called the Sisar decollement, 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, posing the threat of large, multisegment earthquakes. Holocene marine 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 similar to 800 years ago (Rockwell, 2011). Uplift this large would require large earthquakes (M-w 7.7-8.1) involving the entire Ventura/Pitas Point system and possibly more structures along strike, such as the San Cayetano fault. Because of the local geography and geology, such events would be associated with significant ground shaking amplification and regional tsunamis.