A new paper from Tom Rockwell and others model a 3D fault model of the western Transverse Ranges, linking the Ventura–Pitas Point fault to the Red Mountain fault to the west and to the San Cayetano fault to the east. Their analysis reinforces the importance of 3D structural analyses for understanding seismic hazards in the Western Transverse Ranges.
Structure and Seismic Hazard of the Ventura Avenue Anticline and Ventura Fault, California: Prospect for Large, Multisegment Ruptures in the Western Transverse Ranges
Judith Hubbarda, John H. Shawa, James Dolanb, Thomas L. Prattc, Lee McAuliffeb, and Thomas K. Rockwelld
aDepartment of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138
bDepartment of Earth Sciences, University of Southern California, Zumberge Hall, ZHS 111, 3651 Trousdale Parkway, Los Angeles, California 90089
cUnited States Geological Survey, 12201 Sunrise Valley Dr. MS 905, Reston, Virginia 20192
dDepartment of Geological Sciences, MC‐1020, 5500 Campanile Dr., San Diego State University, San Diego, California 92192‐1020
The Ventura Avenue anticline is one of the fastest uplifting structures in southern California, rising at ∼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 ∼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 ∼4.4–6.9 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 ∼800 years ago (Rockwell, 2011). Uplift this large would require large earthquakes (Mw 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.
J Hubbard, J H Shaw, J Dolan, TL Pratt, L McAuliffe, and TK Rockwell, 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, 10.1785/0120130125