Characterizing the Structure and Earthquake Potential of the Blind Ventura and Southern San Cayetano Fault Systems
McAuliffe, Lee J.; Dolan, James F.; Hubbard, Judith; Shaw, John H.; Pratt, Thomas L.; and Rhodes, Ed
The recent occurrence of several destructive thrust fault earthquakes highlights the risks posed by such events to major urban centers around the world. In order to determine the earthquake potential of such faults in the western Transverse Ranges of southern California, we studied the structure and earthquake potential of the blind Ventura and southern San Cayetano faults through a multidisciplinary analysis of high resolution seismic reflection data and shallow depth borehole data. These two thrust faults form the middle section of a >200-km-long, east-west belt of large, interconnected reverse faults that extends across southern California. Although each of these faults represents a major seismic source in its own right, we are exploring the possibility of even larger-magnitude, multi-segment ruptures that may link these faults to other major faults to the east and west in the Transverse Ranges system. The proximity of this large reverse-fault system to several major population centers, including the metropolitan Los Angeles region, and the potential for tsunami generation during offshore ruptures of the western parts of the system, emphasizes the importance of understanding the structure and behavior of these faults for seismic hazard assessment.
Our initial efforts to document the earthquake history and slip-rate of this large, multi-fault reverse fault system focus on two sites - one site above the blind, southern San Cayetano thrust ramp at Briggs Road, and another above the blind Ventura fault at Day Road. At both Day Road and Briggs Road, high-resolution seismic profiles across the locus of recent folding reveals a well-defined north-dipping active synclinal axial surface in growth strata that extends to the surface at a prominent south-facing fold scarp. During May 2012, we drilled 17 hollow-stem boreholes and cone-penetrometer tests along the same transect as the reflection profile at Day Road, providing overlap between the data sets. Preliminary analysis of the borehole data reveals interbedded fine-grained silts with coarse-grained sands and gravels. The identification of two horizons with stratigraphic growth provides possible evidence for two large magnitude ruptures on the Ventura Fault. We anticipate that further analysis of borehole and high-resolution seismic reflection data from both sites will yield a detailed record of the fold growth during recent large earthquakes, which together with 14C and OSL dates from particular stratigraphic units will allow us to reconstruct the paleoseismic history of the underlying blind thrust faults.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013