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Characterization of Western San Cayetano Fault

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Characterization of Western San Cayetano Fault 2011 SCEC Report Defining the slip rate, paleoseismology, and earthquake potential of the blind Western San Cayetano and Ventura fault system: Determining recent slip rates and paleo-earthquake ages and displacements Principal Investigators: James F. Dolan (University of Southern California) John H. Shaw (Harvard University) Collaborator: Thomas L. Pratt (U. S. Geological Survey, Seattle) Participating Graduate Students: Lee McAuliffe (University of Southern California), Judith Hubbard (Harvard University) Proposal Categories: Data Gathering and Products Integration and Theory Science Objectives: A2. Investigate implications of geodetic/geologic rate discrepancies A3. Develop a system-level deformation model A9. Assess predictability of rupture extent and direction on major faults C. Improve and develop community products that can be used in system-level models of the forecasting of seismic hazard Summary Our research has involved a multi-disciplinary effort to characterize the activity and earthquake potential of a series of poorly understood blind-thrust faults that lie at the heart of a major transfer zone connecting some of the largest reverse faults in the western Transverse Ranges (WTR). Specifically, in this collaborative effort (USC, Harvard, USGS – Seattle) we have investigated the Ventura, Southern San Cayetano, and Pitas Point faults, which link rapid north-south shortening accommodated by the San Cayetano fault in the east with active thrust faulting along the Red Mountain fault and other active structures in the Santa Barbara Channel to the west (Figure 1). Although the potential for these faults to link together in potentially very large magnitude earthquakes (Mw≥8) has recently been recognized, relatively little is known about the rates of fault slip, or the ages, repeat times, and displacements that have occurred in ancient earthquakes generated by these structures. Following our 2010 SCEC-funded acquisition of high-resolution seismic reflection data across the loci of active folding at several sites along the western San Cayetano-Ventura thrust system, last year we began our acquisition of borehole data to define the slip rate and paleo-earthquake ages and displacements. Our main 2011 study site was along Briggs Road near Santa Paula, CA, where we drilled a north-south transect of 6 cored boreholes and 4 CPTs across the southern edge of the active fold front of the southern SCF fold front. These boreholes revealed well-bedded stratigraphy along the southern flank of the SSCF forelimb. Together with planned 2012 boreholes, these results will allow us to characterize the geometry and ages of strata that have been deformed by late Pleistocene-Holocene earthquakes on the underlying SSCF blind thrust ramp. Introduction The Ventura basin is the locus of some of the most rapid north-south convergence rates in southern California (Donnellan et al., 1993a; 1993b; Hager et al., 1999). This shortening is accommodated by slip on opposing thrust systems – including the south-dipping Oak Ridge system to the south, and the north- dipping San Cayetano thrust system on the north. Thrust loading of the basin has resulted in deposition of one of the thickest Plio-Pleistocene sedimentary sections in the world, and this extremely deep basin exhibits pronounced basin amplification (prominent in many recent SCEC earthquake simulations). Numerous studies, many of them utilizing the abundant seismic reflection and well data collected by the petroleum industry, have elucidated the geometry and long-term deformation rates of these structures (e.g., Schlueter, 1976; Cemen, 1977; 1989; Dibblee, 1987; 1990a; 1990b; 1991; Namson and Davis, 1988; Rockwell, 1988; Yeats, 1993; Huftile and Yeats, 1995a; 1995b; 1996). These studies show that the eastern part of the San Cayetano fault, known as the Modelo lobe, exhibits an exceptionally rapid slip rate of at least 7.5 mm/yr, and possibly as fast as ~10 mm/yr – the fastest rate documented for any fault in the Transverse Ranges (Huftile and Yeats, 1995a). The San Cayetano is separated into two major segments by a 4-km-wide, right-stepping lateral ramp near the city of Fillmore. The slip rate of the western part of the fault, which is slower than the eastern, Modelo lobe segment, diminishes westward to zero just east of Ojai (Rockwell, 1988). Moreover, the western section of the San Cayetano differs markedly in its geomorphic expression from the eastern segment of the fault. Whereas the eastern Modelo lobe segment extends to the surface at the base of the mountain front along the northern edge of the Ventura basin, the western strand extends to the surface far above the basin floor on the side of the mountains to the north of the valley (Rockwell, 1988). Such a geometry indicates that the western strand of the San Cayetano fault lies entirely within the hanging wall of another thrust fault that lies deeper in the section, beneath the northern edge of the Ventura basin (e.g., Huftile and Yeats, 1995a). This fault, which is not well documented, is referred to here as the southern San Cayetano fault. The blind, southern San Cayetano fault segment is a gently north-dipping thrust ramp that extends upward to a depth of ~ 7 km. At this point, it merges with a south-dipping back-thrust known as the Sisar fault (e.g., Huftile and Yeats, 1995). This type of active wedge folding has been successfully imaged by high- resolution seismic data along the Coyote segment of the Puente Hills blind thrust in Los Angeles (Pratt et al., 2002; Shaw et al., 2002). Our 2011 investigations of this structure are described below. Further to the west in the Ventura area, we used a set of well data, industry seismic reflection profiles, and the seismic profiles acquired by our group to construct a 3D model of the Ventura fault system [see Hubbard et al., 2011 and companion Harvard-lead SCEC report]. The Ventura fault underlies the Ventura Avenue anticline, and is one of the fastest uplifting structures in southern California, rising at a rate of 5 mm/yr [Rockwell et al., 1988]. However, there was persistent disagreement about whether this structure posed a significant hazard, stemming from uncertainty about the fault geometry at depth. Two models were proposed: one suggesting that the Ventura fault extends to seismogenic depths beneath the anticline [e.g., Sarna-Wojcicki et al., 1976; Sarna-Wojcicki and Yerkes, 1982], and an alternative interpretation that the Ventura fault is a shallow, bending-moment fault that does not pose a significant seismic hazard [e.g., Huftile and Yeats, 1995;Yeats, 1982a,b]. Our results suggest that the Ventura fault does extend to seismogenic depths and accommodates uplift of the Ventura Avenue anticline by fault-propagation folding. Specifically, based on dipmeter logs and stratigraphic cutoffs imaged in the seismic reflection profiles, we showed that the north-dipping Ventura fault likely extends to the base of the seismogenic crust (Figure 2). Fault offset increases with depth, implying that the Ventura fault has propagated upwards over time. Thus, we interpret the Ventura Avenue anticline to be a fault-propagation fold underlain by an active thrust ramp. A decrease in the uplift rate of the anticline at 30 ka, as measured from uplifted terraces [Rockwell et al., 1988], is consistent with a breakthrough of the Ventura fault at that time, although the fault remains buried by a thin sedimentary cover and thus is blind. This fault breakthrough resulted in the development of a 500-m-wide monocline (and the Ventura fault scarp) several hundred meters south of the main body of the anticline. If this interpretation is correct, then the uplift rate on the anticline implies that the fault must slip at a rate of ~6.5 mm/yr, significantly faster than previous estimates of fault slip, which have ranged from 0.2 to 2.4 mm/yr [Perry and Bryant, 2002]. Together, the blind southern San Cayetano and Ventura faults appear to represent the linkage between the San Cayetano fault in the east and the Red Mountain and other active faults in the Santa Barbara Channel to the west (Figure 1). Our research seeks to define the activity and earthquake potential of each of both of these potentially independent earthquake sources. In addition, we seek to explore if these faults may serve as a connection between several of the major thrust faults in the Transverse Ranges, thereby enabling extremely large magnitude earthquakes. Available slip-event data suggest that very large earthquakes indeed may have occurred. Specifically, the most recent event, which occurred ~800-1,000 years ago, uplifted the paleo-shore face by ~8 m; the penultimate event, which occurred at 2-2.5 ka, was also an extremely large uplift event (Rockwell, 2011). Uplift of this magnitude would require a very large (M7.7-8.1) earthquake, likely rupturing a fault area equivalent to the entire Ventura-Pitas Point fault combined with the Red Mountain and San Cayetano faults (based on the empirical relations of Wells and Coppersmith, 1994; Biasi and Weldon, 2006; and Hanks and Bakun, 2002, 2008). Our 3D model of the fault system shows that this sort of multi-segmented rupture is permissible, since the deep fault ramp to the north of the Ventura fault may link directly to the San Cayetano and Red Mountain faults (Figure 2). One of the largest of these potential multi-fault earthquakes involves rupture of the rapidly slipping eastern San Cayetano fault westward via the blind, southern San Cayetano fault that forms the focus of this study onto the Ventura thrust fault responsible for uplift of the VAA and correlative faults to the west ( e.g., Lower Pitas Point thrust; Figures 1 & 2). This could produce a 75- to 100-km-long rupture plane on gently dipping thrust faults. The resulting fault- plane area could be as much as several thousand square kilometers – on par with the rupture area of the great 1857 Fort Tejon and 1906 San Francisco earthquakes.
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