2011 SCEC Report

Defining the slip rate, paleoseismology, and potential of the blind Western San Cayetano and system: Determining recent slip rates and paleo-earthquake ages and displacements

Principal Investigators: James F. Dolan (University of Southern ) John H. Shaw (Harvard University)

Collaborator: Thomas L. Pratt (U. S. Geological Survey, Seattle)

Participating Graduate Students: Lee McAuliffe (University of ), 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 (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.

2011 Investigations

In the first phase of this collaborative study, we used SCEC funding to acquire a series of high-resolution seismic reflection profiles across the locus of active folding above the Southern San Cayetano and Ventura faults. These data show discrete fold limbs draped with syntectonic strata that are located above the tiplines of these faults (e.g., Figure 3). Subsequently, during Summer 2011 we excavated a transect of four continuously cored, hollow-stem boreholes and six cone-penetrometer tests (CPTs) across the topographically prominent fold scarp of the Southern San Cayetano fault at a site along Briggs Road 15 km northeast of the city of Ventura. These boreholes were excavated along the easternmost of our high- resolution seismic reflection profiles. Together with a final set of boreholes that will be acquired this year, these results will provide the detailed structural geometry and ages of young folded strata, allowing us to assess the ages of recent folding events and the Holocene-latest Pleistocene slip rate of this part of the Transverse Ranges reverse-fault system.

During the summer of 2011 we used two coring techniques to acquire stratigraphic data from 10 locations along Briggs Road - the easternmost of four studied transects along the Ventura Basin (Figure 1). These 6 hollow-stem auger borehole cores and 4 Cone-Penetration Test (CPT) data were collected across a prominent fold scarp that has developed in response to slip on the underlying thrust ramp of the southern San Cayetano blind thrust fault (Figures 4 & 5).

These data provide us with the ability to correlate the stratigraphy of the uppermost 25 m of the fold, overlapping with the shallowest part of the 2010 high-resolution seismic reflection data collected along Briggs Road (Figures 3 & 4). Both the borehole and CPT data allow us to determine grain size variations with depth and thus correlate the subsurface stratigraphy along the entire profile. Correlation of the various sedimentary layers allows us to determine whether particular units have experienced growth. Stratigraphic growth is expected through the onlapping of material onto a fold scarp formed after slip on the underlying southern San Cayetano fault. The presence of growth with a particular depth interval indicates an event horizon within that interval. Analysis of our initial results indicates a small yet noticeable amount of growth at two locations - between the ground surface and a prominent soil horizon at a depth of 9 m in BR-4, 15 m in BR-3, and 18.3 m in BR-2, and above a coarse sandy layer at a depth of 6 m in BR-2 and 10.7 m in BR-1 (Figure 4). These two periods of growth likely correspond to two separate events on the southern San Cayetano fault. In addition to the correlation of the subsurface stratigraphy, the collection of cores allows us to extract dateable charcoal fragments from different depths within our boreholes. The dating of the charcoal by 14C radiocarbon methods allows us to constrain particular events recorded in the rock record. The dating of the event will allow us to construct a crude recurrence interval for the fault and calculate a minimum slip rate for the southern San Cayetano fault. From our four boreholes we were able to extract 11 charcoal samples (locations indicated by red stars in figure 4). These samples were sent to the UC Irvine, Keck Carbon Cycle AMS laboratory for dating. Of the 11 samples sent for dating, only three were large enough to make it through the pre-treatment phase. The three samples yielded ages ranging from 29300±3100 BP to 50800±3700 BP, all from relatively deep in the exposed stratigraphic section, indicate that the oldest parts of the cores are late Pleistocene in age.

Although we were hopeful that the locus of active folding, in this case the synclinal axial surface, was located south of the busy Foothill Road built along the base of slope (Figure 5), our 2011 borehole data indicate that the axial surface lies beneath the roadway.

Planned acquisition of additional borehole data to the north of Briggs Road will help us constrain the geometry and ages of recent folding through the synclinal axial surface at this site. Specifically, we plan to drill three additional boreholes north of BR-4 along the south-facing hill slope north of Foothill Road. This slope represents the dip slope of the folded strata in the forelimb of the fold (Figures 3 & 5). We anticipate that by including the data from these three additional boreholes we will be able to both accurately and tightly constrain the location and structure of the synclinal axial surface. Our initial analysis of the subsurface stratigraphy to the south of the mountain front indicates that two prominent paleosol horizons as well as several distinctive gravel beds that are observed in our 2011 cores will probably also be discernible in the planned northern boreholes, providing a clear marker horizon with which to track the geometry of recent folding.

In addition to our planned boreholes at the northern end of the Briggs Road transect, we also plan to conduct a similar slip rate/recurrence interval study on the Ventura fault, along the Day Road transect in the city of Ventura (Figure 6). The planned Day Road transect, which was one of three seismic reflection profiles that we acquired during the summer of 2010 (Figure 7), was selected for study because it is the only location along the ventura fault scarp that has been recently covered by an active late Holocene alluvial fan; elsewhere along the scarp south-flowing drainages have isolated the alluvial surfaces from active deposition. Our planned acquisition of continuously cored boreholes and CPTs on either side of the fold scarp (i.e., surface manifestation of recently folding through the synclinal axial surface of the Ventura fault), we will be able to constrain the subsurface structure of the fold scarp.

Collectively, the data that we have collected, together with industry provided seismic survey data and planned additional borehole transects, will help define the earthquake potential (location, magnitude, and recurrence) of the southern San Cayetano and Ventura thrust faults so that these sources can be included in regional seismic hazards assessments. As part of this effort, these new fault representations will be incorporated in a subsequent version of the SCEC CFM.

References

Biasi, G. P., and R. J. Weldon, 2006, Estimating surface rupture length and magnitude of paleoearthquakes from point measurements of rupture displacement: Bull Seismol. Soc. Am. 96, 1612-1623. Cemen, I., 1977, Geology of the Sespe-Piru Creek area, Ventura County, California, unpub. M. S. thesis, Ohio University, Athens, Ohio, 69p. Cemen, I., 1989, Near-surface expression of the eastern part of the San Cayetano fault: A potentially active thrust fault in the California Transverse Ranges: Jour. Geophys. Res., v. 94, p. 9665-9677. Dibblee, T. W., Jr., 1987, Geologic Map of the Ojai Quadrangle, Ventura County, California: Dibblee Geol. Foundation Map #DF-13, Santa Barbara, California (1:24,000). Dibblee, T. W., Jr., 1990a, Geologic Map of the Fillmore Quadrangle, Ventura County, California: Dibblee Geol. Foundation Map #DF-27, Santa Barbara, California (1:24,000). Dibblee, T. W., Jr., 1990b, Geologic Map of the Santa Paula Peak Quadrangle, Ventura County, California: Dibblee Geol. Foundation Map #DF-26, Santa Barbara, California (1:24,000). Dibblee, T. W., Jr., 1991, Geologic Map of the Piru Quadrangle, Ventura County, California: Dibblee Geol. Foundation Map #DF-34, Santa Barbara, California (1:24,000). Donnellan, A., Hager, B. H., and King, R. W., 1993a, Rapid north-south shortening of the Ventura basin, southern California: Nature, v. 366, p. 333-336. Donnellan, A., Hager, B. H., King, R. W., and Herring, T. A., 1993b, Geodetic measurement of deformation in the Ventura basin, southern California: Jour. Geophys. Res., v. 98, p. 21727-21739. Hager, B. H., Lyzenga, G. A., Donnellan, A., and Dong, D., 1999, Reconciling rapid strain accumulation with deep seismogenic fault planes in the Ventura basin, California: Jour. Geophys. Res., v. 104, p. 25, 207-25,219. Hanks, T. C., and W. H. Bakun, 2002, A bilinear source-scaling model for M-logA observations of continental earthquakes, Bull. Seismol. Soc. Am. 92, 1841-1846. Huftile, G. J., and Yeats, R. S., 1995, Convergence rates across a displacement transfer zone in the western transverse ranges, Ventura Basin, California: Jour. Geophys. res., v. 100, p. 2043-2067. Huftile, G. J., and Yeats, R. S., 1995b, Cenozoic structure of the Piru 7 1/2-minute quadrangle, California: U. S. Geol. Surv. Open-File Report 95-68, Map Scale 1:24,000, 33 p. Huftile, G. J., and Yeats, R. S., 1996, Deformation rates across the Placerita (Northridge Mw 6.7 aftershock zone) and Hopper Canyon segments of the western Transverse Ranges deformation belt: Bull. Seismol. Soc. Amer., v. 86, p. S3-S18. Namson, J., and Davis, T., 1988, Structural transect of the western Transverse Ranges, California: Implications for lithospheric kinematics and seismic risk evaluation, Geology, v. 16, p. 675-679. Perry, S. S., and W. A. Bryant, 2002, Fault number 91, Ventura fault, in Quaternary fault and fold database of the United States. U.S. Geological Survey website, http: //earthquakes.usgs.gov/regional/qfaults, [Online; accessed May 6th 2011]. Pratt, T. L., Shaw, J. H., Dolan, J. F., Christofferson, S., Williams, R. A., Odum, J. K. and Plesch, A., 2002, Shallow folding imaged above the Puente Hills blind-thrust fault, Los Angeles, California: Geophysical Research Letters, v. 29, p. 18-1 to 18-4 (May 8, 2002). Rockwell, T., 1988, Neotectonics of the San Cayetano fault, Transverse Ranges, California, Geol. Soc. Amer. Bull., v. 100, p. 500-513. Rockwell, T. K., Keller, E. A., and Dembroff, G. R., 1988, Quaternary rate of folding of the Ventura Avenue anticline, western Transverse Ranges, southern California: Geol. Soc. Amer. Bull., v. 100, p. 850-858. Sarna-Wojcicki, A. M., K. M. Williams, and R. F. Yerkes, 1976, Geology of the Ventura Fault, Ventura County, California. U.S. Geological Survey Miscellaneous Field Studies, map MF-781, 3 sheets, scale 1:6,000. Sarna-Wojcicki, A. M., and R. F. Yerkes, 1982, Comment on article by R. S. Yeats entitled “Low-shake faults of the Ventura Basin, California”. In: Cooper, J. D. (Ed.), Neotectonics in Southern California. Geological Society of America, 78th Cordilleran Section Annual Meeting, Guidebook, pp. 17–19. Schleuter, J. C., 1976, Structure of the Upper Ojai-Timber Canyon area, Ventura County, California, unpubl M. S. thesis, Ohio University, 67p. Shaw, J. H., Plesch, A., Dolan, J. F., Pratt, T., and Fiore, P., 2002, Puente Hills blind-thrust system, Los Angeles basin, California: Bulletin of the Seismological Society of America, v. 92, p. 2946-2960. Wells, D. L., and K. J. Coppersmith, 1994, New empirical relationship among magnitude, rupture length, rupture width, rupture area, and surface displacement, Bull. Seism. Soc. Am. 84 (4), 974–1002. Yeats, R. S., 1982a, Low-shake faults of the Ventura Basin, California. In: Cooper, J. D. (Ed.), Neotectonics in Southern California. Geological Society of America, 78th Cordilleran Section Annual Meeting, Guidebook, pp. 3–15. Yeats, R. S., 1982b, Reply to Sarna-Wojcicki, A. M. and R. F. Yerkes. In: Cooper, J. D. (Ed.), Neotectonics in Southern California. Geological Society of America, 78th Cordilleran Section Annual Meeting, Guidebook, pp. 21–23. Yeats, R. S., 1993, Converging more slowly, Nature, v. 366, p. 299-301. Yeats, Robert S., Gary J. Huftile, and Leonard T. Stitt, 1994, Late Cenozoic Tectonics of the East Ventura Basin, Transverse Ranges, California, AAPG Bulletin, V. 78, No. 7, P. 1040–1074.

Figure 1. Map showing locations of major faults in the Ventura basin, including the blind southern San Cayetano and Ventura faults which are the focus of this study. The numbered transects (1-4) are high- resolutions seismic reflection profiles acquired this past summer. Transect 1 is the Briggs road profile and transect 3 is the Day road profile.

Figure 2. 3D perspective view of the Ventura fault system, looking from the southeast. The Ventura-Pitas Point fault flattens to a detachment and then ramps down to the north to the base of the seismogenic crust. The cities of Ventura and Santa Barbara are shown as white ovals. We suggest that the San Cayetano fault to the east and the Red Mountain fault to the west may rise directly from the deep fault ramp, leading to the possibility of large, multi-segmented ruptures.

Figure 3. High-resolution seismic profile acquired this past summer at site 1 (Figure 1) across the active fold limb and synclinal axial surface associated with the tipline of the blind southern San Cayetano Fault. Green and red bars indicate locations of boreholes and CPT’s acquired during the summer of 2011. The new projected synclinal axial surface (~30 meters to the north of the initial location, and locations of three future boreholes are indicated.

N Briggs Road, Ventura - CPT/Borehole S

BR -4

Grain size Color Soil CPT - 6 0 ------BR -3 - - 10 - - - 20 - - Grain size Color Soil - - 0 ------20 - - - - - CPT - 5 ------10 ------30 - - - - - BR -2 ------20 ------40 - - - - - Grain size Color Soil Depth (feet) - - - - 0 - - 30 - - - - CPT - 4 ------50 ------10 - - CPT - 0 - - 40 ------

Elevation (m) Elevation 60 - - - CPT - 3 ------20 - - - - 50 - - - - Projected borehole locations ------BR -1 70 ------30 - - - - 60 ------Grain size Color Soil ------0 80 ------40 - - - - CPT - 2 70 ------0 - - - - 10 ------50 - - - - 80 ------? - - 20 CPT - 1 ------60 ------30 ------70 ------Key ------40 ------80 ------Clay 50 ------? - - Between Clay and silt - - 60 ------? - - silt and silt-v. ne grain - - 70 ------? - - v. ne sand - - No vertical exaggeration 80 ? ne ne-med sand

med-coarse sand

Very coarse sand to gravel Figure 4 (previous page). Preliminary stratigraphic interpretation from borehole and CPT (Cone Penetration Test) data along the Briggs Road transect. Borehole logs (BR-1 to BR-4) show grain size and Munsel color identifications. Dark brown colors next to the borehole grain size show the depth of an argillic horizon which can be traced along BR-2, BR-3 and BR-4. Preliminary correlations show growth strata at several locations including between BR-4 and BR-3 above the soil horizon, and between BR-3 and BR-2 above the yellow highlighted coarse grain material.

Figure 5. Oblique virtual aerial view of borehole and CPT locations from work completed during the summer of 2011 (base image from GoogleEarth). The red line indicates the seismic profile acquired during the summer of 2010 shown in figure 4, and the orange circled indicate the proposed borehole locations on the northern side of the synclinal axial surface. Updated projected synclinal axial surface indicated by the blue line.

Figure 6. Oblique virtual aerial view of projected borehole (yellow circles) and CPT (green circles) locations along Day road (transect 3 in figure 1) (base image from GoogleEarth). This planned transect is located along a Holocene alluvial fan and will allow us to determine a Holocene slip rate and paleo-earthquakes ages and displacements for the Ventura fault. The shaded orange section in the image is the fold scarp associated with uplift on the blind Ventura fault beneath.

Figure 7. High-resolution seismic reflection profile collected in 2010 along Day road (transect 3 in figure 1). The projected synclinal axial surface is shown as the dashed black line. Projected planned borehole (green bar) and CPT (pink bars) locations are shown on the 5x vertically exaggerated topographic profile.