2013 SCEC Project Report

Assessing the earthquake potential of the Ventura-Pitas Point fault system: Characterizing the linkages to other faults in the Transverse Ranges and prospects for

large, multi-segment earthquakes

Principal Investigator: Professor John H. Shaw

Dept. of Earth & Planetary Sciences, Harvard University 20 Oxford St., Cambridge, MA 02138

shaw@eps.harvard.edu //(617) 495-8008

co-Investigator: Andreas Plesch (Harvard)

andreas_plesch@harvard.edu

Collaborators: James F. Dolan (University of Southern California) Thomas L. Pratt (U. S. Geological Survey, Seattle)

Proposal Categories:

A: Data gathering and products C: Special fault Study Areas: Ventura/

Western Transverse Ranges

Science Objectives: 4A, 4C, 4E

Abstract As part of the SCEC Ventura Special Fault Study Area (SFSA), this project developed detailed models for the Ventura-Pitas Point fault system and major thrust faults that appear linked with it along strike, including the Red Mountain and San Cayetano faults. The models consolidate fault representations from the SCEC Community fault model (CFM) (Plesch et al., 2007), the UCSB group (Kammerling et al., 2003), and Hubbard et al. (BSSA, 2014, in press). We provide two sets of fault representations, that differ based on the presence or absence of a mid-crustal detachment level. The fault representations in each scenario are provided in large, contiguous tsurfs with about 1km resolution and quality controlled meshes.

A goal of the SFSA is to encourage SCEC investigators to utilize these representations in their planned studies in the Ventura region, including efforts in fault system modeling, finite rupture simulations, and earthquake hazards assessments. We also encourage interaction with the SCEC USR group to help refine these fault representations. The models were made available by the SCEC CME and our group, and are accessible at: http://isites.harvard.edu/icb/icb.do?keyword=k93600&pageid=icb.page637015 Technical Report Introduction Our research seeks to characterize the activity and earthquake potential of the Ventura-Pitas Point fault system, which lies at the heart of a major transfer zone connecting some of the largest, fastest-slipping reverse faults in the Western Transverse Ranges (Figure 1). The Ventura fault underlies the Ventura Avenue anticline, which is one of the fastest uplifting structures in southern California, rising at a rate of ∼5 mm/yr [Rockwell et al., 1988]. Holocene terraces on the anticline suggest that it deforms in discrete events with 5-10 m of uplift, with the latest event occurring ~800 years ago (Rockwell, 2011). This much uplift would require large earthquakes (M7.7-8.1) that rupture the underlying Ventura-Pitas Point fault in conjunction with other major thrusts along strike, such as the San Cayetano and Red Mountain faults (Hubbard et al., 2011, 2014). Such large, multi-segment events would cause widespread hazardous ground shaking and likely induce strong regional tsunamis. To assess these hazards and better understand the potential for multi-segment thrust fault earthquakes throughout the Transverse Ranges, SCEC has developed a special fault study area in Ventura (SFSA). Our work serves as a primary effort to characterize the subsurface geometry of the Ventura-Pitas Point fault system, and provides products that will be used in a wide range of fault system models, ground motion simulations, and hazard assessments. Specifically, we have developed a set of alternative, 3D subsurface representations of the Ventura-Pitas Point fault and other major structures that may be linked to it along strike. These models are constrained using surface geology, industry seismic reflection profiles and wells, and precisely relocated hypocenter and focal mechanism catalogs provided by SCEC. The alternative models attempt to encompass the range of uncertainty in subsurface fault geometries, in particular by highlighting the implications of the presence or absence of mid-crustal

detachments. These models have been made available through our and the SCEC CME website, and will be incorporated into future versions of the SCEC Community Fault Model (CFM).

Figure 1: Map showing locations of major faults in the Ventura basin and Santa Barbara Channel, including the Ventura, Pitas Point, and Southern San Cayetano faults. The transects are industry (green) and high-resolution (black) seismic reflection profiles assembled and acquired for this project.

Past Efforts Our past efforts in this multi-year project have focused on: 1) using industry seismic reflection and well data to constrain the subsurface architecture of the Ventura-Pitas Point fault system (Harvard lead); 2) acquiring high-resolution seismic reflection data across fault and fold scarps at four sites (USGS, Harvard, USC); and 3) excavating a series of borehole transects at two sites to constrain the paleoearthquake history (USC lead). In this first phase of the Harvard study, we used SCEC funding to develop a comprehensive, 3-D subsurface representation of the Ventura-Pitas Point fault system using 2- and 3-D seismic reflection surveys and well data provided by the petroleum industry. These investigations integrated data and analysis from previous studies (Sarna-Wojcicki et al., 1976; Sarna-Wojcicki and Yerkes, 1982; Huftile and Yeats, 1995;Yeats, 1982a,b) that had debated the presence and activity of the Ventura fault. Our results demonstrate that the Ventura fault is an active blind-thrust ramp that extends from the shallow subsurface to seismogenic depths beneath the Ventura Avenue anticline (Figure 2). We interpret that the Ventura Avenue anticline developed as a fault-propagation fold until about 30ka, when the fault broke upwards and to the south into the Ventura basin. This breakthrough formed the prominent scarp that passes through the city of Ventura. This change in structural kinematics is consistent with the reduction in uplift rate at 30ka based on the terrace uplift record (Rockwell, 2011).

Figure 2: Perspective view of the subsurface geometry of the Ventura – Pitas Point thrust system and Ventura Avenue anticline developed as part of this study.

Results in 2013 The indications of Holocene activity from our excavations and the deformed terraces (Rockwell et al., 1988; 2011), combined with our understanding that the Ventura fault is rooted in the seismogenic crust (Hubbard et al., 2011; 2014), suggest that the fault system poses a considerable seismic hazard. Moreover, the large uplift events recorded by the terraces suggest that the Ventura fault may rupture in large (M7.7-8.1) earthquakes. To reach this earthquake magnitude, the Ventura fault must rupture with other faults systems along strike, perhaps including the San Cayetano and Red Mountain faults. Thus, the focus of this year’s study was to extend our mapping of the faults in the region to develop a comprehensive representation of these potential sources. Originally, the focus of our proposed work was simply to address the Southern San Cayetano fault. However, motivated by the needs of the Ventura SFSA as outlined in the August 2013 workshop, we extended our evaluation to include other major faults in the region. To pursue this study, we georeferenced and combined all of the current fault representations, including those in Community Fault Model (CFM) (Plesch et al., 2007), and others from the UCSB group (Kammerling et al., 2003), and Hubbard et al. (BSSA, 2014, in press). In addition, we continued our collaborations with Thomas Pratt (USGS) to reprocess and interpret additional seismic reflection profiles acquired in the 1980’s across the Southern Cayetano fault (Figure 1). All of these data were depth converted using constraints from the SCEC Community Velocity Model (CVM-H 11.9). We then incorporated SCEC relocated seismicity and focal mechanisms catalogs (Shearer et al., 2005; Lin et al., 2007; Hauksson et al., 2011), and used these constraints to develop new, more precise representations of the faults based on the methods of Plesch et al., (2007). The primary changes represented in these new models relative to the current CFM were the inclusion of a Southern San Cayetano fault representation (which was

absent from the CFM), refinement of the fault linkages and lateral terminations, and extrapolation of the fault surfaces to the base of seismicity surface. We recognized that while many aspects of the current interpretations of the Ventura-Pitas Point fault were compatible, one primary difference emerged – specifically, the presence of absence of a mid-crustal detachment in the Ventura-Pitas Point fault. The detachment was first proposed by Yeats (1982a) and Huftile and Yeats (1995), as a feature localized within the Miocene section that linked the upper and lower ramps of the Ventura-Pitas Point fault. Hubbard et al., (2011, 2014) provided further evidence to support the existence of the detachment, and extended its interpretation into a 3D structural representation. Alternatively, Kammerling et al. (2003) proposed a geometry for the offshore extension of the fault where the detachment is absent. In their interpretation, the Pitas Point fault has a listric shape, decreases its dip with depth. We felt that both interpretations were permissible based on the direct data constraints, and thus developed two complete fault representations, one with and one without the mid-crustal detachment (Figure 3). Each fault representation is provided in large, contiguous tsurfs with about 1km resolution and quality controlled meshes.

Figure 3A: Perspective view looking west of the Ventura-Pitas Point faults system and other structures. This model includes a mid-crustal detachment level (Yeats, 1982a; Huftile and Yeats, 1995; Hubbard et al., 2011, 2014).

Figure 3B: Perspective view looking west of the Ventura-Pitas Point faults system and other structures. This model is based on Kammerling et al., (2003) and components of Hubbard et al., (2011, 2014) and the SCEC CFM. This model does not include the mid-crustal detachment level present in Figure 3A.

A goal of the SFSA is to encourage SCEC investigators to utilize these representations in their planned studies in the Ventura region, including efforts in fault system modeling, finite rupture simulations, and earthquake hazards assessments. The models were made available by the SCEC CME and our group, and are accessible at: http://isites.harvard.edu/icb/icb.do?keyword=k93600&pageid=icb.page637015

Intellectual merit This project investigated the nature of active fault systems in the Transverse Ranges of California, developing comprehensive 3D representations of the Ventura-Pitas Point thrust systems and other faults linked with it along strike. This provides the basis to assess the likelihood of large, multi-segment thrust fault earthquakes based on insights from the geologic record and dynamic rupture simulations. Broader Impacts As exemplified by the 2008 Wenchuan, China (M 7.8) earthquake, large thrust fault systems pose considerable seismic hazards. This study contributes to SCEC efforts to use the Ventura fault as a natural laboratory to study the possibility of large multi-segment thrust fault earthquakes and to assess the hazards posed by these events. Publications Hubbard, J.A., J. H. Shaw, J. Dolan, T. Pratt, L. McAuliffe, and T. Rockwell, 2014, Structure and

seismic hazard of the Ventura Avenue anticline and Ventura fault, California: Prospect for large, multisegment ruptures in the Western Transverse Ranges, BSSA (in press).

References Hauksson, E., W. Yang, and P. Shearer, 2011, Waveform Relocated Earthquake Catalog for

Southern California (1981 to 2011), SCEC Annual Meeting, Palm Springs, CA. Hubbard, J.A., J. H. Shaw, J. Dolan, T. Pratt, L. McAuliffe, 2011, Structural linkage of

onshore and offshore thrust systems across the Ventura fault and prospects for large earthquakes in the Transverse Ranges, SCEC Annual Meeting (invited talk), Palm Springs, CA.

Hubbard, J.A., J. H. Shaw, J. Dolan, T. Pratt, L. McAuliffe, and T. Rockwell, 2014, Structure and seismic hazard of the Ventura Avenue anticline and Ventura fault, California: Prospect for large, multisegment ruptures in the Western Transverse Ranges, BSSA (in press).

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.

Kammerling, M., C.C. Sorlien, and C. Nicholson, 2003, 3D development of an active, oblique fault system, northern Santa Barbara Channel, CA, SSA Annual meeting Abstracts.

Lin, G., P. M. Shearer, and E. Hauksson, 2007, Applying a three-dimensional velocity model, waveform crosscorrelation, and cluster analysis to locate southern California seismicity from 1981 to 2005, J. Geophys. Res., v.112, n.B12, 14 pp, B12309, doi:10.1029/2007JB004986.

Plesch, A., J. H. Shaw, C. Benson, W. A. Bryant, S. Carena, M. Cooke, J. Dolan, G. Fuis, E. Gath, L. Grant, E. Hauksson, T. Jordan, M. Kamerling, M. Legg, S. Lindvall, H. Magistrale, C. Nicholson, N. Niemi, M. Oskin, S. Perry, G. Planansky, T. Rockwell, P. Shearer, C. Sorlien, M. P. Süss, J. Suppe, J. Treiman, and R. Yeats, 2007, Community Fault Model (CFM) for Southern California, Bulletin of the Seismological Society of America, Vol. 97, No. 6, doi: 10.1785/012005021

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.

Rockwell, T.K., and K. Clark, 2011, Large co-seismic uplift of coastal terraces across the Ventura Avenue anticline: Implications for the size of earthquakes and the potential for tsunami generation, SCEC Annual Meeting (invited talk), Palm Springs, CA.

Sarna-Wojcicki, A. M., Williams, K. M., Yerkes, R. F., 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 Yerkes, R. F., 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.

Shearer, P., E. Hauksson, and G. Lin, Southern California hypocenter relocation with waveform cross correlation: Part 2. Results using source-specific station terms and cluster analysis, Bull. Seismol. Soc. Am., 95, 904-915, doi: 10.1785/0120040168, 2005.

Yeats, R. S., 1982a. Low-shake faults of the Ventura Basin, California. J. D. (Ed.), Neotectonics in Southern California. Geological Society 78th Cordilleran Section Annual Meeting, Guidebook, pp. 3-15.

Yeats, R. S., 1982b. Reply to Sarna-Wojcicki and Yerkes. In: Cooper, J. D. (Ed.), Neotectonics in Southern California. Geological Society of America, 78th Cordilleran Section Annual Meeting, Guidebook, pp. 21-23.