Paleoseismic Trenching of the West Napa Fault – Reconciling High Geodetic Slip Rates Using Geologic Data from a Significant Seismic Source in the North Bay Region

Paleoseismic Trenching of the West Napa Fault – Reconciling High Geodetic Slip Rates using Geologic Data from a Significant Seismic Source in the North Bay Region

U.S. Geological Survey (USGS) National Earthquake Hazard Reduction Program (NEHRP) – External Research Grant Award Number: G14AP00035 Term: April 2014 - March 2017

Recipient: California Geological Survey (CGS) 1900 South Norfolk Street, Suite 300 San Mateo, CA 94403

Principal Investigators: Ron S. Rubin

Phone: 650-350-7311 Email: [email protected]

Timothy Dawson

Phone: 650-350-7307 Email: [email protected]

Research supported by the U.S. Geological Survey, Department of the Interior, under USGS award number G14AP00035. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

NEHRP Final Technical Report – Award Number G14AP00035

Abstract

The West Napa Fault Zone (WNFZ) is a dextral shear fault within the greater San Andreas Fault System. The 24 August 2014 South Napa earthquake (SNE) produced at least 12.5 km of clear surface rupture, extending from Cuttings Wharf in the south to north of Alston Park, located in the City of Napa, CA, and ruptured five individual faults within the zone. The SNE demonstrated the earthquake potential of this fault zone, however, little is known about the past behavior of the fault. Basic paleoseismic data such as earthquake recurrence, timing of past earthquakes, and slip rate is needed to provide the inputs essential for seismic hazard models in the region.

This report presents the results of paleoseismic studies conducted at three sites on the WNFZ, with the goal of developing a record of timing and recurrence of past earthquakes. At the Alston Park site, the trenching investigation found no evidence of Holocene faulting that previous investigators identified in a shallow stream bank exposure located adjacent to the trench. Two other sites, named the South Avenue and Buhman Avenue sites, were located on the principal trace of the 2014 surface rupture. At those sites evidence was found of past Holocene-activity, although it was not possible to resolve individual past earthquakes within the trenches.

Taken in context with other studies conducted after the SNE, multiple strands of the WNFZ have ruptured repeatedly during the Holocene. However, the long term spatial and temporal pattern of past earthquakes along this fault system will require additional paleoseismic studies to provide useful constraints for the seismic source characterization for the WNFZ.

Introduction

The West Napa Fault Zone (WNFZ) consists of a group of subparallel northwest-striking faults along the west margin of Napa Valley that accommodates a small fraction of regional San Andreas Fault System dextral shear. The 24 August 2014 Mw 6.0 South Napa Earthquake (SNE) resulted in as much as 20 km of end-to-end surface rupture, and involved seven sub-parallel traces of the WNFZ (Figure 1).

Prior to the SNE, various researchers reached differing conclusions regarding the activity of the WNFZ. Based on geomorphic mapping and a review of site investigations conducted by consulting geologists, Bryant (1982) concluded there was insufficient evidence to show that the WNFZ was Holocene-active north of the Napa River. This mapping focused primarily on the range-front bounding the west side of Napa Valley, which includes an east-facing scarp in Alston Park that other investigators (e.g. Fox et al., 1973; Helley and Herd, 1977; Pampeyan, 1979) had identified as either a Quaternary or Holocene-active fault-related feature.

Bryant (1982) mapped an additional fault about 2.5 km west of the range front based on subtle geomorphic features, but did not consider the evidence conclusive of Holocene activity. However, this fault later ruptured during the SNE as part of the principal rupture trace (Figure 1). Wesling and Hanson (2008) mapped geomorphic features they attributed to Holocene fault activity across an area that includes the City of Napa, and several kilometers to the north and south.

Recent paleoseismic studies have focused on assessing the timing of past activity along different strands of the WNFZ. Wesling and Hanson (2008) interpreted faulted Holocene deposits in a

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trench at the Napa County Airport. Further north along the range front, Wesling and Hanson (2008) also identified recently active faults in an exposure at Napa Creek (Figure 1). Approximately 2.5 km north of Napa Creek, Clahan et al. (2011) interpreted faulted Holocene deposits in a natural exposure within a drainage at eastern Alston Park (Figure 1).

Prior to the SNE, CGS was conducting a re-evaluation of the WNFZ that was motivated by the findings of Wesling and Hanson (2008) and Clahan et al. (2011). Based on CGS site visits to the Napa Creek and Alston Park exposures, corroboration of the presence of active faults was not possible. However, at the Alston Park site, the potential to both confirm the presence of faults, as well as possibly extend the paleoseismic record by trenching across the scarp immediately adjacent to the natural exposure was recognized.

This report presents paleoseismic results from three trench sites investigated both before and after the SNE. The report also includes discussion of related post-earthquake paleoseismic studies and mapping conducted by CGS and other investigators. At the Alston Park site, CGS conclusively found no evidence of faulting in a trench adjacent to the exposure logged by Clahan et al. (2011). These results demonstrate the need for subsurface paleoseismic studies, because near-surface fault-like features can be misinterpreted without a clear exposure of geologic relationships.

Following the SNE, trenches were excavated across the principal rupture trace (Rupture Trace A) at the South Avenue and Buhman Avenue sites (Figure 1). At both sites, evidence was found for pre-SNE late Pleistocene to Holocene earthquakes demonstrating that, with hindsight, the subdued geomorphic expression as mapped by Bryant (1982) was not sufficient evidence to preclude Holocene-activity. This research shows that characterization of fault activity should combine geomorphic mapping with subsurface paleoseismic studies. This is particularly important along faults with low rates of activity where surface processes may mask the geomorphic expression of recent tectonics.

This report first summarizes previous fault mapping and paleoseismic investigations along the WNF. It then describes the results from the three paleoseismic sites excavated for this study. Finally, discussion of the significance of these results in the context of other post-SNE paleoseismic research, and what can be inferred regarding the behavior of the WNF from these studies.

Summary of Previous West Napa Fault Mapping and Paleoseismic Investigations

Previous Mapping and Paleoseismic Investigations

Researchers’ understandings of the location and activity level of the WNFZ has evolved over time. Early regional maps (e.g., Fox et al., 1973; Helley and Herd, 1977; Pampeyan, 1979) depict faults along portions of the range front of western Napa Valley as Holocene-active, primarily based on a prominent ~7-m-high scarp in eastern Alston Park. In an evaluation conducted under the State of California’s Alquist-Priolo Earthquake Fault Zoning Act (A-P Act), Bryant (1982) considered the WNFZ Holocene-active south of the Napa River using the criteria of being sufficiently active and well-defined (CGS, 2018). The evaluation of Bryant (1982) was based primarily on geomorphic

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mapping and a compilation of unpublished site investigations by consulting geologists. However, relative to the clear geomorphic expression of the fault to the south, Bryant (1982) did not consider the faults north of the Napa River Holocene-active due to more subdued geomorphology.

Based on observations of subtle geomorphology, such as tonal lineaments and aligned notches, Bryant (1982) also mapped a fault approximately 2.5 km west of the range front fault. Bryant (1982) concluded that this fault was not sufficiently active, reasoning it could be mapped only a short distance and that it could be an expression of differential weathering rather than recent tectonics. This fault later ruptured with the SNE as part of the more extensive Rupture Trace A.

Wesling and Hanson (2008) mapped a system of geomorphic lineaments north of the Napa River along the range front of western Napa Valley, along similar alignments to some of the previously mapped faults. The lineaments include scarps, right-laterally offset drainages, and linear fronts. Wesling and Hanson (2008) classified many of the lineaments along the range front as faults with “Probably Holocene” activity. Wesling and Hanson (2008) also confirmed Holocene fault activity in a trench exposure south of the Napa River near the Napa County Airport, consistent with the conclusions of Bryant (1982) in that area. North and south of their trench, Wesling and Hanson (2008) mapped a ~6-km-long alignment of scarps, linear drainages, right-laterally offset drainages, and tonal lineaments within inferred Holocene deposits.

Paleoseismic investigations along the range front in Napa utilized exposures within natural drainages at fault crossings to address evidence for recency of fault activity. Wesling and Hanson (2008) logged a fault zone that juxtaposes Tertiary bedrock against alluvium in Napa Creek (Figure 1), with faults extending into overlying fluvial deposits. Radiocarbon dating shows these deposits are Holocene age and Wesling and Hanson present this as evidence of Holocene faulting along the range front. In eastern Alston Park (Figure 1), Clahan et al. (2011) logged an approximately 1-meter deep exposure where it crosses a mapped fault at the base of a large scarp. They interpreted faults within Holocene deposits, although they could not identify individual paleo-earthquakes in the exposure.

Post-SNE WNFZ Evaluation

A re-evaluation of the WNFZ (Rubin, 2018) addresses fault activity north of the Napa River. Rubin (2018) concluded Holocene activity is evident in that area, along some previously mapped faults which did not rupture during the SNE, as well as on faults that ruptured during the SNE. The re- evaluation was based on: an assessment of previous mapping; original mapping with lidar imagery; Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) imagery; mapping of the SNE ruptures; a comprehensive compilation of consulting geologists’ site investigation reports; and the new paleoseismic investigations presented in this report. Though the consultants’ investigation reports do not provide direct age control, two provide evidence of fault offsets of the bedrock-soil interface. The reports were key to making judgements about the activity of mapped faults and geomorphic features along the range front (Rubin, 2018).

CGS Collaborative Efforts following the South Napa Earthquake

CGS allocated considerable internal resources in the form of scientific input, field labor, and funds

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for radiocarbon dating to work collaboratively with others following the SNE. Combining resources with other investigators advanced the goal of developing a better understanding of the WNFZ.

At the Hendry Winery site (Figure 1), CGS participated in paleoseismic trenching investigations led by Carol Prentice (USGS) across Rupture Traces A and C. Results summarized in Prentice et al. (2015) show that the SNE ruptures were only weakly expressed as shallow fissures in the trenches, with no evidence of prior ruptures within approximately 6 ka-old deposits. However, cumulative apparent vertical separation, on the order of about two meters, is exposed in trenches across the ruptures on an elevated, likely Pleistocene surface.

Using characterization of shallow SNE deformation at the South Avenue and Buhman Avenue sites, Brooks et al. (2017) (USGS) developed a model to explain buried shallow fault slip using the pattern of deformation at the surface collected from terrestrial lidar surveys. The model suggests near-surface displacement can underestimate fault slip at depth by as much as 30%.

Finally, CGS conducted a paleoseismic study across Rupture Trace F at the Napa Oaks site (Figure 1) (Seitz et al., 2015). The trench was part of a site-specific fault investigation for a proposed development led by Kevin Ryan, a local consulting geologist. It became a target of opportunity and the property owners, at the request of Kevin Ryan, granted CGS access and time to document a portion of the exposure. The trench provided evidence of 3 or 4 earthquakes during the past 14 ka years, with two Holocene events at 6.5 ka and 4 ka (Seitz, pers. comm., 2017). The previous ruptures have apparent vertical displacements significantly larger than found on that fault in the SNE (Seitz et al., 2015). This suggests previous earthquakes on Rupture Trace F may have a greater length and displacements than in the 2014 SNE, and implies that past earthquakes on the WNFZ were fundamentally different than the 2014 earthquake.

Results

This section presents the results of paleoseismic investigations at the three sites preformed under this research grant: eastern Alston Park, South Avenue, and Buhman Avenue (Figure 1).

Eastern Alston Park

The paleoseismic trench in eastern Alston Park (Figure 1) (Rubin et al., 2014) crossed a previously mapped trace of the WNFZ at the base of a prominent scarp. Clahan et al. (2011) interpreted evidence of Holocene fault activity at the site based on a 1-meter-deep natural drainage exposure where it crosses the scarp. During two field visits to the exposure, CGS could not verify faults depicted on the log with the exposure, although a possible step in gravels within the terrace deposits were observed. However, the exposure was too shallow to conclusively determine if faulting formed the step in the gravels.

The trench (Plate 1) was located approximately 3 - 4 meters south of the natural exposure, to shadow the fault zone interpreted by Clahan et al. (2011) and extended to a depth of approximately 3 meters. The trench exposed the same surficial units (Plate 1) observed in the drainage. These surficial units lie above an unconformity with Pleistocene and possibly older stratigraphy at depth.

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In the trench, a series of escarpment-parallel channel deposits that were correlated to the deposits exposed in the channel, are buried at the base of the slope. These unbroken depositional contacts preclude the presence of faults along the entire length of the trench. Based on the stratigraphic relationships exposed in the trench, CGS believes that Clahan et al. (2011) misinterpreted depositional channel contacts and channel bottom irregularities as faults, likely due to the limited depth of the exposure logged in their study.

South Avenue

The trench at the South Avenue site (Figure 1) (Dawson et al., 2016) lies about 50 meters northwest of the intersection of Los Carneros and South Avenues and spans Rupture Trace A (Plate 2). The rupture produced approximately 10-20 cm of coseismic and post-seismic right lateral displacement, measured at a location about 30 meters southeast of the trench (Ponti et al., in press). The fault exhibits little geomorphic expression at the site; however, about 350 meters to the north, an approximately 1-km-long pre-existing scarp is coincident with the SNE rupture (Plate 2).

The deposits exposed in the trench consist of a well-developed argillic horizon in the near surface, overlying fine sandy clay alluvium with local fine gravelly beds that extend to the depth of the trench (Plate 2). An absolute age of the deposits was not obtained; although, they are likely Pleistocene in age based on the soil development and geomorphic position of the site.

The SNE rupture was expressed as open fissures at the ground surface, within an approximately 2-meter-wide fault zone. A truncated gravel bed is juxtaposed against the southwest side of the fault zone (Plate 2), indicating an unknown amount of horizontal displacement across the fault. It was not possible to determine the number and age of pre-SNE events due to a lack of stratigraphic resolution in the trench. However, the recognition of several faults, pervasively sheared deposits, and mismatches in stratigraphy across the fault zone are indicative of repeated offsets prior to the SNE.

Buhman Avenue

The Buhman Avenue trench site lies approximately 1.5 km northwest of the intersection of Buhman Avenue and Old Sonoma Road (Figure 1). The site was compelling based on its position adjacent to a small drainage, which increased the probability of finding stratigraphy favorable to preserving evidence of past earthquakes. Two trenches were excavated (Plate 3): Trench 1 was located across a short, pre-existing 2-meter-high scarp (Plate 3), across Rupture Trace A (Dawson et al., 2016). Trench 2 was located about 25 meters northwest of Trench 2, near a small drainage where it was expected to find younger, stratified stream deposits. Trench 1 was logged on large-scale (1:10) photomosaics (Plate 3). Trench 2 was not logged because distinct depositional units could not be identified, nor could the SNE rupture from dozens of shrink/swell cracks exposed in the massive clayey deposits be differentiated.

Although surface deformation could not be recognized from the SNE rupture at the exact location of Trench 1, it was possible to map clear ruptures to within a few meters of the trench. Based on a terrestrial lidar survey, approximately 30-35 cm of right-lateral slip from the SNE rupture was measured on a right-laterally deflected fence (Plate 3) (Stephen DeLong, USGS, pers. comm., 5 California Geological Survey

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2016). The trench spanned the entire field of deformation observed along the fence.

The deposits exposed in Trench 1 consisted entirely of colluvium. The section is poorly stratified, but local lenses of coarse sandy clay to fine gravelly clay are present within a massive clayey matrix (Plate 3). The lenses are not laterally continuous for more than a couple meters, and typically could not be correlated between trench walls.

Radiocarbon ages range between 9,295-7,845 Cal BP, based on 5 detrital charcoal samples (see Appendix for individual sample results). The maximum age of the section is approximately 8 ka, based on the deepest sample (Plate 3). The ages do not provide a clear stratigraphic ordering, which is not unexpected due to the colluvial nature of the depositional environment.

The SNE rupture is weakly expressed in the near surface and, due to the poor stratification, the number of earthquake events in the trench is unresolved. However, faults are better expressed lower in the section, with some having well-developed shear surfaces. Some faults offset and juxtapose different stratigraphic units indicating some unknown amount of lateral offset that appears larger than offsets from the SNE. These observations suggest the faults have greater cumulative displacement and are likely the result of at least one or more pre-SNE earthquakes.

Significance of Results Repeated offsets from surface rupturing earthquakes have occurred on the WNFZ, based on results from CGS paleoseismic investigations. The SNE rupture, combined with paleoseismic data, also provides insight regarding how faults with presumed low slip rates and uncertain recency of activity are expressed geomorphically (Rubin, 2018). The paleoseismic data from this study suggests the faults that ruptured during the SNE, while not well-expressed geomorphically, may represent the more active strands of the WNFZ relative to faults along the range-front.

The lack of faulting in the eastern Alston Park trench demonstrates that depositional features in the drainage exposure were misinterpreted as faults by Clahan et al (2011). The large scarp implies a history of significant fault movement, but if so, the fault would be in a location other than what previous workers interpreted based on geomorphology and the stream exposure logged by Clahan et al. (2011). The fault may exist several meters upslope and west of the trench, based on a subtle UAVSAR lineament interpreted from interferograms (Ponti et al., in press; Rubin, 2018) collected as part of post-SNE studies. Alternatively, the scarp could be a result of fluvial and differential erosion, which is permissive based on the existence of channels along the base of the slope in our trench. This study illustrates the value of paleoseismic investigations to accurately assess geomorphic features that may, or may not, be formed by tectonic processes.

The trenches at the South Avenue and Buhman Avenue sites demonstrate Rupture Trace A has ruptured multiple times previously. The Buhman Avenue site confirms evidence for pre-SNE movement in the Holocene. The comparatively faint expression of the SNE rupture in the subsurface at South Avenue and Buhman Avenue, as well as at the Napa Oaks (Seitz et al., 2015) and Hendry Winery (Prentice et al., 2015) sites, indicates SNE-type events may not be preserved in the paleoseismic record. Therefore, earlier deformation observed in the trenches at these sites is possibly the result of previous ruptures with relatively greater amounts of

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displacement than the SNE. However, additional SNE-type ruptures are also permissible based on the available data.

Radiocarbon dates from the sites discussed above, in aggregate, suggest several possible spatial and temporal fault interactions within the WNFZ. Combining the records from the Buhman Avenue site and the nearby Hendry Winery site (Prentice et al., 2015), located 4 km to the north (Figure 1), pre-SNE ruptures on Rupture Trace A are constrained between 8 ka and 6 ka. This assumes that there have been no earthquakes that ruptured as far north as the Hendry Vineyard site after 6 ka. Alternatively, if displacements at Hendry were relatively small, as they were in 2014, they may not be preserved at all, as suggested by Prentice et al. (2015). In this case, the radiocarbon dating only provides a one-sided constraint along Strand A at Buhman Avenue for earthquakes prior to the SNE, with one or more prior 2014 ruptures since ~ 8 ka.

Pre-SNE ruptures on Trace F include events at 6.5 ka and 4 ka (Seitz, pers. comm., 2017), based on the Napa Oaks study. These events are both permissible for Rupture Trace A based on the deformed 8 ka section at Buhman Avenue, which suggests Rupture Traces A and F could have ruptured in the same event. However, the 4 ka event is not evident on either Traces A or C at the Hendry Vineyard, which conversely indicates faults within the WNFZ can rupture independently. A new paleoseismic site with favorable Holocene stratigraphy such that a comparison of event ages with the Napa Oaks record is needed along Rupture Trace A to test these models.

Topics for Possible Future Work Several outstanding questions regarding the WNF exist, including: slip rate and recurrence; how many individual faults in the WNF system are seismic sources, and if they can significantly rupture in the same event; and the overall extent of the fault beyond the northern limits of the SNE rupture. A site with favorable stratigraphy is needed to address the slip rate and recurrence, and one has yet to be identified along the rupture, or along any other strand of the WNFZ.

Subtle geomorphology, landslide complexes, mountainous topography, and dense tree cover makes mapping the WNFZ to the north challenging. Recent unpublished work by UC Davis, as well as the USGS, at locations approximately 25 km northwest of the northern end of the SNE rupture in Napa Valley, may provide an opportunity to map southward and better constrain the extent of the WNFZ (Belle Philibosian, pers. comm., 2018).

Presentations and Dissemination Results of this NEHRP-funded study have been presented at several outlets including two NEHRP Northern California regional workshops, three scientific meetings (e.g., Rubin et al., 2014; Dawson et al., 2016), a journal publication (Brooks et al., 2017), and as supporting data for an official CGS regulatory hazard zone product under the A-P Act (Rubin, 2018).

Each of this study’s trenches available for community reviews in the field. Reviewers include combinations of the following individuals: USGS - Ben Brooks, Rufus Catchings, Steve DeLong, Suzanne Hecker, Alexandra Pickering, Dan Ponti, Carol Prentice, Carla Rosa, Katherine Scharer, David Schwartz; CGS - Dave Branum, Marc Delattre, Wayne Haydon, Peter Holland, Jeremy Lancaster, Tim McCrink, Brian Olson, Jennifer Thornburg; Private Sector - Cooper Brossy

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(Fugro), Ryan Coppersmith (Coppersmith Consulting), Mike Jewett (Miller Pacific), Steve Korbay (Geokor), Jared Pratt (RGH), Kevin Ryan (Ryan Geological Consulting), Kelly Shaw (AMEC), Donald Wells (AMEC).

Acknowledgements CGS Engineering Geologist Maxime Mareschal participated in logging and interpretation at all of the trench sites except the Napa Oaks site. CGS Engineering Geologist Gordon Seitz and Lawrence Livermore National Laboratory provided radiocarbon dating services for the Hendry Winery investigation. Danielle Madugo of Earth Consultants International assisted with trench logging at the South Avenue site. Thanks also to Steve DeLong (USGS), who conducted the terrestrial lidar survey at the Buhman Avenue site.

Success of these investigations was dependent on permission to access property and excavate across the faults. The City of Napa, Eric Luce and the Eric Ross Winery, and the Furloine family all generously accommodated our unique inquiries and allowed this scientific effort to move forward, while asking for nothing in return.

References Cited Brooks, B. A., S. E. Minson, C. L. Glennie, J. M. Nevitt, T. Dawson, R. Rubin, T. L. Ericksen, D. Lockner, K. Hudnut, V. Langenheim, A. Lutz, M. Mareschal, J. Murray, D. Schwartz, D. Zaccone, 2017, Buried shallow fault slip from the South Napa earthquake revealed by nearfield geodesy: Science Advances, v. 3, e1700525, 12 p.

Bryant, W.A., 1982, West Napa fault zone and Soda Creek (east Napa) fault, Napa County: California Division of Mines and Geology Fault Evaluation Report FER-129, 9 p. in Fault Evaluation Reports Prepared Under the Alquist-Priolo Earthquake Fault Zoning Act, Region 1 – Central California: California Geological Survey CGS CD 2002-01 (2002).

California Geological Survey, 2018, Earthquake Fault Zones, a guide for government agencies, property owners / developers, and geoscience practitioners for assessing fault rupture hazards in California: California Geological Survey Special Publication 42, 83 p. Available on-line at: http://www.conservation.ca.gov/cgs/Documents/CGS_SP42_2018.pdf

Clahan, K.B., Wesling, J.R., and Brossy, C., 2011, Paleoearthquake Chronology along the Northern West Napa Fault Zone, Napa County, California: Final Technical Report submitted to the U.S. Geological Survey NEHRP, Award no. 07HQGR0081.

Dawson, T.E., Rubin., R.S., and Mareschal, M., 2016, Paleoseismic Results from Two Sites on the Principal Strand of the August 24, 2014 South Napa Earthquake Rupture: Seismological Research Letters, v. 87, n. 2B, p. 536.

Field, E.H., Biasi, G.P., Bird, P., Dawson, T.E., Felzer, K.R., Jackson, D.D., Johnson, K.M., Jordan, T.H., Madden, C., Michael, A.J., Milner, K.R., Page, M.T., Parsons, T., Powers, P.M., Shaw, B.E., Thatcher, W.R., Weldon, R.J., II, and Zeng, Y., 2013, Uniform California earthquake rupture forecast, version 3 (UCERF3)—The time-independent model: U.S. 8 California Geological Survey

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Geological Survey Open-File Report 2013–1165, 97 p., California Geological Survey Special Report 228, and Southern California Earthquake Center Publication 1792, http://pubs.usgs.gov/of/2013/1165/.

Fox, K.F., Jr, Sims, J.D., Bartow, J.A., and Helley, E.J., 1973, Preliminary geologic map of eastern Sonoma County and western Napa County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-483, scale 1:62,500.

Helley, E.J. and Herd, D.G., 1977, Map showing Quaternary displacement, northeastern San Francisco Bay area region: U.S. Geological Survey Miscellaneous Field Studies Map MF- 881, scale 1:125,000.

Hudnut, K.W., Brocher, T.M., Prentice, C.S., Boatwright, J., Brooks, B.A., Aagaard, B.T., Blair, J.L., Fletcher, J.B., Erdem, J.E., Wicks, C.W., Murray, J.R., Pollitz, F.F., Langbein, J., Svarc, J., Schwartz, D.P., Ponti, D.J., Hecker, S., DeLong, S., Rosa, C., Jones, B., Lamb, R., Rosinski, A., McCrink, T.P., Dawson, T.E., Seitz, G., Rubin, R.S., Glennie, C., Hauser, D., Ericksen, T., Mardock, D., Hoirup, D.F., and Bray, J.D., 2014, Key recovery factors for the August 24, 2014, South Napa earthquake: U.S. Geological Survey Open-File Report 2014–1249, 51 p., http://dx.doi.org/10.3133/ofr20141249

Pampeyan, E.H., 1979, Preliminary map showing recency of faulting in coastal north-central California: U.S. Geological Survey, Miscellaneous Field Studies Map MF-1070, 13 pp. pamphlet, 3 sheets, scale 1:250,000.

Ponti, D.J., Rosa, C., and Blair, J.L., (in press), Observations of ground deformation from the August 24, 2014 South Napa Earthquake: U.S. Geological Survey Open-File Report 2XXX-XXXX.

Prentice, C. S., Sickler, R. R., Scharer, K. M., Hecker, S., DeLong, S., Rubin, R., Lienkaemper, J. J., Dawson, T. E., Rosa, C. M., Pickering, A., Page, A., Mareschal, M., Ponti, D. J., Schwartz, D. P., 2015, Preliminary Paleoseismic Results from Excavations across the Surface Rupture Associated with the 2014 South Napa Earthquake, Abstract T31A-2820 presented at 2015 Fall Meeting, AGU, San Francisco, CA, December 14-18.

Rubin, R.S., 2018, West Napa Fault in the Napa and Cuttings Wharf 7.5-minute Quadrangles, Napa and Solano County, California: California Geological Survey, Fault Evaluation Report FER-256, 21 p.

Rubin, R.S., Dawson, T.E., Mareschal, M., 2014, Pre-Earthquake Paleoseismic Trenching in 2014 Along a Mapped Trace of the West Napa Fault, Abstract S33F-4934 presented at 2014 Fall Meeting, AGU, San Francisco, CA, December 15-19.

Seitz, G., Ryan, K., and Rosa, C., 2015, Multiple Holocene-age events on the easternmost surface rupture of the August 24, 2014 South Napa Earthquake: Seismological Research Letters, v. 86, no. 2B, p. 634.

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Wesling, J.R., and Hanson, K.L., 2008, Digital compilation of West Napa fault data for the Northern California Quaternary Fault Map Database: Final Technical Report submitted to the U.S. Geological Survey NEHRP, Award no. 05HQAG0002, 61 pp.

U.S. Geological Survey and California Geological Survey (2006), Quaternary fault and fold database for the United States, accessed March 21, 2018, from USGS web site: http//earthquake.usgs.gov/hazards/qfaults/.

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FIGURE 1 and PLATES 1-3

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122°20'0"W

ALSTON PARK ! H Dry Creek Rd NAPA Hendry Winery Redwood Rd Napa River

Napa County Airport

Napa Creek

B E F

BUHMAN AVE H! Buhman Ave Napa Oaks

³ A Explanation A 2014 South Napa Rupture traces. (USGS/CGS 2006) Letters indicate individual rupture traces. (Ponti et 38°15'0"N 38°15'0"N al., in press) Quaternary Faults (USGS/CGS 2006) Inset 2014 South Napa Earthquake G Ruptures (Ponti et al., in press) Base Map from ESRI. SOUTH AVE H! South 0 1 2 3 Ave

Kilometers

122°20'0"W Figure 1. Location map of study sites. Faults shown are modi ed from the USGS/CGS (2006) compilation, which includes mapping from Fox et al. (1973), Helley and Herd (1977), Pampeyan (1979), Wes- ling and Hanson (2008), and Hudnut et al. (2014). SW

10 20

50 20

26 NE 10 30 27 10 40 30 50 30 40 60 25 40 70 60 1 m 80 70 80 90 1 m 80 90 Notes: • All contacts depositional. • Heavy line represents prominent unconformity. Vertical natural exposure in foreground. Trench • Black box correlates with photo of natural exposure, at right. behind spoil pile. Black box approximates area of faulting interpreted by Clahan et al. (2011) and correlates with box on the trench log. Vertical height of exposure ~ 1 m. View to southeast.

Unit Descriptions 10

Unit 10 - Clay. Dark grayish brown 10 YR 4/2, trace Unit 26 - Gravelly clay. Very dark grayish brown 10 YR 3/2, 30 – Unit 50 - Claystone. Light olive brown 2.5 Y 5/4, hard, moderately to severely gravel, subangular to rounded, abundant roots up to 1cm 50% gravel up to 25 cm dia., clasts subangular to rounded, clast weathered, locally lithi ed, strati cation poorly preserved, local sub-parallel fabric 10 dia., abundant pores <1 mm dia., thin clay lms, weak lithologies mostly hard volcanics with on distinctive friable of dark greenish gray 10 Y 4/1 clay seams and calcite near trench bottom may 20 blocky ped structure, peds up to 5 cm dia., abundant sandstone, trace roots up to 1 cm dia., abundant pores < 1 mm, clay represent bedding plane fractures but do not project far and display no osets, burrows. Unit distinctive based on relative lack of gravel. A lms ~ 1 mm, weak ped structure. Unit distinctive based on color fabric spaced 10’s of cm apart, unit is mostly oxide stained, trace red ne gravel, 20 horizon/colluvium. contrast relative to adjacent units. Colluvium/channel ll. trace hard volcanic cobbles, trace roots < 5 cm dia., clay seams appear wavy or vein-like, dip of fabric appears to shallow upwards in trench wall.

Unit 20 Gravelly clay. Dark grayish brown 10 YR 4/2, 30 Unit 27 - Gravelly clay. Dark yellowish brown 10 YR 4/4, 20 – 40% - Conglomerate. Light olive brown 2.5 Y 5/3 to olive brown 2.5 Y 4/3, - 50% gravel commonly 5 – 15 cm dia. and up to 0.5 m, gravel commonly < 10 cm dia. and up to 25 cm, clasts subangular to Unit 60 - 40% gravel and cobbles, 30% sand, 30% clay, clast sizes commonly < 5 cm dia. and clasts commonly rounded but includes some subangular rounded, roots up to 5 cm, few pores, thin clay lms, weak blocky ped range < 1 cm up to 30 cm, clasts subangular to rounded and commonly at, 30 and trace at, clast lithologies are primarily volcanic, some structure, peds < 5 cm. Unit distinctive based on relatively lighter generally poorly sorted, clast lithologies include white tu and trace sandstone, roots up to 1 cm dia., abundant pores <1 mm, clay lms up color and inclusion of slump blocks of Unit 50. Colluvium/channel ll. includes local clay pockets possibly derived from Unit 70 below, local darker to 2 mm thick and same color as matrix, moderate blocky 26/27 brown clay lms up to 2 mm thick, larger clasts generally aligned parallel to ped structure, peds up to 5 cm dia. Unit distinctive based bounding contacts. Fluvial deposit. 40 on abundance of rounded gravel and gradation to Unit 30 - Clay. Dark grayish brown 10 YR 4/2, trace gravel claystone near base. Colluvial/residual soil. commonly ne and up to 5 cm dia., clasts subangular, clast lithology 50 red and white tu, abundant roots up to 5 cm dia., trace clay lms, Unit 70 - Clay. Olive brown 2.5 Y 4/3, hard to very hard, trace gravel < 1 cm dia., minimal soil structure, common burrows. Unit distinctive based on variably oxidized with local mottling, contacts locally marked by dark grey clay. Unit 25 - Gravelly clay. Dark yellowish brown 10 YR 4/4, subtle color relative to Unit 40 below, stiness, and relative lack of Unit distinctive based on lack of gravel and strong color. Paleosol? 80 40 -50% gravel commonly 5 – 15 cm dia. and up to 25 cm, gravel. AB horizon. 60 clasts angular to subrounded with many at, common roots < 1 cm, clay lms < 1 mm thick, matrix ped structure Conglomerate. Brown 7.5 YR 4/4 to dark brown 7.5 YR 3/4, very hard, blocky locally and generally weak, peds < 3 cm dia. Unit Unit 80 - 60 Unit 40 - Gravelly clay. Dark yellowish brown 10 YR 4/6, 40% clay supported matrix, 40% grave/cobbles, clast sizes commonly < 3 cm dia. and distinctive based on yellowish colored and friable matrix, gravel and cobbles, clast sizes grouped between 5 – 10 cm dia. and over 15 cm with some up to 30 cm, clasts angular to subrounded and commonly 70 unit is channeled into Unit 40 below. Debris ow >15 cm with one 70 cm, clasts angular to subrounded and commonly broken, clast lithologies include abundant red ne gravel and friable white tu in 90 deposit/channel ll. at or broken, clast lithologies volcanic with common white friable sizes, trace clay lms, black staining common locally, with dark grayish brown 10 tu, trace roots < 1 cm dia., gray clay lms <1 mm thick, matrix ped YR 4/2 mottling, unstrati ed, upper and lower contacts gradual. Unit distinctive structure blocky, peds < 2 cm. Unit distinctive based on several based on strong color and hardness. Debris ow deposit. characteristics: gray 10 YR 5/1 mottling; lower hardness relative to View of NW trench wall from top of trench. Unit 80 below; includes locally preserved distinctive light colored sandy silty clay paleosol. Debris ow deposit/channel ll. Unit 90 - Clay. Brown 7.5 YR 4/4 to dark yellowish brown 10 YR 4/4, hard to very hard, with local dark gray 7.5 YR 4/1 mottling, massive. Unit is distinctive based on relative lack of gravel. Paleosol? Plate 1 - Alston Park. Log of southeast trench wall. NE SW

N Note: red lines are faults. 1 m

1 m

Pre-existing scarp

Los Carneros Ave

Trench Photomosaic of fault zone. Orange ags represent faults. South Ave 0 100 200 m

Aerial photo map of site area. South Napa EQ rupture in red (Ponti et al., in press). Plate 2 - South Avenue. Log of southeast trench wall. SW

1N29 1N15 NE

1N3 1S23 (opposite wall)

Notes: • Red lines indicate faults. • Numbers refer to radiocarbon samples. See Report for details. 1N13

1 m ³ 1 m

Trench 2

Trench 1

50 Right-lateral displacement measurement by Stephen DeLong, USGS. Terrestrial Pre-existing Eastward LiDAR scan of trench site and multiple Facing Scarp 40 Metal ranch fence adjacent to linear regression of fence posts (pink) trench at CGS Buhman Road site model the co-seismic offset of metal rail fence adjacent to trench. 30 Dextral Slip = 31.5 +/- 3.2 cm 20 September 3, 2015 Distance oﬀ line (cm) 10

Calculated (cm) Slip Calculated 05 10 20 30 40 Multiple Linear Regression Fit m 0 Site map. South Napa EQ rupture (not shown), mapped along scarp. LIDAR data provided by Stephan DeLong, USGS. Log for Trench 2 not available. -10 Plate 3 - Buhman Avenue. 0 10 20 30 40 50 Meters Along Fence Line Photomosaic log of northwest wall of Trench 1. DRAFT NEHRP Final Technical Report – Award Number G14AP00035

APPENDIX

Beta Analytic, Inc. Radiocarbon Dating Results

California Geological Survey March 11, 2016

Mr. Ron Rubin California Geological Survey 345 Middlefield Road MS -520 Menlo Park, CA 94025 United States

RE: Radiocarbon Dating Results For Samples 1N3, 1N13, 1N15, 1N29, 1S23

Dear Mr. Rubin:

Enclosed are the radiocarbon dating res ults for five samples recently sent to us. The report sheet contains the Conventional Radiocarbon Age (BP), the method used, material type, and applied pretreatments, any sample specific comment s and, where applicable, the two -sigma calendar calibration range. The Conventional Radiocarbon ag es have been corrected for total isotopic fractionation effects (natural and laboratory induced).

All results (excluding some inappropriate material types) which fall within the range of available calibration data are calibrated to calendar years (cal BC/AD) and calibrated radiocarbon years (cal BP). Calibration was calculated using the one of the databases associated with the 2013 INTCAL program (cited in t he references on the bottom of the calibration graph page provided for each sample.) Multiple probability ranges may appear in some cases, due to short -term variations in the atmospheric 14 C contents at certain time periods. Looking closely at the calibr ation graph provided and where the BP sigma limits intercept the calibration curve will help you understand this phenomenon.

Conventional Radiocarbon Ages and sigmas are rounded to the nearest 10 years per the conventions of the 1977 International Rad iocarbon Conference . When co unting statistics produce sigma s lower than +/ - 30 years, a conservative +/ - 30 BP is cited for the result.

All work on t he se sample s was performed in our laboratories in Miami under strict chain of custody and quality contro l under ISO/IEC 17025:2005 Testing Accreditation PJLA #59423 accreditation protocols. Sample, modern and blanks were all analyzed in the same chemistry lines by qualified professional technicians using identical reagents and counting parameters within our own particle accelerators . A quality assurance report is posted to your directory for each result.

Our invoice has been sent separately. Thank you for your prior efforts in arranging payme nt. As always, if you have any questions or would like to discuss the results, don’t hesitate to contact me.

Sincerely,

Digital signature on file

Page 1 of 8 Mr. Ron Rubin Report Date: 3/11/2016

California Geological Survey Material Received: 3/4/2016

Sample Data Measured d13C Conventional Radiocarbon Age Radiocarbon Age(*)

Beta - 432992 7070 +/ - 30 BP -25.3 o/oo 7070 +/ - 30 BP SAMPLE : 1N3 ANALYSIS : AMS -Standard delivery MATERIAL/PRETREATMENT : (charred material): acid/alkali/acid 2 SIGMA CALIBRATION : Cal BC 6010 to 5895 (Cal BP 7960 to 7845) ______

Beta - 432993 7220 +/ - 30 BP -26.8 o/oo 7190 +/ - 30 BP SAMPLE : 1N13 ANALYSIS : AMS -Standard delivery MATERIAL/PRETREATMENT : (charred material): acid/alkali/acid 2 SIGMA CALIBRATION : Cal BC 6075 to 6010 (Cal BP 8025 to 7960) ______

Beta - 432994 7300 +/ - 30 BP -25.7 o/oo 7290 +/ - 30 BP SAMPLE : 1N15 ANALYSIS : AMS -Standard delivery MATERIAL/PRETREATMENT : (charred material): acid/alkali/acid 2 SIGMA CALIBRATION : Cal BC 6225 to 6070 (Cal BP 8175 to 8020) ______

Beta - 432995 8230 +/ - 30 BP -25.2 o/oo 8230 +/ - 30 BP SAMPLE : 1N29 ANALYSIS : AMS -Standard delivery MATERIAL/PRETREATMENT : (charred material): acid/alkali/acid 2 SIGMA CALIBRATION : Cal BC 7345 to 7140 (Cal BP 9295 to 9090) ______

Page 2 of 8 Mr. Ron Rubin Report Date: 3/11/2016

Sample Data Measured d13C Conventional Radiocarbon Age Radiocarbon Age(*)

Beta - 432996 8150 +/ - 30 BP -24.2 o/oo 8160 +/ - 30 BP SAMPLE : 1S23 ANALYSIS : AMS -Standard delivery MATERIAL/PRETREATMENT : (charred material): acid/alkali/acid 2 SIGMA CALIBRATION : Cal BC 7285 to 7275 (Cal BP 9235 to 9225) and Cal BC 7250 to 7230 (Cal BP 9200 to 9180) and Cal BC 7185 to 7065 (Cal BP 9135 to 9015) ______

Page 3 of 8 CALIBRATION OF RADIOCARBON AGE TO CALENDAR YEARS

(Variables: C13/C12 = -25.3 o/oo : lab. mult = 1)

Laboratory number Beta-432992 : 1N3

Conventional radiocarbon age 7070 ± 30 BP

Calibrated Result (95% Probability) Cal BC 6010 to 5895 (Cal BP 7960 to 7845)

Intercept of radiocarbon age with calibration Cal BC 5985 (Cal BP 7935) curve

Calibrated Result (68% Probability) Cal BC 5990 to 5975 (Cal BP 7940 to 7925) Cal BC 5950 to 5920 (Cal BP 7900 to 7870)

7070 ± 30 BP CHARRED MATERIAL 7175

7150

7125

7100

7075

7050

7025 Radiocarbon age (BP) 7000

6975

6950 6025 6000 5975 5950 5925 5900 5875

Cal BC

Database used INTCAL13 References Mathematics used for calibration scenario A Simplified Approach to Calibrating C14 Dates, Talma, A. S., Vogel, J. C., 1993, Radiocarbon 35(2):317-322 References to INTCAL13 database Reimer PJ et al. IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP. Radiocarbon 55(4):1869–1887., 2013.

Beta Analytic Radiocarbon Dating Laboratory 4985 S.W. 74th Court, Miami, Florida 33155 • Tel: (305)667-5167 • Fax: (305)663-0964 • Email: [email protected] Page 4 of 8 CALIBRATION OF RADIOCARBON AGE TO CALENDAR YEARS

(Variables: C13/C12 = -26.8 o/oo : lab. mult = 1)

Laboratory number Beta-432993 : 1N13

Conventional radiocarbon age 7190 ± 30 BP

Calibrated Result (95% Probability) Cal BC 6075 to 6010 (Cal BP 8025 to 7960)

Intercept of radiocarbon age with calibration Cal BC 6050 (Cal BP 8000) curve

Calibrated Result (68% Probability) Cal BC 6065 to 6020 (Cal BP 8015 to 7970)

7190 ± 30 BP CHARRED MATERIAL 7300

7275

7250

7225

7200

7175

7150 Radiocarbon age (BP) 7125

7100

7075 6090 6075 6060 6045 6030 6015 6000 5985

Cal BC Database used INTCAL13 References Mathematics used for calibration scenario A Simplified Approach to Calibrating C14 Dates, Talma, A. S., Vogel, J. C., 1993, Radiocarbon 35(2):317-322 References to INTCAL13 database Reimer PJ et al. IntCal13 and Marine13 radiocarbon age calibration curves 0–50,000 years cal BP. Radiocarbon 55(4):1869–1887., 2013.

Beta Analytic Radiocarbon Dating Laboratory 4985 S.W. 74th Court, Miami, Florida 33155 • Tel: (305)667-5167 • Fax: (305)663-0964 • Email: [email protected] Page 5 of 8 CALIBRATION OF RADIOCARBON AGE TO CALENDAR YEARS

(Variables: C13/C12 = -25.7 o/oo : lab. mult = 1)

Laboratory number Beta-432994 : 1N15

Conventional radiocarbon age 7290 ± 30 BP

Calibrated Result (95% Probability) Cal BC 6225 to 6070 (Cal BP 8175 to 8020)

Intercept of radiocarbon age with calibration Cal BC 6205 (Cal BP 8155) curve Cal BC 6190 (Cal BP 8140) Cal BC 6185 (Cal BP 8135) Cal BC 6165 (Cal BP 8115) Cal BC 6160 (Cal BP 8110) Cal BC 6140 (Cal BP 8090) Cal BC 6105 (Cal BP 8055)

Calibrated Result (68% Probability) Cal BC 6215 to 6085 (Cal BP 8165 to 8035)

7290 ± 30 BP CHARRED MATERIAL 7400

7375

7350

7325

7300

7275

7250 Radiocarbon age (BP) 7225

7200

7175 6250 6225 6200 6175 6150 6125 6100 6075 6050

Beta Analytic Radiocarbon Dating Laboratory 4985 S.W. 74th Court, Miami, Florida 33155 • Tel: (305)667-5167 • Fax: (305)663-0964 • Email: [email protected] Page 6 of 8 CALIBRATION OF RADIOCARBON AGE TO CALENDAR YEARS

(Variables: C13/C12 = -25.2 o/oo : lab. mult = 1)

Laboratory number Beta-432995 : 1N29

Conventional radiocarbon age 8230 ± 30 BP

Calibrated Result (95% Probability) Cal BC 7345 to 7140 (Cal BP 9295 to 9090)

Intercept of radiocarbon age with calibration Cal BC 7295 (Cal BP 9245) curve Cal BC 7220 (Cal BP 9170) Cal BC 7195 (Cal BP 9145)

Calibrated Result (68% Probability) Cal BC 7320 to 7180 (Cal BP 9270 to 9130)

8230 ± 30 BP CHARRED MATERIAL 8350

8325

8300

8275

8250

8225

8200 Radiocarbon age (BP) 8175

8150

8125 7375 7350 7325 7300 7275 7250 7225 7200 7175 7150 7125

Beta Analytic Radiocarbon Dating Laboratory 4985 S.W. 74th Court, Miami, Florida 33155 • Tel: (305)667-5167 • Fax: (305)663-0964 • Email: [email protected] Page 7 of 8 CALIBRATION OF RADIOCARBON AGE TO CALENDAR YEARS

(Variables: C13/C12 = -24.2 o/oo : lab. mult = 1)

Laboratory number Beta-432996 : 1S23

Conventional radiocarbon age 8160 ± 30 BP

Calibrated Result (95% Probability) Cal BC 7285 to 7275 (Cal BP 9235 to 9225) Cal BC 7250 to 7230 (Cal BP 9200 to 9180) Cal BC 7185 to 7065 (Cal BP 9135 to 9015)

Intercept of radiocarbon age with calibration Cal BC 7135 (Cal BP 9085) curve Cal BC 7100 (Cal BP 9050) Cal BC 7085 (Cal BP 9035)

Calibrated Result (68% Probability) Cal BC 7175 to 7075 (Cal BP 9125 to 9025)

8160 ± 30 BP CHARRED MATERIAL 8275

8250

8225

8200

8175

8150

8125 Radiocarbon age (BP) 8100

8075

8050 7300 7250 7200 7150 7100 7050 7000

Cal BC

Beta Analytic Radiocarbon Dating Laboratory 4985 S.W. 74th Court, Miami, Florida 33155 • Tel: (305)667-5167 • Fax: (305)663-0964 • Email: [email protected] Page 8 of 8