Tearing the Terroir: Details and Implications of Surface Rupture And

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RESEARCH ARTICLE Tearing the terroir: Details and implications of surface 10.1002/2016EA000176 rupture and deformation from the 24 August 2014 Key Points: M6.0 South Napa earthquake, California • The 24 August 2014 South Napa earthquake ruptured the Earth’s Stephen B. DeLong1, Andrea Donnellan2, Daniel J. Ponti1, Ron S. Rubin3, James J Lienkaemper1, surface in a complex manner both 1 3 3 1 4 spatially and temporally Carol S. Prentice , Timothy E. Dawson , Gordon Seitz , David P. Schwartz , Kenneth W. Hudnut , 1 1 2 • Advanced remote sensing techniques Carla Rosa , Alexandra Pickering , and Jay W. Parker enhanced field measurements made to map and quantify surface 1U.S. Geological Survey, Menlo Park, California, USA, 2Jet Propulsion Laboratory, California Institute of Technology, deformation Pasadena, California, USA, 3California Geological Survey, Menlo Park, California, USA, 4U.S. Geological Survey, Pasadena, • The fault zone was previously mapped California, USA as complex, but there were differences between mapped faults and the surface rupture Abstract The Mw 6.0 South Napa earthquake of 24 August 2014 caused slip on several active fault strands within the West Napa Fault Zone (WNFZ). Field mapping identified 12.5 km of surface rupture. These field observations, near-field geodesy and space geodesy, together provide evidence for more than ~30 km of Correspondence to: S. B. DeLong, surface deformation with a relatively complex distribution across a number of subparallel lineaments. Along a [email protected] ~7 km section north of the epicenter, the surface rupture is confined to a single trace that cuts alluvial deposits, reoccupying a low-slope scarp. The rupture continued northward onto at least four other traces

Citation: through subparallel ridges and valleys. Postseismic slip exceeded coseismic slip along much of the southern DeLong, S. B., et al. (2016), Tearing the part of the main rupture trace with total slip 1 year postevent approaching 0.5 m at locations where only a few terroir: Details and implications of centimeters were measured the day of the earthquake. Analysis of airborne interferometric synthetic surface rupture and deformation from the 24 August 2014 M6.0 South Napa aperture radar data provides slip distributions along fault traces, indicates connectivity and extent of earthquake, California, Earth and Space secondary traces, and conﬁrms that postseismic slip only occurred on the main trace of the fault, perhaps Science, 3, doi:10.1002/2016EA000176. indicating secondary structures ruptured as coseismic triggered slip. Previous mapping identiﬁed the WNFZ as a zone of distributed faulting, and this was generally borne out by the complex 2014 rupture pattern. Received 19 APR 2016 Accepted 22 SEP 2016 Implications for hazard analysis in similar settings include the need to consider the possibility of complex Accepted article online 28 SEP 2016 surface rupture in areas of complex topography, especially where multiple potentially Quaternary-active fault strands can be mapped.

1. Introduction In the greater San Francisco Bay area of California, relative motion of the North American and Paciﬁc plates is accommodated across an up to 100 km wide, spatially complex array of faults that comprise the San Andreas Fault System (SAFS). Knowledge of fault slip rates and earthquake histories varies signiﬁcantly between individual faults and fault segments [Jennings and Bryant, 2010; Field et al., 2014]. In the greater SAFS, damaging earthquakes do not always manifest as surface-rupturing events on the primary, geomor-

phically apparent faults. The Mw 6.0 South Napa earthquake of 24 August 2014 occurred in the West Napa Fault Zone (WNFZ), which was recognized as a potential hazard and previously mapped as a zone of several subparallel fault traces occurring over ~46 km from Vallejo to Saint Helena, California (Figure 1). The WNFZ may continue northwest through hills west of Saint Helena based on geomorphic evidence of Pliocene- Pleistocene offset [Wesling and Hanson, 2008; U.S. Geological Survey and California Geological Survey, 2006, hereafter USGS-CGS] or may merge with the Maacama Fault based on geophysical data [Langenheim et al., 2010]. To the south, it may connect with the Calaveras Fault via the Contra Costa

Shear Zone [Brossy et al., 2010]. The 2000 Mw 5.0 Yountville earthquake was the most recent prior damaging earthquake on the WNFZ. Its epicenter was 20 km north-northwest (along an azimuth of 325°) from the ©2016. The Authors. 2014 event epicenter but did not produce surface rupture [Langenheim et al., 2006]. The slip rate of the This is an open access article under the terms of the Creative Commons WNFZ is inferred to be low, with estimates ranging from 0.2 to 1 mm/yr based on geomorphic evidence Attribution-NonCommercial-NoDerivs [USGS-CGS, 2006; Field et al., 2014] and perhaps up to 4 mm/yr based on geodetic modeling [Field et al., License, which permits use and distri- 2014; d’Alessio et al., 2005]. bution in any medium, provided the original work is properly cited, the use is The 24 August 2014 earthquake nucleated at a depth of ~8.8 km [Brocher et al., 2015; Hardebeck and non-commercial and no modiﬁcations or adaptations are made. Shelly, 2016]. Fault modeling indicates that the rupture propagated updip and principally to the north

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on a near-vertical fault plane [Brocher et al., 2015; Wei et al., 2015; Dreger et al., 2015; Floyd et al., 2016]. The ground motions associated with the 2014 earthquake caused widespread damage, especially to masonry buildings on alluvial soils. Surface faulting caused localized damage to roads, buildings, and other infrastructure [Bray et al., 2014; EERI, 2014; Hudnut et al., 2014; Brocher et al., 2015]. Because of its location in the densely populated San Francisco Bay area and the resulting surface rupture which affected resi- dential neighborhoods, surface rupture associated with the South Napa earthquake was Figure 1. Setting of West Napa Fault zone relative to other faults studied in unprecedented detail using a wide in the San Francisco Bay area. Red traces are Quaternary-active faults from USGS-CGS [2006]. Green traces are the 2014 surface range of geophysical, geodetic, and geological rupture as indicated by field mapping (this study). Geographic methods. Here we summarize the fault surface locations mentioned in text: SH, Saint Helena; R, Rutherford; V, rupture associated with the South Napa earth- Vallejo; and SPB, San Pablo Bay. Tectonic features mentioned in quake in the context of prior understanding text: WNFZ, West Napa Fault Zone; S, Southampton Fault; F, from Quaternary-active and bedrock fault Franklin Fault; and CCSZ, Contra Costa Shear Zone. mapping and discuss the implications for seismic hazard assessment in similar settings. We focus on direct observations made in the field including geologists’ observations and measurements, near-field geodetic measurements (alignment arrays and lidar), and observations made from Uninhabited Aerial Vehicle Synthetic aperture Radar (UAVSAR) collected by NASA (uavsar.jpl.nasa.gov, last accessed March 2016). The data presented here are not intended to be a comprehensive summary of all data related to surface deformation from the South Napa Earthquake. Rather, they represent a synthesis and interpretation of several data sources that provide wide spatial and partial temporal understanding of surface deformation from this intriguing and complex, but moderate magnitude, surface-rupturing earthquake.

2. Methods and Data Much of our understanding about the effects of the 2014 South Napa Earthquake is from direct observations made in the hours, days, and weeks following the event. Geologists from the U.S. Geological Survey, the California Geological Survey, academia, NASA, consulting firms, and other government agencies quickly responded to the earthquake. Airborne reconnaissance and photography were performed by USGS in cooperation with the California Highway Patrol on 24 and 25 August 2014 [Hudnut et al., 2014]. Google acquired aerial photography on the day of the earthquake, made it available for viewing on Google Earth, and shared orthometrically corrected mosaic images with the USGS (Google, written communication, 2014). Geologists identified the location of surface rupture, measured surface slip amount, documented faulting-related surface effects, and shared observations through the California Earthquake Clearinghouse [Rosinski et al., 2015]. Airborne laser scanning was performed over the rupture area on 9 October 2014 [Hudnut et al., 2014; www.opentopography.org, 2015, last accessed December 2015]. Many of geologists’ earliest measurements of surface slip were highly uncertain, especially as to whether they captured the full width of the deformation zone or if some amount of deformation extended beyond the measured offset features [Bray et al., 2014]. Furthermore, rapid onset of postseismic slip overprinted the coseismic slip within hours of the earthquake [Lienkaemper et al., 2016]. Near-field geodetic methods were employed to quantify postseismic slip including the establishment of alignment arrays [Lienkaemper et al., 2016], terrestrial laser scanning [DeLong et al., 2015], and mobile laser scanning [Brooks et al., 2015]. Cultural features such as fences, curbs, and vineyard rows were measured to determine total coseismic and postseismic slip at several locations [Brooks et al., 2015; DeLong et al., 2015; Lienkaemper et al., 2016].

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UAVSAR data collected by NASA 87 days prior to the earthquake (29 May 2014), 5 days postearthquake (29 August 2014), and 59 days postearthquake (22 October 2014). These data, processed as interferograms, therefore quantify understanding of coseismic combined with early postseismic deformation and an interval of postseismic deformation. NASA/Jet Propulsion Laboratory’s UAVSAR airborne repeat-pass interferometric synthetic aperture radar (InSAR) system is mounted beneath a NASA Gulfstream III aircraft. L-Band (24 cm wavelength) SAR images with a resolution of 1.9 m in range and 0.8 m in the azimuth direction have been collected over the Napa area since November 2009. A precision autopilot with real-time differential GPS maintains repeat flight paths within a 10 m diameter, and typically achieves a repeat-track baseline of less than 5 m. Image pairs were differenced to create interferograms, which reflect the change in distance (range) between a point on the ground and the airborne instrument [Rosen et al., 2006]. The elevation angle between the ground and the instrument at specific ground locations was used to convert line of sight ground range change to horizontal motion perpendicular to the flight path and then projected that onto the average trend of the fault (158.3°). The difference is an angle of 13.3° between the look direction and the strike of the fault. These data were produced along both fault-parallel and fault-normal profiles. Data gaps due to decorrelation were present in particular in areas of changed land use, vegetation, water, or marshland. Error inherent in such an analysis is mostly caused by uncertainties in topographic models and refractive variability of the atmosphere that introduces random phase distortions, these errors may total up to low tens of millimeters in deformation measurements [Rosen et al., 2006]. Measurement noise of this magnitude in areas without cor- roborating evidence of deformation support this error budgeting, but the errors can also limit interpretation along subtle features with possible total slip magnitudes within the margin of error.

3. Coseismic Surface Rupture and Deformation 3.1. Field Observations Observations of surface rupture were made within hours of the 2014 South Napa earthquake led to a rapid first-order understanding of the complex surface rupture [Bray et al., 2014; Morelan et al., 2015]. Slip within the complex pattern of surface rupture can be classified with respect to amount of coseismic displacement, amount of postseismic displacement, and continuity of surface rupture (Figure 2). Observations the morning of the earthquake indicated coseismic slip along two subparallel NW-striking fault traces (traces A and C; Figure 2) of no greater than 0.1–0.2 m except along a ~2 km long section of trace A north of Henry Road, where the coseismic slip peaked at 0.46 m [Bray et al., 2014; Brocher et al., 2015; Morelan et al., 2015]. One day after the earthquake, postseismic slip was observed on trace A, especially south of Henry Road, and additional ruptures with dextral displacements of less than 0.1 m were identified (traces B, D, and E in Figure 2). Postseismic slip occurred only on trace A, with the relative amounts of coseismic and postseismic slip varying along strike (Figure 3). The locations with the highest coseismic slip experienced less postseismic slip than did the locations closer to the epicenter that experienced lower coseismic slip. The largest measured coseismic slip (0.46 m) was located 10.4 km north of the epicenter and is equal to the peak combined coseismic and postseismic slip (0.47 m) measured 1 year postevent at a location 6 km north of the epicenter [Lienkaemper et al., 2016; Wei et al., 2015; DeLong et al., 2015] (Figure 3). Most postseismic slip occurred 3–9 km north of the epicenter, in areas underlain by alluvial deposits more than 60 m thick (Figure 2) [Kunkel and Upson, 1960]. No interseismic creep had been detected along the WNFZ prior to 2014 [Galehouse and Lienkaemper, 2003]. Field measurements of dextral slip on secondary strands B-E do not exceed 0.08 m (Figures 2 and 3). Trace C is the longest of the secondary ruptures with observable surface slip occurring over a length of ~7.5 km. Rupture lengths on strands B, D, and E were less than 1.5 km, and surface fractures were discontinuous and observed primarily within pavement and sidewalks with displacements less than 5 cm. The surface rupture characteristics depended on displacement magnitude. Most surface rupture was manifest as a series of left-stepping en echelon breaks with measureable dextral slip. Where slip exceeded 0.3 m the surface rupture was manifest as connected, but left-stepping en echelon breaks. Where slip was less than 0.1 m, observed surface rupture was commonly discontinuous. Surface rupture tended to be more observable on hard surfaces such as roads and sidewalks and less visible on weak sediments such as young

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Figure 2. Map of surface rupture from ﬁeld observation associated with 2014 South Napa Earthquake classiﬁed according to relative amounts of coseismic and postseismic slip.

river terraces and surﬁcial soils. The various remote sensing methods deployed after the earthquake, including airborne and ground-based laser scanning and air photo collection, resolved surface rupture and were used to supplement ﬁeld observations (Figure 4).

3.2. Surface Deformation From Remote Sensing More comprehensive mapping of the details and extent of surface rupture in the days and weeks following the earthquake was facilitated by several remote sensing data sets that covered the rupture area. These include photography from helicopter overﬂights on 24 and 25 August 2014, aerial orthoimagery generated by Google from images taken on 24 August 2014, airborne UAVSAR data collected on 29 August 2014, and airborne lidar and imagery data collected on 9 September 2014 (Figure 4). UAVSAR interferograms from the interval 29 May 2014 to 29 August 2014 conﬁrm that surface deformation was essentially continuous along mapped fault traces (Figure 5). These observations are consistent with, but add detail to, those made from spaceborne SAR as reported in Floyd et al. [2016]. They use lower resolution SAR data, GPS inversions, and less complete surface rupture mapping to generate 3-D models of fault rupture that are simpler than, but do not contradict, our reported observations.

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50 The initial time interval measured by UAVSAR includes coseismic and the ﬁrst

40 5 days of postseismic slip. The UAVSAR data also reveal other extensive, but

Henry Road subtle, lineaments that indicate surface 30

CA Highway 12 deformation. These lineaments do not

Redwood Road necessarily indicate that surface rupture Cuttings Wharf 365-day postseismic 20 was missed in the ﬁeld or that the sur- Surface slip (cm) Coseismic face failed in distinct brittle fractures or 10 Trace E that surface rupture even occurred along

Trace A Trace C Trace A these trends. They may indicate very 0 shallow fault rupture manifest as defor- 0 51015mation over a width of up to several Distance NNW from epicenter meters, which is potentially undetect- fi Figure 3. Distribution of coseismic and postseismic slip on Traces A, C, able in the eld. Their locations along and E (see Figure 2) of the 2014 South Napa Earthquake from field mapped fault traces and/or as exten- observations and alignment arrays. Traces B and D exhibited less than sions of observed fault rupture is com- 5 cm of slip and are not plotted. Coseismic slip magnitudes are from pelling evidence that these are observations within 24 h of the earthquake. coseismic tectonic features. However, alternative origins for these features include differential subsidence due to variable subsurface materials, incoherence/decorrelation of SAR data, and tropospheric effects. A lineation indicating clear evidence for greater length of surface deformation than was clearly observed in the fieldisthesouthwardextensionoftraceA.Thepresenceoftidalsaltmarshandtidallyinfluencedchannelsofthe

Figure 4. Details of rupture pattern ~0.5 km south of California Highway 12, along Cuttings Wharf Road. (a) Google orthoimagery from 24 August 2014; (b) terrestrial lidar-derived hillshade map from 26 August 2014; (c) orthoimagery collected 9 October 2014; and (d) hillshade map from airborne lidar data collected 9 October 2014.

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Figure 5. Interferograms from UAVSAR data collected 29 May 2014 and 29 August 2014. This interval includes coseismic deformation as well as the first 5 days of postseismic deformation. (a) Interferogram with location of 24 August 2014 Mw 6.0 main shock, aftershocks, and the observed distribution of surface rupture. Thin lines are roads in area of main fault rupture. SP, Slaughterhouse Point. (b) Uninterpreted interferogram and (c) interpreted interferogram including observed surface rupture in black, lineaments from UAVSAR data interpreted as surface deformation as solid white lines were certain, dotted lines were uncertain, and queried were ambiguous, and cross sections from which displacement data were extracted (shown in Figure 6). Thin grey lines parallel to faults are locations of along-fault profiles used to generate along-fault displacement measurements (shown in Figure 9). The northernmost along-fault profiles are wider than the others due to extensive UAVSAR decorrelation in the hills west of the surface rupture in that area.

Napa River precluded definitive field observations to the south of Cuttings Wharf, where the seawall was dis- tinctly ruptured. However, UAVSAR interferograms reveal displacements consistent with dextral slip across brush-covered islands in the Napa River, and near Slaughterhouse Point, ~7 km south of the epicenter (Figure 5). Displacements extracted from UAVSAR south of Cuttings Wharf (profiles D-D′ and F-F′ in Figure 6) are 10 cm or less and may occur as deformation over a width of several meters. Deformation associated with this lineament south of Cuttings Wharf was confirmed by Morelan et al. [2015] at their Green Island location. These lineaments interpreted from UAVSAR can be explained by shallow slip and are corroborated by joint inversion of seismic waveforms, GPS, and InSAR data that indicate shallow slip at a depth above 2 km occurred 5–10 km southeast of the hypocenter [Dreger et al., 2015; Floyd et al., 2016] Evidence from UAVSAR for distinct surface deformation at Slaughterhouse Point coincides with the south end of the after- shock zone (Figure 5a). These observations indicate that coseismic slip extended southeast of the epicenter along trace A for a distance of as much as 10 km. Slip estimates from Dreger et al. [2015] indicate that this southeast continuation of the rupture contributes to ~3% of the total earthquake moment. Additional lineaments visible in UAVSAR interferograms include a potential connection from fault trace C to the observed surface rupture south of the Napa Airport, and a possible continuation of that trace southward for several kilometers south of the airport, perhaps as far south as the southern extent of trace A (Figure 5c). These lineaments are subtle, and while cross-fault displacement profiles have inflections at these lineaments, displacements appear to be no greater than a few centimeters. In the northern part of the surface rupture extent, east of the zone of distributed faulting, other subtle lineaments are visible in the UAVSAR interferogram (Figure 5c). These lineaments are aligned with the base of a topographic scarp that marks the edge of the extensive low-relief surface underlying Napa Valley previously

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Figure 6. Cross-fault profiles of horizontal displacement from 29 May 2014 to 29 August 2014 UAVSAR interferograms. The displacement values estimated from profiles include coseismic and the first 5 days of postseismic deformation following the 24 August 2014 earthquake and are given with error based on data scatter. Thin arrows indicate position of field- mapped surface ruptures and are labeled with the rupture trace name from Figure 2. Wider, open arrows indicate locations of lineaments from UAVSAR from Figure 5. Data gaps indicate decorrelation of UAVSAR data, generally from presence of water, marsh, vegetation, or land use change in the survey interval.

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Figure 7. Interferograms from UAVSAR data collected August 29, 2014 and October 22, 2014. This interval includes postseismic deformation from 5 to 54 days after the earthquake. a) Interferogram with location of August 24, 2014 Mw 6.0 mainshock, aftershocks and the observed distribution of surface rupture. b) uninterpreted interferogram, c) interpretedinterferogram including observedextent of postseismicslip in black, lineamentsfrom UAVSARdata interpreted as extent of postseismic surface deformation in white, and cross sections from which displacement data were extracted (shown in Figure 8). Thin lines parallel to faults are locationsof along-fault proﬁles used to generate along-fault displacement measurements (shown in Figure 9).

mapped as a potential location of a Quaternary-active fault [Bryant, 1982; Wesling and Hanson, 2008]. Although some fracturing was visible in the field along this trend, no field observations indicated that through-going tectonic surface rupture occurred. A cross-fault horizontal displacement profile across this lineament from UAVSAR data (profile A-A′ in Figure 6) indicating ambiguous evidence for tectonic movement, raises the possibility that vertical motion, either tectonic or nontectonic (differential settlement across an apparent bedrock-basin contact) or other minor kinematic phenomena occurred along this trend (Figure 6). To the north of the mapped surface rupture, deformation across the fault zone continues and does not decay to zero until ~23 km north of the epicenter and well beyond the extent of lineaments in UAVSAR (Figures 5 and 6). No distinct surface rupture was observed in this area in the field, although cracking and surface damage was observed in the hills north of Hendry Winery and Alston Park. It is thus possible, perhaps likely, that the deformation north of the end of observed surface rupture was distributed over a wider area and could not be detected in the field or as a distinct lineament in the UAVSAR data.

4. Postseismic Surface Slip From Field Observation and UAVSAR Postseismic slip was measured in the field along much of trace A using alignment arrays and terrestrial lidar [Lienkaemper et al., 2016; DeLong et al., 2015] (Figure 3). UAVSAR data confirm extensive postseismic slip along trace A but no apparent postseismic slip along any of the mapped secondary rupture traces (Figure 7). Furthermore, deformation appears to have continued along this southern extent of trace A through the postseismic interval (profiles D-D′ and F-F′ in Figure 8). The absence of postseismic slip on traces B-E and the northernmost part of trace A raises the possibility that displacement on these traces resulted from triggered shallow slip [e.g., Donnellan et al., 2014] rather than slip connected at depth to the seismogenic fault. Dreger et al. [2015] conclude that secondary fault trace connectivity is unresolved but propose that the rupture configuration could be compatible with a flower structure at

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Figure 8. Cross-fault profiles of horizontal displacement from 29 August 2014 to 22 October 2014 UAVSAR interferograms. The displacement values estimated from profiles include postseismic deformation from 5 to 54 days after the 24 August 2014 earthquake and are given with error based on data scatter. Data gaps indicate decorrelation of UAVSAR data, generally from presence of water, marsh, vegetation, or land use change in the survey interval. Thin arrows indicate position of field-mapped surface ruptures and are labeled with the rupture trace name from Figure 2. Wider, open arrows indicate locations of lineaments from UAVSAR in Figure 5.

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depth. The rupture traces approach each other at the northern end of the rupture suggesting that they may connect at shallow depths. Floyd et al. [2016] interpret geodetic data that indicate minor postseismic slip at depth along the fault trace at the Napa airport and suggest this is delayed deeper slip that follows immediate triggered slip. The small amount of displacement on all secondary traces makes the determination of connected versus triggered slip difﬁcult. It is, however, apparent that the slip on the primary rupture did not step over to secondary traces in a signiﬁcant manner because the slip magnitudes were comparatively low on secondary traces.

5. Comparison of Slip Distribution From Field and Remote Sensing Measurements Analysis of UAVSAR interferograms allows for extraction of total slip distributions along the fault traces from the coseismic and early postseismic period (29 May 2014 to 29 August 2014), and the postseismic period from 29 August 2014 to 22 October 2014 (Figure 9). These include a component of vertical deformation that indicates centimeter-level uplift on the west side of the fault, consistent with long-term west side up displacement man- ifested by higher topography and gravity values west of the fault zone [Langenheim et al., 2006]. This is sup- ported by analyses of lidar data [DeLong et al., 2015] and spaceborne SAR and GPS data [Polcari et al., 2016]. The slip distribution from UAVSAR is somewhat different than that generated from sparse field measurements and alignment arrays but they tend to agree within a decimeter (Figure 9c). The UAVSAR-derived slip distribution has a smoother shape than the field-measured slip. Important differences include (1) the absence of a local slip minimum at Henry Road in the UAVSAR profile and (2) absence of the sharp peak derived from a single slip measurement of 46 cm observed north of Henry Road on the day of the earthquake in the UAVSAR profile. The UAVSAR data also show that the extent of slip extends farther to the north and south than observed in the field. The UAVSAR displacement measurements integrate across a wider area and integrate hundreds to thousands of independent measurements. As such, the UAVSAR measurements are less suscep- tible to fine-scale variability that are common within several meters of a surface rupture, such as local slip variability due to block rotation, as was demonstrated by laser scanning by DeLong et al. [2015]. Furthermore, the UAVSAR data are more likely to capture deformation that is distributed farther from the fault trace than indicated by commonly measured cultural features.

6. Napa Valley Fault Mapping: A Retrospective Look and Comparison to 2014 Surface Rupture The pattern of surface rupture in 2014 did not precisely match the fault pattern from any single previous map of the WNFZ (Figure 10). Existing maps did indicate a network of NNW-SSE striking faults on the western margin of the Napa Valley in an area of hilly topography. The rupture in 2014 was distributed among several traces that have weak or no geomorphic expression but are generally parallel to the ridge-valley topography (Figure 10). Prior to the earthquake, the primary rupture trace had been only partially mapped, and the secondary traces were largely unrecognized. Previous mapping was produced as part of (1) regional geological mapping, which is focused on identifying bedrock, surficial, and structural geology; and (2) Quaternary fault mapping. Regional geologic mapping identifies faults on the basis of lithologic contrasts across discontinu- ities with kinematic indicators or by identification of deformed surficial materials and usually does not classify faults according to relative activity (Figure 10c) [Graymer et al., 2007; Wagner and Gutierrez, 2010]. Fault hazard mapping integrates observations of a wide range of tectonic-geomorphic features and visible lineaments in the landscape to map potential traces of Quaternary faults [e.g., Helley and Herd, 1977; Bryant, 1982; Wesling and Hanson, 2008]. This mapping includes identification of features such as scarps in alluvium, offset stream channels, linear valleys, lineaments in vegetation, sag ponds, linear terrace scarps and mountain-fronts, and other indications of potentially recent tectonic activity. Synthesis efforts such as the USGS Quaternary Fault and Fold Database [USGS-CGS, 2006] are a result of interpretation of these various mapping efforts and categorization of mapped fault traces by recency of activity. Previous fault mapping indicated that the best geomorphically expressed trace of the WNFZ crosses the Napa Airport at the north end of an ~8 km long set of discontinuous fault traces. This section of the fault zone displays geomorphic evidence of Holocene faulting including lineaments in vegetation, topographic sags, and subtle linear troughs as indicated in Figure 10b [Bryant, 1982; USGS-CGS, 2006; Wesling and Hanson, 2008]. Farther north, the WNFZ is characterized by branching and stepping faults mostly

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Figure 9. Along-fault displacements from UAVSAR data. (a) Displacements measured from 29 May 2014 to 29 August 2014 UAVSAR interferograms, (b) displacements measured from 29 August 2014 to 22 October 2014 UAVSAR interferograms, and (c) comparison of total coseismic and postseismic slip from UAVSAR 54 days postearthquake and our best estimate for ﬁeld measurements of coseismic and postseismic slip as measured or interpolated at ~54–60 days post earthquake. Geographic locations mentioned in the text and in Figure 2 are indicated. The locations of the along fault proﬁles that were used to generate these data are on Figures 5 and 7. Up-to-the-west vertical deformation was also extracted for the coseismic period and is plotted as a dotted line.

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Figure 10. (a) Surface rupture from 2014 South Napa earthquake. (b) Map of potential Quaternary faults, lineaments, and geomorphic features as mapped prior to 2014 earthquake. Numbers 1–3 correspond to features indicating active faulting mentioned in text: (1) topographic saddles, (2) deﬂected streams, and (3) a low-angle scarp in alluvium [Bryant, 1982]. (c) Faults from regional geologic mapping done prior to 2014 earthquake. Base for maps in Figures 10a and 10b, and Figure 10b is a hillshade map from airborne lidar data [National Center for Airborne Laser Mapping, 2003].

concentrated within a ~0.6 km wide zone trending along the west side of Napa Valley from the Napa River to Rutherford, California (Figure 1) [USGS-CGS, 2006]. Geomorphic evidence of Quaternary faulting is less apparent along this part of the fault zone, and the 2014 surface rupture occurred ~1 km or more to the west of some previously mapped faults. Previously mapped fault traces coincident with the 2014 rupture trace A were identified as Quaternary active based on presence of features including a subdued scarp, linear and deflected drainages, and a topographic saddle (as indicated in Figure 3) [Bryant, 1982; Wesling and Hanson, 2008]. These features led to 4.5 km of the central portion of trace A being mapped as potentially Quaternary active. The only rupture observed in 2014 on a fault strand mapped as Holocene active was a 0.6 km long rupture with <0.01 m slip at the Napa County Airport, 2 km east of trace A. Evidence for Holocene fault activity near the Napa County Airport includes a distinct tonal lineament in soils and vegetation and deformation of Holocene sediments expressed as subtle topographic scarps and valleys. Regional geologic mapping [Wagner and Gutierrez, 2010; Graymer et al., 2007] identifies faults displacing bedrock units that align well with the northern portions of traces A and C (Figure 10c). The regional geologic maps do not indicate the specific criteria used for each mapped fault segment, but it is apparent that along the northern part of traces A and C, distinct lithological contrasts, mostly between Eocene sandstones and Cretaceous Great Valley Sequence, indicate faults. Farther south along trace A, Wagner and Gutierrez, 2010 mapped a concealed fault at the location of the scarp, but Graymer et al. [2007] did not, possibly because no apparent lithologic contrast could be identified, confounding a genetic interpretation. While interpretation of regional geologic maps can be uncertain, this example illustrates that detailed regional geologic mapping is a critical foundation for seismic hazard analysis, especially in areas where geologic materials that otherwise might preserve evidence of young faulting are scarce. Comparison of previous mapping to interpretations made from UAVSAR in 2014 indicates surface deformation along several previously mapped fault traces. Although these subtle patterns of deformation may be triggered slip and may not indicate that these structures are capable of generating damaging

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earthquakes, they provide a more complete picture of how complex faults zones behave in moderate earthquakes. It may be the case that future, perhaps larger, earthquakes within the West Napa Fault Zone or similar fault zones elsewhere could lead to extensive fault rupture along multiple connected or distributed fault traces in a manner suggested by the subtle patterns of surface deformation revealed by the 2014 South Napa earthquake.

7. Implications for Seismic Hazard The 2014 South Napa earthquake reinforces that geological mapping, paleoseismology, and probabilistic seismic-hazard assessment provide motivation for risk mitigation and hazard preparation but do not predict specific events in detail. Previous study of the WNFZ indicated the likelihood of moderate earthquakes and previous mapping indicated that the WNFZ was spatially complex, but the fine details of the spatial and temporal characteristics of the 2014 fault rupture were unanticipated, in part due to its generally poor geomorphic expression. While the extent of surface rupture and amount of slip exceeded many other earthquakes of similar moment magnitude, it was not completely unexpected in terms of rupture length, slip magnitude, or spatial complexity [Brocher et al., 2015]. Interpretation of existing maps would suggest fault rupture on the WNFZ should occur along the well- defined Napa County Airport section of the fault and perhaps along the western margin of the Napa Valley where geomorphic interpretation led to mapping of potential Quaternary active fault strands. Instead, primary rupture occurred along a fault trace that was only partially identified and located ~2 km west of strands of the WNFZ thought to be most recently active. This fault was characterized prior to the South Napa earthquake by subtle, sparse geomorphic evidence including a low scarp, a short linear valley, and possibly offset streams [Bryant, 1982] but was not mapped as a continuous structure. The subtle scarp in otherwise flat alluvial terrain is compelling because scarps in strike-slip environments are often formed by juxtaposition of dif- fering topography from lateral motion or by local vertical movement on the fault plane. This results in short, sharp scarps that may have considerable spatial variability. Furthermore, low-gradient scarps, most often studied in normal-fault settings, are generally considered to be evidence of lack of tectonic activity for significant periods of time [Hanks, 2000]. DeLong et al. [2015] identified that both the coseismic and postseismic patterns of vertical deformation along this fault in 2014 occurred over a width of ~50 m with no more than 15–20 cm of elevation change. Polcari et al. [2016] also identify east downward relative vertical motion of ~8 cm across the main fault trace along this part of the fault. This indicates wide, low-angle scarps may, in some settings, be diagnostic of Holocene strike-slip faulting rather than evidence for tectonic quiescence. The South Napa earthquake, by rupturing multiple subparallel traces through complex ridge-and-ravine topography, revealed the existence of other previously unmapped fault strands that have the potential for future surface displacements. The prior geomorphic evidence indicating the strand of the WNFZ trending through the Napa Airport is the most active fault remains compelling [Bryant, 1982]. There is no evidence that the very small displacements on this fault trace during the 2014 earthquake have lowered its seismic potential. The South Napa earthquake revealed one scenario for surface rupture through Napa Valley, but the potential for rupture by other mapped faults in the area still exists. Observation of the complexity of the 2014 surface rupture was facilitated by advanced remote sensing, especially UAVSAR and lidar. UAVSAR in particular was used to measure coseismic and postseismic slip distributions, vertical deformation, and fine-scale details of fault zone kinematics [Hudnut et al., 2014; DeLong et al., 2015; Brooks et al., 2015]. UAVSAR helped identify ruptures with low-slip magnitude and revealed deformation at Slaughterhouse Point in northern San Pablo Bay, lending support to models that propose the WNFZ may connect southward to the Southampton and/or Franklin Faults and from there via the Contra Costa Shear Zone to the Calaveras Fault [Graymer, 2014; Baltay and Boatwright, 2015] . The South Napa earthquake raises questions about the WNFZ and complex low-slip rate faults in general, and provides targets for future study. Paleoseismic and geomorphic investigations are underway to determine prehistoric rupture history in space and time. These may aid in refining how we identify late Quaternary faults in similar low slip-rate settings. The ongoing detailed field and geodetic observations of surface deformation from this event have provided a comprehensive approach for scientific earthquake response that promises a richer understanding of future earthquakes as well.

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How the multiple fault strands involved in the 2014 earthquake interact with each other, with previously mapped faults nearby that did not rupture, and how these structures may continue to the north and south are fundamental unanswered questions. The complexity revealed by this rupture complicates our prior understanding of possible fault connectivity with other structures, which in turn has implications for potential maximum magnitude of earthquakes involving the WNFZ and nearby faults. Resolving these issues will require further geologic mapping, paleoseismological excavation and interpretation, geophysical investigations, and collection of high-resolution topographic data. The South Napa Earthquake adds to the list of his- torical earthquakes in California that have provided new scientiﬁc questions that are being addressed in order to deepen our understanding of faulting phenomena and seismic hazard. More generally, the complex rupture pattern, some of which has only been revealed by advanced remote sensing methods, provides unprecedented detail on the surface effects of even moderate earthquakes away from primary rupture traces. Accommodation of primary moment release in moderate to large earthquakes may be focused along fairly simple-geometry fault traces, but surface deformation can occur along nearby fault traces, either through connection at depth or due to triggered slip of preexisting structures. In larger events, dynamically triggered slip and complex patterns of surface deformation may be more damaging than that which occurred in the 2014 South Napa earthquake.

8. Conclusions

The 2014 South Napa earthquake was a damaging Mw 6.0 earthquake on the West Napa Fault Zone. Considering its moderate size, the earthquake was associated with unusually complex coseismic and postseismic surface faulting. Extensive documentation of the surface rupture and interpretation of UAVSAR data reveal many previously unrecognized complexities and secondary rupture traces and indicate surface deformation extending at least 7 km south of the epicenter through marshy areas not conducive to field investiga- tion, along subparallel fault traces east of the main rupture traces, and to the north to ~23 km north of Acknowledgments We thank the landowners in Napa Valley the epicenter. that provided land access and the Patterns of surface rupture are somewhat discordant with prior fault mapping but are in accordance with the contributions of many other researchers who responded to this earthquake. This general interpretation that the WNFZ is complex and multistranded in the hills of western Napa Valley. The work was funded by FEMA, the National conservative approach to future hazard analyses in similar settings is to anticipate the possibility of multi- Earthquake Hazards Reduction stranded surface rupture in areas of complex topography especially where multiple possibly Quaternary- Program, and the State of California. The JPL UAVSAR team collected and active fault strands can be mapped. While it is important to focus attention on those faults that are likely processed the UAVSAR data. NASA’s to produce frequent large earthquakes, the South Napa earthquake is a reminder that moderate magnitude GeoGateway interface was used for events can produce both seriously damaging surface rupture and ground motion. California’s recent history analysis of the UAVSAR data products. Portions of this work were carried out at of earthquakes on “secondary” faults, e.g., the 1971 Mw 6.6 San Fernando, 1983 Mw 6.4 Coalinga, 1989 Mw 6.9 the Jet Propulsion Laboratory, California Loma Prieta, 1994 Mw 6.7 Northridge, and 2003 Mw 6.5 San Simeon earthquakes reminds us that the cumula- Institute of Technology under contract tive impacts of moderate earthquakes can be significant. This motivates the need for directing additional with NASA. Helpful reviews of an earlier version of the paper by K. Scharer, resources to ongoing fault and tectonic geomorphic mapping, paleoseismology, shallow geophysics, synop- K. Knudsen, and S. Bennett significantly tic remote sensing measurements, and continued development of new methods for identification and char- improved the work. Airborne lidar data acterization of seismogenic structures that may not be well expressed in the landscape. from 2003 to 2014 are available at www.opentopography.org, terrestrial lidar data are available from first author, air photos (orthometric and References oblique helicopter) are available at Baltay, A. S., and J. Boatwright (2015), Ground-motion observations of the 2014 South Napa Earthquake, Seismol. Res. Lett., 86(2A), 355–360, hddsexplorer.usgs.gov, Google air doi:10.1785/0220140232. photos are available in Google Earth Bray, J., J. Cohen-Waeber, T. Dawson, T. Kishida, and N. Sitar (Eds.) (2014), Geotechnical engineering reconnaissance of the August 24, or from the first author, UAVSAR data 2014 M6 South Napa Earthquake, 415 pp., GEER Association. are available at uavsar.jpl.nasa.gov, Brocher, T., et al. (2015), The Mw 6.0 24 August 2014 South Napa Earthquake, Seismol. Res. Lett., 86(2), 309–326, doi:10.1785/0220150004. postseismic creep measurement data Brooks, B. A., et al. (2015), Uncorking shallow slip and the slip history of the 2014 South Napa Earthquake, Abstract G21C-04 presented at are available at http://pubs.usgs.gov/ 2015 Fall Meeting, AGU, San Francisco, Calif., 14-18 Dec. of/2009/1119/, and field observational Brossy, C., K. Kelson, and M. Ticci (2010), Digital compilation of data for the Contra Costa Shear Zone for the Northern California Quaternary data are available from the third Fault Map Database: Collaborative research with William Lettis & Associates, Inc. and the U.S. Geological Survey—Final technical report author and are planned to be submitted to the U.S. Geological Survey NEHRP, Award no.07HQGR0063, edited. published separately as a U.S. Bryant, W. A. (1982), West Napa fault zone; Soda Creek fault (East Napa fault): California Division of Mines and Geology Fault Evaluation Geological Survey Open-File Report. Report FER-129, pp. 9. Any use of trade, firm, or product d’Alessio, M., I. Johanson, R. Burgmann, D. Schmidt, and M. Murray (2005), Slicing up the San Francisco Bay Area: Block kinematics and fault names is for descriptive purposes only slip rates from GPS-derived surface velocities, J. Geophys. Res., 110, B06403, doi:10.1029/2004JB003496. and does not imply endorsement by DeLong, S., J. Lienkaemper, A. Pickering, and N. Avdievitch (2015), Rates and patterns of surface deformation from laser scanning following the U.S. Government. the South Napa earthquake, California, Geosphere, 11(6), 2015–2030, doi:10.1130/GES01189.1.

DELONG ET AL. NAPA EARTHQUAKE 14 Earth and Space Science 10.1002/2016EA000176

Donnellan, A., J. Parker, S. Hensley, M. Pierce, J. Wang, and J. Rundle (2014), UAVSAR observations of triggered slip on the Imperial, Superstition Hills, and East Elmore Ranch Faults associated with the 2010 M 7.2 El Mayor-Cucapah earthquake, Geochem. Geophys. Geosyst., 15, 815–829, doi:10.1002/2013GC005120. Dreger, D., M. Huang, A. Rodgers, T. Taira, and K. Wooddell (2015), Kinematic Finite-Source Model for the 24 August 2014 South Napa, California, Earthquake from joint inversion of seismic, GPS, and InSAR data, Seismol. Res. Lett., 86(2), 327–334, doi:10.1785/0220140244. Earthquake Engineering Research Institute (EERI) (2014), M 6.0 South Napa earthquake of August 24, 2014, EERI Special Earthquake Report (October 2014), 27 pp. [Available at http://www.eqclearinghouse.org/2014-08-24-south-napa/preliminary-reports.] Field, E. H., et al. (2014), Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3)—The time-independent model, Bull. Seismol. Soc. Am., 104(3), 1122–1180, doi:10.1785/0120130164. Floyd, M. A., et al. (2016), Spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 South Napa, California, earthquake, Geophys. Res. Lett., 43, 6808–6816, doi:10.1002/2016GL069428. Galehouse, J. S., and J. J. Lienkaemper (2003), Inferences drawn from two decades of alignment array measurements of creep on faults in the San Francisco Bay Region, Bull. Seismol. Soc. Am., 93(6), 2415–2433. Graymer, R. W. (2014) Three-dimensional fault framework of the 2014 South Napa Earthquake, San Francisco Bay region, California: AGU 2014 Fall Meeting: San Francisco, Calif. Graymer, R. W., E. E. Brabb, D. L. Jones, J. Barnes, R. S. Nicholson, and R. E. Stamski (2007), Geologic map and map database of eastern Sonoma and western Napa Counties, California, U.S. Geological Survey Scientific Investigations Map 2956, scale 1:100,000. Hanks, T. C. (2000), The age of scarplike landforms from diffusion-equation analysis, in Quaternary Geochronology, pp. 313–338, AGU, Washington, D. C., doi:10.1029/RF004p0313. Hardebeck, J., and D. Shelly (2016), Aftershocks of the 2014 South Napa, California, Earthquake: Complex faulting on secondary faults, Bull. Seismol. Soc. Am., 106(3), 1100–1109, doi:10.1785/0120150169. Helley, E. J., and D. G. Herd (1977), Map showing faults with Quaternary displacement, northeastern San Francisco Bay region, California, U.S. Geological Survey Miscellaneous Filed Studies Map MF-881, scale 1:125,000. Hudnut, K. W., et al. (2014), Key recovery factors for the August 24, 2014, South Napa earthquake: U.S. Geological Survey Open-File Report 2014–1249, 51 pp., doi:10.3133/ofr20141249. Jennings, C. W., and W. A. Bryant (2010), Fault activity map of California: California geological survey geologic data map no. 6, scale 1:750,000. Kunkel, F., and J. E. Upson (1960), Geology and ground water in Napa and Sonoma Valleys, Napa and Sonoma Counties, California, U.S. Geological Survey Water Supply Paper 1495 Langenheim, V., R. Graymer, and R. Jachens (2006), Geophysical setting of the 2000 ML 5.2 Yountville, California, earthquake: Implications for seismic hazard in Napa Valley, California, Bull. Seismol. Soc. Am., 96(3), 1192–1198, doi:10.1785/0120050187. Langenheim, V., R. Graymer, R. Jachens, R. McLaughlin, D. Wagner, and D. Sweetkind (2010), Geophysical framework of the northern San Francisco Bay region, California, Geosphere, 6(5), 594–620, doi:10.1130/GES00510.1. Lienkaemper, J. J., S. B. DeLong, C. J. Domrose, and C. M. Rosa (2016), Afterslip behavior following the 2014 M 6.0 South Napa Earthquake with implications for afterslip forecasting on other seismogenic faults, Seismol. Res. Lett., 87(3), 11, doi:10.1785/0220150262. Morelan, A., C. Trexler, and M. Oskin (2015), Surface-rupture and slip observations on the day of the 24 August 2014 South Napa Earthquake, Seismol. Res. Lett., 86(4), 1119–1127, doi:10.1785/0220140235. NASA (2015) UAVSAR data, (accessed 10/2015 at uavsar.jpl.nasa.gov). National Center for Airborne Laser Mapping (2003), Napa Watershed, CA, edited, doi:10.5069/G9BG2KW9. Polcari, M., M. Palano, J. Fernandez, S. Samsonov, S. Stramondo, and S. Zerbini (2016), 3D displacement field retrieved by integrating Sentinel-1 InSAR and GPS data: The 2014 South Napa earthquake, Eur. J. Remote Sens., 49,1–13, doi:10.5721/EuJRS20164901. Rosen, P. A., S. Hensley, K. Wheeler, G. Sadowy, T. Miller, S. Shaffer, R. Muellerschoen, C. Jones, H. Zebker, and S. Madsen (2006), April. UAVSAR: A new NASA airborne SAR system for science and technology research. In 2006 IEEE Conference on Radar. 8p. Rosinski, A., et al. (2015), “California earthquake clearinghouse after-action report: South napa earthquake” (California Geological Survey and Earthquake Engineering Research Institute, April 17, 2015). [Available at http://www.eqclearinghouse.org/2014-08-24-southnapa/files/ 2015/04/California_Earthquake_Clearinghouse_After_Action_Report-South_Napa_Earthquake-2015.04.17.pdf.] U.S. Geological Survey and California Geological Survey (2006), Quaternary Fault and Fold Database for the United States, accessed 31 July 2015, from USGS. [Available at http://earthquake.usgs.gov/hazards/qfaults/.] Wagner, D. L., and C. I. Gutierrez (2010), Geologic map of the Napa 30′x60′Quadrangle, California. California Geological Survey. [Available at http://www.consrv.ca.gov/cgs/rghm/rgm/Pages/northern_region_quads.aspx.] Wei, S., S. Barbot, R. Graves, J. Lienkaemper, T. Wang, K. Hudnut, Y. Fu, and D. Helmberger (2015), The 2014 Mw 6.1 South Napa Earthquake: A Unilateral Rupture with Shallow Asperity and Rapid Afterslip, Seismol. Res. Lett., 86(2), 344–354, doi:10.1785/0220140249. Wesling, J. R., and K. L. Hanson (2008), Mapping of the West Napa fault zone for input into the northern California Quaternary fault database, Final Technical Report for USGS External Award Number 05HQAG0002Rep.,61pp.

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