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Tearing the Terroir: Details and Implications of Surface Rupture And PUBLICATIONS Earth and Space Science 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 DELONG ET AL. NAPA EARTHQUAKE 1 Earth and Space Science 10.1002/2016EA000176 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 asso- ciated 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 asso- ciated 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 seis- mic 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 obser- vations 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 per- formed 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 avail- able 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 geo- detic 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 deter- mine total coseismic and postseismic slip at several locations [Brooks et al., 2015; DeLong et
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