Geodetic Constraints on the 2014 M 6 . 0

On 24 August 2014, the M 6.0 South Napa earthquake shook much of the San Francisco Bay area, leading to significant damage in the NapaValley. The earthquake occurred in the vicinity of the West Napa fault (122.313° W, 38.22° N, 11.3 km), a mapped structure located between the Rodger’s Creek and Green Valley faults, with nearly pure right-lateral strike-slip motion (strike 157°, dip 77°, rake –169°; http://comcat.cr.usgs.gov/ earthquakes/eventpage/nc72282711#summary, last accessed December 2014) (Fig. 1). The West Napa fault previously experienced an M 5 strike-slip event in 2000 but otherwise exhibited no previous definitive evidence of historic earthquake rupture (Rodgers et al., 2008; Wesling and Hanson, 2008). Evans et al. (2012) found slip rates of ∼9:5 mm=yr along the West Napa fault, with most slip rate models for the Bay area placing higher slip rates and greater earthquake potential on the Rodger’s Creek and Green Valley faults, respectively (e.g., Savage et al., 1999; d’Alessio et al., 2005; Funning et al., 2007). High-quality geodetic observations from both continuous and campaign Global Positioning System (GPS) networks in the Bay area, as well as Interferometric Synthetic Aperture Radar (InSAR), were collected in the days immediately following the earthquake (Fig. 2; E Fig. S1, available in the electronic supplement to this article). These observations recorded surface displacements generated by the earthquake, including both the coseismic surface displacement field and early postseismic deformation. In this study, we analyze these geodetic observations to map the fault location and static slip distribution (SD) of the South Napa earthquake and to assess associated static stress changes on neighboring faults. The location of the earthquake inferred from geodetic observations coincides with both mapped and unmapped sections of theWest Napa fault, in agreement with our own and other’s field observations of surface rupture (http://www.geerassociation.org/GEER_Post%20EQ%20Reports/ SouthNapa_2014/index.html; last accessed December 2014). The earthquake propagated northward from the epicenter with the majority of slip immediately adjacent to the city of Napa. Static stress change calculations show increased Coulomb stress on both the northern and southern continuations of theWest Napa fault and through a releasing bend of the adjacent Rodgers Creek fault. In addition to exploring the source properties of the Napa earthquake, this study details the methods used for U.S. Geological Survey National Earthquake Information Center (USGSNEIC) geodetic-based finite-fault inversions and to highlight the importance of rapidly updated geodetic observations, both in situ and remotely sensed, for event source characterization and response. Finite-fault SDs derived from teleseismic observations are an integral response product of the NEIC, providing critical spatial information for other products such as ShakeMap, ShakeCast, and Prompt Assessment of Global Earthquakes for Response (PAGER). Where available, geodetic observations complement these finite source models, providing additional constraint on slip location and complexity and expanding the characterized magnitude range of events (Figs. 2, 3). Events like the South Napa earthquake demonstrate the utility of geodetic observations in constraining slip complexity and resulting stress changes from small to moderate magnitude earthquakes. As a part of the response to the South Napa earthquake, these geodetic data sets were progressively incorporated into NEIC response products and made available to the public. The first GPS-based source model was released within 36 hours of the earthquake and then progressively revised as additional GPS displacements and InSAR observations became available (Fig. 3).