A Unilateral Rupture with Shallow Asperity and Rapid Afterslip

The Mw 6.1 South Napa earthquake occurred near Napa, California, on 24 August 2014 at 10:20:44.03 (UTC) and was the largest inland earthquake in northern California since the 1989 Mw 6.9 Loma Prieta earthquake. The first report of the earthquake from the Northern California Earthquake Data Center (NCEDC) indicates a hypocentral depth of 11.0 km with longitude and latitude of (122.3105° W, 38.217° N). Surface rupture was documented by field observations and Light Detection and Ranging (LiDAR) imaging (Brooks et al., 2014; Hudnut et al., 2014; Brocher et al., 2015), with about 12 km of continuous rupture starting near the epicenter and extending to the northwest. The southern part of the rupture is relatively straight, but the strike changes by about 15° at the northern end over a 6 km segment. The peak dextral offset was observed near the Buhman residence with right-lateral motion of 46 cm, near the location where the strike of fault begins to rotate clockwise (Hudnut et al., 2014). The earthquake was well recorded by the strong-motion network operated by the NCEDC, the California Geological Survey and the U.S. Geological Survey (USGS). There are about 12 sites within an epicentral distance of 15 km that had relatively good azimuthal coverage (Fig. 1). The largest peak ground velocity (PGV) of nearly 100 cm=s was observed on station 1765, which is the closest station to the rupture and lies about 3 km east of the northern segment (Fig. 1). The ground deformation associated with the earthquake was also well recorded by the highresolution COSMO–SkyMed (CSK) satellite and Sentinel1A satellite, providing independent static observations. Intriguingly, photos taken at the same location along the southern segment, but at different times, indicate a large amount of afterslip (∼20 cm) occurring within the first 24 h following the earthquake (Brooks et al., 2014). Alignment arrays installed after the earthquake by theUSGS were able to measure the temporal decay of surface afterslip and also indicate variation of aseismic slip distribution along the fault strike, with the largest afterslip occurring along the southern segment (Lienkaemper et al., 2014). Reports from previous events indicate some temporal afterslip changes (i.e., Hsu et al., 2002, 2006; Freed, 2007) yet lack complete spatial coverage and cross dataset verification. The high-quality datasets for the South Napa earthquake provide a detailed picture of both the spatial and temporal distribution of afterslip and help to distinguish the relative contributions of coseismic and postseismic slip in the observed Interferometric Synthetic Aperture Radar (InSAR) and Global Positioning System (GPS) data. Our analysis begins with a relocation of the mainshock epicenter, which is done to help constrain the fault location and geometry. Then, to obtain the coseismic slip distribution we rely on the strong-motion waveform data, because they are located close to the fault and provide the best resolution in resolving the rupture details. Although there are also some high-rate GPS sites in the vicinity of the rupture, these data were not fully processed at the time of our work and thus were not included in the current analysis. Once a reliable coseismic slip distribution is obtained, we compare this with the static-only slip model derived from InSAR and GPS data, which includes contributions from both coseismic and postseismic slip and thus allows us to identify potential afterslip regions. These results, combined with the postseismic deformation recorded by the alignment array, are then used to guide dynamic afterslip modeling, which provides insights on the frictional properties along the fault.