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Published March 2015 | Published
Journal Article Open

The 2014 M_w 6.1 South Napa Earthquake: 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 high resolution COSMO–SkyMed (CSK) satellite and Sentinel-1A satellite, providing independent static observations.

Additional Information

© 2015 by the Seismological Society of America. Strong-motion waveform data was downloaded from the Northern California Data Center, California Geological Survey, and U.S. Geological Survey (USGS) (strongmotioncenter.org; last accessed September 2014). Interferometric Synthetic Aperture Radar data were obtained from the Jet Propulsion Laboratory (JPL), static GPS data were made available by Tom Herring from the Massachusetts Institute of Technology. Constructive reviews by Annemarie Baltay, Art Frankel, and an anonymous reviewer led to improvements in the manuscript. Figures were made using Generic Mapping Tool (Wessel and Smith, 1991). Part of this research was carried out at JPL (California Institute of Technology), sponsored by the National Aeronautics and Space Administration.

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August 20, 2023
August 20, 2023