Coseismic Slip and Early Afterslip of the M6.0 24 August 2014 South Napa, California, Earthquake

• Published in: Journal of geophysical research. Solid earth 2019-11, Vol.124 (11), p.11728-11747
• Main Authors: Pollitz, Fred F., Murray, Jessica R., Minson, Sarah E., Wicks, Charles W., Svarc, Jerry L., Brooks, Benjamin A.
• Format: Article
• Language: English
• Subjects: afterslip; Asperity; coseismic slip; Dipping; Earth; Earth surface; Earthquakes; Fault lines; Geophysics; Global positioning systems; GPS; Interferometric synthetic aperture radar; Inversions; Offsets; Planes; Positioning systems; Rupture; Rupturing; SAR (radar); Seismic activity; Seismicity; Seismograms; Slip; South Napa earthquake; Synthetic aperture radar
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2019JB018470

We employ strong motion seismograms and static offsets from the Global Positioning System, Interferometric Synthetic Aperture Radar, and other measurements in order to derive a coseismic slip and afterslip model of the M6.0 24 August 2014 South Napa earthquake. This earthquake ruptured an ∼13‐km‐long portion of the West Napa fault with predominantly right‐lateral strike slip. In the kinematic seismic slip inversions, we couple the coseismic slip and afterslip distributions by requiring both distributions to involve right‐lateral strike slip with positive amplitude, with the net static slip being the sum of the two. We consider several candidate fault geometries: a first involving two steeply east dipping fault planes that reach Earth's surface at the western surface trace (STW), where most surface rupture was observed, a second involving a steeply west dipping plane that also reaches Earth's surface at the STW, and a third involving a combination of two variably west dipping planes constrained to pass through the locus of postseismic seismicity located ∼1 km west of the STW. The data are best fit using the model of two east dipping fault planes, with coseismic slip up to ∼1.2 m on a dominant shallow asperity about 10 km north of the hypocenter and on deeper asperities on the southern part of the rupture. Afterslip up to 1 m is concentrated along the southern part of the rupture at depths ≲5 km, consistent with surface observations of afterslip. Seismic moments associated with coseismic slip and afterslip are 1.13×1018 N m (Mw 6.00) and 3.64×1017 N m, respectively. Key Points Fault slip of the South Napa earthquake is constrained by seismic waveforms, geodetic data, and surface observations of slip Models designed to fit the seismic and geodetic data favor a steeply east dipping fault geometry The earthquake was followed by afterslip amounting to about one third of the seismic moment of the mainshock