Surface Fault Rupture from the M6.0 South Napa Earthquake of Aug. 24, 2014

The South Napa earthquake produced the largest and most extensive coseismic surface rupture of any documented California earthquake of similar magnitude. More than 14 km of complex surface faulting, extending from the Napa River at Cuttings Wharf northward beyond the north boundary of Alston Park in the city of Napa, occurred on two principal sub-parallel N-NW trending fault strands. Other minor sub-parallel rupture zones (≤1.5 km in length with ~1-3 cm displacements) were identified near the principal strands. The surface rupture lies primarily NW of the epicenter and W of most of the mapped traces of the West Napa fault zone, but rupture was locally coincident with portions of some mapped late Quaternary and older fault traces. Geomorphic expressions of prior faulting are observed intermittently along the main traces. Surface displacements are predominantly right lateral and typically expressed as discontinuous en echelonleft-stepping fractures within zones that range from <1 m, to tens of meters in width. The largest lateral offsets occurred on the western principal strand; coseismic slip was minimal near the epicenter but increased to 40-45 cm ~10 km to the NW. Farther north, in the Browns Valley area, offsets of 10-20 cm across roads, pipelines, and residential structures produced significant damage. The ~7 km-long eastern strand had coseismic dextral offsets of 2-8 cm. Its southern end lies 7.5 km NW of the epicenter and 1.1 km E of the western strand, while its northern end approaches the western strand where the two appear to merge a few hundred meters south of Alston Park. Afterslip has been documented along the western strand but was not observed on the eastern strand. It was most rapid in the middle third of the western strand, increasing initial slip by ≥20 cm one day after the mainshock. Repeated measurements suggest total slip may reach ~40 cm along half of the western strand. The complex character and locations of surface rupture produced by this event have significant implications for current approaches to fault hazard mapping in California. Additional contributors: USGS: N. Avdievitch, M. Bennett, B. Collins, T. Holzer, A. Pickering, J. Tinsley. CGS: D. Branum, B. Bryant, C. Davenport, M. Delattre, W.Haydon, J. Lancaster, M. Mareschal, A. Perez, C. Pridmore, M.Silva, J. Thornburg , J.Treiman, M. Wiegers, C. Wills.