The Mw 6.0 South Napa earthquake of August 24, 2014—Observations of surface faulting and ground deformation, with recommendations for improving post-earthquake field investigations

Abstract

The M w 6.0 South Napa earthquake of August 24, 2014, produced complex and extensive surface faulting and other ground deformation features. Following the event, geologists made more than 1,200 field observations at locations where tectonic faulting and ground failure produced visible deformation that fractured and disturbed the ground surface. At a few locations, large-scale, detailed, field-based maps of fault rupture and ground deformation were produced. The South Napa earthquake response was one of the first times when post-earthquake reconnaissance data were mostly collected and disseminated electronically. The advantages and opportunities these new methods bring to our research also pose new challenges to large-scale compilation efforts and demonstrate the value of developing guidelines and better standardization across the community to more optimally utilize developing technology in future post-earthquake investigations. Some suggestions for standardizing the collection and dissemination of post-earthquake field reconnaissance data are provided herein. Field observations and maps were integrated with airborne imagery, lidar, and InSAR to produce a comprehensive, large-scale digital map of fault rupture and zones of ground deformation. The map, observations, and photo database are summarized here in appendixes and figures and are also available as a series of digital data products within a companion U.S. Geological Survey data release ( Ponti and others, 2019 ); the characteristics of fault rupture and ground deformation features are summarized in detail in the body of this report. The results of this compilation reveal that faulting occurred within a 2-km-wide zone on six, roughly parallel traces within the West Napa Fault System. Most of the fault slip, and all the afterslip, occurred on the 21-km-long westernmost trace (Trace A). Maximum coseismic slip was greater than 40 cm and possibly as great as 60 cm, with the slip maximum located about 10 km north of the epicenter. Extensive ground deformation also occurred off the principal fault traces. Deformation characteristics of these features were not consistent with either primary faulting or shaking-induced ground failure and remain enigmatic, although this report includes speculation about possible origins. The use of InSAR was invaluable for identifying and mapping secondary traces with small displacements, and for delineating the overall details of the extensive rupture. InSAR data also highlighted other areas with possible ground deformation—some of which are found coincident with previously mapped fault traces, whereas others are in areas where no faults were previously mapped. Several of these regions had no visible ground deformation, whereas others did produce features that were inconsistent with tectonic faulting, so care must be taken not to over interpret the InSAR data without careful, corroborating field investigations.