Surface‐Rupture and Slip Observations on the Day of the 24 August 2014 South Napa Earthquake

Abstract

Online Material: Color versions of Figures 1–4; Google Earth file with imbedded field photos. Surface‐rupture observations constrain the orientation, length, and direction of fault slip and are especially useful in the information‐limited immediate aftermath of a large earthquake. The extent of surface rupture and preliminary slip observations are critical for rapid assessment of damage to fault‐crossing lifelines and provide context for understanding strong ground motions resulting from rupture directivity. Some of the most important information that surface‐rupture observations may constrain are the locations of the ends of fault ruptures. Both aftershocks and postseismic deformation at fault tips are especially revealing of rheological properties (Hearn, 2003) and earthquake‐triggering mechanisms (Dieterich, 1994; Stein et al. , 1997). Rates of aftershocks and postseismic deformation, including fault creep at the surface, decline quickly (Omori, 1894; Marone et al. , 1991). Thus, it is important to rapidly obtain and openly disseminate surface‐rupture observations, preferably within hours of the mainshock, to deploy instrumentation most effectively. Here, we report our observations of surface rupture and slip within the hours following the 24 August 2014 M w 6.0 South Napa earthquake. The mainshock occurred at 3:20 a.m. local time with an epicenter near American Canyon, California (Brocher et al. , 2015). Our team was one of several that mobilized that morning to document surface rupture (Bray et al. , 2014). By monitoring social media in the hours following the event, we identified photographs on Twitter with indications of right‐lateral tectonic offset of State Highway Route 12/121 (SR 12/121; Fig. 1). In response, two members of our team (A.M. and C.T.) mobilized from the University of California, Davis, about a one‐hour drive to the epicenter. Our observations were collected specifically with the goal of quickly defining the length and magnitude of surface rupture, focusing on displaced cultural features vulnerable to repair. Information …