Ground Deformation in the Very Near Fault Region during the Mw 6.0 West Napa Earthquake

The Mw 6.0 West Napa earthquake on August 24, 2014 provides a unique case history to study very near fault deformation in infrastructure. Initial reconnaissance of the residential neighborhoods in the Browns Valley area in western Napa following the earthquake noted extensive zones of surface deformation, consisting of compression features including buckled sidewalks, curbs and pavement. Whereas some deformation was associated with surface faulting, much of the observed deformation consisted of these compressional features, which occurred away from surface faulting, could not be attributed to any particular faulting mechanism. One neighborhood with significant effects of surface faulting and well expressed compressive features was chosen for detailed mapping over a three day period, August 26 to 28, 2014. Specifically, compression and extension features along streets and sidewalks parallel to the fault trace and along cross streets roughly orthogonal to the fault trace were measured in detail. A summary of the observations and recordings, as well as the potential deformation mechanisms, are discussed.