Tectonic Setting of 24 August, 2014 M = 6.0 South Napa Earthquake

The 24 August 2014 M=6.0 South Napa earthquake had strike-slip surface deformation extending over some 15 km with a maximum offset of 46 cm. The San Andreas fault (SAF) system in this region is relatively simple, the deformation zone is about 100 km wide and is bounded by the rigid Pacific Plate and the rigid Sierra-Nevada Plate. The deformation in this zone has some 33 mm/yr of right lateral strike-slip motion, some 20 mm/yr of this motion is accommodated on the SAF. There are a number of other right-lateral strike-slip faults in this zone. The South Napa Earthquake occurred on a branch of one of these, the West Napa Fault. The deformation zone has high heat flow indicating a thin lithosphere. This thin lithosphere is attributed to the recent subduction of the Farallon Plate beneath the Sierra-Nevada Plate. This subduction left a gap known as a slab window. Extensive GPS observations are available in the deformation zone, these data are well approximated by a near uniform shear strain between the two rigid boundary plates. We suggest that this near uniform surface shear can be attributed to viscoplastic deformation of the lower crust in the slab window. The thin brittle upper crust responds to this deformation. Because of the geometrical incompatibilities between the orientation of the San Andreas Fault and the imposed deformation, the brittle upper crust is subjected to distributed internal deformation. The 2014 South Napa earthquake is an example of this deformation, the localized co-seismic deformation is redistributed over the slab window by viscoplastic deformation of the lower crust.