Post-Earthquake Characterization of Surface Displacements and Spatial Variation of Deformation Kinematics near the Southern End of the Recent West Napa Fault Rupture

We performed detailed mapping along more than 1.3 km of surface rupture between Cuttings Wharf and CA-12 in order to characterize the location, width, orientation, amount, direction, and kinematics of permanent ground deformation produced in the hours, days, and weeks following the August 24 M6.0 South Napa earthquake.

Surface rupture along a 550 m long reach between Middle Ave and South Ave near Los Carneros Ave is expressed as a series of right-stepping, 337° striking intervals roughly 2 to 6 m wide and 30 to 60 m long. The dextral component of displacement produced localized transpressional strain and formed narrow pressure ridges 15 to 20 cm high and 0.5 to 1.5 m wide. Each of these right-stepping reaches are composed of an irregular pattern of smaller scale (0.3 to 8 m long) left-stepping en echelon fractures striking 355° to 030°, with more easterly oriented fractures tending to be longer. In additional to pressure ridges, the deformation field exhibits a broad west-up sense of vertical displacement. Total station survey data from two faulted fence lines indicate 34 ± 4 cm and 30.5 ± 4.5 cm of dextral displacement and 15 ± 4 cm and 12 ± 2 cm of net west-up vertical displacement. The average direction of dextral displacement is 330° ± 5° along this reach.

Surface rupture along a 800 m long reach from Las Amigas Road south to Los Carneros Creek is expressed as a series of left-stepping, N20°W striking intervals roughly 1.5 to 4 m wide and 40 to 70 m long. Each of these intervals are composed of a regular pattern of smaller scale (4 to 6 m long) left-stepping en echelon fractures striking 000° to 015°. Multiple measurements indicate a gradual southward decrease in the amount of dextral surface displacement from 12 ± 2 cm near Las Amigas Road to 7.5 ± 1.5 cm near Los Carneros Creek; these dextral displacements are consistently 2 to 3 times larger than co-located net west-up vertical displacements. The dominant direction of dextral displacement is 325° ± 5° along this reach, mostly accomodated by oblique expansion of tensional cracks. Pressure ridge deformation commonly observed along the northern reach is absent here, indicating a transition from transpressional deformation (north) to transtensional deformation (south) that is coincident with the spatial pattern of compound scarp production/preservation shown on many pre-earthquake fault maps.