Structural Control on South Napa Earthquake Sequence and Its Effects

Potential-field data indicate that the M6.0 South Napa earthquake and its aftershocks were influenced by complex structure among the West Napa-Carneros-Pinole-Franklin faults system. The West Napa and Carneros faults, although considered to be minor Quaternary players within the San Andreas fault system, are major faults that in part bound the Mayacamas Mountains, an uplifted block of Mesozoic basement between Sonoma and Napa Valleys, and that produce prominent potential-field gradients. These two faults may account for as much as 30-50 km of right-lateral offset since ~6 Ma based on offset magnetic anomalies, length of pull-apart basins, and correlation of Neogene units. Uplift of the block may be Quaternary in age because gravels found to the west of the range contain clasts of 2.8 Ma obsidian derived from the east of the range. The southern extent of the South Napa earthquake rupture and seismicity is bounded on the west by the north end of a pull-apart basin between the Carneros and Pinole faults reflected in gravity data. The prominent gravity and magnetic gradients associated with the West Napa fault weaken where mapped traces of the fault begin to splay at the southern extent of Mesozoic and Paleogene outcrops. This region also encompasses multiple linear north-northwest-trending features identified in UAVSAR and is where the Carneros and West Napa faults are closest at the surface. We suggest that the interaction of these two faults not only affects the South Napa earthquake, but also influenced the extent of seismicity following the 2000 Yountville earthquake as well as the diffuse zone of seismicity generally confined between these two faults as they curve northwestward towards the southern end of the Maacama fault. Damage produced by the South Napa and Yountville earthquakes may have been focused by the 2-3 km-deep basin beneath the city of Napa, concentrated not over the deepest parts of the basin, but rather along the periphery or over an intra-basin arch. Red-tagged buildings from the South Napa event are located over a slight arch where magnetic anomalies attributed to the Sonoma Volcanics appear to diminish, suggesting a thicker sedimentary fill overlies the volcanic rocks in the arch. Red-tagged buildings from the Yountville event are also located over the arch and over the western and eastern basin margins.