The cause of the California earthquake on August 24, 2014. First, to put the South Napa earthquake in context, the West Napa fault, which was the causative fault, is part of the large San Andreas Fault Zone (SAFZ) system. The West Napa fault extends along the western margin of the basin that underlies much of Napa Valley. The relative motion of this fault is not well known but thought to be about 1 mm/yr (0.04 in/yr). The SAFZ is not a simple and single fault, but a complex collection of smaller fault segments that collectively accommodate the horizontal movement between the North American Plate and the Pacific Plate.
Sometimes a single fault segment will slip and cause an earthquake, and other times many fault segments slip in unison, causing a larger earthquake. The South Napa earthquake included a few small segments that ruptured together to produce the M 6 earthquake. The only other noteworthy earthquake in the Napa area in the recent past (since 1973) was the M4.9 Yountville earthquake on September 3, 2000. The epicenter of that earthquake was 20 km (12 mi) NNW of the South Napa epicenter, and also on the West Napa fault system.
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