ABSTRACT

The 24 August 2014 M 6.0 South Napa, California, earthquake exhibited unusually large slip for a California strike‐slip event of its size with a maximum coseismic surface slip of 40–50 cm in the north section of the 15‐km‐long rupture. Although only minor (<10  cm) surface slip occurred coseismically in the southern 9 km section of the rupture, considerable postseismic slip occurred, so that the maximum total slip one year after the event approached 40–50 cm, approximately equal to the coseismic maximum in the north. We measured the accumulation of postseismic surface slip on four ∼100‐m‐long alignment arrays for one year following the event. Because prolonged afterslip can delay reconstruction of fault‐damaged buildings and infrastructure, we analyzed its gradual decay to estimate when significant afterslip would likely end. This forecasting of Napa afterslip suggests how we might approach the scientific and engineering challenges of afterslip from a much larger M∼7 earthquake anticipated on the nearby urban Hayward fault. However, we expect its afterslip to last much longer than one year.

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