Afterslip hazard map of the Browns Valley neighborhood and surrounding area. A detailed map explanation is presented on the following page. vi Caption for map on previous page: Levels of Afterslip Hazard for the Browns Valley Neighborhood, City of Napa, California: All fault traces shown on this map face potential future earthquake fault surface rupture hazard and other earthquake-related hazards such as shaking, liquefaction, and landslides; these hazards are treated separately in other publications and maps from CGS and USGS (with preliminary updates provided in this report). For all levels of afterslip hazard, the afterslip amount that is measured 90 days after the earthquake can be expected to as much as double by 10 years after the earthquake (less than double is also possible). Red Fault Trace-High level of afterslip hazard; very likely to experience more than 15 cm of afterslip during the 3 years after the earthquake. (Red is intentionally included, even though none is indicated on this map.) Yellow Fault Trace-Moderate level of afterslip hazard; likely to experience less than 15 cm, but more than 5 cm, of afterslip during the 3 years after the earthquake. (Additional afterslip accumulation is likely to gradually accumulate an additional 5 cm during the 10 years after the earthquake and an additional 5 cm 30 years after the earthquake.) Green Fault Trace-Low level of afterslip hazard; very unlikely to experience more than 5 cm of afterslip during the 3 years after the earthquake. (Faults that experienced <10 cm of coseismic offset and <5 cm of afterslip within the 3 months after the earthquake are included in this category. Some faults or lineaments shown as green had no measurable coseismic slip or afterslip associated with the August 24, 2014, earthquake. Faults and lineaments of several categories are shown for completeness. Some are previously mapped strands (U.S. Geological Survey and California Geological Survey, 2006); others represent preliminary mapping based on a combination of imagery interpretation and field mapping that has taken place since the August 24, 2014, earthquake. All of the faults and/or imagery lineaments shown as heavy green lines on this map may be considered to have a low level of afterslip hazard. Subsequent ongoing mapping, that is, work still in progress, may reveal that certain lineaments shown here are not actually faults.) Map orientation: North direction is toward top of map.

Abstract-Rayleigh wave group velocities obtained from ambient noise tomography are inverted for an upper crustal model of the Central Valley, California, centered on the Sacramento/San Joaquin Delta. Two methods were tried; the first uses SURF96, a least squares routine. It provides a good fit to the data, but convergence is dependent on the starting model. The second uses a genetic algorithm, whose starting model is random. This method was tried at several nodes in the model and compared to the output from SURF96. The genetic code is run five times and the variance of the output of all five models can be used to obtain an estimate of error. SURF96 produces a more regular solution mostly because it is typically run with a smoothing constraint. Models from the genetic code are generally consistent with the SURF96 code sometimes producing lower velocities at depth. The full model, calculated using SURF96, employed a 2-pass strategy, which used a variable damping scheme in the first pass. The resulting model shows low velocities near the surface in the Central Valley with a broad asymmetrical sedimentary basin located close to the western edge of the Central Valley near 122°W longitude. At shallow depths, the Rio Vista Basin is found nestled between the Pittsburgh/ Kirby Hills and Midland faults, but a significant basin also seems to exist to the west of the Kirby Hills fault. There are other possible correlations between fast and slow velocities in the Central Valley and geologic features such as the Stockton Arch, oil or gas producing regions and the fault-controlled western boundary of the Central Valley.

If shaking from a local or regional earthquake in the San Francisco Bay region were to rupture levees in the Sacramento/San Joaquin Delta, then brackish water from San Francisco Bay would contaminate the water in the Delta: the source of freshwater for about half of California. As a prelude to a full shear‐wave velocity model that can be used in computer simulations and further seismic hazard analysis, we report on the use of ambient noise tomography to build a fundamental mode, Rayleigh wave group velocity model for the region around the Sacramento/San Joaquin Delta in the western Central Valley, California. Recordings from the vertical component of about 31 stations were processed to compute the spatial distribution of Rayleigh wave group velocities. Complex coherency between pairs of stations was stacked over 8 months to more than a year. Dispersion curves were determined from 4 to about 18 s. We calculated average group velocities for each period and inverted for deviations from the average for a matrix of cells that covered the study area. Smoothing using the first difference is applied. Cells of the model were about 5.6 km in either dimension. Checkerboard tests of resolution, which are dependent on station density, suggest that the resolving ability of the array is reasonably good within the middle of the array with resolution between 0.2 and 0.4°. Overall, low velocities in the middle of each image reflect the deeper sedimentary syncline in the Central Valley. In detail, the model shows several centers of low velocity that may be associated with gross geologic features such as faulting along the western margin of the Central Valley, oil and gas reservoirs, and large crosscutting features like the Stockton arch. At shorter periods around 5.5 s, the model's western boundary between low and high velocities closely follows regional fault geometry and the edge of a residual isostatic gravity low. In the eastern part of the valley, the boundaries of the low‐velocity zone and gravity anomaly are better aligned at longer periods (around 10.5 s) suggesting that the eastern edge of the gravity low is associated with deeper structure. There is a strong correspondence between a low in gravity near the Kirby Hills fault and low velocities from the ambient noise tomography. At longer periods, higher velocities creep in from the east and narrow the overall dimension defined by the lower velocities. Overall, there is a strong correspondence between the shape and location of low velocities in the Rayleigh wave velocity images, and geological and geophysical features.

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