Key recovery factors for the August 24, 2014, South Napa Earthquake

Abstract

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.

3D displacement field retrieved by integrating Sentinel-1 InSAR and GPS data: the 2014 South Napa earthquake

“…1), and ~340°/165° strike depending on the dip (east-dipping or westdipping, respectively). This is also in agreement with previous works showing a rupture propagating NNW for about 15 Km [Barnhart et al, 2015] and reaching the surface with a maximum slip of about 46 cm near the city of Napa [Hudnut et al, 2014]. Also he E-W component is significant, especially along the southern part of the fault (Fig.…”

“…2B). Indeed, as pointed out in previous studies [Hudnut et al, 2014;Wei et al, 2015], the fault strike rotates by about 15° counterclockwise from N to S along the rupture, near the sites where slip peaks. In addition, it is worth noting a small, but not negligible, vertical displacement detected in the north-east side of the fault, where a subsidence of about 8 cm occurred (Fig.…”

“…The lack of GPS stations results in a deformation mainly derived from the InSAR data that are not fully suitable in constraining the N-S component due to the orbital configuration. However, the deformation gradient is located close to the city of Napa, where field investigations by Hudnut et al [2014] revealed a strike rotation that might have led to the estimated pattern. Table 1 provides an evaluation of the performance of the adopted method.…”

Proceedings of the 11th United States-Japan natural resources panel for earthquake research, Napa Valley, California, November 16–18, 2016

“…Slip on multiple fault strands was observed during the South Napa earthquake others, 2015, Hudnut, andothers, 2014; see figure below]. Simulations of earthquakes using viscoelastic models may help us understand how faults root at depth and how this is reflected at the surface, particularly in complex events such as the South Napa earthquake.…”

“…The method is particularly suited for applications in regions where the tectonic deformation is complex and poorly understood, and we illustrate its utility using the GPS velocity field in northeastern California, where subduction, strike-slip faulting, back arc extension and volcanism all occur. An update will be provided on the USGS forecast and assessment of ongoing fault afterslip (Hudnut and others, 2014), especially in the Browns Valley residential neighborhood of the City of Napa. At the time of the M6.0 South Napa earthquake, the fault ruptured the ground surface right away, as is typical.…”