Rates and patterns of surface deformation from laser scanning following the South Napa earthquake, California

Abstract

The A.D. 2014 M6.0 South Napa earthquake, despite its moderate magnitude, caused significant damage to the Napa Valley in northern California (USA). Surface rupture occurred along several mapped and unmapped faults. Field observations following the earthquake indicated that the magnitude of postseismic surface slip was likely to approach or exceed the maximum coseismic surface slip and as such presented ongoing hazard to infrastructure. Using a laser scanner, we monitored postseismic deformation in three dimensions through time along 0.5 km of the main surface rupture. A key component of this study is the demonstration of proper alignment of repeat surveys using point cloud-based methods that minimize error imposed by both local survey errors and global navigation satellite system georeferencing errors. Using solid modeling of natural and cultural features, we quantify dextral postseismic displacement at several hundred points near the main fault trace. We also quantify total dextral displacement of initially straight cultural features. Total dextral displacement from both coseismic displacement and the first 2.5 d of postseismic displacement ranges from 0.22 to 0.29 m. This range increased to 0.33-0.42 m at 59 d post-earthquake. Furthermore, we estimate up to 0.15 m of vertical deformation during the first 2.5 d post-earthquake, which then increased by ~0.02 m at 59 d post-earthquake. This vertical deformation is not expressed as a distinct step or scarp at the fault trace but rather as a broad up-to-the-west zone of increasing elevation change spanning the fault trace over several tens of meters, challenging common notions about fault scarp development in strike-slip systems. Integrating these analyses provides three-dimensional mapping of surface deformation and identifies spatial variability in slip along the main fault trace that we attribute to distributed slip via subtle block rotation. These results indicate the benefits of laser scanner surveys along active faults and demonstrate that fine-scale variability in fault slip has been missed by traditional earthquake response methods.

Tearing the terroir: Details and implications of surface rupture and deformation from the 24 August 2014 M6.0 South Napa earthquake, California

“…The locations with the highest coseismic slip experienced less postseismic slip than did the locations closer to the epicenter that experienced lower coseismic slip. The largest measured coseismic slip (0.46 m) was located 10.4 km north of the epicenter and is equal to the peak combined coseismic and postseismic slip (0.47 m) measured 1 year postevent at a location 6 km north of the epicenter [ Lienkaemper et al ., ; Wei et al ., ; DeLong et al ., ] (Figure ). Most postseismic slip occurred 3–9 km north of the epicenter, in areas underlain by alluvial deposits more than 60 m thick (Figure ) [ Kunkel and Upson , ].…”

“…These include a component of vertical deformation that indicates centimeter‐level uplift on the west side of the fault, consistent with long‐term west side up displacement manifested by higher topography and gravity values west of the fault zone [ Langenheim et al ., ]. This is supported by analyses of lidar data [ DeLong et al ., ] and spaceborne SAR and GPS data [ Polcari et al ., ].…”

Spatio-Temporal Mapping of Plate Boundary Faults in California Using Geodetic Imaging

“…These include the 2004 M6.0 Parkfield, California earthquake; the 2004 M6.6 Niigita Ken Chuetsu earthquake in Japan [62]; the 2010 M7 El Mayor-Cucapah earthquake in Baja California, Mexico [63]; the 2009 M6.3 L'Aquila earthquake in Italy [64]; and the 2014 M6.0 South Napa earthquake [40,65].…”

“…For more complete descriptions of the processing required to acquire and analyze these data, we direct the reader to a recent review of lidar and other high-resolution topographic data [53], and, for an example of terrestrial laser scanner collection and processing, a recent study of surface rupture associated with the South Napa earthquake [65].…”

“…This highlights the challenge of mapping fine scale tectonic features and the need for topographic and imagery data with submeter and finer spatial resolution. Though vertical deformation over the coseismic interval was measureable using repeat airborne lidar, and some coarse estimations of horizontal measurement were made using repeat airborne lidar [72,73] the most precise (but spatially limited) quantifications of horizontal slip magnitudes were made during the postseismic interval using terrestrial lidar [65,74]. Figure 6 displays results from identifying geometric primitive objects from terrestrial lidar point clouds and tracking them through time.…”