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

Abstract

The M w 6.0 South Napa earthquake of 24 August 2014 caused slip on several active fault strands within the West Napa Fault Zone (WNFZ). Field mapping identified 12.5 km of surface rupture. These field observations, near-field geodesy and space geodesy, together provide evidence for more than~30 km of surface deformation with a relatively complex distribution across a number of subparallel lineaments. Along ã 7 km section north of the epicenter, the surface rupture is confined to a single trace that cuts alluvial deposits, reoccupying a low-slope scarp. The rupture continued northward onto at least four other traces through subparallel ridges and valleys. Postseismic slip exceeded coseismic slip along much of the southern part of the main rupture trace with total slip 1 year postevent approaching 0.5 m at locations where only a few centimeters were measured the day of the earthquake. Analysis of airborne interferometric synthetic aperture radar data provides slip distributions along fault traces, indicates connectivity and extent of secondary traces, and confirms that postseismic slip only occurred on the main trace of the fault, perhaps indicating secondary structures ruptured as coseismic triggered slip. Previous mapping identified the WNFZ as a zone of distributed faulting, and this was generally borne out by the complex 2014 rupture pattern. Implications for hazard analysis in similar settings include the need to consider the possibility of complex surface rupture in areas of complex topography, especially where multiple potentially Quaternary-active fault strands can be mapped.

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

“…It has been deployed globally, but the main focus has been in California where it supports a number of investigations. UAVSAR has been used to image deformation after three earthquakes ranging in magnitude from 5.2 to 7.2 [38][39][40]. UAVSAR produces images with 1 m spatial resolution for phase change images and 7 m for images that have been unwrapped.…”

“…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].…”

“…A primary challenge after the earthquake was rapid and accurate surface rupture mapping, quantification of coseismic slip, and subsequent identification and monitoring of potentially damaging postseismic slip [72]. Analysis of UAVSAR data provides slip distributions along multiple fault traces, indicates connectivity and extent of secondary traces, and confirms that postseismic slip only occurred on the main trace of the rupture [40].…”

“…UAVSAR interferograms were used iteratively with field observation to understand the distribution of surface deformation. Furthermore, UAVSAR provided the most robust means of generating full coseismic and postseismic slip distributions [40] (Figure 7). The wrapped interferogram shows considerable surface disruption, but the product is useful for identifying the location of fault rupture from discontinuities in the image.…”

“…When combined with GPS measurements, a much better assessment can be made of how deeply shallow slip extends and the extent of off fault deformation. Though UAVSAR measurements are not as accurate as GPS they have proven useful for identifying localized slip across faults [38][39][40], including faults triggered by distant earthquakes [72,112]. In general, significant triggered slip is several cm, outside the error of the UAVSAR measurements.…”