Seasonal Nontectonic Loading Inferred From cGPS as a Potential Trigger for the M6.0 South Napa Earthquake

Abstract

AbstractWe analyze crustal strain corresponding to transient continuous Global Positioning System (cGPS) horizontal displacements in Northern California, detecting a seasonal positive dilatational strain and Coulomb stress transient in the South Napa region peaking just before the 24 August 2014 M6.0 South Napa earthquake. Using data from 2007 to 2014, we show that average dilatational strain within a 500‐km2 region encompassing South Napa and northern San Pablo Bay peaks in late summer at 76 ± 17 × 10−9, accompanied by a Coulomb stress change of 1.9 ± 0.8 kPa. The situation reverses in winter, with an average dilatational strain of −51 ± 17 × 10−9 and Coulomb stress change of −1.4 ± 0.8 kPa. Within a smaller 100‐km2 area centered on the South Napa rupture, peak values are considerably higher, including a summer Coulomb stress peak of 5.1 ± 1.6 kPa. We examine regional seismicity but see no statistically significant correlation with seasonal Coulomb stressing in the declustered earthquake catalog. Using western U.S. vertical cGPS displacements, we estimate that strain from hydrologic loading explains ≤10% of the observed long‐wavelength strain and only 2–3% of peak strains around the South Napa rupture. Thermoelastic crustal strain estimated from temperature gradients between the San Francisco Bay and Sacramento Valley reaches values as high as 15% of the observed strain, but the strain patterns are not spatially consistent. Vertical deformation within the Sonoma and Napa Valley subbasins inferred from interferometric synthetic aperture radar explains large horizontal motions at nearby cGPS stations and suggests that seasonal changes in groundwater may contribute to observed strain and stress transients.