Tectonic Seasonal Loading Inferred from cGPS Measurements as a Potential Trigger for the M6.0 South Napa Earthquake

Abstract:

Measurements from continuous global positioning system (cGPS) networks continue to unfold details about transient strain signals [Mavrommatis et al., 2014; Heki, 2003]. Linking these transient strain signals to seismic events remains elusive, as it requires detailed information about the steady-state tectonic loading sources, faulting geometries, and strain distribution with depth. Here we use cGPS measurements to uncover a regional strain transient peaking just prior to the M6.0 August 24, 2014 South Napa earthquake. This signal appears to have produced a coulomb stress increase, favoring slip on the West Napa faulting system. Analysis of cGPS time series during the interseismic period from 2006 to 2014 shows a stacked summer dilatational lobe of +142 ± 64 x 10^-9 in the 100 km² earthquake region.

The Napa region is part of a broad, long wavelength, zone of positive dilatational strain and coulomb stress increase peaking each summer season. Summer transients are associated with horizontal displacements of 3–5 mm directed eastward toward the Sacramento Basin and of 1–3 mm directed southwest toward the San Francisco Bay and Pacific Ocean. Winter transients involve the opposite of these motions, causing negative dilatational strains and negative coulomb stress changes in the Napa region.

We observe a significant increase in summer seismicity rates (greater than 95% confidence for a Chi-square test) within regions of positive coulomb stress change in Northern California. Large scale models of vertical hydrologic loading predict some components of the long-wavelength horizontal signal in Northern California, but this loading accounts for only 20 – 30% of the total anomalous signal. We hypothesize that the remaining signal is associated with smaller-scale seasonal groundwater fluctuations in local basins (e.g., the Sonoma and Napa sub-basins) along with thermoelastic effects. We provide details regarding the amount of thermoelastic strain from the elastic portion of the Earth’s crust that contributes to our signal [Prawirodirdjo et al., 2006; Ben Zion and Allam, 2013]. Our results suggest that densely instrumented geodetic networks are capable of monitoring subtle strain changes within the crust and have the potential to improve region-specific seismicity forecasts.