Drought-induced stress changes on faults associated with the 2014 South Napa earthquake

The western United States has been experiencing severe drought since 2013, with some areas affected since 2012. The immediate solid earth response to the loss of water mass from the drought should be elastic rebound whose magnitude varies with load changes in both time and space. We have identified surface uplift from the current drought at continuously operating GPS stations in the EarthScope Plate Boundary Observatory and inverted for the spatiotemporal pattern of mass loss consistent with the vertical GPS displacements. The mass loss is greatest in California's Sierra Nevada and Coastal Ranges, where it ranges up to 50 cm of water equivalent.

Since the change in water loading due to the drought is not uniform, it induces spatially varying crustal strains. Some investigators have suggested that strains from changes in water loading place significant stresses on seismogenic faults, and we investigate this hypothesis for drought-induced load changes on the faults associated with the 2014 Mw6.0 South Napa earthquake. We calculate the Coulomb and other stress changes on these faults and assess whether they could have brought these faults closer to failure prior to the South Napa earthquake. We also extend this analysis to a broader selection of California faults to assess whether drought-induced load changes might have significantly changed stresses on faults elsewhere in the region.