It is well established that earthquakes occur on fault systems where stress has accumulated over periods of centuries to millennia. The specific factors that trigger individual earthquakes are typically unknown, but researchers have recently found links between seasonal variations in earthquake occurrence and local changes in water storage and temperature. Using data from high-precision continuous GPS stations in Northern California, we observed a small 3 mm horizontal expansion of the Earth’s crust prior to and in the vicinity of the August 2014 M6.0 South Napa earthquake. By analyzing the previous eight years of GPS data, we additionally found that a similar pattern of crustal motion repeats every summer. We have determined that this crustal expansion releases pressure on nearby faults, including those in the South Napa fault system, making them more likely to slip during the summer months. Large seasonal variability in the amount of groundwater in the Napa Valley and Sonoma subbasins may contribute to the observed changes.