Abstract
Many streams and springs, which were dry or nearly dry before the 2014 Mw6.0 South Napa earthquake, started to flow after the earthquake. A United States Geological Survey stream gauge also registered a coseismic increase in discharge. Public interest was heightened by a state of extreme drought in California. Since the new flows were not contaminated by pre-existing surface water, their composition allowed unambiguous identification of their origin. Following the earthquake we repeatedly surveyed the new flows, collecting data to test hypotheses about their origin. We show that the new flows originated from groundwater in nearby mountains released by the earthquake. The estimated total amount of new water is ∼106 m3, about 1/40 of the annual water use in the Napa–Sonoma area. Our model also makes a testable prediction of a post-seismic decrease of seismic velocity in the shallow crust of the affected region.
Highlights
Many streams and springs, which were dry or nearly dry before the 2014 Mw6.0 South Napa earthquake, started to flow after the earthquake
A United States Geological Survey (USGS) stream gauge on Sonoma Creek documented a coseismic increase of discharge, showing that increased groundwater flow occurred across the basin after the earthquake
Water is 200 to 300 deep and flowingy’ Water in the stream was 800 deep when we visited on 28 August, suggesting a gradual increase in discharge, similar to that documented at the Sonoma Creek by the USGS stream gauge (Fig. 2a)
Summary
Many streams and springs, which were dry or nearly dry before the 2014 Mw6.0 South Napa earthquake, started to flow after the earthquake. At least nine tributary streams of the Napa River and Sonoma Creek and several springs in the Napa– Sonoma area, which were dry or had little flow before the earthquake, started to flow after the earthquake.
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