Figure 2: Changes in discharge after the South Napa earthquake.

(a) Daily averaged discharge of Sonoma Creek documented by a USGS gauge at Agua Caliente before and after the South Napa earthquake. Measurement errors are similar to the size of symbols used. Short duration increases of discharge, indicated by thick arrows, were due to precipitation both inside and outside of the studied area, and do not necessarily correspond to the precipitation in the valley (b). Thin arrow shows the time of the earthquake. Simulations of the flow are based on the model discussed in the text and Supplementary Note 2. Two simulations were made; the first simulation (black line) is based on data for the first 17 days after the earthquake (open squares) to exclude the first incursion of precipitation and the second simulation (red line) is based on all data before significant precipitation in the valley (∼60 days after the earthquake). The similarity between the two results suggests robustness of model. (b) Daily precipitation in Sonoma Valley; precipitation in Napa Valley (not shown) was similar. Little precipitation occurred for 60 days after the earthquake, but significant precipitation started afterwards. (c) Discharges as a function of time in new streams and Spencer Spring in Napa County. Discharges in different streams and spring are shown by different coloured symbols; measurement errors are shown as error bars except where the error bars are smaller than the symbols. Symbols in brackets show conditions reported by local residents and one discharge data converted from early depth measurements, with depth-to-discharge conversion calibrated during subsequent surveys. Coloured curves show simulated stream discharges based on 60 days of data after the earthquake, using the same model as used in a; dashed curves show extrapolations from the simulated discharges. Measurement at Spencer Spring started 21 days after the earthquake; this flow is not simulated. (d) Discharges and simulated discharges as a function of time in new streams in Sonoma County. Upward arrow indicates that measured discharges were off scale.