Origin of postseismic streamflow changes inferred from baseflow recession and magnitude‐distance relations

The effects of earthquakes on baseflow, the component of streamflow provided by groundwater, are analyzed for streams that exhibit large coseismic increases in streamflow and discharge large volumes of excess water. Although discharge Q may increase rapidly, the rate of baseflow recession d log Q / dt is unchanged suggesting that the hydraulic conductivity of groundwater system providing baseflow does not change. For (at least) the studied streams, the coseismic increase of discharge therefore requires increased hydraulic head gradients resulting from the rapid release of water from some source of storage. The relationship between the response of streams, earthquake magnitude, and the distance of the drainage basin from the epicenter, is consistent with subsurface liquefaction being the mechanism that increases head gradients.