Don,

The best review I've read recently was this one by Michael Manga of UC Berkeley. Precursory changes have not been shown to be present, but postseismic changes are often observed. Here's the somewhat technical jargon.

John

Streamflow and Water Well Responses to Earthquakes
David R. Montgomery and Michael Manga
Science, Vol 300, Issue 5628, 2047-2049 , 27 June 2003

Abstract

Earthquake-induced crustal deformation and ground shaking can alter stream flow and water levels in wells through consolidation of surficial deposits, fracturing of solid rocks, aquifer deformation, and the clearing of fracture-filling material. Although local conditions affect the type and amplitude of response, a compilation of reported observations of hydrological response to earthquakes indicates that the maximum distance to which changes in stream flow and water levels in wells have been reported is related to earthquake magnitude. Detectable streamflow changes occur in areas within tens to hundreds of kilometers of the epicenter, whereas changes in groundwater levels in wells can occur hundreds to thousands of kilometers from earthquake epicenters.

Key Conclusion

Different mechanisms of hydrologic response to earthquakes should have different ranges of characteristic response times, and one also should expect to see a wide range of time scales for hydrologic response to earthquakes, depending on the local geological and geomorphological contexts. Streamflow response to the Nisqually earthquake, for example, occurred within 12 hours, indicating a shallow source, and therefore precluding expulsion of overpressured fluids in the seismogenic zone and pore-pressure diffusion after coseismic strain in the upper crust as mechanisms for the observed streamflow changes associated with the earthquake. Sustained streamflow response, such as the greatly increased stream flow that persisted for many months after the Loma Prieta earthquake, indicates a deeper source, whereas rapid transient streamflow response to earthquakes may reflect the expulsion of water from surficial deposits in response to consolidation during ground shaking.