CHANGES in hydrology, usually involving increases in stream and spring flow, occur in response to large earthquakes. These changes have been attributed to two very different mechanisms : the expulsion of water from the upper or middle crust due to elastic compression(1), or near-surface permeability enhancements(2-4). If the former mechanism is correct, then sampling streams and springs affected by earthquakes may provide information about the nature of fluids at depth. Alternatively, if the changes in hydrology reflect only shallow processes, then the behaviour of these fluids provides insight into the rheological response of the shallow crust to earthquakes. Studies following the 17 October 1989 Loma Prieta earthquake in California provided a wealth of information regarding changes in stream and spring bow, groundwater how and stream chemistry in the region around the earthquake epicentre(1-4) Here we show that both the initial hydrological response acid the hydrology of the region several years after the earthquake are more readily explained by earthquake-induced enhancements of permeability in the shallow crust that are persistent and widespread.