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EARTH AND WATER
DURHAM, NC- When the earth quakes, water also moves. Recent studies shed new light on how, why and where underground water moves. The earliest reports of earthquakes note marked hydrological effects seen after major temblors. These changes usually manifest as increases in the flow from springs and in streams near the surface. However, there has been considerable disagreement in the scientific community as where the water comes from and how it makes its way to the surface. One theory is that increased pressure from below brought on by seismic activity could squeeze water out of the crust, pumping up flow near the surface. Another theory holds that the principal effect of earthquakes is to break up the ground, increasing the permeability of surface rocks. If the first theory is correct, samples of water taken after earthquakes should yield information about the composition of fluids normally found at greater depths. If the latter hypothesis is correct, then the behavior of surface waters should provide insights into changing permeability of rock during earthquakes. A team of researchers from Duke University teamed up with investigators from the US Geological Survey to study hydrological changes following the Loma Prieta earthquake that struck Northern California in 1989. They studies changes immediately following that quake and in the several years since then. The researchers studied hydrological changes in the San Lorenzo River and Pescadero Creek. They found that significant drops in water elevation seen in wells in these areas corresponded closely with significant increases in flow in the rivers. Follow-up studies conducted in 1992 and 1993 suggested that these changes were long lasting. Further studies indicated that ionic concentrations of stream water in the San Lorenzo river increased significantly and remained elevated three years after the earthquake. The team sampled 100 streams and springs in an attempt to identify sources of the water with high ionic concentration. The researchers concluded that much of the sampled water could be attributed to water table discharge following the quake. Based on these findings, the researchers believe the second hypothesis to be valid, that earthquakes exert effects on the permeability of surface rocks that are both persistent and widespread. This study was reported in Nature, 1/19/95, v.373, Rojstaczer et al. pp 237-239.
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Science Updates Index
