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Floodwaters may trigger fault motion

Geologists find evidence for natural disaster one-two punch

9:36am, June 27, 2011

Before Hoover Dam was built, Colorado River floodwaters may have triggered small earthquakes in southern California. These tremors coincided with — and perhaps helped kick off — at least one big temblor on the nearby San Andreas fault, researchers report online June 26 in Nature Geoscience.

Daniel Brothers, a marine geologist at the U.S. Geological Survey Woods Hole Coastal and Marine Science Center in Massachusetts, and his colleagues mapped out several faults beneath southern California’s Salton Sea, which sits in a basin at the southern tip of the San Andreas fault. They also used sound waves to probe layers of lake bed sediment that record ancient floods from the nearby Colorado River. Sharp tilts in the sediment revealed past motion on the faults, including four slips that happened at the about the same time as quakes on the San Andreas.

At least four out of 17 small fault slips in the lake bed records coincided with floods, though

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