Floodwaters may trigger fault motion

Geologists find evidence for natural disaster one-two punch

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 whether earthquake or deluge came first is anyone’s guess. In at least one case, a big quake on the San Andreas occurred at about the same time.

“Overall, it’s a story that makes sense,” says Ken Hudnut, a geophysicist at the USGS Pasadena field office in California.

In the geologic past, floods periodically transformed the area studied by Brothers’ team from a dry basin into Lake Cahuilla, Salton Sea’s ancient predecessor that was at one point more than six times deeper than the present-day lake. The weight and pressure of this water could have stressed the underlying faults, just as reservoirs in dammed-up rivers can trigger seismic events. In worst-case scenarios assessed by the team’s computer simulations, this stress was enough to rupture the faults.

“The physics supports the idea that Lake Cahuilla would promote failures of these faults,” says Brothers.

But several researchers say they were unconvinced that floods have ever affected activity on the San Andreas.

“When you take a hard look at the data,” says Ray Weldon, a geologist at the University of Oregon, “you have to ask yourself: ‘Is this just a coincidence?’”

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