Episodic flooding of an ancient lake at the southern end of the San Andreas fault triggered a series of small earthquakes adjacent to the fault, reports a paper published online this week in Nature Geoscience. In turn, the earthquakes could have altered the state of stress on the nearby San Andreas fault, causing it to rupture.

Daniel Brothers and colleagues present seismic images of sediments deposited in the ancient Lake Cahuilla in the Salton Trough, California. The data show that episodic flooding of the lake by the Colorado River during the past 1,200 years, triggered earthquakes on small faults underlying the lake. The timing of the flood events correlates with large earthquakes on the southern San Andreas fault, to within a few hundred years, and modelling shows there could be a link. It is therefore possible that human engineering of the Colorado River to prevent flooding of the Salton Trough over the past century has contributed to the lull in seismic activity currently experienced by southern parts of the San Andreas fault.