A string of quakes ending on Sept. 22, 2005 struck in Kern County near the southern end of California’s Central Valley  – and the new study, published in Geophysical Research Letters, concluded that the odds that those quakes might have occurred by chance were just 3 percent.

Instead, the researchers honed in on a very specific set of culprits: three wastewater injection wells in the Tejon Oil Field. Between 2001 and 2010, the rate of wastewater injection at that oil field quintupled, and up to 95 percent of that wastewater was sent to just that trio of closely-spaced wells, the scientists noted.

The largest of the earthquakes in the swarm measured magnitude 4.6 on the Richter scale meaning that the quakes were relatively small, unlikely to have done any damage to buildings but significant enough to be felt by those in the area.

To be sure, natural earthquakes have always far outnumbered human-caused quakes in California – but the researchers warned that even if the number of industry-caused quakes is small, wastewater injection could be responsible for larger, more dangerous quakes in the future.

“Based on our empirical results, injection-induced earthquakes are expected to contribute marginally to the overall seismicity in California,” the researchers from the California Institute of Technology, University of California, University of Southern California and two French universities, wrote. “However, considering the numerous active faults in California, the seismogenic consequences of even a few induced cases can be devastating.”

The researchers also warned that the number of California quakes tied to oilfield activities has been little-studied compared to other parts of the country and that natural quakes may have “masked” the oil industry’s impacts.

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