A new study suggests some Los Angeles-area earthquakes in the 1920s and 1930s could have been caused by the oil boom at the time.

The paper, scheduled to be published online Tuesday in the Bulletin of the Seismological Society of America, presents evidence that drilling around Los Angeles between 1915 and 1932 could have been associated with damaging earthquakes in the area, including the magnitude 6.4 Long Beach quake in 1933 that killed 120 people.

Although the modern practice of hydraulic fracturing has been tied to small man-made earthquakes, right off the bat, the study authors — Susan E. Hough and Morgan Page of the U.S. Geological Survey — make clear this is not a study to draw lessons from today. The authors write in the opening paragraph:

"Our results suggest that significant earthquakes in Southern California during the early twentieth century might have been associated with industry practices that are no longer employed (i.e., production without water reinjection), and do not necessarily imply a high likelihood of induced earthquakes at the present time."

As the Los Angeles Times points out, "Nowadays, water is carefully used to replace the pumped-out oil, which prevents land from sinking and also helps extract more oil," something that was not being done in the '20s and '30s in the oil fields of the Los Angeles basin.

Rather than a model for the present, the new paper is a lens through which to view the seismic past of Southern California, using scientific history.

The authors note that earthquake-sensing technology was neither as sensitive nor as widespread in the early 20th century as it is today. Most earthquakes in the Los Angeles basin in the first third of the century "caused light shaking reported by at most a few witnesses," they write.

A few witnesses are not the most reliable source of data, so Hough and Page limited themselves only to the 22 biggest, most widely felt earthquakes of the period, and compared those quakes to oil industry records from the time.

Of those earthquakes, they found 13 possibly or likely were associated with "oil production activities," including the drilling of deeper and deeper oil wells near hidden faults that are known to cause earthquakes. The authors based their findings on the best information available about when and where the earthquakes and drilling occurred and the kind of drilling being conducted.

Last year, Hough and Page published evidence that midcentury earthquakes in Oklahoma were triggered by the injection of wastewater during oil production in that part of the country. That historical finding was more relevant to today's oil production activities than the California data. The USGS announced earlier this year that pumping wastewater underground during oil production has caused parts of Oklahoma to be as seismically active as parts of California.

Hough and Page used a similar historical record analysis for the Oklahoma and California papers, but Thomas Heaton, director of the Earthquake Engineering Research Laboratory at the California Institute of Technology, told NPR he thought the Oklahoma research was more convincing than the new California findings.

"There is a long history of earthquakes being triggered by increased fluid pressure," he wrote in an email after reviewing the new paper. "Although we don't understand [the association] very well, it's as close as our community gets to consensus that fluid induction triggers earthquakes."

"The tricky part of the equation in California is that earthquakes occur regularly without fluid injection," he wrote.

He did not go so far as to call into question the associations suggested by the paper — he thought it's certainly possible that oil drilling played a part in some or all of the earthquakes Hough and Page identified. But it's hard to be sure about the reason for any one quake.

"It would be no surprise to discover that some of our California earthquakes were triggered by oil production," Heaton wrote. "What would be surprising would be to discover that a team of scientists was able to present a compelling case that it had occurred in any particular example (e.g. 1933 Long Beach)."

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