Did human activity trigger California earthquakes nearly 100 years ago?

On March 10, 1933, the ground beneath Californian's feet shook for 15 seconds. The quaking may have been momentary in Long Beach, but it was powerful enough to cause massive buildings to collapse. More than 100 people died.

It's not hard to chalk that 6.4 magnitude earthquake up to natural disturbance along California's infamous fault lines. But that might be misguided, suggests Susan Hough of the US Geological Survey. 

Humans may have actually had a hand in that disaster, as well as some other significant earthquakes in the Los Angeles basin in the early 20th century, according to Dr. Hough's research published Monday in the Bulletin of the Seismological Society of America.

Hough points to parallels between oil drilling activity and earthquake records in the early 1900s as clues that the petroleum industry may have played a role in shifting the Earth's crust and thus inducing some earthquakes in the early 1900s.

At the time, oil was extracted from the Earth by "just pulling it out of the ground," Clifford Frohlich, a geophysicist at the University of Texas at Austin who was not part of the research, explains in an interview with The Christian Science Monitor. This straightforward extraction left the Earth's crust susceptible to slumps and cracks with the changing pressure, which likely triggered tremors. 

It's difficult to "prove beyond all shadow of a doubt that an earthquake was induced because you basically have to show that the earthquake would not have happened otherwise," Hough tells the Monitor. So Hough and her colleague Morgan Page have assembled multiple lines of circumstantial evidence that collectively suggests some earthquakes in the Los Angeles basin, including the 1920 Inglewood and 1929 Whittier earthquakes, in addition to the 1933 Long Beach quake.

One thing the researchers looked at was what was happening in nearby oilfields when the earthquakes occurred. They found that earthquakes commonly followed when older oil wells were being deepened. Furthermore, these quakes seemed to be occurring quite close to where the wells were being deepened. For example, the epicenter of the 1929 Whittier quake was right on top of that activity, Hough says.

Another clue that an earthquake may have been induced by drilling is if it was particularly shallow. Natural earthquakes tend to occur perhaps 6 miles below the surface, whereas human-induced quakes may be just a mile deep, Hough says. And those shallow earthquakes can cause a lot more damage at the epicenter.

It wasn't just the characteristics of individual earthquakes that tipped the researchers off. They also found that when oil production ramped up in the 1920s, ground motion seemed to increase as well. 

"Frankly, from what we've seen other places, it would not surprise me if there was a link here. And this paper brings to light some places that we should be looking pretty hard," Geoffrey Abers, director of graduate studies for geological sciences at Cornell University, who was not part of the study, tells the Monitor.

Previous research has suggested a link between oil production and earthquakes in places like Texas and Oklahoma. But, Dr. Abers points out, the geology of those states is quite different than in California. In Texas and Oklahoma naturally occurring earthquakes are very rare, so it's easy to notice an uptick. But in California, earthquakes are much more common, so sorting out any possible human influence is much trickier.

"I think there's sort of been a sense that California is immune to induced earthquakes," Hough says. And this research suggests it might be necessary to reconsider that.

But if there is a link, this could paint a more positive picture of California's natural earthquake risk, as hazard predictions are based off historical quakes. "The rate of natural earthquakes in the Los Angeles basin might actually be lower than we've estimated," Hough says.

Just because some of these early 20th century earthquakes may have been triggered by oil production doesn't mean that that is still the case in California, Dr. Frohlich points out. Practices in the industry have changed. By the middle of the century, it was common practice to pump water, or some other fluid, into the the oil wells to drive more out. This also would serve to maintain the pressure underground and may leave less opportunity for the rocks to shift underground and cause tremors.

But more recently, petroleum engineers have been employing hydraulic fracturing to access oil and gas locked deep within the Earth. The disposal process for the water used in that technique has been associated with earthquakes in Texas in Frohlich's own research, but the process is different from the simpler extraction that was occurring in the Los Angeles basin in the early 20th century.

Although Hough insists that her research isn't the smoking gun proving that any of these earthquakes were indeed human-caused, it does suggest that it is possible. And, she says, further research could help prevent disasters.

"For natural earthquakes, there's nothing you can do about them," Hough says. "But if people are causing the earthquakes, and if we understand them, potentially we can actually mitigate the hazards."