Earthquake Swarm Reveals Complex Structure of a California Fault Line

Over the course of four years, one region of Southern California experienced over 22,000 earthquakes, most too small for people to notice. Unlike classic earthquakes that arrive in one large event followed by fading aftershocks, the thousands of earthquakes under the Cahuilla Band of Mission Indians reservation were a drawn-out swarm.

Now, researchers at California Institute of Technology applied machine learning to years of seismic data to pinpoint the source of 22,698 tremors to tease out an explanation. Based on the data, which show not only where each quake started but also when, the researchers identified a likely culprit: fluid, like water or liquid carbon dioxide, entered a complex system of cracks in the earth. As the fluid moved through the cracks, it set off the years-long seismic swarm. The research was published on June 18 in the journal Science.

“Swarms have been somewhat enigmatic for quite a while,” U.S. Geological Survey geophysicist David Shelly, who was not involved with the study, tells Carolyn Gramling at Science News. “…This one is particularly cool, because it’s [a] rare, slow-motion swarm. Most might last a few days, weeks or months. This one lasted four years. Having it spread out in time like that gives a little more opportunity to examine some of the nuances of what’s going on.”

The research team learned of the interesting seismic event in 2017 when they received an email from a citizen who had noticed a cluster of tiny earthquakes, Maya Wei-Haas reports for National Geographic. When they looked into the data, they realized that the swarm began a year earlier. As Shelly explains to Science News, swarms are most common in areas with volcanic and hydrothermal activity, and they are harder to interpret when, like the Cahuilla swarm, they aren’t in those areas.

But California Institute of Technology geophysicist Zachary Ross and his colleagues had been developing an algorithm that can pick earthquake signals out of seismic noise.

“We decided to turn this loose on [the Cahuilla swarm] dataset,” says Ross to National Geographic.

The result is a three-dimensional map of tremors as they move through the fault system. The evidence suggests that, based on the way the earthquakes spread over time, some fluid was injected into the cracks of the fault from underneath. The fluid changed speed during its travels, occasionally getting stuck behind a rocky barrier and pushing through or rushing around an obstacle.

That triggered a 4.4 magnitude quake—strong enough to be felt on the surface—in 2018 that set off a final flurry of tremors. As Ross tells Science News, the swarm then slowed down, but it’s not over just yet.

“The detail here is incredible,” University of Puerto Rico at Mayaqüez seismologist Elizabeth Vanacore, who wasn’t involved in the study, tells National Geographic. “This type of work is cutting edge and really where the science is going.

While fault lines are often explained as a large crack in the ground where two plates of the Earth’s crust either meet or separate, the data show that the true story is more complicated.

“You can see that the sequence [of earthquakes] originated from a region that’s only on the order of tens of meters wide,” Ross tells Science News. But after four years, the tremors occurred over an area 30 times larger than the original zone, Ross notes. “Typically, we have very limited explanations that we can provide to the public on what’s happening. It gives us something that we can explain in concrete terms.”

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Theresa Machemer is a freelance writer based in Washington DC. Her work has also appeared in National Geographic and SciShow. Website: tkmach.com