I (Don’t) Feel the Earth Move: Study Detects 2 Million Quakes in California Over 10 Years

Southern California experiences a very small quake every three minutes, says Caltech big-data study that weeded out the noise from modern society

Ruth Schuster
Ruth Schuster
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The Dumont Dunes are pictured in the Mojave desert in California, April 8, 2019.
Southern California, which turned out to have experienced almost 2 million quakes in ten years' time.Credit: AFP
Ruth Schuster
Ruth Schuster

You might assume you would notice earthquakes — but you would be wrong. Southern California experienced nearly 2 million quakes from 2008 to 2017, most too small to be discerned by us coarse human beings, seismologists from Caltech revealed in Science last week. On average, there are 495 earthquakes a day in southern California, occurring roughly three minutes apart.

Maybe that's why cats are so crazy. Maybe they can sense them all. 

Haaretz Weekly Ep. 24

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Seismologists know perfectly well that the earth is moving — let’s say, all the time — and that a lot of little quakes pass beneath our internal radar. But now they checked more closely, the sheer magnitude of the phenomenon is staggering.

>> Read more: Israel is hundreds of years overdue for a massive earthquake

Scientists knew for sure there had been around 180,000 quakes in southern California during those 10 years. But the actual number, they now realize, is 10 times greater.

Hidden Earthquakes in Southern California

We are talking about very small movements of the earth, but movements of the earth they are.

“It’s not that we didn’t know these small earthquakes were occurring. The problem is that they can be very difficult to spot amid all of the noise,” says Zachary Ross, lead author of the study.

Detecting the micro-quakes in past data involved overcoming the low signal-to-noise ratio. The problem is that a seismometer worth its salt picks up everything from trains passing in the night to aircraft touching down to passing trucks and the pounding of heavy machinery, which can drown out quake signals.

The team solved that conundrum by using the signals of slightly larger, more easily identifiable earthquakes as examples of what an earthquake’s signal at a given location looks like.

When likely candidates with suitable waveforms were identified, the researchers scanned records from nearby seismometers to see whether the earthquake’s signal had been recorded elsewhere and could be independently verified. This can only work when there are seismometers all over the place, the team explains.

A worker onitoring water tanks at a Hess fracking site near Williston, North Dakota, November 2014.Credit: Andrew Cullen / REUTERS

In other words, they tapped absolutely massive data processing, working over about three years — and their results were stunning. They found new information about foreshocks (small quakes that precede a “relatively big one,” which naturally can only be defined as “foreshocks” after the event).

Until now, the conventional wisdom had been that not all major temblors are preceded by foreshocks. The new data sheds fresh light on the situation: There can be, simply, shocks all the time in lively seismic zones.

Ross collaborated with Egill Hauksson of Caltech, Daniel Trugman of Los Alamos National Laboratory and Peter Shearer of Scripps Institution of Oceanography at UC San Diego.

It makes sense that the propensity to seismic action along a given fault will affect faults around it. Hauksson explains that the new data reveal “previously undetected foreshocks” that precede major earthquakes and shed light on the development of earthquake swarms.

Shaking canaries in the coal mine

One immediate application of the new technique could be to debunk, or validate, concerns about fracking as a cause of tangible geological destabilization.

Fracking is a nickname for “hydraulic fracturing,” which achieves exactly what the name indicates: fissures, for instance in shale layers, are forced wider by means of injecting liquid at high pressure in order to extract fossil fuels. The practice has provoked concern at several levels, one being that it may cause quakes, another being that the chemicals injected with the water may poison the groundwater.

While at first the industry may have been in denial, the consensus has been widening that fracking causes at least little quakes. Even the U.S. Geological Survey is straightforward that fracking — deep, not shallow — causes quakes, though it suggests it doesn’t cause many of the types one feels without scientific aid.

In southern California, where one man hopes not to feel anything move under his feet.Credit: Jae C. Hong/AP

But there is thinking that this hydraulic fracturing may cause big earthquakes as well. Scientists at Stanford reported in December 2017 that small earthquakes at fracking sites could portend bigger tremors to come: “Tiny tremors caused by hydraulic fracturing of natural gas near the surface could be early signs of stressful conditions deep underground that could destabilize faults and trigger larger earthquakes,” wrote Mark Shwartz in the university’s journal.

The small quakes at fracking sites could be like canaries in the coal mine, co-author William Ellsworth told the Stanford journal. And they’re singing a message: If your company is fracking at a site and a swarm of small quakes ensues, have somebody credible investigate the conditions hidden beneath our feet.

The news should be of vast interest to Israel’s geologists and public at large, given the existence of the active Dead Sea Rift — a great crack in the earth that stretches from the Gulf of Eilat to Turkey.

The Dead Sea and Lake Kinneret both sit in this giant fault, which produces not only numerous little swarms of wee quakes, but also major ones that have leveled cities and the Temple in Jerusalem itself in historic times. The most recent major quake generated by the fault was in 1927, and it measured about 6.2 on the Richter scale. Geologists warn that a massive quake originating in the Dead Sea Transform is overdue — even hundreds of years overdue in some sections.

If there’s one thing Israel doesn’t need, it is to provoke the fault into action. While the National Infrastructure, Energy and Water Ministry would dearly love to split apart rocks to find oil and gas, especially in the Negev and the Golan Heights, the Environmental Protection Ministry is appalled by the notion. It has tabled a demand to ban all terrestrial oil and gas exploration in this tiny land, noting, among other things, that encouraging more fossil fuel exploitation flies in the face of policy to reduce greenhouse gas emissions.

The energy ministry argues that Israel is importing oil, which is causing more emissions than if it were to produce the oil at home. The battle is on. Stay tuned.