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Spike in Earthquakes? An 'Illusion' Raises New Questions

Some of the best minds in earthquake science have been analyzing seismic waves to see if the largest recent quake might have triggered others.
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Does it seem as if there have been more earthquakes in recent weeks? Some scientists thought so.

Some of the best minds in earthquake science have been counting quakes and analyzing seismic waves to see if the largest in a string of recent quakes — the magnitude-8.2 tremor in Chile on April 1 — might have triggered others far, far away.

Ross Stein, a senior U.S. Geological Survey geophysicist who studies how quakes interact, got so excited that on April 12 he fired off an email to colleagues that started with this: "Guys, seems like a lot of big quakes have been popping off around the globe over the past week."

Experts for years have known that the seismic waves from one quake can trigger a quake somewhere else — a process known as "dynamic triggering."

Stein himself co-authored a study tying a magnitude-8.7 Indian Ocean quake in 2012 to a spike in quakes globally in the days after. That increase lasted about a week, and a few days after the spike, the rate for larger quakes fell to below average.

"It's as if the Indian Ocean quake had shaken the tree, causing the apples ready to fall out to do so," said Thorne Lay, a seismology professor at the University of California at Santa Cruz.

Image: Earthquakes over 30 days
This chart from the U.S. Geological Survey shows where earthquakes measuring magnitude-4.5 or greater have been reported over the past 30 days. The size of the circle indicates the strength of the quake. Orange dots indicate quakes that have occurred over the previous 24 hours. Yellow dots indicate quakes over the past week.USGS

Lay's own research has found that over the last decade the number of major quakes, those measuring 8.0 or bigger, is nearly triple the rate for the 1900s, but whether that's just a random cluster or a sign of dynamic triggering is unclear.

In any case, the latter possibility is what got experts like Stein and Lay wondering if the Chilean quake had triggered others. A 7.2 on Mexico’s Pacific Coast on April 18 and a 7.5 off Papua New Guinea two days later were the latest to get attention.

Doing a quick review of quakes magnitude 6 or larger and which struck within the upper 43 miles (70 kilometers) of Earth’s crust, Lay found an uptick when comparing April 1-18 to the first three months of 2014.

But the increase wasn’t significant, so the question remained: Did dynamic triggering play a role, or was that just random chance? "That’s a harder problem to answer," Lay said.

Seismic wave data from the Chilean quake was studied by UC Santa Cruz grad students to see if it might have triggered one in Nicaragua nine days later. "We did not see anything obvious," Professor Emily Brodsky said of her team’s work.

Stein, like Lay, also counted quakes but looked instead at moderate and large quakes (4.5 magnitude or greater) in the 10 days before and after the April 1 quake that struck Iquique, Chile. In his email to peers, Stein concluded: "I do not see a global increase in activity post-Iquique, at least for moderate and larger quakes — the ones that matter for hazards."

"[It] seemed like a lot of big quakes" after Iquique, he later told NBC News, "but it's largely an illusion."

That letdown is part of the reality of earthquake science, which is still in its infancy. "We have false hunches all the time," Stein said. "We don’t want to miss something."

Making more sense of quakes, especially dynamic triggering, could be helped by a wider network of monitoring equipment. But that’s no easy task.

“We need seismometers close to the faults that make the world’s biggest earthquakes and most of those faults are underwater."

The U.S. has "dense networks" in active quake areas, noted Jill McCarthy, head of the USGS Geologic Hazards Science Center. "But networks are expensive, and it's unlikely we're going to have the same dense coverage globally, at least not anytime soon."

Expanding that network would also mean going underwater. "We need seismometers close to the faults that make the world's biggest earthquakes, and most of those faults are underwater,” said Brodsky. "This would take a substantial investment."

Whether that added monitoring would have much pay back, especially in preventing deaths, is uncertain.

Less than three percent of quakes have any measurable effect in a given spot, according to USGS geophysicist Tom Parsons. Of that tiny percentage, any quakes that might have been triggered by others have not caused serious damage.

On top of that, seismic waves don’t provide great clues. "There don’t appear to be any obvious characteristics about the seismic waves that trigger other earthquakes in terms of amplitude or frequency," said Parsons.

For all the uncertainties of quake science, and false hunches, the scrambling for answers can lead to new avenues for research.

Stein, for example, told peers in his email that he had found something "very unusual" about the Iquique quake: The area near the epicenter saw an unusually high number of "foreshocks" to the mainshock on April 1.

"Maybe only 5 percent of quakes have what, in retrospect, we would call foreshocks," he says.

It turns out that the Iquique foreshocks covered an area almost as large as the area of the Iquique aftershocks. That, Stein adds, is "extremely rare and worthy of study."