Below the rumble of passing cars, chirping birds, and rustling leaves, the Earth is constantly humming. This geologic symphony is driven by the ever-sloshing oceans that blanket nearly three-quarters of our planet, but tracing individual refrains from this watery orchestra has long posed a challenge.
Now, researchers have done just that, picking out a previously unknown seismic phenomenon that they have dubbed stormquakes. These events, described this week in Geophysical Research Letters, are pulses of seismic waves birthed from the ferocious energy in massive storms, and they can radiate thousands of miles across continents. (Learn about a different kind of strange seismic wave that rippled around the world.)
“I was surprised by what they saw,” says Göran Ekström, a seismologist at Columbia University who specializes in unusual earthquakes. Big storms are thought to produce a lengthy jumble of rumbles that radiate from coastlines. But in the new study, the team identified a discrete “burst of wiggles” from each stormquake that they can trace back to its origin off shore.
The find joins a number of recent studies that are applying new methods to sort through the noise recorded on the world’s growing network of seismometers. These signals can help scientists better understand the world around us, from deciphering our planet’s inner structure to tracking ocean or ice dynamics and even monitoring climate change. (Read about how a groundbreaking earthquake catalog may have solved a seismic mystery.)
A lot of this information has previously been discarded as noise in our seismic readings, but scientists are now seeing how that “noise” may be providing useful records of environmental happenings, says study leader Wenyuan Fan, a seismologist at Florida State University.
“We just didn’t know where to look and what to look for,” he says.
The seismic search
Similar to many scientific advances, stormquakes were discovered by accident. In summer 2018, Fan and his colleagues were developing a method to study what are known as very low frequency earthquakes. These are not the sudden, intense jolts we usually think of when a temblor unzips our planet’s surface. Instead these tremors shake the surface in a low-frequency side-to-side warble at intensities below what humans can detect without instrumentation. Geologists can identify these events by the seismic waves they generate, picked up on sensitive instruments known as seismometers.
“Seismometers are basically like little ears pressed to the ground,” explains Wendy Bohon, an earthquake geologist at the Incorporated Research Institutions for Seismology who was not part of the study team. The devices can pick up all sorts of vibrations, from boisterous sports fans jumping up and down and airplanes passing overhead to distant earthquakes rattling the ground.
Very low frequency earthquakes, however, are tough to trace over vast distances, since the wiggles from these events don’t always look the same from one seismometer to the next, Fan says. So he and his team devised a method to track them, piecing together the signals from smaller regions like a seismic puzzle. But during this process, an unusual set of events emerged that looked similar to, but not exactly like, the earthquakes Fan was chasing.
Surprisingly, the events were seasonal, never occurring between May and August. Earthquakes that release energy from Earth’s shifting crust, however, are usually indifferent to the changing seasons. What’s more, the curious quakes radiated from both the east and west coasts of North America. Earthquakes are common out west, rumbling as the earth shifts along a spidery network of fractures in the surface, but the eastern coast largely lacks these quake-generating features.
Baffled, Fan and his team turned to models to suss out what was going on—and that was when they realized the connection: Many of these tremors coincided with massive storms or even full-fledged hurricanes. By digging through data largely from EarthScope’s USArray, a series of hundreds of seismometers temporarily placed across the country, the team unearthed 14,077 of these stormquake events between 2006 and 2015. (Learn more about Earth’s hum.)
The making of a stormquake
Not every big storm, however fearsome, can produce a stormquake. For example, the signals were conspicuously absent during Hurricane Sandy, even with its wind gusts topping 90 miles an hour in some locales. Instead, it seems a particular underlying geology is required to spawn a stormquake.
For one, the quakes came from regions with a broad continental shelf, an underwater section of a continent just off the coast that is relatively shallow. This likely allows the waves from stormquakes time to build up, Fan explains. Most wind-borne ocean waves generate signals at a frequency higher than the 20- to 50-second cycle of a stormquake. But a broad shelf gives the waves time to interact with each other, perhaps stretching into a longer, lower frequency wave.
Tempestuous temblors also seem to turn up only around ocean banks. These flattened underwater hills can focus the energy so that the pressure from the waves transfers to the ground to create the uniform burst of wiggles—similar to a hammer striking in the ocean, Fan says.
However, more work is needed to precisely untangle the mechanism behind these discrete pulses of energy, says Ekström, who was one of the reviewers for the study.
Looking beyond the wiggles
Fan and his colleagues are hoping to continue the hunt for the mechanism behind the curious temblors. And now that the study is out in the world, Bohon is excited to see what scientists from other fields can add to the find.
“This paper is laying the foundation for building up new information about how the world works,” she says.
Fan and his colleagues hope that stormquakes could help future researchers better understand ocean dynamics or even earth structure. Scientists can already use more traditional temblors like a planetary x-ray, tracking the seismic waves to visualize Earth’s innards. (Learn how big earthquakes helped scientists find rugged ‘mountains’ deep underground.)
Low-frequency waves, like those found in stormquakes, won’t give the most clear signal from inside the planet, notes geophysicist Jonathan Berger of Scripps Institution of Oceanography. But the events might help fill some gaps in the record for areas like New England, where earthquakes are scarce.
And there may be other applications yet to be imagined: “Scientists are inherently creative people,” Bohon says. “Who knows what some inspired young students are going to be able to use these for.”