Powerful hurricanes can whip the ocean into a frenzy. That wave energy hammers the seafloor. Sometimes it can be strong enough to produce a novel kind of quake, new research reveals.
Wenyuan Fan led the new research. He’s a seismologist at Florida State University in Tallahassee. His team described these stormquakes online October 14 in Geophysical Research Letters. These tremors are a newly identified type of interaction between Earth’s atmosphere, ocean and crust.
Stormquakes differ from earthquakes. Earthquakes are triggered by subsurface shifting within the solid Earth. The driving force behind stormquakes are ocean waves that have been whipped into deep swells by a hurricane or nor’easter. Stormquakes can be as powerful as a magnitude 3.5 earthquake, the new report says. That’s a level barely noticeable to people but detectable by seismometers.
The new analysis is “a really great first start” at understanding a little-studied part of the seismic record, says Fabrice Ardhuin. “It brings something really new,” he says. Ardhuin is a physical oceanographer in Brest, France. He works at the Ocean Physics and Satellite Oceanography laboratory.
Scientists have long known that the constant sloshing of ocean waves produces seismic waves at frequencies of about once every few minutes. This phenomenon has come to be known as “Earth’s hum.” Waves can also produce high-frequency signals called microseisms. They tend to occur about once every five seconds.
In between those types of seismic noise is another band of signals. It’s generated in the ocean and occurs once every 20 to 50 seconds or so. That’s a frequency of 0.02 and 0.05 hertz. What produces seismic signals within that band hasn’t been so well understood.
Fan and his colleagues initially looked for possible triggers for the signals they saw coming from within the Earth. They analyzed such seismic waves picked up between 2006 and 2015 by the USArray. This was a network of moveable seismometers. They marched across the country from west to east.
The team started by focusing on data from the Pacific Northwest. Fan became excited when he found what he thought were previously undetected offshore earthquakes. They occurred in a mysterious seismic band.
Then Fan noticed something weird. These data “were seasonal,” he says. They showed up only during winter months. “Earthquakes do not have seasonality,” he notes. “But weather does.” The driving force behind the mysterious quakes came into clearer focus once Fan began looking at seismic data from the U.S. East Coast. That area is prone to powerful storms such as hurricanes and winter nor’easters.
Recipe for a stormquake
To be a stormquake, the team determined, the source of the seismic data had to meet several criteria. It had to occur during a stormy day. It could not be part of a known earthquake event. And it had to belong to a swarm of similar quakes on the same day. In all, the team identified more than 14,000 stormquakes for its study. Those quakes occurred from September 2006 to February 2015 along the eastern coasts of Canada and the United States, as well as the U.S. Gulf Coast.
Oddly, not every powerful storm that pounded along that eastern seaboard produced stormquakes. Such quakes seemed confined to certain regions of the coast. These were places where maps of the seafloor’s structure showed small raised areas known as ocean banks.
For example, 2012’s Hurricane Sandy roared toward land from off the coast of New Jersey. But off New Jersey, Fan notes, the seafloor descends in a gentle slope along the continental shelf toward the deeper ocean. The conditions weren’t right to produce a stormquake. And they didn’t.
But other storms triggered many stormquakes. These included 2009’s Hurricane Bill. That storm moved farther north than Sandy. It ultimately made landfall in Newfoundland, Canada as a tropical storm. This indicates that seafloor topography plays a role in generating the quakes, Fan says. Deep swells form as storm-swept ocean waves interact and transfer energy. Those swells, the team suggests, may in turn interact with these elevated parts of the seafloor. They essentially pound at them like a hammer.
Potential uses for stormquake data are still coming into focus, Fan says. “It’s still very new. We didn’t know such things exist in nature,” he says. But, he adds, these seismic waves form in parts of the planet that are, tectonically, relatively inactive. Scientists use tectonic activity to “see” into the planet. So stormquakes may be able to illuminate structures in the continent that had previously been seismically invisible.
In addition to the U.S. East Coast, Fan notes, other parts of the world have the right kind of seafloor structure and storm activity to produce stormquakes. These include the west coasts of Europe and India.
It’s interesting that the researchers were able to track the sources of some of the seismic signals back in time to the storms, says Lucia Gualtieri. She’s a geophysicist at Stanford University in California. But, she points out, big moving storms impact a large portion of the seafloor. So it’s hard to see how those impacts might be considered to be point sources like the hypocenter of an earthquake. (The hypocenter is the location in the subsurface where a rupture takes place.) “More work is likely needed to precisely understand the mechanisms behind these seismic records,” she says.
The “source” of a seismic signal can vary in size. It might be a tiny slip of a fault line or an ocean swell hammering at an ocean bank hundreds of kilometers across. Ardhuin agrees that the size of the source of the signals remains an open question. But it is possible that the shape of the seafloor in these locations might be creating an effective point source.
In other words, a special combo of hurricane, ocean waves and seafloor shape may be required to produce a perfect stormquake.