Scientists Discover ‘Stormquakes’: You have never seen before
New Study shows, strong hurricanes can produce “stormquakes”. Mighty hurricanes can frenzy the ocean – and this wave energy can be strong enough to hit the ocean floor and cause a new kind of earthquake.
These stormquakes, as described online in the Geophysical Research Letters of October 14, are a newly identified type of interaction between the Earth’s atmosphere, the ocean, and the Earth’s crust. Unlike earthquakes caused by subterranean shifts within the solid earth, the driving forces behind these seismic signals are ocean waves hit in deep waves by a hurricane or not by a hurricane. Storm quakes can be as strong as a 3.5 magnitude earthquake, a strength that is barely perceptible to humans but can be perceived with seismometers, report seismologist Wenyuan Fan and colleagues.
The work is “really a good first start” to understand a little-studied part of the seismic record, says the physical oceanographer Fabrice Ardhuin from the Laboratory of Ocean Physics and Satellite Oceanography in Brest, France. “It brings something very new.”
Scientists have long known that the constant waves of ocean waves produce seismic signals at frequencies about every few minutes, a phenomenon known as “ground noise” (SN: 9/29/04). Waves can also produce high-frequency signals, called microseisms, that occur every five seconds.
Between this seismic noise, however, is another signal band that is generated in the ocean and that occurs every 20 to 50 seconds or at a frequency between 0.02 and 0.05 hertz. What seismic signals are generated in this band, is not yet known exactly.
First, fans of Florida State University in Tallahassee and his colleagues were looking for possible triggers for these signals from Earth. They analyzed seismic data collected from 2006 to 2015 from a network of moving seismometers that marched through the country from west to east as part of the US array. The fan initially focused on data from the Pacific Northwest. He was excited when he found out that what he had previously discovered undetected in an offshore earthquake was in this mysterious seismic tale.
Then he noticed something strange about the data.
“They were seasonal,” says Fan; The signals occurred only in the winter months. “Earthquakes have no seasonality. But the weather works. The driving force behind the mysterious earthquakes became more apparent as he started to study seismic data from the US East Coast – sensitive to strong storms such as hurricanes, and neither in winter nor in eastern countries.
To be a stormquake, the seismic data source had to meet several criteria. It had to happen on a stormy day; not part of a known earthquake; and belong to a flock of similar earthquakes on the same day. Overall, from September 2006 to February 2015, the team identified more than 14,000 assault strikes along the east coast of Canada and the United States and the US Gulf Coast.
Strangely enough, not every powerful storm that stormed this eastern coast caused earthquakes. Instead, stormquakes confined themselves to certain regions along the coast where topographical maps of the seafloor indicated that there were small elevated areas known as ocean shores.
Hurricane Sandy, for example, caused no stormquakes in 2012 as it roared off the coast off New Jersey. According to Fan, the seabed descends gently from New Jersey along the continental shelf into a deeper ocean.
But storms such as the 2009 Hurricane Bill, which moved further north and eventually landed as a tropical storm in Newfoundland, caused deep stormquakes. This shows that the topography of the seafloor also plays a role in the generation of the earthquakes, says Fan. The team suggests that deep waves that occur when stormy ocean waves interact and transmit energy can in turn communicate with these elevated parts of the seafloor and basically hit like a hammer.
According to Fan, the potential use of storm-to-earth data is ongoing. “It’s still very new, we did not know that such things exist in nature.” However, these seismic data come from parts of the planet that are relatively tectonically inactive and therefore seismic have been invisible, which means that stormquakes might be able to “illuminate turbulent structures on the continent that have never been lit before.”
Besides the US East Coast, Fan says, other parts of the world have the right kind of seabed topography and storm activity to trigger stormquakes, including the west coast of Europe and India.
It is interesting that researchers were able to track the sources of some of the seismic signals from time to time, says geophysicist Lucia Gualtieri of Stanford University. But she says large, moving storms hit a large part of the seabed. It is therefore difficult to see how these consequences can be considered as point sources, such as the hypocenter of an earthquake – the location in the subsurface where a fracture occurs. “More work is likely to be needed to fully understand the mechanisms behind this seismic data,” she says.
The size of the “source” of a seismic signal – whether it is a small slip-up of a fault line or an ocean swell hammering hundreds of miles over an ocean bank – is important when it comes to using such signals, a high-resolution 3D Image of the seafloor, a technique called seismic tomography.
Ardhuin agrees that the size of the signal source remains an open question. “Is it still useful for tomographic applications when it’s 100 or 400 kilometers long?” He asks. In this case, however, it is possible that the shape of the seabed at these locations is an effective point source.
In other words, a very special combination of hurricane, ocean waves, and seabed may be needed to create a perfect stormquake.