Scientists Discover Stormquakes: Earthquakes Caused By Storms Seismologist Wenyuan Fan explains the accidental discovery — buried deep in seismic and meteorological data — that certain storms over ocean water can cause measurable seismic activity, or 'stormquakes.' He says this phenomenon could help scientists better understand the earth below the sea.

The original paper Wenyuan co-authored on stormquakes is here.

Follow host Maddie Sofia on Twitter @maddie_sofia. Email the show at shortwave@npr.org.
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Discovering 'Stormquakes'

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Discovering 'Stormquakes'

Discovering 'Stormquakes'

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(SOUNDBITE OF MUSIC)

MADDIE SOFIA, HOST:

You're listening to SHORT WAVE from NPR.

If you were in Seattle, Calgary or Juneau, you might have felt it. October 28, 2012 - a 7.8 magnitude earthquake struck off the coast of Canada, near the archipelago of Haida Gwaii. Earthquakes aren't unusual in that area because there's a big fault line nearby. But it turned out buried in the data associated with this earthquake was something unusual.

WENYUAN FAN: We were not particularly looking for something, but we were trying to look for earthquake evolution processes.

SOFIA: Wenyuan Fan is a professor at Florida State University.

FAN: I am a seismologist, or you can say a geophysicist - you know, a more general term.

SOFIA: And so a couple of years back, when he was looking at the data from the 2012 earthquake off the coast of Canada, he noticed that before that big earthquake struck, there were records of what looked like smaller earthquakes nearby.

FAN: Just migrating from north to south. And by noticing that, I thought I found a precursor to the earthquake. And that was quite exciting to me.

SOFIA: And that would have been pretty cool on its own - a previously unknown precursor to a big 7.8 earthquake. But just to make sure it was a real thing, Wenyuan looked at some data from two years before and the year after.

FAN: So not only 2012, but 2010, 2013. And what we started to find is that such activities would happen every year, but only happened during wintertimes.

SOFIA: So those smaller earthquakes in the ocean weren't a precursor to the big earthquake after all because they happened before it and after it, too. Now, you don't have to be a seismologist to know that earthquakes don't exactly know when it's wintertime.

FAN: That's right. That's right.

SOFIA: Right. Yeah. Earthquakes aren't seasonal, right? They kind of just happen when they happen.

FAN: But the weather is seasonal.

(LAUGHTER)

SOFIA: Sure is.

FAN: So that was a eureka moment in seeing, well, the seasonality of the seismic activity obviously is quite important.

SOFIA: What Wenyuan and some of his colleagues would later confirm is that storms out in the ocean were causing what they say is a seismic phenomenon that they never knew about before.

FAN: Instead of finding earthquakes, we found stormquakes.

SOFIA: Stormquakes - this episode - how they happen and what they could teach us. I'm Maddie Sofia, and this is SHORT WAVE, the daily science podcast from NPR.

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SOFIA: So stormquakes - we're not talking huge quakes; we're talking small quakes beneath the ocean floor, maybe 3.5 or so in magnitude, not something you're necessarily going to feel from land, but definitely a new phenomenon to scientists. And to get why they're important, let's go back when Wenyuan Fan first stumbled across them in the Pacific Northwest. He and his fellow researchers had records of storms synced up with records of seismic activity beneath the ocean floor.

So maybe by now you're thinking what Wenyuan was thinking...

FAN: I do have to say...

SOFIA: ...Waves.

FAN: ...The name of the show is quite compatible with the study.

(LAUGHTER)

SOFIA: That's so true.

As Wenyuan and his colleagues outlined in their paper in the journal Geophysical Research Letters, stormquakes all come down to waves.

FAN: Because when you have large storms, it will couple with the ocean and make high waves.

SOFIA: Gotcha.

FAN: And by doing the cross-examination of the ocean waves and the seismicity, we start to see a clear correlation between the occurrence of stormquakes and also the high-wave conditions.

SOFIA: Gotcha. And what was your reaction when you first kind of realized that correlation? Had anybody, you know, hypothesized about that possibility before?

FAN: I was quite confused, to be honest.

(LAUGHTER)

SOFIA: That's the beginning of science. You know what I mean?

FAN: That is correct.

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SOFIA: So one reason that this was a confusing result is that, like we said, seismic activity is not unusual in the part of the Pacific Northwest that Wenyuan was looking at. There was a chance the data could kind of be contaminated by other seismic activity in the region. But it turned out...

FAN: Storm days correlating with stormquakes is also observed offshore New England.

SOFIA: OK.

FAN: For example, near Cape Cod, where I did my postdoc. So in that region, there are no faults. We don't really have earthquakes there.

SOFIA: Right. OK.

FAN: And during storm days, we see such activities; during calm days, we do not.

SOFIA: But the closer he looked, Wenyuan saw that a stormy day doesn't always equal a quaky day, even if the storm happens to be a powerful hurricane. For example, Hurricane Sandy off the coast of New Jersey, 2012 - no stormquakes. Hurricane Bill in 2009, also a Northeast coast storm, generated hundreds of stormquakes. The difference, Wenyuan says, is that you need a special combination of strong ocean waves interacting with the right type of topography on the seafloor.

FAN: And one thing particularly important that is required to excite stormquakes is a seafloor topography feature called ocean banks.

SOFIA: OK.

FAN: So an ocean bank is like a hill on land. And when such seafloor topography feature is present, there is a continental shelf of interaction between the ocean waves. And the seafloor topography would produce a stormquake.

SOFIA: Gotcha. So, like, OK, this is - this might be hard. But so I'm underwater (laughter) - so I'm underwater.

FAN: Yes.

SOFIA: I'm looking at, like, this ocean bank. Is the idea that the storm kind of, like, pushes the water into that bank and the way that the water interacts with that seafloor pushes and creates - I don't know. Help me out, Wenyuan. I'm...

FAN: No, you're almost there.

(LAUGHTER)

FAN: So what's really happened is actually an interesting unknown process, but our current understanding of the process is when storms are approaching the coastlines, they start to drag the ocean's surface. That's why we have high waves. When these high waves propagate towards the coastline, they start to create a new type of ocean waves, which is called infragravity waves. This kind of infragravity waves would have a much longer wavelength.

SOFIA: Gotcha.

FAN: And because of that, it can touch the seafloor.

SOFIA: Oh.

FAN: Once it's able to touch the seafloor, that's the initiation of interaction between these ocean waves and the seafloor topography.

SOFIA: So, OK, all right. All right, I'm going to try it. So, you know, like, a hurricane moves through, and as it moves across the water, it drags the water on the surface, creating a wave. And those waves are big enough or powerful enough that when they crash back down, they can actually interact with the seafloor, and that is what, you know, like, initiates a stormquake.

FAN: Yes.

SOFIA: Hey. Not bad. I'm getting it.

FAN: Very good.

SOFIA: I'm getting it (laughter).

FAN: Very good.

SOFIA: As you can hear, y'all, physics - not my strong suit.

FAN: So, you know, in the physics world, everything is a wave, right?

SOFIA: OK.

FAN: So the seafloor topography, if you think about that, it is also a type of wave; it's just this wave doesn't move. It has the shape of up and down, but it is a frozen wave.

SOFIA: You're blowing my mind right now.

FAN: Think about that. It makes sense, right?

SOFIA: (Laughter) Yeah.

FAN: So it has ups and downs. It has the shape of a wave. It just does not move back and forth. But the ocean waves can move around. And because of that, the interaction between the seafloor topography and the ocean wave is almost inevitable.

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SOFIA: In the datasets that Wenyuan sampled, they found over 14,000 stormquakes off both U.S. coasts over roughly a nine-year period, beginning in 2006. That's more than a thousand each year, which raises the question - shouldn't somebody have noticed this before?

FAN: You know what? We asked the same question as well.

(LAUGHTER)

FAN: We can't be the first. So...

SOFIA: The answer has something to do with how seismic data is gathered - via thousands of seismic sensors all over the world that are constantly recording huge amounts of data.

FAN: In reality, only about 5% of those records are about earthquakes, and the remaining ones are recording everything around us - a car driving by, the ocean and someone doing a dance lesson. So without knowing where to look for or what to look for, it is actually kind of hard to harness the power of the seismic data to extract truthful information.

SOFIA: Gotcha.

FAN: It is only because we designed a new method that deals with continuous data to process them in large scales, that is how - eventually, led us to to the finding of stormquakes.

SOFIA: I see. So it's almost like there was too much data out there to be able to find a pattern like this without, you know, some, like, big computational abilities.

FAN: Yes. Although, to a seismologist, there is never such a thing of too much data.

SOFIA: (Laughter) You know what. I get it. I get it.

(LAUGHTER)

SOFIA: Wenyuan and his colleagues still have a lot of questions about stormquakes, like how exactly the water interacts with the seafloor to produce them. But simply knowing that stormquakes exist could provide an entirely new source of information about the Earth below the seafloor.

FAN: Traditionally, when we image the Earth structures, we need a signal that is clean, that is simple and that is recognizable, right? So these stormquakes are like flashlights traveling through the Earth and get to be picked up by seismometers. And now by using these stormquakes like lights, illuminating the subsurface, we will be able to know a little bit more about the subsurface structure and, potentially, the long-term tectonics.

SOFIA: So using information that we can get from stormquakes, that will help us kind of understand what the Earth looks like even below the ocean floor.

FAN: Yes.

SOFIA: Awesome. That's cool. I like that - shining a little light all the way down to that crust.

FAN: Yes.

SOFIA: What are you doing down there? What do you look like?

FAN: Yes.

SOFIA: (Laughter) Do you talk to the Earth like that all the time when you're...

FAN: Yes (laughter).

SOFIA: Because I think if you haven't, you should start trying it, you know.

FAN: That's right. That's right. Hopefully, it will give me some answers.

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SOFIA: Wenyuan Fan at Florida State University. We've got a link to his paper on stormquakes in the episode notes.

This episode was produced by Brent Baughman, fact-checked by Emily Vaughn and edited by Viet Le, who is celebrating a birthday today. Happy birthday, Viet. We are so glad you were born. I'm Maddie Sofia, and we're back with more SHORT WAVE from NPR tomorrow.

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