Forecasting the future with hurricanes' seismic fingerprints
Saturday, February 24, 2018, 3:56 PM - It's not a new question, but in the wake of last year's record-breaking hurricane season, it may be one we're thinking about more often - does climate change mean hurricanes are getting stronger?
Unfortunately, there's no straight answer here as far as climatology is concerned. While there are reasons to believe that a warming ocean and atmosphere will lead to hurricanes with more intense winds and behaviors that are more difficult to forecast, it's hard to give any definitive answer given that the hurricane record is full of gaps in data. Before satellites, we didn't have a reliable means of surveying hurricanes and typhoons that were out at sea, and 'hurricane hunter' flights only began around 1950. But there is another way to track these strong, massive storms, and it comes from an unlikely source - Earth's seismic record.
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If you hear 'seismic' and think 'isn't that for earthquakes?', you're not wrong. Seismometers record the movement of the ground, which is why they pick up earthquakes. Since there's no way (yet) to forecast when an earthquake will occur, they run all the time. Because of this, they pick up other vibrations in the ground, too, from trains, to sonic booms, to - as it turns out - typhoons and hurricanes.
A seismograph at work. In addition to capturing earthquakes, the machines capture background 'noise' from sources such as industrial activity, trains, sonic booms, and hurricanes. Image: Getty Images
In a study published in Earth and Planetary Science Letters, a group of researchers has presented the results of looking back at how 13 years of seismic data recorded the passage of tropical cyclones by the way they shake the seafloor. "Typhoons show up very well in the record," Dr. Lucia Gualtieri - one of the researchers involved in assembling the group - told Phys.org. The recordings once dismissed as 'noise' by geophysicists, are in fact the effect of ocean storms all over the planet, as waves hit the beach, or collide with one another over the open ocean.
"The main mechanism to generate seismic abnormalities from a typhoon is to have two ocean waves interacting with each other," Gualtieri says. Generally speaking, garden-variety waves themselves aren't strong enough to shake the seafloor, but large waves colliding with one another can be enough to disrupt the 'column' of ocean below them all the way to the seafloor, where their passing can be picked up by the local seismic network.
Click play below to watch: 'Listening' to 12 years of hurricanes (sound on)
This technique has actually been used in the past to track specific large storms, but Dr. Gualtieri and her colleagues decided to reverse the question: can you detect past storms we might have missed before satellites using seismographic data?
Turns out, it looks like you can.
The research group looked at 13 years of data from the northwest Pacific Ocean, using data from seismometers in eastern Asia and on Pacific islands. They used the first 11 years of data to construct a computer algorithm that could recognize a tropical cyclone in the seismograph and estimate its intensity. For the final two years, they let the algorithm work out the intensity on its own, and the results were encouraging. The computer's estimates matched up well with recorded measurements from satellites.
The team's next step is to look at other storm basins, including the Caribbean.
"If all this data can be made available, we could have records going back more than a century, and then we could try to see any trend of change in intensity of tropical cyclones over a century or more," Salvatore Pascale, another researcher on the team, told Phys.org. "Models and theories suggest that [tropical storms] should become more intense [due to global warming], but it's important to find observational evidence.'