Hurricane Andrew hit Category 5 as it approached Cuba and Florida in August 1992.
Credit: NASA
PORTLAND, OREGON: Hurricanes can be detected on seismometers, says an expert, who claims that we may be able to extend the historic North Atlantic hurricane record using seismic noise records.
When people think about a seismic record, they think about an earthquake record, but there is also background vibrational "noise" that's always there, said Carl Ebeling, a geophysicist at Northwestern University. "It doesn't matter where you're standing on the globe; you can see it everywhere. It's very pervasive."
"We've always been interested in the earthquakes, but there's a lot that can be learned just by listening to the noise," said Ebeling, who suggested the technique last week at the Geological Society of America's annual meeting held in Portland, Oregon.
Filling gaps in the data
By analysing this noise from digital seismic monitoring stations, Ebeling has been looking for hurricanes that were missed using other methods or took place before detailed records began in the 1940s. He has found patterns that corresponded with well-known hurricane periods.
Preliminary results from studying seismic data taken during Hurricane Andrew in 1992, indicated spikes in microseismic power - faint tremors caused by natural phenomena such as waves - which corresponded with the low barometric pressure and high wind speeds associated with hurricanes.
"Seismologists and climatologists tend to do their own things, but combining these methods seems to work," said Ebeling. "The results of this research have the potential for impacts reaching far beyond the seismological community."
Wave action
When most people think of seismometers, they think of the analogue versions with their old-fashioned needles, bobbing up and down on a paper roll recorder, he said.
But new digital seismometers use electronic sensors, amplifiers, and recording devices so that information can automatically be uploaded to a computer. "A number of North Atlantic hurricanes have occurred since the digital recording era began, allowing a depth of analysis not possible with analogue records" said Ebeling.
What researchers are seeing is atmospheric energy coupled into the water column and, ultimately, the ocean floor to produce waves which show up on seismic records.
"The pitches are comparable to frequencies in music," said Ebeling. "The more energetic the storm, the higher the microseismic amplitude will be. There's also a shift into the longer periods. Think in terms of base versus soprano - the more energetic the storm, the lower the frequency."
He hopes the findings can be applied to older analogue records so that a more complete hurricane history can be reconstructed. "Old [storm] records were based entirely on ship logs and land observations before aircraft reconnaissance began in the mid-1940s," he says. "It's possible that hurricanes may have gone undetected before then."
Independent verification of data
Ebeling noted that the research is still very preliminary and began only in June: "The observation record's too short and the data set isn't complete." But, he said, the correlations are still exciting.
Phil Klotzbach, an atmospheric scientist at Colorado State University, in Fort Collins, applauded the effort to obtain a longer, more detailed hurricane record. Atlantic hurricane data extends as far back as 1851, he said, "[but] there is increasing uncertainty as one goes back in time with the quality of this dataset. This seismological dataset may result in a useful independent verification."
He said he was dubious, however, that seismic data can be well enough correlated to storm strength to add much to our knowledge of climate trends. "Most modeling studies argue that if global warming does make storms stronger, it's only going to be 2 to 4 mph [3 to 6 km/h] over the next 100 years," he said. "Any signal will probably be buried in the noise of the observations."
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