When storms become stormquakes


Geophysicists link wild weather to seismic waves.


Hurricane Bill, then at Category 4, over the Dominican Republic on 19 August 2009.

Jeff Schmaltz, NASA, MODIS Rapid Response Team, Goddard Space Flight Centre.

By Richard A Lovett

There is more to hurricanes and typhoons than wind and rain, scientists say. Big enough storms can generate offshore earthquakes large enough to rattle windows if they occurred on land.

The phenomenon came to light when geophysicist Catherine de Groot-Hedlin, from Scripps Institution of Oceanography, US, and colleagues noticed odd signals in 2009 data from an array of 400 seismometers deployed across the western interior of North America.

Curious, they traced the signals to their source, and found that the source had been moving northward along the US/Canadian Atlantic seaboard, directly in tandem with one of the year’s strongest storms, Hurricane Bill, which at its peak reached Category 4.

In a study presented on Saturday at a meeting of the Acoustical Society of America, the team looked back over 10 years of seismic data, hunting for similar traces of what de Groot-Hedlin is now labelling “stormquakes”.

Hurricane Bill, they found, wasn’t the only source of such tremors: other hurricanes and powerful winter storms known in New England and Canada as Nor'easters had the same effect, producing seabed temblors measuring as large as magnitude 3.5 on the Richter scale.

But stormquakes didn’t occur for all storms, nor did they occur along the entire storm track of the storms for which they did occur.

Rather, they appeared to be largely confined to parts of the ocean with wide continental shelves (shallow seabed extending outward from the continent), such as occur in New England, Canada’s Georges Bank, and parts of Florida.

The best guess for what’s happening, de Groot-Hedlin says, is that alternating giant waves and deep troughs created by the storm are changing water pressure at the seabed by enough to make it vibrate, with a frequency of about one vibration every 20 to 50 seconds.

“The seismometers are recording that pounding,” she says.

Stormquakes don’t occur in deep water, however, because there the seabed is more isolated from the beating going on at the surface.

“Most hurricanes we studied were strong as they passed north along Georgia to New Jersey [where the seabed is deep] but weakened by the time they hit offshore New England,” de Groot-Hedlin says. “Despite this, stormquakes were only detectable off the coast of the New England region and Georges Bank, when the storms were weaker.”

In theory, stormquakes could be used to help track major storms, but the reality is that there are a lot of simpler ways of doing that.

Instead, stormquakes might better be used as a way of investigating ocean wave dynamics during large storms.

In addition, de Groot-Hedlin says, seismic waves are valuable to geophysics seeking methods of probing the Earth’s interior structure. “Now there’s another source [of seismic waves] that could be useful,” she says.

Contrib ricklovett.jpg?ixlib=rails 2.1
Richard A. Lovett is a Portland, Oregon-based science writer and science fiction author. He is a frequent contributor to COSMOS.
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