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Sandy Shook the Earth, Helping Scientists Study Crust and Upper Mantle

Before Superstorm Sandy struck New Jersey and New York, the storm made a turn to the northwest toward land, rattling the ocean floor and setting off seismometers across the U.S., according to University of Utah researchers.

Sandy was not the only storm to do this, the researchers say. Keith Koper, director of the University of Utah Seismograph Stations, says “microseisms,” or relatively small seismic waves, were detected before and after Sandy from North Pacific and North Atlantic storms that never hit land. 

And Hurricane Katrina in 2005, Koper adds, “was recorded by a seismic array in California, and they could track the path of the storm remotely using seismometers.”

With Sandy, the readings grew stronger when the storm turned to the northwest due to waves interacting with each other, generating pressure at the seafloor. Koper explains, “When Sandy made that turn to the northwest, although wind speeds didn’t get dramatically bigger, the seismic energy that was created got tremendously bigger because the ocean’s standing waves were larger from the wave-wave interaction.”

He notes that normal ocean waves “decay with depth very quickly,” but when Sandy made its turn, the sudden increase in waves hitting waves created “standing waves,” and Koper says, “Pressure generated by standing waves remains significant at the seafloor.”

The data gathered has practical purposes. “The energy generated by Sandy is going to be used to image the crust and upper mantle under North America,” says Koper. “We are using seismic waves created by ocean waves to make images of the continent.”

It is all part of a project called Earthscope, which the University of Utah describes in a statement as “a National Science Foundation-funded array of about 500 portable seismometers” located throughout the country. 

The idea is to use seismic waves from quakes and other sources to make images of Earth’s crust and upper mantle beneath North America – “similar to how X-rays are used to make CT scans of the human body,” the university says. 

Koper points out that the seismic waves created by Sandy and other storms are not earthquakes. “Seismic waves can be created by a range of causes. We have beautiful seismic records of the meteor that hit Russia. That’s not an earthquake, but it created ground motion.”

This technology could also help scientists observe storms in ways satellites can miss. Seismologists did not track Sandy in real time by monitoring the seismic waves the storm created, but Koper says the seismographic data of the storm suggests it might be possible to help track storms in the future using their seismicity.

Oner Sufri, a University of Utah geology and geophysics doctoral student, says seismic tracking of storms might one day help researchers “understand how climate is changing and how it is affecting our oceans – are we seeing more intense storms and increasing numbers of storms?”

Koper says the Sandy study “is exploratory science where we are trying to learn fundamental things about how the atmosphere, oceans and solid Earth interact.”

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