When Superstorm Sandy struck the United States on 30 October, it didn't just devastate the Eastern Seaboard, it shook the ground as far away as the West Coast, producing tiny vibrations in Earth's crust that were picked up by seismometers there. Scientists can use this activity to track the path of the storm. Now, they say that analyzing past records of these vibrations may help them discern whether climate change has influenced the amount of storminess over the world's oceans in recent decades.
Hurricane Sandy swept northward a few hundred kilometers off the East Coast, hooked left, and then slammed into the shore just northeast of Atlantic City, New Jersey. Long before the storm struck land, however, minuscule vibrations triggered in Earth's crust could be picked up on instruments onshore, says Oner Sufri, a seismologist at the University of Utah in Salt Lake City. While some of the motions were produced by surf pounding beaches, a larger fraction came from large storm waves far offshore that smashed into each other.
The most intense ground motions were triggered as Sandy swerved toward shore—which also happened to be toward the nationwide network of seismometers shaken by the storm, Sufri reported today at the annual meeting of the Seismological Society of America in Salt Lake City. For the new study, Sufri and university colleague Keith Koper analyzed data recorded across the continental United States between 18 October and 4 November, an interval that also included at least two significant earthquakes.
Storm-induced seismic vibes aren't a newly recognized phenomenon. In 2005, ground motions triggered by Hurricane Katrina were picked up by seismometers in California. And even storms that remain far from land can trigger ground motions, Sufri and Koper note.
Because the strongest ground motions are typically created at or near a storm, researchers can track its progress using seismic data alone. That offers opportunities for scientists to delve through old data sets—especially those from the presatellite era—to look for signs of storms that might have been missed by earthbound observers, or to better estimate their paths and intensities, Sufri says.
The ground motions generated by strong storms over the sea typically cycle at low frequency, outside the range of most vibrations produced by earthquakes, says Daniel McNamara, a seismologist with the U.S. Geological Survey in Golden, Colorado. "Previously, such vibrations were thought of as seismic noise and they were filtered out, but now they're recognized as useful," he notes.
For one thing, the waves can be used to make images of large structures within Earth's crust and the underlying mantle—using the same sort of mathematical techniques incorporated into the machines that make CT scans of the human body. (Consider the seismic waves bouncing through Earth in all directions as analogous to x-rays, and seismometers as individual sensors in the CT-scan machine.) Because these waves have a tremendously long wavelength, only large structures can be mapped: those measuring tens of kilometers across or larger, says Richard Aster, a seismologist at the New Mexico Institute of Mining and Technology in Socorro. But that's precisely the size range of slabs of tectonic plates that have sunk toward Earth's core at deep-sea trenches, he notes. Mantle plumes, the slowly rising blobs of hot material within Earth's mantle thought to feed major spates of volcanic activity across broad regions at the surface, also fall within that size range.
By poring through seismic data gathered in recent decades, researchers might gain insight into whether and how changing climate has influenced long-term trends in the storm-induced waviness of Earth's oceans. "This is just another piece of the puzzle that can help researchers resolve storminess trends in the global oceans," Aster says.
Although it's now possible to track major storms at sea using images from space, seismic data could help fill in any gaps created if a weather-gazing satellite fails before its replacement is traveling in the same orbit, Sufri says.