April 19, 2013
Superstorm Sandy Made A Seismic Impact, Shaking Up North America
Lawrence LeBlond for redOrbit.com - Your Universe Online
There´s no doubt that last autumn´s superstorm Sandy left a trail of destruction as it churned up the eastern seaboard, making a bulls-eye run at New York City. But a new study from researchers at the University of Utah has found that the storm also shook things up a bit.
"We detected seismic waves created by the oceans waves both hitting the East Coast and smashing into each other," Keith Koper, director of the university´s Seismograph Stations, said in a statement. He noted that the strongest seismic measurements occurred after Sandy took that turn toward the coast.
Oner Sufri, a geology and geophysics doctoral student at Utah and first author of the study, added that the team was “able to track the hurricane by looking at the ℠microseisms´ [relatively small seismic waves] generated by Sandy.” He noted that the seismic activity increased as the storm started its west-northwesterly transition.
The preliminary findings of the study were presented by Sufri at the Seismological Society of America´s annual meeting in Salt Lake City on Thursday (Apr 18).
Koper noted that there´s no magnitude scale for microseisms generated by the storm, but did say they ranged from 2 to 3 on the magnitude scale. He said conversion is difficult at best because earthquakes occur over relatively short periods, while the energy from hurricanes last for hours upon hours.
The team also noted that some of the seismic readings came from waves crashing the coast but the bulk came from waves slamming into each other, creating “standing waves” that penetrate downward through the water column to the seafloor, where the energy is released.
Most people associate seismology with just earthquakes. But Sufri and Koper said there are several factoring events that lead to seismic activity. Besides temblors and powerful ocean storms, seismic waves can be picked up by mining, mine collapses, tornadoes, meteoric impacts, construction activities and even traffic.
So, Koper explained, not all seismic waves are earthquakes. Just this past February, the Russian meteor explosion, as powerful as it was over the Ural Mountains skies, created “beautiful seismic records,” he said. “That's not an earthquake, but it created ground motion.”
As for Sandy, she was not the first storm to produce widespread seismic activity. Koper said that strong microseisms have been recorded from before and after North Pacific and North Atlantic superstorms that never reach land but still create “serious ocean wave action.”
“Hurricane Katrina in 2005 was recorded by a seismic array in California, and they could track the path of the storm remotely using seismometers,” Koper added.
The microseismic detections were picked up by Earthscope, a National Science Foundation-funded array of seismometers that were first placed in California in 2004. But since have been placed across the US with highest concentrations east of an imaginary line running from Minnesota to east Texas and west of Lake Erie to Florida.
The main goal of the Earthscope array, which consists of some 500 seismometers, is to use seismic waves to make detailed images of the Earth´s crust and upper mantle. The process is similar to how X-rays are used to make CT scans.
Sufri noted that the Earthscope data was compiled from October 18 to November 3, 2012, “which coincides with the passage of Hurricane Sandy.”
While Hurricane Sandy dropped in intensity from a Cat 3 in Cuba to a Cat 2 when making landfall in the Northeast, its immense size (nearly 1,100 miles in diameter) was the driver for the damaging storm surge that devastated the east coast of New York and New Jersey.
Koper said that they will be able to use the data they collected from Sandy´s energy to “image the crust and upper mantle under North America.” With earthquakes, it can take years to generate images. With Sandy, the team has been able to do that in a heartbeat, relatively speaking.
Koper explained that while wind speeds didn´t grow stronger when Sandy turned northwest, the “seismic energy that was created got tremendously bigger because the ocean's standing waves were larger from the wave-wave interaction.” Not only did these seismic waves get more energetic, "but the periods got longer so, in a sense, the sound of those seismic waves got deeper — less treble, more bass — as the storm turned," he added.
If anything, the team noted that their seismographic data of Hurricane Sandy suggests that future storms may be able to be tracked through seismology, even though the team had not tracked Sandy in real time.
Such tracking could allow observations of storms that satellites don´t pick up, said Sufri. Perhaps, this technique could also 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?”
Image Below: This map, taken from a University of Utah video, shows colored dots to represent the locations of portable seismometers in the Earthscope array, which is funded by the National Science Foundation. Most are now located in the eastern part of the United States. Blue-green dots indicate low seismic activity, while yellow-orange-red dots indicate stronger seismic activity. The map shows that when superstorm Sandy turned west-northwest toward Long Island, New York City and New Jersey on Oct. 29, 2012, the seismometers “lit up” because of ground shaking by certain ocean waves imparting energy to the seafloor. Credit: Keith Koper, University of Utah Seismograph Stations.