Superstorm Sandy Shook the U.S.,
Literally
 |
| This map shows colored dots to
represent the locations of portable seismometers in the Earthscope array. 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. 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.) |
When superstorm Sandy
turned and took aim at New York City and Long Island last October, ocean waves
hitting each other and the shore rattled the seafloor and much of the United
States -- shaking detected by seismometers across the country, University of
Utah researchers found.
"We detected
seismic waves created by the oceans waves both hitting the East Coast and
smashing into each other," with the most intense seismic activity recorded
when Sandy turned toward Long Island, New York and New Jersey, says Keith
Koper, director of the University of Utah Seismograph Stations.
"We were able to
track the hurricane by looking at the 'microseisms' [relatively small seismic
waves] generated by Sandy," says Oner Sufri, a University of Utah geology
and geophysics doctoral student and first author of the study with Koper.
"As the storm turned west-northwest, the seismometers lit up."
Sufri was scheduled to present the preliminary, unpublished findings in Salt Lake City Thursday, April 18 during the Seismological Society of America's annual meeting.
There is no magnitude
scale for the microseisms generated by Sandy, but Koper says they range from
roughly 2 to 3 on a quake magnitude scale. The conversion is difficult because
earthquakes pack a quick punch, while storms unleash their energy for many
hours.
The shaking was caused
partly by waves hitting the East Coast, but much more by waves colliding with
other waves in the ocean, setting up "standing waves" that reach the
seafloor and transmit energy to it, Sufri and Koper say.
While many people may
not realize it, earthquakes are not the only events that generate seismic
waves. So do mining and mine collapses; storm winds, waves and tornadoes;
traffic, construction and other urban activities; and meteors hitting Earth.
"They are not
earthquakes; they are seismic waves," says Koper, a seismologist and
associate professor of geology and geophysics. "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."
While Sandy's seismicity
may be news to many, Koper says microseisms just as strong were detected before
and after the superstorm from North Pacific and North Atlantic storms that
never hit land but created "serious ocean wave action."
Koper adds:
"Hurricane Katrina in 2005 was recorded by a seismic array in California,
and they could track the path of the storm remotely using seismometers."
In a related study set
for presentation on Friday at the seismology meeting, Koper and geophysics
undergraduate student YeouHui Wong found preliminary evidence that seismometers
near Utah's Great Salt Lake are picking up seismic waves generated either by
waves or winds on the lake.
Koper says researchers
wonder if microseisms from storms and other causes might trigger tiny but real
earthquakes, but "that hasn't been investigated yet," he says.
Earthscope Picks up Seismic Waves from Ocean Wave Collisions
The microseisms
generated by Sandy were detected by Earthscope, a National Science
Foundation-funded array of about 500 portable seismometers that were first
placed in California in 2004 and have been leapfrogging eastward so that most
now are located east of line running from Minnesota to east Texas, and west of
a line from Lake Erie to Florida. Some remain scattered across the Midwest and
West, with a heavier concentration in the Pacific Northwest.
Earthscope's purpose 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. To do it accurately, scientists must
understand all sources of seismic waves.
Sufri says the new study
included Earthscope data from Oct. 18 to Nov. 3, 2012, "which coincides
with the passage of Hurricane Sandy, and we tried to understand microseisms
that were generated."
Sandy caused a damaging
storm surge due to its size -- almost 1,100 miles in diameter for
tropical-storm-force winds -- more than its intensity, which was 3 when it hit
Cuba and 2 off the Northeast coast.
"The energy
generated by Sandy is going to be used to image the crust and upper mantle
under North America," says Koper, noting that Earthscope uses years of
seismic data to construct images. "We are using seismic waves created by
ocean waves to make images of the continent."
Normal ocean waves
"decay with depth very quickly," says Koper. But when Sandy turned,
there was a sudden increase in waves hitting waves to create "standing
waves" like those created when you throw two pebbles in a pond and the
ripples intersect. "Pressure generated by standing waves remains
significant at the seafloor," he says.
"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
adds.
Not only did the seismic
waves become 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," Koper says.
Seismic Tracking of Hurricanes
Seismologists can track
Sandy and other big storms because seismometers detect three components of
motion: one vertical and two horizontal. If most of the energy on a seismometer
is detected with a north-south motion, it means the source of the energy is
north or south of the device.
"If you have enough
seismometers, you can get enough data to get arrows to point at the
source," Koper says.
He says the seismologists
didn't track Sandy in real time, but the seismographic data of the storm
suggests it might be possible to help track storms in the future using their
seismicity.
Sufri speculates that
seismic tracking of storms might allow observations that satellites can miss,
and perhaps could 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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University of Utah
(2013, April 18). Superstorm Sandy shook the U.S., literally. ScienceDaily.
Retrieved April 21, 2013, from http://www.sciencedaily.com/releases/2013/04/130418213919.htm
