The topography of the sea floor around Florida takes a pounding during a hurricane, and the Earth feels the jolt.

The hard crust of Earth’s surface doesn’t know the difference between winter or summer, or spring or fall, so Florida State University earthquake researchers were surprised to find a seasonal rhythm in seismic tremors.


In areas offshore of Florida and in the Gulf of Mexico, deep shudders equivalent to a 3.5 magnitude earthquake were found to coincide with the ultimate in low pressure systems — hurricanes.


The pummeling of the sea floor repeatedly by tropical cyclone-forced waves produces an intense seismic source activity that scientists are calling "stormquakes."


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Wenyan Fan, an FSU assistant professor of Earth, Ocean and Atmospheric Science, said he was looking for something else when he stumbled on an annual cycle of seismic signals that he was able to tie to hurricane season, and even individual storms.


His peer-reviewed study was published this month in the journal Geophysical Research Letters.


"It was just an accident," Fan said about the discovery. "There was a very strong correlation that whenever extreme storms are present and they are traveling through certain regions, the stormquakes would light up."


Nor’easters have also been tied to the seismic jostles, which were measured near New England, Florida and in the Gulf of Mexico, as well as Nova Scotia, Newfoundland and British Columbia in Canada.


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The magnitude quake is nothing people would feel, but is a reaction to ocean energy being transferred to the Earth as waves interact with sea-floor topography. Florida has shallow hills offshore called banks that would take a repeated pounding by hurricane-whipped seas. The continental shelf can also be a producer of seismic activity.


Hurricane Ike in September 2008 caused intense seismic activity in the Gulf of Mexico as it became a Category 2 storm before making landfall near Galveston, Texas, Fan said. Hurricane Irene skirted along the Bahamas in 2011 creating stormquakes near Little Bahama Bank off Grand Bahama Island and Great Abaco.


Fan said most of the seismic stations he looked at for his study have since been moved to Alaska, so there was no data on September’s Category 5 Hurricane Dorian.


Still, more than 10,000 stormquakes were identified from 2006 to 2015 in the study with no correlation between the size of the storm and the quakes measured.


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"We don’t understand much about how the energy transfers," said Jeff McGuire, a U.S. Geological Survey researcher at the Earthquake Science Center in Moffett Field, Calif. "There’s pressure on the sea floor from the wave field all the time and that’s particularly true in storms. What’s new is the ability to track individual episodes that generate large seismic activity."


Fan is not the only researcher looking at a tie between hurricanes and earthquakes.


Florida International University professor Shimon Wdowinski has been studying whether earthquakes follow hurricanes that cause landslides in areas where there is a fault line.


His theory is a landslide takes pressure off the top of a mountain, reducing the weight along a fault that can then trigger an earthquake. It’s difficult to prove because it could be years between the hurricane and earthquake.


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A study published over the summer in Nature Communications linked tremors in some regions to lower tides. Researchers found when there is less pressure from water sitting on pockets of magma, the magma expands, putting pressure on fault lines.


"It’s interesting how situations on the surface affect the Earth’s interior," Wdowinski said. "We like to find answers that explain unusual phenomenon because we are curious creatures."


The significance of the stormquake discovery and how it can be used to further seismic study is yet to be determined.


"We have lots of unknowns," Fan said.


Kmiller@pbpost.com


@Kmillerweather