Energetic storms generate strong ocean waves, which can interact with shallow seafloor features located near the edge of continental shelves known as ocean banks; such interactions produce seismic sources — named ‘stormquakes’ — with equivalent earthquake magnitudes that can be greater than 3.5, according to a Florida State University-led study.
Stormquakes offshore North America and seismic stations from 2006 to 2015. The located stormquakes are shown as the red circles, and the seismic stations are shown as triangles. Due to the location uncertainties, some of the apparent stormquake locations are beyond the coastlines. The yellow contours show regions where stormquakes were detected. The orange contour shows a comparison region offshore Mexico that has been struck by hurricanes but without excitation of stormquakes. The insert shows earthquakes (blue dots) reported in the International Seismological Centre catalog with magnitudes greater than 3 and shallower than 40 km from 2006 to 2015. Image credit: Fan et al, doi: 10.1029/2019GL084217.
“We’re calling them stormquakes,” said Florida State University’s Dr. Wenyuan Fan.
“This involves coupling of the atmosphere-ocean and solid Earth. During a storm season, hurricanes or nor’easters transfer energy into the ocean as strong ocean waves, and the waves interact with the solid earth producing intense seismic source activity.”
In the study, Dr. Fan and colleagues analyzed ten years of seismic and oceanographic records.
They found a connection between strong storms and intense seismic activity near the edge of continental shelves or ocean banks.
Specifically, they found evidence of more than 10,000 stormquakes from 2006 to 2019 offshore of New England, Florida and in the Gulf of Mexico in the United States, as well as offshore of Nova Scotia, Newfoundland and British Columbia in Canada.
“We can have seismic sources in the ocean just like earthquakes within the crust. The exciting part is seismic sources caused by hurricanes can last from hours to days,” Dr. Fan said.
The team developed a novel approach to detect and locate seismic events and determine whether the seismic event is a stormquake.
It must occur during a stormy day and meet other geophysical standards to determine the robustness of the correlation between the storm and the seismic event. Additionally, other seismic events such as earthquakes must be ruled out.
According to the researchers, Hurricane Bill is an example of a typical source of stormquakes.
The hurricane originated in the eastern Atlantic on August 15, 2009 and strengthened into a major Category 4 hurricane before ultimately striking Newfoundland as a tropical storm. The storm weakened into a Category 1 hurricane when it approached offshore New England on August 22.
Upon the hurricane’s arrival, numerous seismic sources were located offshore New England and Nova Scotia.
Similarly, Hurricane Ike in 2008 caused stormquake activity in the Gulf of Mexico and Hurricane Irene in 2011 did the same near Little Bahama Bank off Florida’s shore.
However, not all hurricanes generate stormquakes. In fact, stormquakes were repeatedly observed only in a few particular regions around North America.
No stormquakes were detected offshore Mexico or the east coast of the USA from New Jersey to Georgia despite many major hurricanes striking these regions during the study period.
For example, Hurricane Sandy, the most destructive hurricane of the 2012 Atlantic hurricane season, moved ashore in New Jersey, but did not excite any stormquakes along its northward movement. Similarly, Hurricane Bill in 2009 did not excite any stormquakes as it passed by the shelf-break offshore of Maryland and New Jersey.
“This suggests that stormquakes are strongly influenced by the local oceanographic features and seafloor topography,” Dr. Fan said.
“We have lots of unknowns. We weren’t even aware of the existence of the natural phenomenon. It really highlights the richness of the seismic wave field and suggests we are reaching a new level of understanding of seismic waves.”
The study was published in the journal Geophysical Research Letters.
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Wenyuan Fan et al. Stormquakes. Geophysical Research Letters, published online October 14, 2019; doi: 10.1029/2019GL084217
