強烈風暴可以產生猶如地震般的活動
By Kathleen
Haughney
一名佛羅里達州立大學的研究人員發現了新的地球物理現象：颶風或是其他強烈風暴可以在附近的海中產生地震波，強度可以等同於規模3.5的地震。
「我們把這些事件稱為『暴風震』(stormquake)。」研究主要作者，地球、海洋與大氣科學系的助理教授Wenyuan
Fan表示。「這牽涉到大氣―海洋與固體地球之間的交互作用。在颶風季，颶風或是東北大風(nor’easter)會把能量以巨浪的形式傳到海中，這些波浪跟固體地球作用之後會產生強烈的震源活動。」
研究發表在期刊《地球物理研究通訊》(Geophysical
Research Letters)。
Fan和同僚分析了2006年9月至2019年2月這十幾年來的地震波與海洋紀錄，他們發現在大陸棚邊緣或是暗沙附近，強烈的風暴與劇烈的地震波活動之間有所關聯。
確切來說，研究人員發現的證據顯示2006年至2019年總共發生了超過10000起的暴風震，地點為美國新英格蘭、佛羅里達和墨西哥灣的近海地區，以及加拿大新斯科舍省、紐芬蘭省和英屬哥倫比亞的近海地區。
「就跟發生在地殼中的地震一樣，海洋裡面也會有震源出現。」Fan表示，「有趣的是，這種由颶風產生的震源可以持續數個小時到數天之久。」
Fan和同僚發展出一種新穎的方法，可以偵測並且定位地震波事件的發生位置，接著再判定是否為暴風震。這些事件必須發生在風暴襲來的日子並且符合若干地球物理準則，如此才能確立風暴和地震波事件確實有關。此外，也得剔除其他會產生地震波的事件，比方說真正的地震。
研究人員引用的例子之一為比爾颶風。這個颶風於2009年8月15日在大西洋生成，之後強度提升為四級颶風，最後到達紐芬蘭時已經弱化成熱帶風暴。它在2009年8月22日接近新英格蘭時還是一級颶風。
颶風抵達時，新英格蘭和新斯科舍省的近岸地區發生了許多地震波事件，產生了穿越陸地的表面波。
同樣的，2008年的艾克颶風也在墨西哥灣造成了暴風震；2011年的艾琳颶風則在佛羅里達近海的小巴哈馬淺灘周遭造成了類似狀況。
Fan和同僚指出並非所有颶風都會造成暴風震，它們特別容易發生在某些地方。科學家在墨西哥以及美國紐澤西至喬治亞的近岸地區都沒有偵測到暴風震的跡象。既使是在美國紀錄中造成損害最重的颶風之一――珊迪颶風來襲時，也沒有引發暴風震。
Fan說這代表當地的海洋特徵與海床地形對暴風震有相當強烈的影響。
「我們還有很多不了解的地方，」Fan表示，「在此之前我們甚至沒有察覺到這種自然現象的存在。研究凸顯出地震波研究領域的內容相當豐富，而我們對地震波的理解正到達新的層級。」
伍茲霍爾海洋研究所、斯克里普斯海洋研究所和美國地質調查局的研究人員也參與了這項研究。
Strong storms often generate
earthquake-like seismic activity
A Florida State University researcher has
uncovered a new geophysical phenomenon where a hurricane or other strong storm
can spark seismic events in the nearby ocean as strong as a 3.5 magnitude
earthquake.
“We’re calling them ‘stormquakes,’” said lead author
Wenyuan Fan, an assistant professor of Earth, Ocean and Atmospheric Science.
“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.”
The research is published in the journal Geophysical Research Letters .
Fan and his colleagues analyzed nearly a decade of
seismic and oceanographic records from September 2006 to February 2019 and
found a connection between strong storms and intense seismic activity near the
edge of continental shelves or ocean banks.
Specifically, researchers 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,” Fan said. “The exciting part is seismic sources
caused by hurricanes can last from hours to days.”
Fan and his colleagues 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.
One example the researchers cited was Hurricane Bill,
an Atlantic hurricane that originated on Aug. 15, 2009, strengthened into a
Category 4 hurricane and ultimately struck Newfoundland as a tropical storm. It
was a Category 1 hurricane when it approached offshore New England on Aug. 22,
2009.
When the hurricane arrived, numerous seismic events
were located off the New England and Nova Scotia coasts, which produced
transcontinental surface waves.
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.
Fan and his colleagues noted that not all hurricanes
cause stormquakes. There are hotspots. Scientists detected no evidence of stormquakes
off of Mexico or from New Jersey to Georgia in the United States. Even
Hurricane Sandy, one of the costliest storms on record in the United States,
did not spur stormquakes.
This suggests that stormquakes are strongly
influenced by the local oceanographic features and seafloor topography, Fan
said.
“We have lots of unknowns,” Fan said. “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.”
Researchers from Woods Hole Oceanographic Institution
as well as Scripps Institution of Oceanography and the U.S. Geological Survey
contributed to this research.
原始論文：Wenyuan Fan, Jeffrey J. McGuire, Catherine D. Groot‐Hedlin, Michael A.H. Hedlin, Sloan Coats, Julia W.
Fiedler. Stormquakes. Geophysical Research Letters,
2019; DOI: 10.1029/2019GL084217
引用自：Florida State University. "
FSU Research: Strong storms often
generate earthquake-like seismic activity"
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