Powerful storms may be causing offshore ‘stormquakes’

Hurricane Isabel, seen from the International Space Station. (NASA)

Strong ocean swells hammer ridges in the seafloor and produce the earthquake-like shaking

Science News - October 18, 2019

Powerful hurricanes can whip the ocean into a frenzy — and that wave energy can be strong enough to hammer the seafloor, producing a novel kind of quake.

These stormquakes, as described online October 14 in Geophysical Research Letters, are a newly identified type of interaction between Earth’s atmosphere, ocean and crust. Unlike earthquakes, which are triggered by subsurface shifting within the solid Earth, the driving force behind these seismic signals are ocean waves that have been whipped into deep swells by a hurricane or nor’easter. Stormquakes can be as powerful as a magnitude 3.5 earthquake, a level barely noticeable to people but detectable by seismometers, seismologist Wenyuan Fan and colleagues report.

The work is “a really great first start” at understanding a little-studied part of the seismic record, says physical oceanographer Fabrice Ardhuin of the Ocean Physics and Satellite Oceanography laboratory in Brest, France. “It brings something really new.”

Scientists have long known that the constant sloshing of ocean waves produces seismic signals at frequencies of about once every few minutes, a phenomenon known as “Earth’s hum” (SN: 9/29/04). Waves can also produces high-frequency signals called microseisms, occurring every five seconds or so.

But in between that seismic noise is another band of signals generated in the ocean that occur once every 20 to 50 seconds or so, or at a frequency of between 0.02 and 0.05 hertz. What produces seismic signals within that band hasn’t been so well understood.

Initially, Fan, of Florida State University in Tallahassee, and his colleagues set out to look for possible triggers for these signals coming from within the Earth. They analyzed seismic data collected from 2006 to 2015 by a network of moveable seismometers that marched across the country from west to east as part of the USArray. Fan started out by focusing on the data from the Pacific Northwest. He became excited, he says, when he found what he thought were previously undetected offshore earthquakes occurring in that mysterious seismic band.

But then he noticed something weird about those data.

“They were seasonal,” Fan says; the signals occurred only during winter months. “Earthquakes do not have seasonality. But weather does.” The driving force behind the mysterious quakes came into clearer focus once he began looking at seismic data from the U.S. East Coast — prone to experiencing powerful storms such as hurricanes and winter nor’easters. ...
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