Around the world earlier this year an eerie quiet filled the streets when lockdowns were implemented to stop the spread of the coronavirus.
Now an international team of researchers says that quiet extended to the ground beneath our feet, with seismic instruments detecting a sharp reduction of the background “noise” caused by humans.
“Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record,” researchers said in a study published Thursday in the journal Science.
The research team was led by Thomas Leco of the Royal Observatory of Belgium. Team members included Alan Kafka, director of Boston College’s Weston Observatory.
The researchers looked at seismic observations from 268 stations around the world. The study cited places ranging from Boston to Barbados, the Namibia-Angola border, Germany’s Black Forest, and the uninhabited island of Motutapu off Auckland, New Zealand.
Seismometers can pick up human-generated signals such as nuclear test explosions and activities like the fluid injection associated with fracking, but they also record everyday activity, especially in urban environments, researchers said.
“These complicated signals are the superposition of a wide variety of activities happening at different times and places at or near the Earth’s surface, but are typically stronger during the day than at night, weaker on weekends than weekdays, and stronger near population centers,” researchers said, noting studies of seismic signals from road traffic, public transportation, and people gathering for soccer games.
Typically, seismic quiet periods include weekends and the Christmas/New Year holidays, wherever those holidays are celebrated, researchers said.
“We found a near-global reduction in noise, commencing in China in late Jan 2020, then followed by Europe and the rest of the world in Mar to Apr 2020. The noise level we observe during lockdowns lasted longer and was often quieter than the Christmas to New Year period,” the researchers said.
Other studies have looked at how the lockdowns and other measures affected the environment, reducing air pollution, for example, but this study highlighted how the lockdowns “impact the solid Earth,” researchers said.
Kafka said the quieting of the Earth was noticeable on the sophisticated instruments at Weston Observatory, but even more dramatic at two inexpensive seismometers known as Raspberry Shakes set up in buildings at the Boston College campus.
Kafka has posted on Twitter charts from the two “citizen science” seismometers. They show a decline in vibrations in the Earth beginning when Boston’s restrictions were put in place, a flat period, and now a gradual rise.
“You have that shape all around the world,” he said. “It’s started to come back again.”
The study noted that the background noise interferes with the work of detecting what’s going on under the earth, including signs of earthquakes and volcanic eruptions, and said that the quiet period provided “an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise.”
The study also noted, “The seismic observations of human activity during the COVID-19 lockdown allow us to assess the impact of mitigation policies on daily life, especially the time to establish and recover from lockdowns.”
The idea, Kafka said, is that monitoring seismic activity “might be a way of monitoring human activity that is not invasive” that could be used by other scientists, such as epidemiologists studying how much people are limiting their mobility to prevent the spread of the coronavirus.
Martin finucane can be reached at firstname.lastname@example.org