The coronavirus pandemic and the associated economic shutdowns led to an unprecedented drop in the seismic “noise” caused by human activity, scientists reported Thursday.
The shutdown effect registered by seismometers is akin to that typically seen in the middle of the night and during holiday periods, only this one lasted from March to May and encompassed almost every corner of the planet - a stunning global quiescence never before seen in the history of earthquake science.
The report, published in the journal Science, has an unusually vast set of authors: 76 scientists from 27 countries, ranging from Norway to New Zealand. The instruments tell a consistent story: The surface of the Earth became unusually calm and quiet as the virus raced across the planet earlier this year. Constrained by the coronavirus pandemic, human activity diminished when countries began shuttering their economies and urging people to engage in social distancing.
"You wouldn't expect a disease to show up on a seismometer," said report co-author Celeste Labedz, a doctoral candidate in geophysics at the California Institute of Technology.
The study gathered data from 268 research stations and saw a shutdown effect almost everywhere. Reports came in from Turkey, Chile, Costa Rica, Canada, Australia, Iran and even from tiny Luxembourg, as well as many other nations. Some of the seismometers are in crowded urban centers and college campuses; but others are in remote desert or mountain locations.
The effect was most dramatic in Sri Lanka, where a station saw a 50% reduction in background noise. In New York City's Central Park, the decrease at night was 10%. Remote locations did not see a shutdown effect because they don't get pummeled by human activity normally.
This new database could help scientists better distinguish faint natural tremors from those caused by human activity, the researchers wrote. Beyond that, it could be a tool for monitoring activity during pandemics.
Cellphone mobility data provides a powerful way of monitoring the degree to which people are sheltering in place, social distancing and limiting their interactions. But that has raised privacy concerns, even though the data is anonymized. The seismic data is inherently anonymous because there's no way to know who or what is generating the waves of energy.
Such seismic monitoring can serve as a technique for monitoring human activity "with fewer potential privacy concerns than mobility data," the authors write. "In addition, industrial activities may not be captured in mobility data, but leave a seismic noise signature."
Susan Hough, a U.S. Geological Survey seismologist who was not part of the research, said previous studies have looked at human sources of seismic noise, including trucks, trains and factories, and so this latest report is not surprising: "We're impacting the planet. We're not treading lightly. We're inducing earthquakes by various activities. And generating vibrations that you can pick up on seismometers."
Geophysicists use seismometers to study ground motion. When a fault on or below the Earth's surface breaks and triggers an earthquake, energy is released in waves that cause the ground to shake. To a lesser extent, something similar happens when a FedEx truck slams on the brakes - it sends a pulse of energy into the ground.
Someone walking by a seismometer will leave a signature spike on the seismograph. The spikes on these graphs invariably shorten in the wee hours of the night as the hurly-burly of human activity gives way to slumber.
At Yellowstone National Park, a seismograph near the Old Faithful visitor center faithfully records the vibrations of the subsurface water in the roughly 20 minutes before an eruption. But that natural signal is largely drowned out by the vibrations from human beings as they race to watch the geyser's eruption and then head afterward toward the visitor center, said geologist Robert Smith of the University of Utah.
Smith, who was not part of the research, said the Science report is "a very unusual paper" and he has never seen geophysical research coupled in such a way with a biological event.
Scientist studying the Earth's natural tremors have to subtract the background noise from all the humans stomping around. The installation of the instruments is typically accompanied by a "stomp test" to get a sense of how readily the gadget picks up the energies unleashed by people in the vicinity, Labedz said.
The fact that the shutdowns led to a drop in "noise" is not in itself surprising. But the scale of the effect was eye-opening and inspired a spontaneous, collective effort by the scientific community.
Thomas Lecocq, a scientist at the Royal Observatory of Belgium and the lead author of the Science report, posted on Twitter the decrease in seismic noise on instruments he was monitoring. Other scientists from around the world, including Labedz, followed suit.
"It started on social media with some very social seismologists and moved to a really awesome global data set," Labedz said.
Koen Van Noten, another geologist at the Royal Observatory of Belgium and a co-author of the study, said in an email that the findings have united the highly dispersed seismological community.
“For seismology, the lockdown period is now a baseline to show the impact humans can have on our seismic data,” he wrote. “Although staying at home isolated, in my profession I’ve never felt so connected globally.”