Seismometers may be built to detect earthquakes, but their mechanical ears hear so much more: hurricanes thundering hundreds of miles away and meteoroids exploding in the skies on the other side of the planet. Even the everyday hum of humanity — people moving about on cars, trains and planes — has a seismically detectable heartbeat.
But coronavirus has upended our lives. Hoping to curtail the pandemic’s spread, nations have closed their borders, cities have been shut down, and billions of people have been instructed to stay home. Today, in cities large and small, the thumping pulse of civilisation is now barely detectable on many seismograms.
“It did make the scale of the shutdowns a bit more real to me,” said Celeste Labedz, a graduate student in geophysics at the California Institute of Technology.
In person, you can see only your neighbourhood’s dedication to remaining home. With seismometers, Labedz said, you can see the collective willingness of millions of the world’s urban dwellers to hunker down. As a result, the planet’s natural quavering is being recorded with remarkable clarity.
This seismological experiment began with Thomas Lecocq, a seismologist at the Royal Observatory of Belgium in Brussels. He wanted to see what happened to his city’s anthropogenic hiss after its lockdown began in mid-March. His finding, that it had declined precipitously, was shared on Twitter and via news organisations, prompting seismologists elsewhere to look at their own city’s lack of shakes. Many used Lecocq’s bespoke coding to eke out the human noise in their seismic data.
It quickly became apparent that the roar of urban life had turned into a whisper all over the world, in places as far-flung as California and Croatia. “It’s crazy,” Lecocq said.
London is no longer buzzing. Paula Koelemeijer, a seismologist at Royal Holloway, University of London, said the seismometer in her suburban house was clocking a 20% to 25% reduction in average weekly noise, compared with the week before Britain began its lockdown. Another seismometer nearer the city centre registered a 30% drop. In both areas, the cacophonous rhythm of the morning rush has nearly evaporated.
Noise levels on some seismic stations in Los Angeles have dropped to below half of what they normally are, Labedz said. Weekdays sometimes have a quieter seismic signal than pre-pandemic weekends.
Claudio Satriano, a seismologist at the Paris Institute of Earth Physics, detected a 38% drop in the average daytime noise in his city. The oscillation of commuting workers has faded, and thundering nightlife on the weekends has fizzled.
Stephen Hernandez, a seismologist at the Geophysical Institute in Quito, Ecuador, said that a seismic station in the northern part of the city had previously recorded noise declines during times of upheaval, including during perilous social unrest in October 2019. That station is now registering a staggering 60% decline in noise, with any peaks during the city’s lockdown barely surpassing the minimum noise levels observed during normal times.
The bustle of college students the world over has gone. David Cornwell, a geophysicist at the University of Aberdeen in Scotland, has been watching the seismometer in a colleague’s office. Compared with peak noise during a normal day, the noise levels on the university’s King’s College campus have dropped by 60% since its students were sent home.
The pitter-patter of children has also been quieted. A seismic station in a Nepalese school that Lecocq has observed normally reverberates with the pandemonium of children running to and from their place of learning. Now, he said, “those noise levels have completely vanished.”
The characteristic cultural fingerprints of some cities are notable by their absence on seismograms. For example, the vibrations generated in Barcelona by enormous crowds leaping up and down at Camp Nou — the stadium home to the FC Barcelona soccer team — have disappeared. “No more ‘Messiquakes’ for a while,” said Jordi Díaz, a seismologist at the Institute of Earth Sciences Jaume Almera in that city, referring to little temblors set off by fans celebrating the goals of soccer star Lionel Messi.
Seismograms also have shown how some cities quieted down even before mandatory lockdowns took effect. New Zealand’s national order was put in place just before midnight March 25, but people were advised that a lockdown was imminent two days beforehand. Seismographs in Auckland, a city of 1.6 million people, show a clear decline in noise before the first day that the mandate took effect, said Geoff Kilgour, a volcanologist at GNS Science, a geoscientific research organisation.
On March 22, a 5.4-magnitude earthquake rocked Croatia, the most severe the capital city of Zagreb has experienced in 140 years. Aftershocks have been shaking the city ever since, and people would usually be able to feel only the more potent ones. But temblors typically imperceptible to humans are now being reported by hundreds of residents, said Rémy Bossu, secretary general of the European-Mediterranean Seismological Centre. This uptick in individual seismic sensitivity may be explained by the fact that the city’s residents are being cooped up at home.
Scientists, too, are now able to better hear the planet’s natural tectonic soundtrack. With the volume of humanity reduced, “we can detect smaller earthquakes, just like how it’s easier to hear a phone ring in a library than at a rock concert,” Labedz said.
Hernandez has also been able to hear more rumblings from the active volcano that Quito straddles, something attributed to the reduction of city noise. And on Koelemeijer’s household seismometer, earthquakes of middling magnitudes emanating from distant continents, no longer buried by the rumble of rush hour traffic, are now showing up in the day.
A cleaner and more frequent detection of Earth’s seismic activity grants seismologists a less filtered look into the planet’s interior. Although many seismometers are purposefully located far from cities, plenty of urban areas — especially those in seismically hyperactive parts of the planet — are peppered with seismometers. In this time of human quiescence, the creaking of some potentially dangerous faults may be detected better than ever.
Academic benefits aside, many scientists are heartened to see that the lockdowns’ seismic signatures are visible in dozens of populous cities across the world. Noise levels in many of them show no sign of increasing, and in some places, like London, they have dropped over time. That “indicates more and more people are taking it seriously and thus doing their best to flatten the curve,” Koelemeijer said.
But footsteps are far quieter than traffic, so seismometers won’t necessarily betray people who are evading the lockdown measures. “Whenever I look out onto the balcony these days, I still see too many people walking around,” Satriano said.
Don’t be tempted by sunnier days ahead. “Stay home, stay safe,” he added.
