Lockdown was the longest period of quiet in recorded human history

When lockdown started in March, the world went instantly, strangely silent. City streets emptied. Joggers and families disappeared from parks. Construction projects froze. Stores closed.

Now a network of seismic monitoring stations around the world has quantified this unprecedented period of quiet. The resulting research into seismic silence," published in Science today, has shown just how much noise we contribute to the environment. It has also let scientists get an unparalleled listen to what's happening beneath our feet.

We can safely say that in modern seismology, we've never seen such a long period of human quiet," says Raphael De Plaen at the Universidad Nacional Autonoma de Mexico in Queretaro, one of the paper's 76 authors.

Seismic noise, or vibrations of the earth, is most often associated with earthquakes. But seismology also listens to the interplay of earth and water, tracking things like ocean swells and atmospheric pressure changes.

Humans are the third-biggest source of seismic noise. Everyday urban activities like commutes, or stadiums full of fans simultaneously going wild in football quakes," are strong enough to register on seismometers.

It's transport, like cars, trains, traffic, buses," says coauthor Paula Koelemeijer of the Royal Holloway University of London. It's retail and recreation-not just people going shopping, but also going to parks. It's workplaces and residences."

Under normal circumstances, this human noise merges with and muffles natural seismic activity. Exactly how much our behavior affects the levels of background noise has been hard to work out until now. Lockdown presented a unique opportunity for researchers not only to control for human activity but also to hear seismic noise that otherwise gets drowned out.

The researchers-who included academics and citizen scientists-used data collected from 268 seismic monitoring stations around the world. These devices included highly technical seismographs housed in academic institutions. But about 40% of the data also came from Raspberry Shakes, personal seismographs that are built and used by hobbyists. The group used these devices in tandem with anonymized mobility data collected from Google and Apple to detect human movement. They were then able to match those observations with seismic noise reports in order to figure out whether seismic events were likely to be human-caused or natural.

Of the 268 stations, 185, or 69%, showed significant reductions in high-frequency seismic ambient noise, the cocktail of human-produced and natural ambient noise that surrounds us. This silence began in late January in China; by mid-March, it had descended on the world.

While the periods between Christmas and New Year and over Chinese New Year are usually the quietest, the difference was even starker this time. Sri Lanka, for example, saw a 50% reduction in noise, the largest the researchers measured. Sundays in New York's Central Park are usually lively, but lockdown numbers registered a 10% reduction from the weeks immediately before. Even sensors buried deep under the surface picked up on the sudden lack of human activity above. A German observatory that lies almost 500 feet beneath rock was able to detect a drop in vibrations once lockdown kicked in.

Courtesy T. Lecocq et al.

The noise patterns also highlighted human migration. De Plaen says that the Mexico-US border showed an increase in human seismic noise, even though both sides of the border were otherwise still. Citizen seismographs were able to pick up the noticeable drops in noise around schools and universities.

The study itself was a product of the pandemic. Lead author Thomas Lecocq, a seismologist at the Royal Observatory of Belgium, had been writing code to better understand how to tease apart human-generated and seismic noise. He and some colleagues had initially exchanged Twitter direct messages and coordinated through WhatsApp groups before creating a Slack group to coordinate their research on April 1. The Slack group-combined with the accessibility of Raspberry Shakes-expanded the data and made it stronger. It's not every day that you publish results after less than four months of work," De Plaen says.

The fall-off in human noise also gave scientists a chance to listen to the earth's inner workings more closely than ever before-without humans drowning them out. This might add to our knowledge of earthquakes, particularly small ones in urban centers that are often masked by human seismic noise. Smaller earthquakes are key to being able to monitor fault lines, and they act as predictors of bigger quakes to come; scientists now have a baseline data set to work with. We can [now] study relationships between human activity and seismology," De Plaen says. We can now understand with a high level of resolution what is generating noise: the earth or humans."

As we emerge from lockdown, scientists hope this understanding of human-caused noise will also help us better understand how we're moving and living-just by listening.

Not that the human noise ever truly went away. Koelemeijer's seismograph would spike some mornings, when a neighbor's washing machine would hit the spin cycle. Even in the depths of the quietest period of human history, human environments aren't really ever completely silent," she says. You'll always still pick up some noise."