Measuring seismic activity often requires locating instruments away from population centers. During COVID-19, the seismic activity caused by humans dropped a lot:

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Writing today in the journal Science, dozens of researchers from around the world show that the seismic activity from our civilization plummeted as lockdowns went into effect. This “anthropogenic seismic noise,” as seismologists call it, comes from all manner of human activities, whether that’s running factories, operating cars or trains, or even holding concerts. Seismometers pick up these activities as a kind of constant din, which actually peaks on weekdays, when more people are moving around, and falls on weekends when economies slow down. All this activity that seismometers detect mixes with the natural rumbles that scientists are really interested in, like earthquakes, volcanoes, and landslides.

But boy, are they interested in the anthropogenic seismic noise now—or the lack thereof, as humans began to shelter in place. “We see it effectively moving around the globe as a seismic lockdown wave,” says Royal Holloway University of London seismologist Paula Koelemeijer, one of the paper’s coauthors. “So starting in China originally, then in different places in Italy, and then going through Europe. And whenever lockdowns happened in different countries, we see the effect that’s up to an 80 percent reduction in the amplitude of the seismic noise in some places.” The average was about 50 percent…

Normally, seismologists don’t bother monitoring urban environments for just this reason: There’s way too much noise muddying the signals of Earth’s natural processes. But for the past few years, citizen scientists have been collecting signals thanks to a clever little device called the Raspberry Shake, a Raspberry Pi computer outfitted with sensors to create an internet-connected seismometer. “Those instruments have been popping up more and more in people’s homes,” says Koelemeijer. “And so about 40 percent of our data stations that we looked at have been these citizen science instruments. It’s just people finding it funny, geeky, to have one of them. Like, I’m one of those people. I have one in my house.”…

But it was in a remote part of Germany where seismologists recorded perhaps the most surprising lockdown data. The Black Forest Observatory is not only isolated, and thus considered to be a reference low-noise laboratory, but its instruments are stowed over 150 meters below the surface, in bedrock. Yet they, too, picked up a small noise reduction at night during the lockdown. “Germany was a big surprise for us, because that station is very much a remote station, and seen as a very good seismic station for looking at natural signals,” says Koelemeijer. “So the fact that we saw it there was quite remarkable.”

Human activity influences all sorts of spheres that we do not often consider. Here, humans create a lot of noise and activity below the surface of the earth. There are occasionally complaints about noise pollution but most people put up with a certain amount of noise and vibrations in the places where they live.

This reminds of several different aspects of working with “background noise.” Working in radio, I often used sound effects for promotional material. The sound effect CDs we had contained a variety of background noise or din. For example, the city options largely contained traffic noise with vehicle rumbling and honking mixed together.

More recently, sports broadcasts have adjusted to teams playing in stadiums with no fans present in the stands. They often use recordings of fans where a sound technician has the ability to respond to play on the field (such as cheering for a good play for the home team or a negative response from fans). Without such background noise, the broadcast sounds very different: voices echo, the players can be heard, and a comforting or familiar element of the broadcast is lost.

All together, the background noise of our lives matters in everyday activity and in scientific measurement. If COVID-19 changes human movement and interactions, what we hear by ear and scientific instruments also changes.