Equipment stashed around Southern California to measure earthquakes like early Wednesday’s 3.7 temblor detected the sudden decline in human activity that occurred last month when safer-at-home orders to hinder the COVID-19 outbreak shut down restaurants, closed schools and virtually eliminated traffic.
So-called "seismic noise" -- unnoticeable ground shaking created by cars, industry and even pedestrian walking -- dropped three decibels, or about half, when residents began staying home, said Celeste Labedz, a geophysics PhD student at Caltech in Pasadena who normally studies glaciers.
"The quietest day I've seen so far was Easter Sunday," Labedz said. "The week before that, was the quietest I've seen so far. This past week was louder."
The quiet on land, which Labedz said can help researchers study the smallest of earthquakes under our feet, also might be happening at sea, where a decline in cargo and cruise shipping in the ocean might be making life a little easier for whales and other marine life.
Marine life scientists also are interested in how less noise in the water affects marine life behavior.
"Noise can be a real problem for the whales in terms of their ability to communicate with one another," said Dr. Valeria Vergara, a researcher with Ocean Wise’s Marine Mammal Conservation Research Program in Vancouver. "It can only be good news. Noise pollution affects animals ranging from fish and shrimp to mussels and frogs."
Since humans began staying home in mid-March, Labedz has kept close watch on seismometers near USC and Pasadena. She quickly noticed changes in what the machines used to detect earthquakes were uncovering about ground motion and human activity.
She’s been publishing her observations on Twitter, including the fact that seismometers are proving scientifically residents are staying home.
"We can keep an eye on how well people are doing by looking at seismometers all around the city," Labedz said. "You can see it definitely drops when the lockdown starts...We are doing a pretty good darn job."
Labedz likened seismic noise to the rumble one feels on a platform when a train rolls by or a large truck passes outside a window.
During an earthquake, she said, the ground shakes dramatically. But it actually moves all the time, too slight for humans and animals to feel.
Sensitive seismometers, however, pick up small earthquakes and the constant ground motion created by humans. Human-created seismic noise comes from cars, planes, construction sites, mines, factories and pedestrian traffic.
"That continuous hum of tiny motion is what we call 'seismic noise,'" she said. "You can think of it like background audio noise. No one is talking or playing music in my house right now, but there's still a little bit of noise like the fridge running, the wind in the trees outside, etc."
Since governments instituted stay-at-home orders except for essential tasks, scientists’ recording devices have shown them the declines. Normally similar drops occur from daytime to nighttime, and on Christmas, the quietest day of the year.
"A decrease in transit, industry, and general hustle and bustle means the ground is moving a bit less," Labedz said. "The natural sources like the ocean are still there, but not the (human-made) stuff."
Labedz is not the only scientist keeping watch on seismometers during the pandemic. Thomas Lecocq, a seismologist at the Royal Observatory of Belgium in Brussels, reported on Twitter that seismic noise in his country dropped by one-third after schools, restaurants and other businesses closed and non-essential travel was ordered halted in mid-March.
So, what does this mean for all of us? Not too much.
That continuous hum of tiny motion is what we call 'seismic noise'. You can think of it like background audio noise. No one is talking or playing music in my house right now, but there's still a little bit of noise like the fridge running, the wind in the trees outside, etc."
"It’s just kind of neat," Labedz said as she walked across campus.
The decrease in ground noise, however, makes it easier for seismometers to detect the miniscule temblors that occur all the time, but are so small they cannot be felt. That information helps earthquake researchers keep better track of the faults across the area.
Lower background noise allows the machines to detect small seismic waves. Labedz likened it to hearing your phone better in a library than at a rock concert.
"More earthquakes, even very tiny ones, means more information about what’s going on below our feet," Labedz wrote to her Twitter followers. "Earthquakes too small for any person to feel can still give us more information about potential hazards. When you live in earthquake country, the more information the better."
Meanwhile, in the ocean, worldwide COVID-19 shutdowns have resulted in significant decreases in cargo, oil transport and cruises. Researchers wonder how whales and other marine life react.
Dr. Tyler Helble, an oceanographer with the U.S. Navy Naval Information Warfare Center Pacific in San Diego, said the noise from shipping and other loud events in the water impacts animal behavior, including how they communicate.
"If you infer that in humans and other mammals there’s various studies that noise can cause stress, you can infer that if there is less noise and less disruptions, the animals probably have less stress," Helble said, adding .
He suggested marine mammals and fish in the listening range of commercial and pleasure shipping likely will notice a change, but how their behavior is affected won’t be known for some time.
"I would not be surprised if research shows that some of those stress levels have gone down or their behavior has changed in response to shipping changing and ocean noise changing," he said.
In an article on Ocean Wise’s website, Vergara, who focuses on beluga whales, called it an "unprecedented hiatus in ocean noise." Although she hasn’t begun studies, Vergara said she hopes to use hydrophones – underwater recording devices – to study changes in noise in the St. Lawrence River should the pandemic shutdowns continue into the summer.
"Shipping noise will transmit really well," she said. "The noise pollution becomes a problem. They can hear ships or other human activities from very far away."
Studies to be done can corroborate previous work that shows human activities "introduce a tremendous amount of noise into the environment." Vergara wants to see how a reduction in noise affects the whales way of life.
"It’s a wonderful opportunity," she said, "to look into whale behavior and potentially learn a little bit more about what this quiet world might be doing to them, by how they might be affected positively," she said.