‘Bored’ seismologists find a new hobby: Tracking silence
DALLAS — If you’ve been noticing more birds chirping and more frogs singing, it’s probably not your imagination.
Noise created by humans, such as car and truck traffic, quieted by about 30 percent between late March, when Gov. Greg Abbott closed schools and restaurants across Texas, and early May, according to a new analysis by researchers at Southern Methodist University.
“There was quite a big change in some areas,” said Stephen Arrowsmith, a seismologist at SMU, who took on the project with a class of undergraduate and graduate students this spring.
Arrowsmith and his students looked at data from a dozen seismometers across North Texas, including in Irving, Dallas, Farmers Branch, Richardson and Fort Worth. Seismometers are used to detect earthquakes, but they are sensitive to just about everything that makes the ground vibrate, such as strong winds, ocean waves, passing trains, construction and traffic.
Each instrument contains a small weight on a spring. As the earth moves, the weight remains in place, allowing the seismometer to measure the amount of movement relative to the weight.
Scientists usually avoid placing seismometers in cities, because the racket of human activity makes earthquake signals hard to detect. But North Texas is an exception.
Scientists at SMU and at the University of Texas at Austin’s Bureau of Economic Geology placed seismometers in the Dallas-Fort Worth area after Texas experienced a surge of small to moderate-sized quakes beginning in 2013. The stations are now part of the state-wide TexNet seismic monitoring program. Researchers linked the North Texas quakes, which peaked in 2015 before dying down, to oil and gas activity.
Arrowsmith and his students saw a range of noise reductions at the various stations. The biggest changes, perhaps not surprisingly, occurred on college campuses. At the University of Texas at Dallas’ campus in Richardson, a TexNet seismic station registered students’ comings and goings as classes started and ended each weekday.
“After the shutdown, that totally disappears, and the background noise goes way down,” said Arrowsmith.
Stations in Irving also registered significant noise reductions, mostly likely due to fewer cars and trucks on the roads, he said.
A seismic station at a firehouse near Farmers
Branch, though, registered no change in noise levels, presumably because first responders have remained active throughout the pandemic.
“Each station is telling you about quite local things that are going on near that station,” Arrowsmith said. “So you need a good network of stations to fully understand what the effect of the shutdown is on a city as a whole.”
The idea of using seismometers to track urban noise gained popularity in March when Belgian seismologist Thomas Lecocq posted some of his urban noise data from Brussels on Twitter.
Lecocq, of the Royal Observatory of Belgium, received such an overwhelming response from scientists that he launched the group “Lockdown Seismology” on the Slack communications platform. “It’s where bored seismologists around the world are collaborating,” Arrowsmith joked.
Lecocq wrote in an email to The Dallas Morning News that he wanted to document the noise levels to show how small changes in personal behavior can make an international impact.
Cities have seen a wide range of noise reductions, ranging from 20% to 90%, Lecocq said. Dallas has seen among the smaller reductions, said Arrowsmith, which he attributes to stricter lockdown measures in Europe.
He’s working to correlate noise levels with mobility data from companies that use cellphone signals to track how far people travel and how much they interact with one another.
Now that Texas has loosened restrictions and businesses have started to reopen, noise levels have crept up but have not yet reached pre-shutdown levels, Arrowsmith said.
He hopes his findings will contribute to a growing list of creative ways in which researchers are using seismometers. In his spring forensic seismology course Arrowsmith teaches students how seismic stations can help investigators solve crimes like terrorist bombings, aid scientists in tracking nuclear tests in countries like North Korea, or assist inspectors in investigating accidents like space shuttle explosions and chemical plant blasts.
Recently, researchers have begun using seismic stations to track storms over the ocean, because large crashing waves register on the instruments, too.
Even in Dallas, Arrowsmith said, seismometers clearly register sounds from the Atlantic Ocean.
One potential application of his research is to better understand the shallow layers of Earth beneath cities. “That could be useful in places where there’s a real seismic hazard like San Francisco or Los Angeles,” he said, “where just knowing what that shallow structure is tells you a lot about how it would respond in a big earthquake.”