Seismic noise is a relatively persistent vibration of the ground that is usually an unwanted component of signals recorded by seismometers, according to the scientists.
The protective self-quarantine and lockdown measures around the world due to COVID-19 scare have reduced the human-caused seismic noise, as a result of which earthquakes of smaller magnitudes can be detected with more accuracy and specificity, say scientists, adding however that this has not made the Earth’s crust shake less.
Seismic noise is a relatively persistent vibration of the ground that is usually an unwanted component of signals recorded by seismometers, according to the scientists. Earlier studies have reported that human activity of all kinds generates these vibrations that distort measurements from finely tuned seismic instruments.
While the lockdowns enforced in many parts of the world have reduced these disturbances, seismologists, including Supriyo Mitra, a professor at the Indian Institute of Science Education and Research (IISER) Kolkata, believe it is wrong to say that the Earth’s crust is now “moving or shaking slowly” as has been reported in several media reports.
In Belgium, data shows that measures to curb the spread of COVID-19 in Brussels caused human-induced seismic noise to fall by nearly 30 per cent.
The resulting quiet means surface seismic readings are as clear as the ones scientists usually get from the same instruments buried deep beneath the Earth’s surface, noted the scientists. A noise reduction of this magnitude is usually only experienced briefly around Christmas, said Thomas Lecocq, a seismologist at the Royal Observatory of Belgium in Brussels, who made the new observation.
In India, Mitra is working on similar kind of seismic data and readings. “We want to collect the data from the lockdown and see exactly in what band the seismic noise has gone down,” Mitra told PTI. He noted that the cultural noise or ambient noise because of anthropogenic activities is one hertz and above — and one hertz is the standard frequency at which the earthquake energy comes in.
“So if the noise is high, then you generally have a lower detection of earthquakes,” Mitra noted. What has happened as a result of the lockdown, he explained, is there is less vehicular movement and human activity, causing reduction in the ambient noise.
“The threshold of detection has gone down. So there is more detection of smaller earthquakes,” he said. Seismologist Kusala Rajendran, a professor at the Indian Institute of Science, Bengaluru said that with low levels of human activity, the seismic sensors can now detect smaller earthquakes which would have otherwise been part of the noise.
She explained seismic stations are installed away from traffic, ocean waves, and other kinds of noises. “To keep up with spatial coverage and accessibility many stations are located within the range of human activity. These stations are so sensitive that they can record a person walking within 500 meters,” Rajendran told PTI.
“If the noise level is set very low these sensors would record everything — even a large fruit dropping in the yard, not to speak of cultural noise,” she explained. The leading Indian seismologists also noted that it will be wrong to say that the Earth’s crust is “moving or shaking slowly” due to the COVID-19 lockdowns across the world. “Because that would mean the plate tectonics have slowed down, which is not the case,” Mitra, who is also affiliated to the Centre for Climate and Environment Studies said.
“Saying the crust is moving slowly is a misrepresentation of facts. The Earth’s crust is not moving slowly,” he noted.
Rajendran agreed with Mitra, saying that only anthropogenic noise has reduced due to the lockdown. “Seismic activity does not get affected. Recorders located near cities etc. will record at a quieter levels and hence will pick up microtremors that would otherwise be undetected,” she explained.
“So there is no increase or decrease — just better detection for some stations,” the scientist added. Belgian seismologists are not the only ones to notice the effects of lockdown. Celeste Labedz, a graduate student at the California Institute of Technology in the US, tweeted that a similar fall in noise had been picked up by a station in Los Angeles. “The drop is seriously wild,” she said.