The ongoing Covid-19 pandemic has brought the world to a grinding halt and ushered in a “wave of silence” as high frequency noise generated by industrial plants, traffic and other human activities fell sharply during a period marked by lockdowns and social isolation.
A team of seismologists from universities around the world including the Imperial College, London, studied the impact of the coronavirus pandemic on noise-levels worldwide and found that high frequency noise caused by human activities dropped by as much as 50 per cent between March and May, this year. The period of quiet induced by this global health crisis is the longest and most prominent noise reduction on record, the scientists said.
The team analysed data collected from 268 seismic sensors located across 117 countries, and found that human-generated noise had fallen significantly in 185 of them. The most substantial declines were observed in highly-populous urban areas like Singapore and New York as well as tourist hotspots like Barbados and in European ski resorts.
“You can almost see it as a wave,” Stephen Hicks, a seismologist who worked on the study said. “You can see the seismic quietening spread over time, starting in China in late January and then moving on to Italy and beyond in March and April.”
What is seismic noise?
Seismic noise refers to vibrations within the Earth, which are triggered by natural and man-made phenomena like earthquakes, volcanoes and bombs. Seismometers, specialised devices that record ground motions, also capture seismic noise.
Everyday human activity — such as road traffic, manufacturing in factories, the sound produced by planes roaring overhead, or simply people walking down the street — also generate seismic noise, which is recorded as a near-continuous signal on seismometers. The sound signals created by human beings is often referred to as anthropogenic seismic noise.
Seismic noise acts almost like background sound for seismologists — it is the unwanted component of signals recorded by a seismometer. With human activity at a minimum due to the pandemic, anthropogenic seismic noise has been silenced to a large degree.
What was the impact of the pandemic on seismic noise levels?
Soon after the COVID-19 pandemic was declared a global health emergency earlier this year, countries across the world responded by imposing stringent lockdown measures and enforcing social distancing norms. As a result of this, offices were shut, manufacturing slowed down drastically, and air and road travel was stalled almost completely.
Many scientists have noticed favourable changes in the environment — such as reductions in nitrous oxide emissions and improved air quality — due to the pandemic. But the team of seismologists from around the world has found that the coronavirus outbreak also resulted in unparalleled noise reduction globally.
“We found a near-global reduction in noise, commencing in China in late Jan 2020, then followed by Europe and the rest of the world in Mar to Apr 2020,” the study read. The noise level observed during the lockdown is some locations was even quieter than during public holidays like Christmas and New Years.
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What were the regional variations in noise levels?
The level of anthropogenic seismic noise recorded varies based on a number of factors. Highly-populated urban areas will generate more vibrations from human activity than less densely populated regions. Timing too, plays an important role. The degree of seismic noise is found to be much lower during public holidays.
The study found that global median high-frequency anthropogenic seismic noise (hiFSAN) dropped by as much as 50 per cent in the beginning of the year. The most drastic reduction of hiFSAN since the lockdown began was observed in Sri Lanka, where it went down by half.
Meanwhile, a surface station in Belgium’s capital Brussels registered a 33 per cent drop in hiFSAN after the lockdown. In cities like Boston and Michigan in the US, as well as in the UK’s Cornwall, large hiFSAN reductions were noted in schools and universities following the onset of the pandemic. According to the study, noise levels were as much as 20 per cent lower than during school holidays.
Locations that would otherwise attract hoards of tourists, such as the Caribbean and Barbados, also registered an unprecedented decline in seismic noise levels. In Barbados, hiFSAN decreased by 45 per cent following the lockdown on 28 March 2020. Interestingly, research found that seismic levels had already begun decreasing a week or two before local curfews were enforced.
A less intense reduction was seen in sparsely populated areas. For instance in Rundu, located in the African nation of Namibia, less than 25 per cent hiFSAN reduction was recorded after a complete lockdown came into force in March.
Apart from data collected from surface stations, the researchers also assessed the effect of the pandemic on seismic levels underground. They did this using seismometers installed in boreholes. The researchers found that vibrations from human activity are felt even deep below the surface of the Earth.
Why is this important?
Due to this, scientists say they will be able to spot weaker signals, which were otherwise masked by the din produced by human activity. This means that scientists will have a better shot at monitoring a whole range of seismogenic behaviour, including the smallest earthquakes or the early signs of a volcanic eruption. This will further help by making seismic hazard assessment more accurate.
“Low noise levels during COVID-19 lockdowns could thus allow detection of signals from new sources in areas with incomplete seismic catalogs,” the study states. After the lockdown was imposed in Mexico, a low-frequency earthquake at 15 km depth was detected southwest of Petatlan. According to the researchers, it would have been highly unlikely for the quake to have been registered outside of Mexico’s urban environments before the pandemic.
“It’s important to see those small signals because it tells you if a geological fault, for example, is releasing its stress in lots of small earthquakes or if it’s silent and the stress is building up over the longer term,” Hicks told The Guardian.
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