Scientists hear hidden earthquake signals as coronavirus lockdown reduces human 'noise' by 50% globally
It is the longest and most pronounced quiet period of seismic noise in recorded history. The largest drops were seen in the most densely populated areas like New York City and Singapore
An unprecedented period of silence caused by lockdowns globally due to the Covid-19 pandemic has sharpened the ability of scientists to differentiate between human-caused "noise" and natural signals that might warn of upcoming natural disasters. As lockdowns have caused 50% global reduction in human-linked Earth vibrations, the relative quietness allowed researchers to listen in to previously concealed earthquake signals, says a new study led by the Royal Observatory of Belgium and five other institutions around the world, including Imperial College London, UK.
"With increasing urbanization and growing global populations, more people will be living in geologically hazardous areas. It will, therefore, become more important than ever to differentiate between natural and human-caused noise so that we can listen in and better monitor the ground movements beneath our feet. This study could help to kickstart this new field of study," writes lead author Dr Thomas Lecocq from the Royal Observatory of Belgium in the study published in Science.
Measured by instruments called seismometers, seismic noise is caused by vibrations within the Earth, which travel like waves. The waves can be triggered by earthquakes, volcanoes and bombs, but also by daily human activities like travel and industry. "Like earthquakes and other geophysical processes and events, humans are a major source of seismic signals detected by seismometers worldwide. Everyday human activity — from our involvement in industrial processes and construction projects to our raucous outbursts at football stadiums — generates vibrations in the earth that are recorded as a near-continuous stream of high-frequency seismic waves. In general, this seismic noise closely tracks with human behavior; it's typically stronger during the day than at night and weaker on weekends and holidays than it is on typical weekdays," explain authors.
Over the last few decades, seismic noise has gradually increased as economies and populations have grown. However, the nature of global anthropogenic or human-caused seismic noise remains relatively understudied. The complex high-frequency background signal it produces limits the ability of seismic warning networks to detect the more discrete signals associated with local geologic hazards like earthquakes.
However, the new report suggests that global measures such as quarantines, physical isolation, travel restrictions and widespread closures of services and industries that countries around the world have implemented in 2020 resulted in a month-long reduction in global seismic noise. The "2020 seismic noise quiet period" represents the longest and most prominent global seismic noise reduction in recorded history. Disruptions to human activity thus presented a unique opportunity to evaluate human-induced seismicity, enabling scientists to differentiate between human and natural seismic noise more clearly than ever before.
"This quiet period is likely the longest and largest dampening of human-caused seismic noise since we started monitoring the Earth in detail using vast monitoring networks of seismometers. Our study uniquely highlights just how much human activities impact the solid Earth, and could let us see more clearly than ever what differentiates human and natural noise," says co-author Dr Stephen Hicks, from Imperial's Department of Earth Science and Engineering.
Researchers say while human-generated noise usually dampens during quiet periods like over the Christmas/New Year period and Chinese New Year, and during weekends and overnight, the drop in vibrations caused by Covid-19 lockdown measures eclipse even those seen during these periods. Some researchers are dubbing this drop in anthropogenic noise and pollution the "anthropause". 'This is the first global study of the impact of the coronavirus anthropause on the solid Earth beneath our feet,” says Dr Hicks.
To gather the data, researchers looked at seismic data from a global network of 268 seismic stations in 117 countries and found significant noise reductions compared to before any lockdown at 185 of those stations. Beginning in China in late January 2020, and followed by Europe and the rest of the world in March to April 2020, researchers tracked the "wave" of quietening between March and May as worldwide lockdown measures took hold.
In total, global human-caused seismic noise dropped by as much as 50% from March to May. The research reveals that vibrations from human activity affect larger areas than previously thought, reaching seismometers in remote outposts or installed deep underground.
The strongest drops were found in urban areas, but the study also found signatures of the lockdown on sensors buried hundreds of meters underground and in more remote areas. According to the analysis, densely populated areas such as Singapore and New York City saw the largest drops in vibrations. Drops were also seen in remote areas like Germany's Black Forest and Rundu in Namibia.
Citizen-owned seismometers, which tend to measure more localized noise, noted large drops around universities and schools around Cornwall, UK, and Boston, US — a drop in noise that is 20% larger than seen during school holidays. Countries like Barbados, where lockdown coincided with the tourist season, saw a 50% decrease in noise. This coincided with flight data that suggested tourists returned home in the weeks before the official lockdown.
The changes worldwide allowed researchers to listen in to the Earth's natural vibrations without the distortions of human input. Researchers, for example, report the first evidence that previously concealed earthquake signals, especially during the daytime, appeared much clearer on seismometers in urban areas during the lockdown. Smaller earthquakes that would normally have required computer processing to identify an urban seismometer's data, such as a magnitude 5.0 earthquake that struck near Petatlan, Mexico, on April 7, became much easier to spot due to a 40% reduction in human noise in the region.
The research team hopes that their work will spawn further research on the seismic lockdown, as well as finding previously hidden signals from earthquakes and volcanoes. "The lockdowns caused by the coronavirus pandemic may have given us a glimmer of insight into how human and natural noise interact within the Earth. We hope this insight will spawn new studies that help us listen better to the Earth and understand natural signals we would otherwise have missed," says Dr Hicks.