Mumbai, May 4, 2021: The 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. So, the lockdown imposed by several countries due to the Covid-19 caused a drop in seismic noise levels worldwide.
Reduction in seismic noise
A team of seismologists from universities around the world, including the Imperial College, London, studied the impact of the coronavirus pandemic on seismic 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, last 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 noise, which is recorded as a near-continuous signal on seismometers.
The sound signals created by human beings are often referred to as anthropogenic seismic noise.
It 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 noise has been silenced to a large degree.
How it is formed
Physically, seismic noise arises primarily due to surface or near surface sources and thus consists mostly of elastic surface waves. Primary sources of seismic waves include human activities (such as transportation or industrial activities), winds and other atmospheric phenomena, rivers, and ocean waves.