Deafening silence, varying through regional Lock-Unlock switches, has been the abiding theme as the Covid-19 pandemic enforced a global freeze on human and machine activity. Research shows that the silence was palpable not just across the skies and the surface, but also deep down under.

Writing for Temblor Inc , a catastrophe modelling company specialising in seismic hazard and risk assessment, Elizabeth Goldbaum mentioned the case of seismologist Thomas Lecocq tweeting a request on April 1 for collaborators to help make sense of how the pandemic lockdown policies are affecting seismic noise around the world.

At least 76 scientists and citizen scientists from 66 institutions in 27 countries responded to Lecocq, a seismologist at the Royal Observatory of Belgium in Brussels. The team pooled diverse skill sets and data sources to discover that the Earth’s usual hum of vibrations from activities like driving and construction decreased by as much as 50 per cent from March to May.

Click here to read the study

Human heft to seismic noises

This universal stillness is the longest and most coherent global seismic noise reduction in recorded history; it shows that humans’ contributions to specific frequencies of seismic noises are higher than previously thought, the team said in its recently published paper in Science . This unexpected lull in human activity allows scientists to detect otherwise hidden seismic noises that could indicate potentially harmful sub-surface hazards. But this new data comes with a significant caveat.

“We can get excited about science and what it allows us to do, but we need to keep in mind that we’re in the midst of a global pandemic,” said Paula Koelemeijer, a co-author and seismologist at the Royal Holloway University of London. Seismologists are used to that since they rely on earthquakes and volcanic eruptions for valuable data while understanding that those natural disasters can impact and devastate lives.

Moderate to large earthquakes produce low frequency seismic vibrations. Instead of focusing on those, the team examined small earthquakes that aren’t often felt by people and produce high frequency seismic vibrations. These high frequency vibrations can fall into a similar range as human activities, making it difficult for sensors to differentiate between man-made and natural vibrations in the Earth’s crust.

Additional data sets for asking

Anthropogenic seismic noise is considered High-Frequency (4 – 14 Hertz) Seismic Ambient Noise (hiFSAN). Lecocq and his team analysed hiFSAN data from 337 broadband and individually operated citizen seismometer stations, and found a significant lull in hiFSAN during Covid-19 lockdown measures.

“Seismologists are like astronomers,” said Rick Aster, a geophysicist at Colorado State University who was not involved with the study. However, they seek quiet instead of dark places. “We usually use holiday and weekends to compare our data,” he said, because seismic noise typically subsides when people are not commuting and are more likely to stay put for extended periods of time.

However, in January during Chinese New Year in Hubei, China, where the coronavirus outbreak began, hiFSAN in 2020 was markedly lower than in prior years, the team found. “The lockdown gives us additional datasets to detangle human from natural noises,” said Aster.

Opening of schools, workplaces

The global quieting of seismic noise even allowed an urban seismometer station to pick up a relatively small and remote earthquake. A station in Querétaro, Mexico picked up a magnitude 5.0 earthquake at 15 km depth that was 380 km away in Petatlan, Mexico on July 4, 2020. “It’s like being on stage at a stadium that suddenly got very quiet,” said Lecocq. If someone at the back said, “hello,” you’d be able to hear them.

Although the pandemic’s lockdown policies were sudden, de-confinement has been much slower, said Lecocq. The slower reopening of primary schools and then secondary schools and workplaces could allow us to tease out specific seismic noise signatures, he said. If we can more precisely define manmade noise sources, we can eliminate them when we’re looking for natural, geologic seismic noises that could indicate an imminent earthquake or volcanic eruption.

A global drop in seismic noise demonstrates how people around the world are staying home to help combat the spread of the virus. The lockdown policies also show how we can do small actions to reduce our impact on the environment, said Lecocq.

“We’re witnessing the actions of billions of people,” said Lecocq.