The shock waves of civilisation travel through rocky ground and, at times, ricochet around the globe, as geologists know from decades of listening for earthquakes with sensitive seismometers. The human pulses come from heavy traffic, football games, rock concerts, fireworks, subways, mine explosions, rock drilling, factories, jackhammers, industrial blasts and other activities. In 2001, vibrations from the collapse of the World Trade Centre in New York registered in five US states. Seismometers even picked up the impacts of the two airplanes.
Now, a team of 76 scientists from more than two dozen countries reports that lockdowns from the Covid-19 pandemic led to a drop of up to 50 per cent in the global din tied to humans. The main quieting, from March to May, was compared with levels in previous months and years.
"The length and quiescence of this period represents the longest and most coherent global seismic noise reduction in recorded history," the scientists reported in the journal Science. The quieting, they added, resulted from social distancing, industrial shutdowns and drops in travel and tourism. The overall decline far exceeded the kind typically observed on weekends and holidays.
Devices for measuring earthquakes go back at least to the early part of the 18th century, when pendulums were used to display ground motions. In 1895, Irish engineer John Milne established on the Isle of Wight a modern seismometer centre that quickly grew into the world's first global network, with 30 overseas branches. By 1957, a global group of seismologists had listed 600 stations. The devices can pick up vibrations not only from earthquakes and human activities but also from hurricanes and the crashing of ocean waves on shorelines, as well as the impacts of rocky intruders from outer space.
The new research was led by the Royal Observatory of Belgium and other institutions, including Imperial College London and the University of Auckland in New Zealand. Participants from the United States included the seismological laboratory in Albuquerque, New Mexico, of the US Geological Survey, as well as Princeton and Stanford universities and the universities of Alaska, Maine and California.
The team assembled data from 337 seismometers run by citizen scientists and 268 stations run by government, university and corporate geologists. It reported that the global quieting began in China in late January and spread to Europe and the rest of the world in March and April. The team said that by the end of the monitoring period, in May, the vibration levels in Beijing remained below those of previous years, suggesting that the pandemic was still restricting activity in the Chinese capital.
In New York City's Central Park, the vibrations on Sunday nights during the peak lockdown period registered as 10 per cent lower than previously measured.
Overall, the greatest reductions were in big cities and other densely populated areas. The team said the quietude let scientists pick up previously hidden earthquake signals and that continued analysis of the data may help geologists learn how to better differentiate between human and natural vibrations.
The findings of the team were so surprising and clear that they generated news accounts in early April, almost two months before closure of the monitoring window.
In its Science paper, the team reported that the quieting was also quite evident at tourist spots. At the lush tropical isle of Barbados in the Caribbean, the lockdown began on March 28 and, compared with previous years, produced a decline in ground vibrations of up to 50 per cent. The team found similar reductions at ski resorts in Europe and the US.
Globally, median levels of shaking fell up to 50 per cent between March and May, "highlighting how human activities impact the solid earth", the authors wrote.