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With Covid-19, a Seismic Quiet Like No Other

2020-07-24 03:15:12

The shock waves of civilization 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 Center registered in five 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 percent in the global din tied to humans. The main quieting, from March through 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 on Thursday 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 an Irish engineer, John Milne, established on the Isle of Wight a modern seismometer center that quickly grew into the world’s first global network, with 30 overseas branches. By 1957, an international group of seismologists listed 600 stations. The devices can pick up vibrations not only from earthquakes and human activities but 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 of the U.S. Geological Survey, as well as Princeton University, Stanford University, the University of Alaska, the University of Maine and the University of 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 there.

In New York’s Central Park, the team reported, the vibrations on Sunday nights during the peak lockdown period registered as 10 percent lower than previously measured.

Overall, big cities and other densely populated areas produced the greatest reductions. 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 destinations. At the lush tropical isle of Barbados in the Caribbean, the lockdown began March 28 and, compared with previous years, produced a decline in ground vibrations of up to 50 percent. The team found similar reductions at ski resorts in Europe and the United States.

Globally, median levels of shaking dropped up to 50 percent between March and May, “highlighting how human activities impact the solid Earth,” the authors wrote.

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