During the COVID-19 lockdown period, global anthropogenic seismic noise dropped 50 percent during the first half of 2020, making it the quietest since there are records.
The quiet period, an anthropause, was likely caused by the global effect of social distancing measures, the closure of industry and services, as well as a drastic drop in tourism and travel.
This resulted in what scientists say is the longest and most pronounced quiet period of seismic noise in recorded history.
This is according to the results of an international study in which scientists monitored anthropogenic seismic noise parameters through seismometers.
We know that human activity causes vibrations that propagate through the ground and that the origin of vibrations with frequencies between 1 and 15-20 Hz that are registered by seismometers more or less continuously is related to traffic, trains or industrial activity, among others, explained Jordi Díaz, a researcher at the CSIC.
For the study, the scientists collected a large amount of seismic data from more than 300 recording stations spread across the planet and analyzed energy variations in that frequency band from four months before confinement began to the present day.
The results were unsurprising; global anthropogenic seismic noise has dropped by as much as 50%, the lowest since records are being kept.
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,” explained Co-author Dr. Stephen Hicks.
“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,” he added.
The first clip shows the ‘wave’ of noise dampening as the world locks down. The second shows the UK’s seismic noise reduction after the lockdown was announced.
The study, which was published in the magazine ‘Science’, has confirmed what was seen on the streets of large cities: Seismic noise caused by human activity was cut in half during the first months of the year as a result of a drastic drop in human activity since the beginning of February in places like Beijing (China) or Hong Kong and from mid-March in the rest of the world.
“It can be seen that there has been a progressive recovery of the noise level in recent months, but the pre-confinement levels have not yet been reached,” explain researchers.
In order to have a global view, researcher Thomas Lecocq, from the Royal Observatory of Belgium and lead author of the new study, developed a system of analysis to unify the criteria for studying the data by the international seismological community.
Thus began a collaboration in which 76 authors from 66 institutions in 27 countries have worked in a coordinated manner.
The decrease in man-made vibrations has allowed us to identify signs of small earthquakes that would have gone unnoticed otherwise.
In addition, this study shows that seismometers can be a good tool to monitor processes that are not essentially related to geology or geological processes.
Since seismometers are easy to install, and their maintenance is relatively cheap, the instruments can become a good tool that allows scientists to study other processes, both artificial and natural in origin.