(Source: Royal Observatory of Belgium
Data from a seismometer at the observatory show that measures to curb the spread of COVID – 020 in Brussels caused human-induced seismic noise to fall by about one-third, says Lecocq. The measures included closing schools, restaurants and other public venues from 18 March, and banning all non-essential travel from 18 March (see ‘Seismic noise’).
The current drop has boosted the sensitivity of the observatory’s equipment, improving its ability to detect waves in the same high frequency range as the noise. The facility’s surface seismometer is now almost as sensitive to small quakes and quarry blasts as a counterpart detector buried in a 100 – meter borehole, he adds. “This is really getting quiet now in Belgium.”
Information boost
If lockdowns continue in the coming months, city-based detectors around the world might be better than usual at detecting the locations of earthquake aftershocks, says Andy Frassetto, a seismologist at the Incorporated Research Institutions for Seismology in Washington DC. “You’ll get a signal with less noise on top, allowing you to squeeze a little more information out of those events,” he says.
The fall in noise could also benefit seismologists who use naturally occurring background vibrations, such as those from crashing ocean waves, to probe Earth’s crust. Because volcanic activity and changing water tables affect how fast these natural waves travel, scientists can study these events by monitoring how long it takes a wave to reach a given detector. A fall in human-induced noise could boost the sensitivity of detectors to natural waves at similar frequencies, says Lecocq, whose team plans to begin testing this. “There’s a big chance indeed it could lead to better measurements,” he says.
Belgian seismologists are not the only ones to notice the effects of lockdown. Celeste Labedz, a graduate student in geophysics at the California Institute of Technology in Pasadena,
tweeted that a similar fall in noise had been picked up by a station in Los Angeles. “The drop is seriously wild,” she said.
However, not all seismic monitoring stations will see an effect as pronounced as the one observed in Brussels, says Emily Wolin, a geologist at the US Geological Survey in Albuquerque, New Mexico. Many stations are purposefully located in remote areas or deep boreholes to avoid human noise. These should see a smaller decrease, or no change at all, in the level of high-frequency noise they record, she says.
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