The quiet Earth

As many daily activities came to a halt during lockdown, the Earth itself became quiet, probably quieter than it has been since humans developed the technology to listen in.

Dr Kasper van Wijk

Seismologists have analysed datasets from more than 300 international seismic stations – including several located in New Zealand - and found the “buzz” of human activity, called anthropogenic noise, dropped dramatically.

They have dubbed this period the ‘anthropause’ because it’s the longest and most prominent anthropogenic noise reduction on record. It was detected not just in cities but in some of the planet’s most remote places including sub-Saharan Africa.

In New Zealand, seismologist Dr Kasper van Wijk from the University of Auckland was busy looking at seismic data from the tragic eruption at Whakaari/White Island and wasn’t really thinking about lockdown until colleague Dr Thomas Lecocq of the Royal Observatory of Belgium got in touch.

“I used the computer code for White Island to analyse Auckland’s seismic data, and to my surprise, within an hour I could confirm that Auckland was not only quiet above ground but also underground,” he says.

Traditionally, measuring seismic waves is focused on detecting earthquakes and volcanic activity but because seismographs are so sensitive, they can also pick up vibrations from humans at the surface as we walk around, drive cars and catch a train. Heavy industry and construction work also generate seismic noise.

New Zealand’s strict lockdown measures meant a lack of anthropogenic seismic noise was detected at places like Eden Park where a seismograph is buried 380m beneath the sports grounds, and even on Motutapu Island in the Hauraki Gulf.

Because it is situated on a volcanic field, Auckland is a key focus for seismologists – there are 12 seismographs in and around the city, monitoring for even the weakest signs of earthquakes or volcanic unrest.

During lockdown the seismic signal of an earthquake in Mexico appeared clearer-than-normal as humans were confined to quarters. The project leaders, including Dr van Wijk, hope their work will help improve our ability to detect these previously hidden signals.

“One day a volcano in Auckland’s volcanic field will erupt but it will create seismic signals before that happens and this study reminds us that if humans made less noise, we would get an earlier warning,” Dr van Wijk says.

Findings from the study are published in Science and show a 50 per cent reduction in seismic noise on seismographs around the world from early- to mid-2020. Lead authors were based in New Zealand, Belgium, the United Kingdom and Mexico and overall 76 scientists from 27 countries were involved.

There are thousands of international seismic monitoring stations, some run by enthusiastic amateurs including students. Dr van Wijk heads the Ru Network of Seismometers in Schools programme which has seen dozens of seismographs installed in schools that, while not as powerful as professional seismic stations, detect earthquakes. At St Mary’s School in Rotorua the days of lockdown were quieter than even any day during the summer school holidays.

The study found a strong match between seismic noise reductions and human mobility data from mapping apps on mobile phones, made available by Google and Apple. This correlation allows open seismic data to be used as a broad proxy for tracking human activity in near-real-time, and to understand the effects of pandemic lockdowns and recoveries without impinging on potential privacy issues, Dr Lecocq says.

It is also the first evidence that previously concealed earthquake signals, especially during daytime, are much clearer on seismic sensors with reduced anthropogenic noise and the researchers hope this will help them detect previously hidden signals from earthquakes and volcanoes in future.

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