NOISE POLLUTION
Boost for earthquake, volcano research from fall in noise during Covid-19 lockdowns
Michael Daly Jul 24 2020

LAWRENCE SMITH/JASON DORDAY STUFF.CO.NZ

Auckland has become a ghost town during the level 4 lockdown, with deserted streets, motorways and beaches.

A quietening during Covid-19 lockdowns of the seismic noise created by human activity could lead to new insights into earthquake and volcanic activity, particularly in urban areas.

Seismologists analysed datasets from more than 300 international seismic stations – some in New Zealand - and found the buzz of human activity dropped dramatically during lockdowns, the University of Auckland said.

The study provided the first evidence that previously concealed earthquake signals, especially during daytime, were much clearer on seismic sensors with reduced noise from human activity.

Less seismic noise during lockdowns could help researchers uncover new insights about earthquake and volcanic behaviour, particularly in urban areas.

Researchers hoped the findings would help them detect previously hidden signals from earthquakes and volcanoes in future, the university said.


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University of Auckland seismologist Dr Kaspar van Wijk was among those involved in the study.

“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,” he said.

Professor Martha Savage, from Victoria University of Wellington, also took part. The study could contribute to geological research, she said.

“Earthquake signals can often be concealed or obscured by seismic noise, but they appeared much more clearly on the seismometers during lockdown. This data could be used to find new insights about earthquake activity and volcanic activity, particularly in urban areas.”

DOMINICO ZAPATA/STUFF
The Waikato Expressway during the lockdown for coronavirus.

A team of 76 researchers from 66 institutions in 27 countries worked on the study. They were able to clearly link reductions in human activity with lower seismic readings, Savage said.

“This is exciting for future research, as it gives us a way to broadly track human activity in near real-time without affecting people’s privacy, as we don’t need to track specific people or sources of noise.

“This could be used now to track the effects of pandemics and the recovery from Covid-19 and how it has impacted human activity.”

The quiet period during the lockdowns was the most prominent seismic noise reduction on record.

UNIVERSITY OF AUCKLAND
Seismologist Dr Kasper van Wijk from the University of Auckland

A report on the study, published in the journal Science, said the researchers found a near-global reduction in noise of up to 50 per cent, starting in China in late January, then followed by Europe and the rest of the world in March and April.

Human activity caused vibrations that spread into the ground as high-frequency seismic waves, and the biggest falls in seismic noise from those waves during the lockdowns was found in populated areas.

While lockdown effects were seen most strongly at surface seismic stations, researchers also detected them underground, including on seismometers installed in boreholes in Auckland to minimise the effects of human activity.

One of those seismometers is buried 380 metres under Eden Park and another is 98m down under Motutapu Island.

Measurements of high-Frequency Seismic Ambient Noise (hiFSAN) levels at both stations varied between weekdays and weekends before the lockdown, suggesting both were sensitive to human activity, the report said.

While the island station was quieter overall, the lockdown led to a halving of seismic noise at both stations.

A spike in noise at the Motutapu station during lockdown was thought to have been caused by strong winds and high waves. After restrictions were lifted, seismic noise returned to pre-lockdown levels.

ABIGAIL DOUGHERTY/STUFF
Built on a volcanic field, Auckland is akey focus for seismologists

Understanding the sources of seismic noise in urban areas was vital, with human activity degrading the capability of seismic networks to detect signals associated with earthquakes and volcanic eruptions, the report said.

The ability to analyse the full spectrum of behaviour capable of generating earthquakes was essential for monitoring fault dynamics, and for earthquake forecasting and seismic hazard assessment.

Small earthquakes should dominate datasets but many of the smallest earthquakes were often missing.

“This detection issue is especially problematic in populated areas, where anthropogenic noise energy interferes with earthquake signals,” the report said.

Low noise levels during lockdowns could allow detection of seismic signals from new sources in areas with incomplete seismic catalogues. Those signals could be used to find similar activity before and after lockdown when there was more seismic noise.

The same approach worked for tremor signals masked by human activity that were vital for monitoring potential volcanic unrest.

Any increase in the use of low-cost sensors in urban areas would need a better understanding of the sources of seismic noise caused by human activity to suppress false detections.