Mass Quarantine From COVID-19 Mutes Urban Noise, but Earth Roars On

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Urban noise fades, but the Earth roars on

Social distancing is so endemic to the COVID-19 pandemic that it feels somehow cheap to call it a theme. But the daily noise created by the activity of humans — or anthropogenic seismic noise and vibrations — is fading. In normal times, travel, transits, and concerts send waves of urban noise through the Earth, in what the journal Nature calls the "hum of vibrations in the planet's crust," without which the world experiences less kinetic motion.

However, behind and between the waves of urban noise is not silence, but the Earth itself — vibrating to the slow, sporadic pulse of geological disturbances. Think of a fateful avalanche, wild volcanic eruptions, or even the surreal and potentially world-ending impact of a meteor. These cataclysmic events send low-frequency sounds reverberating through the Earth's crust, but (unless you're near enough for danger) below the threshold of human hearing. This is why we can only listen to these events through instruments specifically designed to detect them — like seismographs — according to NOAA's ESRL Infrasonics Program.

The roar of traffic, humming machinery and electrical power lines — all of these sources of urban noise mask natural seismicity, reports a 2017 study on urban noise. The difference in seismic activity is witnessed between working days and holidays, or between night and day — when people are out living life as usual. Even mainstream rock can rattle the Earth's crust.

For geoscientists, the way to monitor this is with a real-life survey, because it's the most direct way to collect seismological data on urban noise in the Earth's crust. But where there are stay-at-home orders, this isn't always possible because there's no way to physically set up monitoring devices.

This is what happened to a research group called ParkerJones Acoustics based in Bristol, U.K., who had to cancel plans for live surveys in light of the late-March lockdown.

"Last night we were just about to head out to a couple of sites to do some overnight noise monitoring," he began, before lamenting the circumstances of a lockdown.

Of course, there's a way around it. Sort of.

In a Medium post, the group noted that they can use historical data and noise map modeling based on traffic count data to interpolate where on an urban noise scale levels should be. This is thanks to an international noise-mapping infrastructure that's been more or less active for 18 years.

Urban mapping infrastructure

Since 2002, the European Union has actively monitored urban noise as a part of its European Environmental Noise Directive, designed to provide several smart tools to access and manage environmental noise, and enhance city development. According to the directive, the idea behind building urban noise policy is to protect people against the negative effects of noise pollution. Anyone who's tried to make a call on or around a train platform can imagine how diffucult life would be if spikes in urban noise weren't highly localized.

However, urban noise is more than traffic and the hum of electrical infrastructure — it's also railways, airports, and the industrial resources to which modern cities are coupled. This wider cast of activity creates a framework for policy-makers globally, including politicians, transportation (or civil) engineers, urban planners, architects, and even private citizens — so everyone can play a part in building regulations and legal mitigation proceedings.

In other words, the incorporation of urban noise into the legal structure of a city gives everyone a common language to address rising concerns about urban noise. With this sonic infrastructure, geoscientists have monitored some surprising events.

Researchers at the Institute of Geological and Nuclear Sciences of Australia recorded seismic activity during a Foo Fighters concert in 2011, and an AC/DC concert four years later. Their seismographic recording monitored the shakes caused by the "weight of the fans dancing, as well as the sound system."

Low urban noise levels could reveal new seismic activity

However, since the rise of COVID-19, anomalies of excess have been replaced by an excess of dips of urban noise where spikes used to be.

I've updated the graph, including the whole 2020 data, so we can compare with other weeks of lower activity, like school holidays in Feb or XMas holidays. The current mean noise level is 33% lower than before the #StayHomeBelgium measures. https://t.co/mL9j48e134

— Thomas Lecocq (@seismotom) March 27, 2020

For example, in Belgium, seismologist Thomas Lecocq of the Royal Observatory of Belgium in Brussels said the average level of urban noise is 33% lower than it was before self-quarantine measures took effect locally, on March 14. Lecocq set up a coding system to monitor seismic data from other locations to show the effects of social distancing.

Much the same is happening in cities globally; in London, Los Angeles, Paris, Auckland, and London.

"It seems quite clear that over the last few days, the increase in noise level at dawn (blue line) is much less steep than over the past few weeks," wrote Stephen Hicks, a seismologist with Imperial College London, in a March 26 tweet. "I guess this is due to a much weaker morning rush hour — fewer people commuting and no school runs."

Here's daily mode noise power from a station in Los Angeles over the past month; the drop is seriously wild.

Graph made on IRIS MUSTANG: https://t.co/aTfnmL6kBB pic.twitter.com/rdgzZxMcAU

— Celeste Labedz (@celestelabedz) March 26, 2020

While the new reality of social distancing means coping with the drag of perpetual cognitive estrangement, it also presents an unprecedented learning opportunity for geoscientists who, upon placing a seismological ear to the ground in lieu of urban noise, might create a baseline model of seismic information, and greatly advance future studies. Without people running, driving, and flying across the Earth, we might even detect new diminutive levels of micro-earthquakes from deep within the planet's crust, and unlock mysteries of seismic origin never seen, or heard, before.