Every step you take is a micro-earthquake -- it produces tiny vibrations that propagate through the Earth. Other human activities, like construction and traffic, produce stronger vibrations that researchers can detect using instruments called seismometers.
In a new study, researchers report a near-global 50% drop in this seismic noise. This is the first global study of the pandemic on the solid Earth beneath our feet.
I was but a wee student when our university group got to see a seismograph. It was an old unit, hidden in a dusty university room. As we were getting a brief lecture on how it works, the needle suddenly started to move, and we all freaked out a bit.
"That's just the subway," we were told.
Sure enough, a subway station was nearby, but the fact that human activities such as public transit can create seismic vibrations was an eye-opener.
In fact, not only is human activity detectable on seismic sensors, it's an active problem. In some instances, humans produce so much noise that they distort information from actual earthquakes. For the last few months, however, that kind of stopped.
As the pandemic forced human activity to slow or shut down entirely, the noise lessened, says Dr. Stephen Hicks from Imperial's Department of Earth Science and Engineering, co-author of the study:
"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."
"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."
Although the earthquake activity has proceeded as normal, the seismic noise has dropped considerably, particularly in urban areas. We know this type of thing can happen as we've seen it briefly around New Years and Christmas when activity quiets down -- but never on such a scale.
Some researcherrs are already calling this period the 'anthropause', the break in human activity. There are many ways to see the anthropause in action, from a drop in pollution levels to reduced mobility, and now, in seismic activity.
Do the Raspberry Shake
To get a clear picture of how the anthropause is 'visible' in the subsurface, researchers used data from 268 stations in 117 countries, even using some citizen seismometer stations such as Raspberry Shakes. Technology such as the Raspberry Pi (a credit-card sized computer, which the Raspberry Shake is also based on) has opened up a new world of scientific projects, including seismic research.
Researchers observed the effects of lockdowns starting in late January 2020 in China, and then expanding into Europe and the rest of the world from March onwards. The noise level reduction was, at times, larger than that observed over Christmas or New Years. The largest drop in vibrations was observed in the densest-populated areas such as Singapore and New York City, but the team also observed the anthropopause in remote areas such as Germany's Black Forest and the town of Rundu in Namibia.
In addition to being an interesting view at the drop in human activity, this also highlights the problems of the day-to-day noise associated with things like traveling, drilling, and construction.
Researchers often monitor hazardous areas using seismic information. If our activity is concealing these natural signals, it may be increasingly difficult to predict impending hazards (such as landslides or volcanic eruptions).
"With increasing urbanisation and growing global populations, more people will be living in geologically hazardous areas," explained seismologist and lead author Thomas Lecocq of the Royal Observatory of Belgium.
"It will therefore become more important than ever to differentiate between natural and human-caused noise so that we can 'listen in' and better monitor the ground movements beneath our feet. This study could help to kick-start this new field of study."
The study's authors hope that their work will inspire other further research on the seismic lockdown Dr. Hicks concludes:
"The lockdowns caused by the coronavirus pandemic may have given us a glimmer of insight into how human and natural noise interact within the Earth. We hope this insight will spawn new studies that help us listen better to the Earth and understand natural signals we would otherwise have missed."