From March, lockdowns across the globe saw streets become ghost-like relics of familiar, bustling centres of commerce. Travel restrictions grounded airplanes, train travel screeched to a halt and the sound of car engines was largely silenced. The peculiarities of this time are manifold and run deep. Seismologists have discovered that lockdowns across the world resulted in the longest and most pronounced quiet period of ‘seismic noise’ in recorded history.
To detect earthquakes, geoscientists use seismometers to eavesdrop on the seismic noise emitted from tectonic shifts that ripple through the Earth. However, human activity on the surface also causes vibrations that propagate into the ground as high-frequency seismic waves. ‘Whether we drive our car, catch the train, or touchdown on an airport runway, each of us contributes to anthropogenic seismic noise,’ explains Stephen Hicks, a seismologist at Imperial College London.
The Royal Observatory of Belgium, along with five academic institutions, including Imperial, have gathered seismic noise data from a global network of 268 seismic stations in 117 countries. During lockdowns, they recorded a global median reduction in seismic noise of up to 50 per cent – leading some researchers to coin the term ‘anthropause’. Predictably, the strongest reductions in seismic noise occurred in populated environments: a 50 per cent reduction was recorded as tourism was grounded in Barbados and the Sri Lankan city of Kandy; while a 33 per cent reduction was seen in Brussels, where lockdowns were enforced from 18 March.
The pronounced period of quiet has its uses. The muting of anthropogenic noise allows scientists to focus on natural tectonic sounds. ‘Anthropogenic noise has always been an unwanted artefact on seismographs. Smaller signals from natural tectonic sources can get lost in noise resulting from the anthropogenic activity occurring at the surface,’ says Hicks.
Large-magnitude earthquakes are generally accompanied by smaller signatures of tectonic sounds that are often obscured by the hubbub of anthropogenic activity. Lockdowns present the best opportunity to date for seismologists to pinpoint these small signatures. ‘If we can detect the signatures of smaller earthquakes that occurred during lockdown, we might be able to go back through the archives of seismograph data and find similar signatures that might, for example, accompany larger earthquakes,’ says Hicks.
Discoveries are already being made. During lockdown, a magnitude-five earthquake occurred southwest of Petatlán, Mexico. Due to the reduction in anthropogenic noise, seismologists were able to hear its tectonic sound signatures more clearly. The newly identified signals could be used as templates to monitor tectonic unrest in the future, potentially hastening earthquake-prediction methods.
Hicks explains that lockdowns have provided a rare opportunity for his field. ‘With growing urban populations in tectonically active areas, such as Tokyo, San Francisco, or Santiago, anthropogenic noise is going to increase,’ he says. ‘With urbanisation, it’s becoming more important that we understand the small tectonic sound signatures, so that we can better forecast large-magnitude earthquakes. We’ve never really been able to quieten anthropogenic noise because we’ve never had a coherent shutdown – now, new avenues of research are opening up.’