How humans can set off seismometers

Seismic stations record any kind of ground motion, and therefore much more than just earthquakes. Our measurements are dominated by ambient seismic noise. These are vibrations caused by distant ocean waves, atmospheric pressure changes, or anthropogenic events such as blasts in quarries, traffic, construction work or explosions. This noise can be used to image the Earth beneath us, or to estimate the extent to which the local subsurface can amplify seismic waves.

Where relevant or necessary, this data can also be used to draw conclusions about events with a seismic footprint. US singer Taylor Swift's concert in Zurich on 9 June was one such event (others occurred on dates when she performed elsewhere). When a lot of people move rhythmically at the same time, this energy is transferred into the ground as harmonic vibrations and can be measured by nearby seismometers. The vibrations caused by the concert audience were visible at eight stations in the seismic network within a 6 kilometre radius of the Letzigrund Stadium. Beyond that, they got lost in other background noise. Previously, other large concerts or football matches have left similar marks on the seismic data.

How clearly human motion can be made out in the background noise depends on a number of factors. For example, there is the distance from the nearest seismic station. One person jumping immediately next to a seismometer will be enough to bring about visible fluctuations. As the distance from the seismometer increases, a larger number of people moving will be needed to produce this effect. For instance, if Switzerland had won the 2024 European Football Championship, families leaping up from their sofas would not have been reflected in the seismic data, whereas the enthusiasm of fans at large-scale public viewings might have been picked up. The subsurface is also a factor here. If crowds move about rhythmically on a soft subsoil, such as the gravel deposits of the Limmat Valley, the vibrations are stronger than on harder rock, although they are also attenuated more quickly with distance. The ground motion is further amplified when people cause a structure, such as a stand, to vibrate at its resonant frequency. Of course, the opposite effect can also be observed. For instance, the measures taken to combat the spread of COVID-19 led to a worldwide reduction in seismic noise caused by human activity from early to mid-2020 (SED news item).

As to whether Taylor Swift generates more or less enthusiasm than other acts or a football match also resulting in measurable seismic vibrations, this cannot be reliably determined from the seismic data.