Nach etwa 25 Minuten gab es für die Fans kein Halten mehr: Da spielte Taylor Swift im Volksparkstadion ihre zwölf Jahre alten Hits „You Belong With Me“ und „Love Story“. Das zeigen nicht nur Videos der beiden Konzerte am Dienstag und auch Mittwoch in den sozialen Netzwerken, sondern auch seismische Messungen in bis zu vier Kilometer Entfernung. Rund eineinhalb Stunden später vibrierte die Erde zu „Shake It Off“ sogar noch stärker.
„Hamburg hat ordentlich mitgeschwungen“, sagt die Seismologin Céline Hadziioannou von der Universität Hamburg. Gemeinsam mit Kollegen hat sie mittels eines Netzwerks aus Glasfaserkabeln untersucht, welche Bodenvibrationen die Konzerte der Sängerin auslösen – genauer gesagt ihre Fans. Die Energie, mit der diese beim Tanzen und Hüpfen auf den Boden treffen, verbreitet sich wellenförmig im Untergrund. Man könne sich das wie die Kreise vorstellen, die entstehen, wenn man einen Stein ins Wasser wirft, erklärt Hadziioannou.
In their data, the researchers can see not only how much the fans dance along, but also how fast or slow the song was. Swift was able to delight her fans equally on both evenings. “The signals look almost the same, and the set list is also followed exactly,” adds physicist Oliver Gerberding, also from the University of Hamburg.
No reason to worry
The phenomenon has already been observed by researchers in several cities and dubbed “Swift Quake”. In Seattle, a researcher even calculated that the total seismic activity of two concert evenings corresponds to a magnitude of 2.3. Hadziioannou does not think the term “Swift Quake” is entirely appropriate, however, because the vibrations are not earthquakes in the true sense of the word: for that to happen, tectonic plates would have to shift, parts of the earth’s crust or underground rock would have to break. In an earthquake, all the energy is also released all at once, not over several hours like at a concert. These small vibrations caused by dancing fans are not noticeable to humans anyway. So there is no need to worry in Munich, where Swift is performing twice this weekend.
Among other things, the researchers hope to learn from the data how the subsoil in the various concert cities dampens or amplifies vibrations. In Ireland, where Swift performed in Dublin at the beginning of July, a colleague, Eleanor Dunn, was able to measure vibrations even 100 kilometers away, says Hadziioannou. In Hamburg, the vibrations were clearly measurable four kilometers away, but the seismologist does not expect them to be 100 kilometers away because of the softer subsoil, which dampens the vibrations more quickly.
The Wave measurement network is located just two kilometers from the Volksparkstadion. Fiber optic cables are buried there and vibrate with the ground. A measuring device sends laser pulses into the fiber, which are reflected back at certain areas. Depending on how long the light pulse takes to return, the researchers know where and how the fiber moved at a certain point, explains Katharina-Sophie Isleif, a physicist from the Helmut Schmidt University, University of the Federal Armed Forces Hamburg, who is also involved.
“Not just something destructive”
Wave comprises a total of 19 kilometers of fiber optics, which, according to the researchers, corresponds to around 19,000 sensors and enables a high density of ground movement data to be measured. The research team works in an interdisciplinary manner: What happens underground is not only exciting for seismologists like Hadziioannou, who uses the data to investigate how the ground beneath the city is changing – for example, the temperature or the soil moisture. But also for physicists like Gerberding and Isleif, who are researching gravitational wave detection.
It’s not just concerts that cause slight vibrations, but also everyday life: for example, buses and trains rolling through the city. But for the sensitive experiments, it has to be as quiet as possible. Even the smallest fluctuations affect their results. The researchers hope to one day be able to predict the vibrations in order to be able to calculate them out of the results or even actively compensate for them.
With the measurements from the Taylor Swift concert, which were even made available in a live stream, the researchers want to show “that seismic waves are not only something destructive, but also something interesting and beautiful.” So visitors to the upcoming concerts in Munich can only be advised: Shake it off!
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