There’s enough people at this week’s Burning Man festival to make the seismic needle quiver, says our expert from the British Geological Survey.
Based on his research from the UK’s Reading Festival, Paul Denton explained what it takes to make the earth move, by how much, and the amount of energy a surging crowd can produce.
ResearchGate: Your research has shown an audience’s “rhythmic dancing” can cause seismic vibrations. Could this happen at the upcoming Burning Man?
PD: That’s certainly enough people to create a signal measurable from about 20 kilometers away. In our work at Reading Festival there were about 40,000 people dancing at the main stage, and the signal we recorded 1.5km away from that was equivalent to a 0.6 magnitude earthquake. Since Burning Man festival is two or three times bigger you’d expect the seismic signal to be two or three times bigger than that - if all 70,000 people are dancing at the same time to the same rhythm.
RG: Can this crowd-made quake be felt by other people in the area?
PD: Normally people don’t feel an earthquake smaller than a magnitude two, but there are cases where the general public have mistaken crowd-made vibrations at rock concerts for earthquakes. One instance was in Finsbury Park, London during the 1970s. A lot of people called the police because they thought an earthquake was causing the tower block they lived in to sway. We checked our records and seismometers but couldn’t detect any seismic disturbances in that area. We did, however, know a rock band called Madness had played a concert very close to the tower block. They were playing again the next night and sure enough the same thing happened. So in this instance people did feel the crowd’s vibrations: their high rise building was amplifying the tremors and causing it to sway.
RG: How much energy can be produced from all 70,000 ‘burners’ dancing at the same time?
PD: The energy input per person is about 30 kilojoules per minute, so that’s the typical amount of calories you’ll burn while dancing. That turns into a power input of about 20 megawatts for 40,000 people, 40 megawatts for 80,000 people, and so on. But only a fraction of that is released into the ground. Although, if you make some modest calculations about how high people jump and how much they weigh, for example, the amount of available energy from people moving is much greater than the energy input from a sound system at a large concert. As a comparison, a rock concert’s sound system power input would be 50 – 100 kilowatts.
RG: Researchers first thought loud speakers made the earth vibrate at rock concerts. How did you discover it was the crowd?
PD: A previous study looked at the signal from a permanent seismic station near a music festival. They looked at the hour-by-hour amplitude variations of the seismic signal, and it correlated with the start and end of the music festival. They then assumed the vibrations were caused by the sound system transmitting music into the ground, which wasn’t an unreasonable assumption because sound systems have quite a lot of power. However, because the music festival we analyzed was also broadcast on television, we could correlate the seismic recording on a second-by-second basis with the video of the crowd dancing. We could actually see the amplitude varying as the crowd danced more or less enthusiastically to the different parts of the song. The amplitude of a crowd’s dancing does vary considerably throughout a three minute song.
RG: How might Burning Man’s playa setting contribute to the crowd’s vibrations?
PD: It’s possible that dried lake beds get a resonance effect, so the seismic signal will bounce around the surface and enhance its amplitude. But generally speaking, Nevada is a seismically active region and the crowd’s vibrations won’t even be noticed – the seismologists there will see dozens of similar sized vibrations each day.
RG: How close do your sensors need to be to record these vibrations?
PD: It depends on the background seismic noise level. You need to record an earthquake from at least three or four different seismic stations in order to locate its exact spot and calculate the magnitude. At Burning Man they’d need a sensor within five to 10 kilometers of the site to detect the dancing crowd’s signal. But it wouldn’t register as an earthquake on the Nevada network unless they had three or four stations nearby to detect it, which is probably not the case. In the UK, we only detect earthquakes on our network that are bigger than a magnitude of two.
RG: What are the other possible causes of man-made earthquakes?
PD: The vast majority of signals detected on seismologists’ networks that aren’t caused by earthquakes are from quarry blasts. Geographically, there’s a concentration of very small shallow earthquakes in areas with historic underground mining. A lot of the seismic signals detected in these areas come from old coal field areas and are interpreted as unused underground mineshafts collapsing. It’s the same above ground, too: seismologists also detect seismic signals from buildings being demolished or blown up.
We’ve also detected seismic signals from large sporting fixtures. These are typically football matches with tens of thousands of people attending. I’ve got a colleague in Barcelona who takes readings from a seismic station near the football stadium where Barcelona play. He can tell what time in the match Messi scores a goal because of the spike in seismic vibrations on his sensor. They regularly get 60 – 100 thousand people watching their matches and they go crazy when goals are scored.
Finding the source of a signal is interesting from a scientist’s point of view because it’s like being a detective. When you find a signal but you’re not quite sure what’s causing it, then you have to think about all the different possibilities. It’s nice to find its cause.
This story also appeared in the Washington Post.
Feature image courtesy of Neil Girling; aerial view by Duncan Rawlinson
