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| You can almost pickout individual songs on the signal! |
Two seismic stations in the Auckland GeoNet seismic network recorded the ground literally rocking to the Foo Fighters gig at Western Springs on Tuesday night. The Herne Bay -HBAZ and Eden Park -EPAZ (with IESE-Auckland Uni) stations are 1.5 and 2km from Western Springs respectively and recorded a strong low frequency signal associated with the Foo Fighters gig.
The first vibrations were recorded around 7:30pm, part way through the Tenacious D set, but the biggest shakes started at 8:20pm when the Foo Fighters took the stage, and then it all went quiet at 11pm when the gig ended.
The concert vibrations were recorded as a semi continuous harmonic signal with a peak osculation of 3Hz, ie the ground was shaking 3 times per second in a nice rhythmic motion. There are lulls in the signal between the songs and peaks in signal intensity during the songs.
The cause of the shaking is most likely the weight of the 50,000 fans dancing, as 50,000 fans is equal to around 5,000 tonnes of mass moving(or moshing)on the ground for the duration of the concert. This set up a nice harmonic vibration in the ground which was recorded in our nearby borehole seismometers.
The sound system, probably contributed as well, especially the bass frequencies coupling in to the earth. The signal is similar to volcanic tremor that is recorded at places like Mt Ruapehu and White Island, however the strong correlation in time with the Foo Fighters concert and the fact it was only recorded on the 2 stations close to Western Springs assured us it was of man-made origin.
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| Detail of the signal showing the 3Hz vibration |
Thanks to Craig Miller and the Wairakei GeoNet crew for the info!
Nice one! Found this blog via
ReplyDeletehttp://www.theregister.co.uk/2011/12/15/dave_grohl_makes_earth_move/
Are you sure this is not a generator or fans rocking some stands that resonate at 3Hz? Most music produces much less peaked spectra, and spectra with energy at higher frequency, and spectra that change over time.
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ReplyDeleteIn 2001, during a gig by Faithless in the Belgian venue Vorst Nationaal. the dancing created waves in the ground which were recorded on a seismograph in a research center in Uccle 1 mile away.
ReplyDeletehttp://www.cobra.be/cm/cobra/muziek/1.997795
site is in dutch, but in the bottom video clip people living in the area talk about how they perceived this at home (swinging chandeliers, pot plants moving etcetera)
John, there's no stands at Western Spings, it has concrete / grass bank seating
ReplyDeleteHi John, I was somewhat surprised by the strong resonance and suspect it may be a site effect. Western Springs is an old swamp so higher frequencies may be strongly attenuated by the soft ground and lower frequencies amplified. Reports from people at the concert say they could actually feel the ground moving underneath them. I wouldn't think it's a generator as the signal has a strong time correlation with the time the FF's were playing, and with lulls between songs, whereas i'd expect a generator to be running more continuously and to have started well before the FF's took the stage. As Muttley said there's no stadium at Western Springs to act as a resonance source. Thanks for your comments. Cheers Craig Miller
ReplyDeleteI was at the show and was sure I could feel the ground moving as the Foo's went in to overdrive. I actually questioned what I was feeling and wondered if it was just me. Good to hear some evidence backing up the great sensation of the moving ground beneath my feet. Awesome stuff!
ReplyDelete"Oscillation" not "osculation"!
ReplyDeleteI have no doubt that there was plenty of osculation at the concert but seismometers don't measure that.
I was there at the concert, and it was awesome. When you stopped the ground was moving that much, it was like trying to stand in a moving bus
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