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.
Perhaps, in the future, bands will compete to have the most seismically active concerts.
Link -via I Heart Chaos | Image: Wairakei GeoNet