A mystery boom in the Lincoln area of Christchurch has rattled residents there.
Police say an alarm has gone off at Lincoln High School, but they're not sure what caused the noise.
Local man Jerry Shearman says his neighbours have all been out on the street trying to work out what was going on.
He says he was sitting in the house at about 10:15 and a big kind of sonic boom shook the house a bit.
Geonet reports a magnitude 2.4 earthquake struck to the southeast of Christchurch at about 10:30, at eight kilometres deep.
But the mystery boom was a quarter of an hour before that.
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