Did you feel that? Was it an earthquake?
Earthquakes happen all the time all around the globe. And LIGO sees them all.
When a large earthquake happens anywhere in the world, the entire Earth is shaking like a bell. A magnitude 7 earthquake in Japan might not be something that you feel here in the US, but the ground can move as much as a fraction of a mm per second at the peak, even here on the West Coast. The entire Earth keeps ringing like a tuning fork for several hours.
LIGO has very sophisticated seismic isolation systems to make sure that the mirrors in our instruments are moving much less than the ground at frequencies above 10 Hz, where we are sensitive to gravitational waves.
But below that frequency, where the ground is shaking because of local and global seismic activity, our isolation systems aren’t that good, and when the seismic waves of a large earthquake arrives, the motion can be so large that we have to stop operations for a few hours until the ground motion gets low enough to allow us to resume operation.
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