Notice anything odd on November 11? Geologists did. A deep thrum rippled around the world lingering a whole 20 minutes. It wasn’t an earthquake. Nobody knows what it was.
Earthquakes, by their very nature, usually register as short-sharp ‘cracks’. As tensions in the Earth’s crust suddenly release, pulses of clearly identifiable seismic waves radiate outwards from where the slippage occurs.
The first signal is called a Primary wave: high-frequency compression waves that radiate in bunches.
Then comes a Secondary wave: these high-frequency waves tend to ‘wiggle’ somewhat more.
Only then comes the surface waves: these slow, deep rumbles tend to linger, and can circle the Earth several times.
The November 11 event is notable in that no Primary or Secondary waves were detected.
All that registered was the deep resonate surface wave. And it didn’t ‘rumble’ as an earthquake’s surface wave tends to. Instead, it maintained a much cleaner — almost musical — frequency.