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Shaking along coast Tuesday wasn't an earthquake, seismologists say

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WESTPORT, Wash. -- Social media started percolating Tuesday afternoon with a number of reports along the central Washington coast of mild shaking, and wondering if they just had experienced an earthquake.

However, none of the sensors at the Pacific Northwest Seismology Network triggered an earthquake alert -- usually those alerts happen within moments of a quake.

So, was it a quake?

A letter from PNSN seismologists to the Grays Harbor Emergency Management said that two seismographs on either side of Ocean Shores about 10 miles apart did pick up some mild shaking, but it was not a classic quake signature.

Instead, there was a 20-second delay between when the two seismographs started squiggling. The speed of sound is about 10 times slower than the speed of quake energy spreading through the ground, and the 20-second delay suggests it was a sound event, said state seismologist John Vidale. An earthquake would have shown up nearly simultaneously on the graphs.

Bottom line: The seismologists' hypothesis was that it was caused by airplanes -- possibly sonic booms, maybe from offshore military exercises.

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