Seemorerocks: Update on Califonia CO spike

Tuesday, 1 March 2016

Update on Califonia CO spike

EARTHQUAKE ALONG THE COAST YESTERDAY AFTERNOON?

THE PACIFIC NORTHWEST SEISMIC NETWORK SAYS NOT SO, HOWEVER ….

Via Facebook

29 February, 2016

Here are responses from Bill Steele & John Vidale from the Pacific Northwest Seismic Network about the ground shaking, window rattling event we had along the Washington Coast, including Grays Harbor County yesterday afternoon @ 3:30pm. The professionals believe it was NOT an earthquake and was possibly generated from some type of offshore military training. They have evidence an event DID OCCUR (see chart below) but it was not an earthquake and likely some air event.
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Chuck,

Here is a recording of the boom at two stations close to Ocean Shores. Air waves travel at ~ 1/10 the speed that earthquake P waves move through rock which is why the arrivals are seconds apart. My guess is the Navy was busy off shore though they will never admit it. (at least they haven't in the past.) Here is a comment from John Vidale who also looked at this.

Thanks-

Bill

John Vidale Clearly recorded on the nearby seismograms, that must have been LOUD. But an air wave, not an earthquake. Likely airplanes.

Those tick marks are each second - the sound is 10s long.
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Chuck,

We’re not saying the ground didn’t move. We’re saying that the ground only moved in such a small area that the energy is mainly being propagated in the air, not in the ground. Ground shaking from an earthquake with such intensity and duration would show up on many stations across the region.

This is confirmed by the observations from OCEN and CORE. Two stations about 10 miles apart show the energy arriving 20 seconds apart. The speed of sound in the air is 300 m/s, the speed of seismic waves is 3 km/s, 10 times faster. These waves are traveling in the air, shaking the ground and our instruments. Or the source is flying through the air, taking 20 seconds between passing by the stations. If the waves were travelling in the ground, the energy would arrive at the two stations more nearly simultaneously.

John

prof john vidale@uw.edu
PNSN Director
Washington State Seismologist
PNW-ANSS Regional Co-ordinator