BINGO. It was Pacific Aerospace. I just got off the phone with Sequim office and yes, just before 10 a.m. they did a big detonation. And yes, they typically do it around the same time on Thursdays, so mystery solved. Their business is welding different metals, like aluminum and steel.
They bury the metals, load about 500 lbs. of explosives, detonate them and the gigantic blast and ensuing heat, weld two metals that couldn’t be combined any other way. Some of the metal is used for rocket nozzles, which is an indication of how high the heat is.
Shannon Moneo Sooke