Thursday, April 16, 2009

Did you ever wonder?

Earthquake, on a boat... did you ever think about it? I know it was something I wondered about, tho in an entirely theoretical way, even before we moved aboard.

During the February 28, 2001 Nisqually earthquake here in Seattle, I was home sick with the flu. It was a quiet day - no wind, and no activity on the water. And I was laying on the settee in the saloon, kind of dozing, half awake.

Then the boat began to bump up and down sharply. Not really like a wake had passed under us. This was a much sharper movement. Eolian is heavy. It takes a lot to move her quickly.

I jumped up and looked out the window to see what the heck was happening. In the back of my head I was already thinking, "This is unusual... this... is an earthquake?"

The pilings at the ends of the finger piers were all waving around in circles, making circular wave patterns. Masts were all waving in random patterns, as they all have different natural frequencies. And strange waves, maybe 6" high were moving around everywhere. And Eolian was going bump, bump, bump.

And then it was over. It lasted maybe 10 seconds at the outside. On shore, the reported length ranged from 45 to 60 seconds. Why the discrepancy? Well, although water carries compression waves just fine, transverse waves just can't be coupled to it. Imagine the seabed moving back and forth, but not up and down. It would just move beneath the water, hardly affecting it except at the edges. My theory is that most of the motion felt on land was the result of transverse waves, which must have lasted a lot longer.

Or maybe it was the Nyquil.

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