Seaquakes - again

Scuttlebutt
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Posted about seaquakes before.

Was reading Bowditch last night on waves, was wondering about max SWH (Significant Wave Height) and saw this:

In addition to seismic sea waves, earthquakes below the surface of the sea may produce a longitudinal pressure wave that travels upward at the speed of sound. When a ship encounters such a wave, it is felt as a sudden shock which may be so severe that the crew thinks the vessel has struck bottom.

From the original thread:

We felt one while coming around the SW tip of Japan, a few minutes later the SAT-C spit out the tsunami warning (canceled after a short time) very close to our position.

At first the mate thought the ship had run aground, but we were in deep water, the chief thought it was heavy cavitation, 1 A/E thought something wrong in the E/R.

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I remember some vessels experienced this in the earthquake that flattened parts of Sumatra 20 years ago. I was mid Pacific and felt nothing.

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It’s a “longitudinal pressure wave that travels upward” so I’d think the ship would have to be relatively close to the epicenter at the time the earthquake struck.

It must depend on the depth but presumably only the few ships near the epicenter would feel the pressure wave.

4

I was at the dock in an engine room when the Nisqually earthquake hit Seattle. The epicenter was only about 35 miles away and the first indication we had was a loud bang that sounded exactly like we were in a collision but there was no motion. The rolling began shortly afterward and continued for about a minute.

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