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Mark Borgerson

In article s.com,
llid says...
"thunder" wrote in message
t...
On Sun, 04 Oct 2009 12:36:52 -0400, Wilbur Hubbard wrote:

If you can feel and hear an earthquake on land it generally means it is
probably within a couple hundred miles of you. (in this case it was
about 150 miles away). Intelligence also means you know the typical
tsunami travels about fifty miles per hour.

I don't think that's accurate. Depending on the depth, a tsunami can
travel @600 MPH. As the water gets shallower, the wave slows down.
Perhaps you meant, 50 MPH when the wave had crested in the harbor.

Wave speed is computed from the square root of the quantity water depth
times the acceleration of gravity. The speed at which tsunamis travel
depends on the ocean depth. A tsunami can exceed 500 mph in 15,000 feet of
water but slows to 40 mph or less in 100 feet of water. In less than 24
hours a tsunami can cross the entire Pacific Ocean.

Different types of tsunamis can definitely travel at different speeds
depending on how they originate. The tsunami that struck American Samoa was
caused by a shift in a fault line directly below the origin. This causes a
slower wave train than if, for example, the side of a volcano sloughed off
violently into the ocean. In this case the wave is hastened by the original
push.

I think the water around American Samoa atoll is quite shallow so it slows
down the speed quite nicely. But, the point of origin is quite deep so that
would have sped it up.

But, you are right. I did underestimate the time factor. I should have
called it an hour instead of three - still plenty of time for those losers
to get out of there.

From several reports that I've read, the first large waves struck
American Samoa 15 to 20 minutes after the earthquake. The epicenter
was about 120 miles from Pago Pago, so that means a speed of advance
of about 360 to 480mph.

The average water depth between American Samoa and the earthquake
epicenter is about 13 to 15,000 feet. The water shallows to less
than 3000 just a few miles offshore. With that depth, I would
expect the wave to move near the theoretical maximum speed---which
was apparently the case.

Mark Borgerson

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