From: Marty Michener <>
>
> HI:
>
> In 1975 I taught a course in Oceanography, so this is from memory. Walt is
> right, but tsunami arrivals can be quite complex. Some of this may already
> been gone over, if so, my apologies - I have not read every word of every
> post recently.
My Oceanography goes even farther back in the mists of memory, the 60's.
Yours seems better than mine.
> So, given a 3-D bathymetric map of an ocean floor, the location of the
> bottom shift that causes it, and a bank of computers, it is easy to predict
> exactly WHEN the wave will arrive at each shoreline. WHEN, but not HOW
> BIG, which is the weak point of the famous Pacific Basin alarm system -
> nobody has any idea of the magnitude of these waves, unless one has very
> accurate, timely surface level measurements. With these in place, and an
> elaborate real-time monitoring system, one can do much better. That is
> why, pretty much, the Hawaiian Islands are so safe to surf.
I don't know how it is now, but when I lived in the Hawaiian Islands in
the late 50's there was a system in place. We had waterfront (a company
house) so were periodically run out by the police for a tidal wave
alert. We'd go down the way to where the road went up over a headland
and watch. If you watched the water below very closely with some you
could detect a inch or two of movement. In a year there that was the
most I ever saw. I think some of this may be that the Hawaiian Islands
are a series of peaks on the deep ocean floor. Once you went the 3/4
mile it was to the outer reef the bottom dropped off very fast. I think
the islands are just too small to really set it off normally.
> As you may know, the synch function describes an impulse gone through a
> modest low-pass filter. This often describes the arriving tsunami wave, as
> in Walt's fascinating narration of his Oregon experiences - the bell begins
> to ring more gradually than you might expect, reaches a peak magnitude
> after a few cycles then dies off again slowly.
Remember, I was a couple miles up Yaquina Bay. What I saw was restricted
by what could come through the mouth with it's heavy rock jetties. Onset
contained a very brief slight movement, but was up to essentially full
speed within a couple minutes. Die off was more like 4-5 minutes. Speed
did increase at onset in a slightly pulsed manner, which continued
somewhat throughout.
Reports of Depoe Bay, a tiny rock bound hole of a bay with a 50' wide
entrance at one end the reports seem to be like it blasted in creating a
large swirl (containing boats) and then squirted back out.
Each spot is different. Where theory meets reality.
BTW, the three main wave pattern we got did not follow the pattern of
the actual earthquake. I've always assumed it was multi-path.
>
> This case is a happy instance for the victims, because (and I thought
> EVERYBODY ALREADY KNEW) the water level begins oscillating BEFORE the main
> wave hits, and so if you ever see the water leave the bay - RUN for
> it! Evidently a lot of children went out into the tide pools to catch the
> flopping fish, instead of seeking higher ground. This is the most
> appalling fact I have yet heard about the current tragedy - so preventable,
> yet again I also wonder how many American school kids living on a coast
> know this simple rule? BTW does everyone know why a tsunami is physically
> impossible on the Atlantic coast? The news guy yesterday didn't! It is
> protected by the relatively shallow banks, upon which 99% of the energy
> would be spent.
I've not been following the news reporting much since some of the early
stuff. Video there showed no initial backwash, the folks nearest to the
water were laying around sunbathing. I suppose what you refer to is some
later reporting.
The tidal wave I went through on the Oregon coast also had no backwash
to warn. It would start each wave with forward current. There was then a
following outwash of near equal proportions. Because we were in a bay,
the current from the forward part hit the marina, the outwash current
was on the other side of the bay at our part due to a curve in the bay.
We could hear it rushing out from our side.
I did also talk to some who were in a campground at one of the Oregon
beaches. They too reported no initial outwash. The Coast Guard boat
which was in the entrance channel was hit with inbound first, not
initial outwash there either.
I did know the rule about the initial outwash. If it occurs, you are
right, get to high ground fast. Just don't count on it occurring. And
hardly anybody knows this. We live in a society that thinks all of
nature is just our playground. Nothing to worry about. Things like this
remind us we are trivia against nature.
Walt
________________________________________________________________________
________________________________________________________________________
|