After the big tsunami and disaster in Ache, Indonesia, I asked around if
anyone remembered any tsunamis here in American Samoa. Those I asked didn't.
Yet Hawaii gets them all the time, sometimes they are disasters. Their phone
books have maps of what areas are subject to the tsunamis, they have warning
sirens, they had all of that back in the late 60's.. Why do they get so many and
American Samoa had no history of getting any in recent memory? (American
Samoa is mostly high volcanic islands, which is where the people are, and
actually we get them though ones that cause damage seem to be rare.) One of the
factors is what direction the energy radiates from the fault. The fault has to
be one where one side of the fault rises, the other falls. That moves huge
amounts of water. The energy radiates perpendicular to the direction of the
fault. Some maps of the path of the waves showed in different colors where the
largest waves were. The fault parallels the Japanese coast because it is the
Pacific Plate going under the plate that northern Japan is part of, amazingly it
is the North American plate! So the greatest energy in the waves went eastward
towards Hawaii. Actually, as I look at the NOAA simulation map, looks like the
greatest energy went roughly between Hawaii and the Marshalls, but it is hard to
tell on that map since you can’t see islands other than Hawaii. The NOAA
simulation may well have predicted that the Marshalls got as much energy as
Hawaii, but it is only a simulation, not actual recorded data. Guam which is
much closer, yet had much smaller waves, because it is almost straight south and
in line with the fault line. Hawaii is also in the target direction for the
largest waves from the Aleutian Islands in Alaska. The Marshall Islands appear
to just be far enough away from the direction the greatest power in the waves
went, to not get hit hard. If there were a major earthquake in southern Japan
or the Philippines, they might not be so lucky.
Tsunami waves are not just water motion at the surface like wind waves.
All the water moves, that at the bottom several kilometers deep as well as at
the surface. At the surface the tsunami is only a couple feet high or so in
deep water and has a period of 10 minutes or so, so very hard to notice. If the
bottom rises as it must near land or an atoll, the water movement and energy of
the wave gets crowded into a smaller and smaller space and the height of the
wave increases and it slows (from about 500 miles an hour). Bays also funnel
the energy and water motion laterally into a smaller space. The word "tsunami"
in Japanese means "harbor wave" since often the only damage is in harbors or
bays. Net effect is that to predict the height and destructive power of a
tsunami, you have to have accurate mapping of depths near land and the seacoast
shape, plus a mathematical model of how it works, and probably a fairly fast
computer to run it on. Even then the predictions don't always match the
observed tsunami exactly, and improvements in the depth maps and model are
needed to get a better match.
But as far as I know there is no intrinsic reason atolls should be spared
any more than any other land from tsunamis. It would all depend on the shape of
the underwater topography and magnitude of the tsunami as it approaches the
atoll or land. Before March 29, 2009, we here in American Samoa thought we
didn't get destructive tsunamis. We learned the hard way that day that we do.
Cheers, Doug
To see the NOAA simulations, take a look at
http://nctr.pmel.noaa.gov/honshu20110311/
Douglas Fenner, Ph.D.
Coral Reef Monitoring Ecologist
Dept Marine & Wildlife Resources
American Samoa
________________________________
From: Don Baker <reefpeace at yahoo.com>
To: Carly Kenkel <carly.kenkel at gmail.com>
Cc: coral-list at coral.aoml..noaa.gov
Sent: Fri, March 18, 2011 1:37:37 PM
Subject: Re: [Coral-List] response to tsunami at Kwajalein
Hi Carly,
Atolls have never (to my recollection) been affected by tsunami waves..but only
storm generated waves from Typhoons. Why? Well.....the tsunami is really a
shock wave and rather deeply focused from its source event. To try to note a
case in point to demonstrate here was a tsunami that hit the Federated States of
Micronesia island of Kosrae one evening near sunset. This event happened maybe
in the 1980s. (can someone assist in a date here from the FSM?). My good
friend, Roger Emerson was sitting on his back door step when he saw a wave about
2 feet high quickly run across the shallow reef flat and smack into the road
causeway. Being only about 2 feet high...no one really thought it was anything
to be concerned about. The next day, spearfisher men came back into towen and
reported what they saw on the outer reef front. The coral was totally ripped
apart, huge porites colonies rolled over and the large acropora plate corals
knocked about.
So what happened? The depth issue is the key. Atolls will get hit by the
tsunami for sure but the wave may only be a foot or less high as it comes across
the reef...and may never come across if its low tide. The small wave is the
tsunami's top portion that skimmed off while the main shock wave hit the atoll's
deep reef front wall. Tsunami waves are, again, deep shock waves. The shock
wave is most dangerous when the coastal region affected has relatively shallow
off shore water. This allows the wave to build up and reach hts of 30 + feet.
Hawaii is typical as well as the coasts of Japan. Guam is not like Hawaii and
tsunami waves may hit but be very small as the water gets quite deep off shore
in less then a mile. The biggest waves I saw on Guam was during Typhoon Pamela
down at the Uof Guam Marine Lab.....they were about 25 + feet high as they
slammed the reef margin and then the tops racing across the shallows and slam
into the jungle.
R/Don
--- On Fri, 3/18/11, Carly Kenkel <carly.kenkel at gmail.com> wrote:
From: Carly Kenkel <carly.kenkel at gmail.com>
Subject: [Coral-List] response to tsunami at Kwajalein
To: coral-list at coral.aoml.noaa.gov
Date: Friday, March 18, 2011, 3:25 AM
Hi All -
In case anyone is interested - I've done some work on Kwaj and know one of
the principal dive instructors there. I emailed him this past weekend and
he says Kwaj is fine. He's always told me that Kwaj has never been affected
by major waves...out of curiosity, does anyone know why this might be the
case?
And thanks Dean for all your efforts in monitoring reefs in the RMI!!!
Best,
Carly Kenkel
Graduate Student
Dept. of Integrative Biology
The University of Texas at Austin
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