[Geysers] Tsunami (Reilly)

[Greg Wimpey]

Greg the geophysicist delurks...

I'm not a tsunami expert, but I'll try to provide some insight.

As to your first question, waters will recede if the "first arrival" of
the tsunami wave on land is a trough instead of a crest.  Whether the
first arrival is a crest or a trough depends largely on your location
relative to the earthquake, but that may also be affected by diffraction
of the wave and resulting interference.

As to your second question, the tsunami affected part of the west coast
of India because the wave diffracted around the tip of India. 
Diffraction is the same effect that allows sound to travel around
corners.  Any sharp point (an island, a peninsula) will act as a
secondary source, radiating waves in all directions.

There may also be refraction effects involved related to the water
depth.  Refraction occurs when the velocity of a wave changes along the
wave front.  Since the velocity of a tsunami is related to the water
depth, changes in ocean depth would cause refraction.  In the case of
light waves, refraction can be seen when you put a straw in a glass of
water, and the straw appears to be "broken" at the surface of the
water.  In the case of a tsunami, the path of the tsunami would "bend"
due to refraction.

It is the combination of refraction and diffraction that can make life
difficult for those trying to predict the effects of a particular
tsunami.  Refraction and diffraction can cause focusing (or
"defocusing") of a tsunami.  Modeling this accurately requires a good
elevation model of the seafloor as well as a good model of the
earthquake source.  Additionally, local shoreline topography has a
significant effect.  In the US, there is currently a major effort going
on to model tsunami effects along the Alaskan coastline.  I also suspect
that the Indian Ocean tsunami will intensify efforts to model and
understand the tsunami potential along the Pacific Northwest coast.  The
geologic situation offshore is very similar to the situation offshore
Sumatra.  Geologists now believe that an earthquake of magnitude 9.0
occurred there in 1700 (based in part on Japanese tsunami records).

Not very geyser related, I'm afraid, but I hope I helped answer your
questions.  The NOAA website (www.noaa.gov) has links to a number of
good tsunami resources.

On Sun, 2005-01-02 at 21:35, Lucille Reilly wrote:
> Hi all,
> 
> While this is not geyser related, this group is the only place I can
> think of coming to where there might be some physicists present to answer
> the following questions (which may be of interest to everyone):
> 
> 1. How come the sea pulled back so far from the shoreline before the wave
> hit?
> 
> 2. How was the WEST coast of India hit by the wave?  Did a current go
> around the sub-continent, or did the wave go clear across the country
> from the east?  If the former, please explain how it works.  (Actually,
> I'm not so sure about the latter, either, given the tall mountain ranges
> in Kerala, where at least two people died.)
> 
> Many thanks for explanations.  And if the list manager feels this topic
> to be too far off the mark for this group, I welcome responses privately
> at thedulcimerlady at juno.com.
> 
> Lucille Reilly
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