Re: [Paddlewise] Tsunamis

"Steve Holtzman" <sh_at_actglobal.net>  wrote:

> The following link has some good information on how Tsunamis form, travel,
> and what happens when they get to land.
> http://www.geophys.washington.edu/tsunami/general/physics/physics.html

This is a great resource.  I found the video clip particularly useful (the 
larger clip is much better).  It illustrates that tsunami waves hit land as 
massive surges, and not as the Hollywood-style "breaking wave" of B-grade 
flicks.  These surges are pretty similar in form (but not magnitude!) to what 
passing freighters induce when they pass a shoreline that shallows gradually 
from the much deeper water of a shipping channel.  I have been surprised by 
these surges several times on the Columbia River.  Some are powerful enough 
to sweep a small child off her feet, and to "plant" small boats 2-3 vertical 
feet above the ambient water level.

Where I live (NW coast of Oregon), there is considerable awareness of 
tsunamis/subduction zone earthquakes because the offshore subduction zone 
pops a magnitude 8.0 - 9.0 quake every 300 - 700  (+/-) years.  The quake and 
tsunami are not the entire story, however; along with the quake, there is a 
sudden drop of the near-shore earth's surface, typically about a meter or so, 
which permanently lowers the surrounding land ... flooding near-shore 
lowlands.  These drops are superimposed on the long-term gradual rise of this 
edge of the continent, so that the record of these tsunami/quake events is 
permanently captured in the sediments of marshes in larger bays, and has been 
much studied by geologists, including Brian Atwater of the USGS, and Curt 
Peterson or Portland State University.  Typically, the inundation and drop in 
land level "drown" spruce trees, and geologists have been able to document 
the frequency (but not the magnitude) of these tsunamis.

This story is pretty well know locally; several communities have tsunami 
warning systems, and roadside signs directing folks to "tsunami safe" zones. 
(Never mind that the road systems would not be able to handle the traffic 
jams inherent in an event that would give us only about 20-30 minutes of 
warning.  Life is risky ... and then you die!)

--
Dave Kruger
Astoria, OR 
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