[LINK] Tsunami Warning - contact bom.gov.au

Richard Chirgwin rchirgwin at ozemail.com.au
Sun Feb 28 10:41:00 AEDT 2010


Stephen Wilson wrote:
> Regarding tsunami amplitude, what really matters is the depth of the 
> water where those measurements have been taken. 
>
> I don't think there is much appreciation amongst lay people of how a 
> tsunami operates.  It's very much like a set of ripples in a pond 
> created when you disturb the water from beneath.  In deep water, a 
> tsunami moves at amazing speed: many hundreds of km/h.  The amplitude is 
> usually only a meter or so, and the wavelength many kilometres.  If 
> you're in a boat on the open ocean. you won't notice them as they hoon 
> past.
>
> But when they encounter shallow water, they slow down, bunch up, and 
> grow to tremendous heights that depend on the geometry of the near-shore 
> sea bed and coastline. 
>
> So ... it's hard to interpret the simple tables of wave heights and 
> general locations, to give meaningful predictions of what happens at the 
> beaches.  Maybe that's why the tables are described as for government 
> agencies only?  For laypeople the figures look trivial and could be 
> misleading.
>
> Stephen Wilson
> ex physicist.
>   
Surely "lapsed physicist" would be more descriptive?

:-)

RC

>
> Kim Davies wrote:
>   
>>> Agreed.
>>> http://www.prh.noaa.gov/ptwc/messages/pacific/2010/pacific.2010.02.27.223924.txt
>>>
>>> Note that this link changes as more information is added. So far the 
>>> wave sizes at the farther distances from the quake are below 1 meter.
>>>     
>>>       
>> As the report pains to point out... "TSUNAMI AMPLITUDE MEASURED RELATIVE TO NORMAL SEA LEVEL. IT IS ...NOT... CREST-TO-TROUGH WAVE HEIGHT." Presumably the waves would be larger than what those measurements suggest.
>>
>> kim
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>>
>>   
>>     
>
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