I would suspect destructive interference of the wave fronts
at the quarter-wave point (50 minutes) assuming the resonance
of the "global tank circuit" is 200 minutes. Either that or
some type of phase shift. How about the coriolis effect ? :-)

-- John or Jan Lahr  wrote:
If you haven't taken a look at the IRIS "Image of the Week" yet,
check out this page (with the now correct URL!):
http://www.iris.edu/about/ENO/iow.htm

The image shows seismic records from stations distributed around
the globe and surface waves that traveled twice around!

Questions to think about --  Why does the y-axis stop at 180 degrees?
Why would the amplitude of the surface waves fall off around 90 degrees
and then increase at 180 degrees again?

Cheers,
John 


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