Ok, I'll bite. 

The wave front is always a circle, but the size of the circle varies.  
Near the source, it is a small circle and the energy is concentrated.
At 90 degrees, it is a large (great) circle, the same diameter as the
earth, spreading the energy out.  At the antipodes, the circle shrinks
again, concentrating the energy.  This causes the blip in amplitude at
angles near 180.   The wave is also dissipating energy and dispersing,
so the blip is superimposed on an overall decreasing amplitude.

The same thing happens with VLF radio transmissions traveling around the
globe.  

Jack

On Thursday, January 6, 2005, at 11:10 AM, 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
>
> __________________________________________________________
>
> Public Seismic Network Mailing List (PSN-L)
>
> To leave this list email PSN-L-REQUEST@.............. with the body of 
> the message (first line only): unsubscribe
> See http://www.seismicnet.com/maillist.html for more information.
>

__________________________________________________________

Public Seismic Network Mailing List (PSN-L)

__________________________________________________________

Public Seismic Network Mailing List (PSN-L)