John Taber, below, points out two sources of information on caustics that
are on the web.  


The upper set if figures on Paul Richards's page (second URL below) shows
how caustics can develop when even when the velocity increases
continuously with depth.  


The lower set of figures shows how a shadow zone and caustic are created
when there is a low velocity zone.


Cheers,

John


Date: Sun, 6 Feb 2005 22:17:39
-0500

Subject: Re: Iris Waveform Chart

To: John or Jan Lahr <johnjan@........>

From: John Taber <taber@........>


John, 


That is my understanding as well. It is a point
where energy will be focused.  Rick Aster has a fairly complete
explanation at
www.ees.nmt.edu/Geop/Classes/GEOP523/Docs/rays2003.pdf 
..  One website which shows an example, but without defining it is
http://www.ldeo.columbia.edu/~richards/ARhtml/add_to_Sec9.4.html.
I had unsubscribed from the PSN list while I was away so your email
reminded me to resubscribe. 


John 




On Sunday, February 6, 2005, at 08:13  PM, John or Jan Lahr wrote:



Tim, 


My understanding is that a caustic is formed when the derivative of
Distance with 

Take-off angle is zero.  ("Distance" is the angular
distance from the earthquake 

to the place where the ray reaches the surface of the Earth and Take-off
angle 

measures the direction with respect to vertical that the ray leaves the
earthquake 

source.) 


Other cases of constructive interference are not termed caustics as far
as I 

know.  I'm traveling now, so can't refer to any seismology books and
have 

yet to find a good explanation on a web site.  


Maybe John Taber or Alan Jones can double check me on this! 


Cheers, 

John