PSN-L Email List Message

Subject: RE: Iris Waveform Chart
From: John or Jan Lahr johnjan@........
Date: Sun, 06 Feb 2005 17:13:23 -0800


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

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!


At 04:15 PM 2/6/2005, you wrote:
So, is the term Caustic used only for the special case of the constructive interference at 155-degrees (or 144) or is it used for any case of constructive interference?

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From: "Connie and Jim Lehman" <lehmancj@...........>
To: <psn-l@..............>
Subject: Re: Iris Waveform Chart
Date: Sun, 6 Feb 2005 18:17:50 -0500
X-Mailer: Microsoft Outlook Express 5.50.4133.2400
Reply-To: psn-l@..............
Sender: psn-l-request@..............

Tim & others-
According to Bath, Intro. to Seismology, the author explains a seismic caustic as follows.  In a model of Earth with Core 1/2 dia. of the Earth Sphere, and a ratio of density l:1.2, every point situated more than 155 degrees from the surface event will receive not one but two PKP waves which propigate along different paths.  Exactly at 155 degrees, these waves coincide and result in a great concentration of energy--or a Caustic. 
   For reasons not easily explained, this caustic --in real earth seismicity occurs at or near 144 degrees.  The net effect of the Caustic is a ring of PKP concentration 36 degrees away from the "antipode" of the event.  ( Remember direct P-waves observe quiet time from 103 to 144 degrees )
    Although I have never seen this demo, I understand a sphere of high density glass enclosed with a sphere of lower density glass will show a similar ring of lazer light entering the far side.  Of course acoustic caustics can be demonstrated.
   The few caustics noted here (long Period) have a signature very unique from the usual events of S. Pacific.  The front loading of P related waves are larger than any of the other wave fronts, expecially in deep events where surface waves are minimized.
   No doubt there are better explanations     (Thanks Jorma)  of  this seismic phenomenon.   Do the geometry of P energy through the Core, and one gets an idea of what is going on.
            &nbs= p;     Jim Lehman
----- Original Message -----
From: Timothy Carpenter
To: psn-l@..............=
Sent: Saturday, February 05, 2005 4:33 PM
Subject: RE: Iris Waveform Chart

I should probably know = =96 but I don't. What is the "caustic" distance? And for that= matter, what is the "caustic"?




Timothy= Carpenter, P.E., Pres.,
GeoDynamics Consultants, Inc.
5043 Whitlow Ct.
Commerce Twp., Mi 48382
248-363-4529 (voice & fax)
248-766-1629 (cell)
geodynamics@...........= (primary)
geodynamics@.......= (secondary)



-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@..............] On Behalf Of= Connie and Jim Lehman
Sent: Sunday, January 09, 2005 7:03 PM
To: psn-l@..............
Subject: Iris Waveform Chart


PSN--thanks for the Iris Waveform Chart for the 9.0= Sumatra  12/26/04 event.  The surface wave arrivals of multiple= stations exhibited by distance, and text,  makes a super= graphic.  I was wondering about  the occurrence of a seismic= caustic at the appropriate degree distance.  Was the gap at 160 degree= area due to no reporting station near the "caustic"= distance.  In periodic recording here we've copied three= caustics  in 20 years--I believe the events were southwest of= Australia for us--not a very hot spot.

    The 18 sec long period system at James Madison= Un.  (Virginia) working into a graphic readout read the 8.1 Macquarie= Is. event-(l2/23) nicely, but the 9.0 event read 20 min after P-diff= arrived and then went off scale for 100 minutes and returned to normal= recording for the 7.1 event at 04:21.  One can conclude, surface wave= arrivals for us (at approx. 145 degrees) were obscured.  Keep up the= good work.

            Jim= Lehman

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