## PSN-L Email List Message

Subject: RE: Iris Waveform Chart
From: John or Jan Lahr johnjan@........
Date: Sat, 05 Feb 2005 18:06:53 -0800

```Caustic.

An earthquake sends waves out in all directions.  In ray theory, one=20
considers just one
"ray" at a time.  A ray that starts out going horizontally will soon reach=
=20
the surface of the
earth, where as a ray that starts out going directly downward will go=20
straight through
the Earth and reach the "other side of the Earth" at a distance of 180=20
degrees ( the angular
distance as measured from the center of the Earth).  If we consider an ray=
=20
leaving the
earthquake source in the horizontal direction to have a "take-off" angle of=
=20
zero, while a
ray going straight down has a take-off angle of 90, then, in general, the=20
greater the
take off angle of the ray, the greater the distance the ray emerges.

However, due to the changes in velocity of the Earth with depth,  most=
notably
the decrease in P-wave velocity in the outer core as compared to the lower=
=20
mantle, there
are some distances where, as the take off angle increases the distance=20
where the
ray emerges starts to DECREASE!  At these points, called caustics,  there=20
is a focusing
effect leading to higher amplitude waves being observed.

I'm sure others will add to this!

Cheers,
John

At 03:34 PM 2/5/2005, you wrote:
>Hi Tim,
>
>Now I'm getting a bit puzzled !
>
>Cheers,
>
>Jorma
>
>
>----------
>From: psn-l-request@.............. [mailto:psn-l-request@..................
>On Behalf Of Timothy Carpenter
>Sent: 05 February 2005 21:33
>To: psn-l@..............
>Subject: RE: Iris Waveform Chart
>
>I should probably know =96 but I don't. What is the "caustic" distance? And=
=20
>for that matter, what is the "caustic"?
>
>Regards,
>
>-Tim-
>
>
>
>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=20
>event.  The surface wave arrivals of multiple stations exhibited by=20
>distance, and text,  makes a super graphic.  I was wondering about  the=20
>occurrence of a seismic caustic at the appropriate degree distance.  Was=20
>the gap at 160 degree area due to no reporting station near the "caustic"=
=20
>distance.  In periodic recording here we've copied three caustics  in 20=20
>years--I believe the events were southwest of Australia for us--not a very=
=20
>hot spot.
>
>     The 18 sec long period system at James Madison Un.  (Virginia)=20
> working into a graphic readout read the 8.1 Macquarie Is. event-(l2/23)=20
> nicely, but the 9.0 event read 20 min after P-diff arrived and then went=
=20
> off scale for 100 minutes and returned to normal recording for the 7.1=20
> event at 04:21.  One can conclude, surface wave arrivals for us (at=20
> approx. 145 degrees) were obscured.  Keep up the good work.
>
>             Jim Lehman

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