John,
Thanks for the response. Usually I've had excellent success finding
information on the web using one or the other of the several search =
engines.
But this time, the only things that showed up were excerpts from
professional papers where the authors presumed the reader had known what =
a
caustic was since kindergarten. :-) (Well, actually I had, but that was
caustic soda.)
-Tim-
-----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@............... =
On
Behalf Of John or Jan Lahr
Sent: Sunday, February 06, 2005 8:13 PM
To: psn-l@..............
Cc: taber@......... Alan Jones
Subject: RE: Iris Waveform Chart
=20
Tim,
My understanding is that a caustic is formed when the derivative of =
Distance
with=20
Take-off angle is zero. ("Distance" is the angular distance from the
earthquake=20
to the place where the ray reaches the surface of the Earth and Take-off
angle=20
measures the direction with respect to vertical that the ray leaves the
earthquake=20
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=20
yet to find a good explanation on a web site. =20
Maybe John Taber or Alan Jones can double check me on this!
Cheers,
John
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?
X-Originating-IP: [64.21.22.120]=20
From: "Connie and Jim Lehman" =20
To: =20
Subject: Re: Iris Waveform Chart=20
Date: Sun, 6 Feb 2005 18:17:50 -0500=20
X-Mailer: Microsoft Outlook Express 5.50.4133.2400=20
Reply-To: psn-l@.................
Sender: psn-l-request@.................
Tim & others-
=20
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. =20
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.
Jim Lehman
----- Original Message -----=20
From: Timothy Carpenter =20
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"?
Regards,
-Tim-
=20
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)
=20
=20
-----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
=20
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
John,
Thanks for the response. Usually =
I've had
excellent success finding information on the web using one or the other =
of the
several search engines. But this time, the only things that showed up =
were
excerpts from professional papers where the authors presumed the reader =
had
known what a caustic was since kindergarten. J (Well, actually =
I had,
but that was caustic soda.)
-Tim-
-----Original =
Message-----
From: =
psn-l-request@..............
[mailto:psn-l-request@............... On
Behalf Of John or Jan Lahr
Sent: Sunday,
February 06, 2005 8:13 PM
To: psn-l@..............=
Cc: taber@......... Alan =
Jones
Subject: RE: Iris =
Waveform Chart
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
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?
X-Originating-IP: [64.21.22.120]
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.
&=
nbsp;
Jim Lehman
----- Original Message ----- =
From: Timothy Carpenter
Sent: Saturday, February 05, =
2005 4:33 PM
Subject:
RE: Iris Waveform Chart
I should probably know – but I =
don't. What
is the "caustic" distance? And 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 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.
&nbs=
p;
Jim Lehman