Fascinating. Thanks for the explanation. I went back and re-ran Alan =
Jones's
animation of wave propagation through the earth, set the cursor at about =
145
degrees and watched. That program is an excellent teaching aid.
=20
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?
=20
-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, February 06, 2005 6:18 PM
To: psn-l@..............
Subject: Re: Iris Waveform Chart
=20
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
=20
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
Fascinating. Thanks for the =
explanation. I
went back and re-ran Alan Jones's animation of wave propagation through =
the
earth, set the cursor at about 145 degrees and watched. That =
program is an excellent =
teaching
aid.
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?
-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,
February 06, 2005 6:18 PM
To: =
psn-l@..............
Subject: Re: Iris =
Waveform Chart
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:<=
/font> psn-l@..............
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<=
b> 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