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. =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 - 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.
Jim Lehman
Tim & others-
According to Bath, Intro. to=20
Seismology, the author explains a seismic caustic as =
follows. In=20
a model of Earth with Core 1/2 dia. of the Earth Sphere, and a ratio of =
density=20
l:1.2, every point situated more than 155 degrees from the surface event =
will=20
receive not one but two PKP waves which propigate along different=20
paths. Exactly at 155 degrees, these waves coincide and result in =
a great=20
concentration of energy--or a Caustic.
For reasons not easily =
explained,=20
this caustic --in real earth seismicity occurs at or near=20
144 degrees. The net effect of the Caustic is a ring of PKP=20
concentration 36 degrees away from the "antipode" of the event. ( =
Remember=20
direct P-waves observe quiet time from 103 to 144 degrees=20
)
Although I have never =
seen this=20
demo, I understand a sphere of high density glass enclosed with a sphere =
of=20
lower density glass will show a similar ring of lazer light entering the =
far=20
side. Of course acoustic caustics can be=20
demonstrated.
The few caustics noted here =
(long=20
Period) have a signature very unique from the usual events of S. =
Pacific. =20
The front loading of P related waves are larger than any of the other =
wave=20
fronts, expecially in deep events where surface waves are=20
minimized.
No doubt there are better=20
explanations (Thanks Jorma) of this =
seismic=20
phenomenon. Do the geometry of P energy through the Core, and one =
gets an=20
idea of what is going on.
=
=20
Jim Lehman
----- Original Message -----
From:=20
Timothy=20
Carpenter
Sent: Saturday, February 05, =
2005 4:33=20
PM
Subject: RE: Iris Waveform =
Chart
I should =
probably=20
know – but I don't. What is the "caustic" distance? And for that =
matter, what=20
is the "caustic"?
Regards,
-Tim-
Timothy =
Carpenter , P.E., Pres.,
GeoDynamics =
Consultants, Inc.
5043 Whitlow Ct.
Commerce Twp., Mi=20
48382
248-363-4529 (voice & fax)
248-766-1629 (cell)
geodynamics@...........=20
(primary)
geodynamics@.......=20
(secondary)
-----Original=20
Message-----
From:=20
psn-l-request@.............. [mailto:psn-l-request@............... =
On Behalf Of Connie and Jim=20
Lehman
Sent: =
Sunday, January=20
09, 2005 7:03 PM
To:=20
psn-l@..............
Subject:=20
Iris Waveform Chart
PSN--thanks for the=20
Iris Waveform Chart for the 9.0 Sumatra 12/26/04 event. =
The=20
surface wave arrivals of multiple stations exhibited by distance, and=20
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 =20
in 20 years--I believe the events were southwest of Australia for =
us--not a=20
very hot spot.
=20
The 18 sec long period system at James Madison Un. (Virginia) =
working=20
into a graphic readout read the 8.1 Macquarie Is. event-(l2/23) =
nicely, but=20
the 9.0 event read 20 min after P-diff arrived and then went off scale =
for 100=20
minutes and returned to normal recording for the 7.1 event at =
04:21. =20
One can conclude, surface wave arrivals for us (at approx. =
145=20
degrees) were obscured. Keep up the good=20
work.
=
=20
Jim =
Lehman