There is a travel time plot vs distance which shows some discontinuities in 
waves depending on distance. see 

http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/AoTM_09_3B_Background.pdf


 Regards
Barry






________________________________
From: Jim and Connie Lehman 
To: psnlist@..............
Sent: Mon, July 2, 2012 7:46:50 AM
Subject: Shear waves

 
Geoff--I believe you inquired about the presence or  absence of "s" waves  Chris 
Chapman has a good handle in his  response.  It has been years since my personal 
recording, but I remember  the "P", "S' , and 'L' waves played back & forth in 
expression  depending on direction of arrival,  distance, & depth.   I regret I 
have forgotten some of the  nuances.
 
But I do remember well some cases where there  were no Shear-waves.  Back in the 
1970's there were occasional  underground A-bomb tests in Nevada.  They were 
usually announced publically  ahead of the test.  They were activated on the 
hour or half hour exactly,  so sensors all across the country could use the 
detonation to help calibrate  their system as the magnitude  and times were made 
known.  We could  set here & watch the event come in.  At our location in 
Virginia, the  time for p-waves to arrive was 6 min., 10 sec.  No shear waves 
appeared,  but "L" waves followed on schedule.  After copying a half dozen of 
these,  it was obvious "S" waves were not part of the package.  We were told the  
underground cavity carved out for the bomb package was spherical in shape, so 
the detonation appeared to  eminate from a point source.
 
Those test days are over, but rogue nations can  still carry out underground 
tests in secret, and we were told later if the  contours of the cavity would be 
oblong or football shape, "S" waves would appear  and could be mistaken for a 
natural event.
 
I hope this bit of seismic history will be of  interest----
 
There is a travel time plot vs distance which shows some discontinuities in waves depending on distance. see 
http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/12/AoTM_09_3B_Background.pdf
 
Regards
Barry



From: Jim and Connie Lehman <lehmanjc@...........>
To: psnlist@..............
Sent: Mon, July 2, 2012 7:46:50 AM
Subject: Shear waves




 
 



Geoff--I believe you inquired about the presence or 
absence of "s" waves  Chris Chapman has a good handle in his 
response.  It has been years since my personal recording, but I remember 
the "P", "S' , and 'L' waves played back & forth in expression 
depending on direction of arrival,  distance, & depth.  
I regret I have forgotten some of the  nuances.
 
But I do remember well some cases where there 
were no Shear-waves.  Back in the 1970's there were occasional 
underground A-bomb tests in Nevada.  They were usually announced publically 
ahead of the test.  They were activated on the hour or half hour exactly, 
so sensors all across the country could use the detonation to help calibrate 
their system as the magnitude  and times were made known.  We could 
set here & watch the event come in.  At our location in Virginia, the 
time for p-waves to arrive was 6 min., 10 sec.  No shear waves appeared, 
but "L" waves followed on schedule.  After copying a half dozen of these, 
it was obvious "S" waves were not part of the package.  We were told the 
underground cavity carved out for the bomb package was 
spherical in shape, so the detonation appeared to 
eminate from a point source.
 
Those test days are over, but rogue nations can 
still carry out underground tests in secret, and we were told later if the 
contours of the cavity would be oblong or football shape, "S" waves would appear 
and could be mistaken for a natural event.
 
I hope this bit of seismic history will be of 
interest----
 
JIM