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----








Geoff--I believe you inquired about the =
presence or=20
absence of "s" waves  Chris Chapman has a good handle in his=20
response.  It has been years since my personal recording, but I =
remember=20
the "P", "S' , and 'L' waves played back & forth in expression=20
depending on direction of arrival,  distance, & =
depth. =20
I regret I have forgotten some of the  nuances.
 
But I do remember well some cases where =
there=20
were no Shear-waves.  Back in the 1970's there were occasional =

underground A-bomb tests in Nevada.  They were usually announced =
publically=20
ahead of the test.  They were activated on the hour or half hour =
exactly,=20
so sensors all across the country could use the detonation to help =
calibrate=20
their system as the magnitude  and times were made known.  We =
could=20
set here & watch the event come in.  At our location in =
Virginia, the=20
time for p-waves to arrive was 6 min., 10 sec.  No shear waves =
appeared,=20
but "L" waves followed on schedule.  After copying a half dozen of =
these,=20
it was obvious "S" waves were not part of the package.  We were =
told the=20
underground cavity carved out for the bomb package was=20
spherical in shape, so the detonation appeared =
to=20
eminate from a point source.
 
Those test days are over, but rogue =
nations can=20
still carry out underground tests in secret, and we were told later if =
the=20
contours of the cavity would be oblong or football shape, "S" waves =
would appear=20
and could be mistaken for a natural event.
 
I hope this bit of seismic history will =
be of=20
interest----
 
JIM