On Sat, 12 Aug 2000 sean@........... wrote:
> I have never heard of a Pb phase, either in regional or teleseismic
> studies. I have no idea what Winquake is doing.

Same here, I even asked a few seismo guys...no idea what Pb is.  I looked
through a few books I have, still no idea.  There is a spectrum of possible
phases going from the more well known teleseismic phases to Pg etc. to other
less known phases the closer the station gets to the source.  Perhaps it is an
exploration seismo term?  

Some of the regional complications exist due to a mid-crustal discontinuity. 
This discontinuity represents the depth (pressure and temperature) where
lighter granitic materials transform to denser mineral phases (such as
garnet).  The depth to this boundary varies as a function of the age,
composition, thermal gradient, and thickness of continental crust (which are
all in turn inter-related).  Also, a sinking of subsiding portion of crust may
have a lower boundary than expected, because the reactions that take place to
form the denser materials are "kinetically inhibited," meaning that although
they are at the right pressure the temperature has not yet risen high enough
to give the vibrating atoms enough energy to assume the new crystal geometry. 
Likewise, an uprising portion of crust may have a higher boundary than
expected, because garnet and many other high pressure phases from the
mid-crust are stable at the surface.

At an even smaller level, many areas contain a "great unconformity" where the
initial magmatic rocks which formed the continent are overlain by younger
sediments usually along a some what flat surface of ancient erosion.  This
feature can be observed in some areas, such as the grand canyon in AZ.  This
will also affect the behavior of waves in certain regions.  In general,
sediments are seismically slower than magmatic rocks.  When some waves travel
along the underside of the interface it slowly turns wave fronts up into the
sediments.  Because the waves slow down while in the sediment, the energy per
unit volume increases, which increases the amplitudes.  Anyways, this was one
of the places I was directed to look into if there really was a Pb phase.  If
that is the case, its behavior would be quite variable.  Maybe the exploration
seismo literature would be more relevant?

Good Luck!

******************************************************************************
John Hernlund
Department of Geological Sciences
Arizona State University
E-mail: hernlund@.......
WWW: http://www.public.asu.edu/~hernlund/
******************************************************************************

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