On Mon, 12 Jul 1999, Nick & Sophie Caporossi wrote:
> The recent Quake in Honduras was a classic. The "P" an "S" waves were right
> to the second. The "lq" wave fell right on the largest spike. It wiggled
> the basement floor for over an hour. Does that mean that the entire earth
> was moving for an hour? 

The Earth was dancing a jig for quite a wihle I imagine.  One thing to note
about surface waves (lq, lr) is that they have a stronger variation in
velocity than body waves (p, s, etc).  This is because their velocity
depends on the material in the uppermost reaches of the earth which is a
lot more heterogeneous that the material in the lower mantle.  Also, surface
waves separate out into a continuous wave train because their velocities
depend on their frequency (or period).  This is because longer period waves
oscillate deeper in the earth than shorter period surface waves, and this
causes their velocities to depend on different materials.  So if you get an
original surface disturbance in the frequency range from 1-40 seconds then
these waves will fan out from the source at different rates (called a group
velocity).  A standard reference period for surface waves is usually around 15
seconds because many seismometers respond well in this band.  So I guess the
lesson here is not to rely too much on the lq or lr arrivals being just
right, because they probably will be off.  An interesting thing to try is to
take a good record with surface waves in it and do small, sharp (many poles) 
band pass filters for various periods from 10 to 40 seconds (0.1 to 0.025 Hz)
and see how they appear different.  Keep in mind that your instrument will
also influence the results...

John Hernlund
E-mail: hernlund@.......
WWW: http://www.public.asu.edu/~hernlund/

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