🚀 go-pugleaf

In article <2iv6lq$huj@crl.crl.com>, William H. Rowan originally wrote:

>My understanding of it is that there are two types of waves: the P (pressure)
>wave, which propagates by compression like sound waves, and the S (shear)
>wave, which propagates by side-to-side motion, like waves in a fence when
>you wiggle it.  The P wave travels faster than the S wave.  My memory of the
>P wave in a quake in 1977 was as a sudden bang, after which there were some
>seconds of quietness, and then the shaking started.  That was the S waves,
>which of course cause the destruction and bring buildings down.  This quake
>was a magnitude 4.5, I think, about 10 miles away.

In article <CKrFIt.D0J@unocal.com>, Richard Ottolini replied:

>There are many more than that, including those that propagate along interfaces
>in rock. P and S propagate through the interior of bodies. Two interface modes-
>called surface waves because they propgate at the rock-air interface- can have
>higher amplitudes than P and S waves and cause destruction. At short distances
>the S and surface waves are scrunched together and that is what people feel.
>At medium distances, people experience the surface waves as strongest.

My own comments:

I believe that the surface waves important in earthquakes are "Rayleigh"
waves. In his "Encyclopedic Dictionary of Exploration Geophysics," Robert
Sheriff describes Rayleigh waves in these words:

  A type of seismic wave propagated along a free surface; the dominant
  wave involved in ground roll.  ...  Particle motion is elliptical and
  retrograde (i.e., the particle moves opposite to the direction of
  propagation at the top of its elliptical path) in the vertical plane
  containing the direction of propagation;  ...  Its amplitude
  decreases exponentially with depth, and the elastic properties to
  a depth of about one wavelength determine its velocity.  ...

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*  Stanley J. Sramek                             My posted statements  *
*  Texaco Inc.                                   do not reflect the    *
*  Houston, Texas, U.S.A.                        views of my employer. *
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