🚀 go-pugleaf

In article <2i1out$7n7@seismo.CSS.GOV> stead@seismo.CSS.GOV (Richard
Stead) writes:

>In article <CARASSO.94Jan24112935@Quaestor.Inference.COM>
>carasso@Inference.COM (dr. carasso) writes:
>>An Earthquake travels --what-- about 900 MPH?  The speed of sound in
>
>Strong shaking arrives at the S wave velocity - about 4 km/s, or about
>9000 mph.  You're off by a factor of ten.
>
>>dirt.  Northdrige the quake center is 15 miles away where a lot of
>
>It was also 10 miles down.  If we approximate the wavefront as spherical,
>the first motion arrives 15 miles away only shortly after it arrives at
>the epicenter...

This accords with and explains my own personal experience.  I live in
Sherman Oaks, and it happened that I was on the phone with people as
far as ten miles away during several of the larger aftershocks.  Within
a fraction of a second, we both announced that we were experiencing an
aftershock.  At first, I thought this was incredible, since the wave
has to take finite travel time, and I was thinking in terms of the
speed of sound.  Then I read an article in the paper which cited the
very high speed of the S wave, which -- together with Richard's point
about "phase velocity" of the spherical wavefront's intersection with
the surface -- makes the near simultaneity of its arrival at distant
points quite understandable.
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 |  | /   \ /\                      Max.Stern@TorreyPinesCA.ncr.com