Date: 23 May 89 06:39:41 GMT
From: agate!shelby!Portia!hanauma!joe@ucbvax.Berkeley.EDU  (Joe Dellinger)
Subject: Re: asteroid almost hits earth

In article <8946@xanth.cs.odu.edu> aiko@cs.odu.edu (John K Hayes) writes:
>Would this not create the greatest of all earthquakes? I have read
>people's comments on meteor hits in the past, but have not seen any
>reference to this aspect of it.

	I'm not sure about REALLY big hits, but I can tell you that
with ordinary Earthquakes there is a practical upper limit to the amount
of shaking possible, at around 9.2 or so on the Richter scale. A very
few earthquakes achieve this (Alaska, 1964; Chile, 1963 (hope I have
the dates right)). If you pump in more energy beyond this it goes into
locally tearing up rocks, and doesn't propagate away as seismic waves.

	So I wouldn't expect meteors to cause damage far away from the
impact site by standard earthquake-style shaking.

	Incidentally, some geophysicists are waiting quite impatiently
for another mag 9 mega-quake. Such quakes excite the normal modes of the
Earth. For the Chile quake instruments were still recording the ground
all over the Earth moving up and down, up and down, about a millimeter
every hour, a month after the quake. The instruments are _much_ more
sensitive now! Piddling quakes like magnitude 6 or 7 aren't much good.
All the magnitude 7 quakes together sum to only a tiny fraction of
the energy release of one 8 or 9 quake. (Energy goes up about a factor
of 32 with each step on the Richter scale. Don't believe the papers when
they say 10. That number is for ground motion.)
\    /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
 \  /  \  /  \  /Dept of Geophysics, Stanford University \/\/\.-.-....___
  \/    \/    \/Joe Dellinger joe@hanauma.stanford.edu  apple!hanauma!joe\/\.-._