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Path: ucla-ma!ucla-cs!elroy.jpl.nasa.gov!swrinde!sgiblab!darwin.sura.net!seismo!stead
From: stead@seismo.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Quake detail maps: what do the beachball symbols mean?
Date: 24 Jan 1994 15:17:39 GMT
Organization: Center for Seismic Studies, Arlington, VA
Lines: 38
Message-ID: <2i0oqj$he@seismo.CSS.GOV>
References: <CK01I8.HJI@optilink.com>
NNTP-Posting-Host: skadi.css.gov

In article <CK01I8.HJI@optilink.com> brad@optilink.dsccc.com writes:
>Could someone explain what the beachball symbols (look like oriented dark/light
>quadrants on spheres) mean on the Northridge detail maps?  I assume these have 
>something to do with direction of motion or strain.  The exact meaning isn't  
>obvious.

The beach balls are focal mechanisms.  They show the displacement on the
theoretical "focal sphere".  The focal sphere is a sphere that completely
encloses the source, assuming uniform material and a point source.
The darkened areas are usually motion out from the sphere, the light
areas are motion inward.  Also, (for the US at least), the image is the
bottom half of the beachball (lower hemispheric projection), mainly
for historical reasons.  Ordinary quakes always have a focal mechanism
that is a "double couple".  This results in 4 quadrants on the beach ball,
two with motion out opposite each other, and two with motion in.  The
two circles formed by the lines separating these areas represent where
the sphere is cut by the fault plane and the auxiliary plane.  Which is
which is completely ambiguous from the mechanism alone, which is why Caltech
was unsure for a little while whether the fault dipped north or south.
The double couple can be interpreted in terms of the stress driving the
quake.  The center of the region with motion out would be the axis of
deviatoric tension, the center of motion in would be deviatoric compression.

Not all mechanisms are double couples, however.  An explosion would be a
beachball all colored dark.  Something called the "compensated linear
vector dipole" would be a ball of one color with a wide band of a different
the opposite color around the middle.  It is caused by such things as
dike injection in volcanic areas and landslides.

Let me know if I still haven't explained it very well.  It is sort of
an accepted convention among seismologists.


-- 
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov