Randall,

Re: earthquake prediction, I believe that there are many systems that 
have successfully predicted quakes.  However I also believe that all 
are pretty much equally good at predicting non-quakes.  A useful 
theory would need to be able to reliably discriminate, but so far I 
don't believe any have demonstrated that they can do that 
particularly well.  Aren't there now some theories around that 
suggest that the process by which an initial small fracture in a 
fault either spreads outward to become a great quake.....or doesn't, 
is essentially chaotic in nature, i.e. unpredictable?

You may sing.  There exists, in fact, a force-balance horizontal 
pendulum, designed by Gary Lindgren.  Theoretically, it should work 
best with the pivot exactly vertical with no gravitational restoring 
force, though I think he set his up to have a period of a few 
seconds.  So far as I know it works great, but it is noisier due to 
the site horizontal (tilt) noise being greater than the vertical 
noise.  (It's become better since he cut down the nearby trees in his 
yard.)  I don't believe it can see as much as his vertical.

FBH photos:
https://sites.google.com/site/seismicsensorinfo/fbh_1
FBHorizontal real-time:
http://www.blue-eagle-technologies.com/seismo3.html#FBH_H_EW
FBVertical real-time:
http://www.blue-eagle-technologies.com/seismo3.html#FBV_H
And there's a lot of other interesting stuff on Gary's site.

We have mostly focused on the non-pendulum, i.e. vertical, 
instruments because, as Gary confirmed, the vertical noise at amateur 
installations will be lower than the horizontal, allowing one to see 
much more.  Even the professional installations see a lot more noise 
in the horizontal direction, but for them the difference may not be 
as great since they can put their instruments underground in a 
borehole or mine or cave.  Here is the vertical channel (top) vs a 
horizontal one from Dave's commercial 3-axis instrument.  It was a 
rather windy day.
http://bnordgren.org/seismo/H_vs_V_-_Trillium_120.jpg

In trying to design the perfect instrument you rather quickly run 
into the limits imposed by site-noise.

The problem with using ferrous materials on moving parts is that they 
can become "induced" magnets from nearby stationary magnets (even the 
earth's field?).  Then they exhibit similar problems as if there were 
magnets on the boom.  Even the stainless steel screws we use have to 
be checked with a magnet to be sure they are truly non-magnetic.  It 
appears that even the supposedly non-magnetic alloys can develop some 
magnetic properties depending on how they were processed.  The 
instrument noise is worse if you have accidentally used a magnetic screw.

Magnetically supported verticals are going to suffer more from 
variation in support force with temperature than those using springs.
These numbers x 1g give the variation in apparent vertical 
acceleration due to temperature change.

NdFeB magnet  -1200 ppm/degC  (most commonly used)
SmCo magnet  -300 ppm/degC (more $$ and not as strong)
17-7PH stainless spring -240ppm/degC   (we use)
Special thermally compensated spring alloys (weaker and $$$) 
<10ppm/degC   maybe

Re: air currents.  Verticals often have to settle for a day or two 
inside their insulated pressure-tight cases before the thermal 
environment stabilizes enough for convection noise inside to 
abate.  These animals are exquisitely sensitive to drafts.

Regards,
Brett

At 01:03 PM 9/28/2012, you wrote:
>This message has nearly 'grown without bounds', and so you may want 
>to go straight to the last three paragraphs (if you have any 
>interest whatsoever).


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