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Subject: Cruel world and Wood-Andersons
From: sean@...........
Date: Mon, 15 Jan 2001 15:37:11 -0600 (CST)

Thomas, Darrell, Danie,

Re: "Goodbye cruel ..."; of course we all know that seismographs attract
earthquakes like lightning rods, so your chance of your wallet being
swallowed by a fissure increases exponentially with your success at
amateur seismology. (I have actually encountered this quake attraction
thinking in trying to permit telemetry station sites in the Ozarks.)

Regarding the Wood-Anderson torsional seismometer: I have operated
several versions of this, including the original wooden box designs
and the later compact refinements by Q.E.D. Early instruments were set
to modest periods, with 2.7 seconds being common, but relatively 
unstable. Later a standard of 0.8 seconds and a magnification of 2800
was agreed on for a standard 60mm/min photographic record.  Because 
its magnification is so low, real data was a rarity unless you were
close to an active fault zone. But for larger quakes it was very useful
for calculating the Richter magnitude, which was based on the instrument.

Since they were horizontal sensors, we photo-recorded them on a triple-
record drum along with a high gain 1 second vertical seismometer/galvanometer
system. But photo-recording became costly and was complicated and relatively 
time consuming, so currently the W-As are adornments in a display case, and 
the W-A response can be derived from any broadband signal and scaled to any 
near-field magnitude.

The original article by Anderson and Wood was published in the BSSA,
vol 15, pp 1072, in 1925.



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Larry Cochrane <cochrane@..............>