Bob,

Yor asked about determining the amplitude of ground motion from a seismogram:

For the case of the STM-8: since this is a triple VBB system, the
sensitivity is determined by the transfer function.  For the flat portion
of the response, the constant output level is equal to 1/(G*Cp), where
G = Gn/M and Cp is the feedback capacitor value. 

For the STM-8 (or any other moving coil seismometer, including the SG), Gn
is the constant of the feedback coil, namely the speaker coil and magnet.
It is determined by zeroing the sensor (using the displacement detector),
adding a small mass, like 1 gram, and using a potentiometer to control
a small current (from AA battery) to lift the boom back to center. This 
results (for the B instrument) in 0.830ma to lift 1 gram, so Gn = 
(1gram/0.830ma)*9.8m/sec^2 = 11.815 Newtons/Ampere.

Now M = 0.5kgrams, so G = 23.63. With Cp = 20 microfarads, or 0.00002 farad.
So k = 1/G*C = 1/(23.63*0.00002) = 2116 volts/meter/second. THis is the
output over the whole flat portion of the response, from 40 seconds (or 
whatever the long period corner of the VBB is set at) to 30 hz. So it 
can be used for any period output in between.

The electronics has a "line driver" amplifier with a gain of X5. This
is more than I need to see the background 6-second microseisms, so I
divide it by two with external resistors. So the signal to my digitizer
is 5290 V/m/sec = 5.29V/mm/sec = 5.29millivolt/micron/sec..

SO what did the "seismogram" show? In this case the data is from the RS 
12-bit multimeter digitizer. Operating at a scale of 200mv, full scale is
37.8 microns/second or 75 microns/sec p-p. When I plotted the seismogram,
I could extrapolate the clipped surface waves for an estimate of the
actual p-p value of 150 microns/second. 

Again, note that the flat VBB response means that this value is valid
for any waveform from 40 seconds (the current Tn selection) to 30 hz.
I can measure the period of the maximum sustained waveform (which is
used for the Ms calculation) from the seismogram plot as 5 cycles over
120 seconds = 24 seconds.  The Ms calculation needs the p-p ground
amplitude in nanometers. To convert the velocity of 150microns/second
into displacement, it is divided by (w)omega = 2*pi/P, where P is the
period. So 150/w = 150*24/(2*pi) = 573 microns, or 5.73*10^5 nanometers.
This means that the p-p ground motion at St. Louis from the Izmit event
was about 0.57 millimeters.

Plugging this into the magnitude formula:
	Ms = log(A/P) + 1.66*log(distance) - 0.18
	Ms = log(5.73*10^5/24) + 1.66*log(81degrees) -0.18
	Ms = 4.378 + 3.17 - 0.18
	Ms = 7.37

Hopefully you will get a similar result with your data. For a conventional
moving coil seismometer, the output is not flat, so the sensitivity at
each period must be determined. Often this is determined from a log-log
graph of the calibration, which is usually determined with a calibration
coil or signal bridge and a function generator.

Regards,
Sean-Thomas

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