principles and applications of microearthquake networks

2.2. Central California Microearthquake Network 33
(2.7) S(f) = ( E/2~f) exp(i3~/2)
where E is the voltage step applied to the amplifier.
Using the seismometer mass release test and the amplifier step test,
Bakun and Dratler (1976) developed an interactive computer program to
calculate complex response functions for the complete seismic system, for
the electronics and recording subsystem, and for the seismometer. To
avoid spectral contamination by aliasing, they recommend a digitization
rate of 200 samples/sec. Figure 12 shows the amplitude response of the
complete system, as computed by Bakun and Dratler (1976). The amplitude
response is badly contaminated by noise at frequencies above 20 Hz.
Bakun and Dratler (1976) also developed a method to compute a
smoothed system response. The method retains the same response curve
as originally computed in Fig. 12 for frequencies lower than 2 or 3 Hz, and
it extends an interpolated curve through the noisy portions of the response
at frequencies above 10 Hz. The smoothed system amplitude response is
shown in Fig. 13.
Using these same daily calibration signals, Healy and O'Neill (1977)
applied a least-squares technique to compute the amplitude response.
This technique assumes an analytic form for the input and output calibration
transients. In deriving their analytical model Healy and O'Neill(l977)
assumed that the frequency response for the seismic system (and its indi-
1 0 7 ~ I Ill I I , I I I I I 1-1
/
1 1 1 1 I l l
10-11 I I l l I 1 I l l I
0.01 0.1 1 10 100
FREQUENCY (Hz)
Fig. 12. Amplitude response of the seismic system used in the USGS Central California
Microearthquake Network as determined by Bakun and Dratler (1976) using a Fourier
transform technique.