principles and applications of microearthquake networks

26 2. Itistrutnetztntioti System
5Hz
I
system
-
Modulator (VCO)
0.1 1.0 10 100 c
system
FREQl
ICY (Hz)
Fig. 9. Stylized amplitude response of individual components and combinations of components
used in the USGS Central California Microearthquake Network (modified from
Eaton, 1977). Amplitude attenuation is shown as an integer in dB/octave.
and the Helicorder. The Oscillomink is used for making high-speed
playbacks of earthquakes originally recorded on magnetic tape. It has a
flat frequency response from dc to well over 100 Hz. The Develocorder is
the primary on-line film recording device. In order to suppress lowfrequency
microseismic noise and to prevent trace wandering caused by
VCO drift, a 0.5-Hz low-cut filter has been added to each input terminal of
the Develocorder. The 15.5-Hz galvanometer accounts for the highfrequency
roll-off in the Develocorder response. Because the Helicorder
uses recording speeds as low as 30 and 60 mdmin, its frequency response
roll-off points are set at 0.1 and 5 Hz.
Figure 91-K shows the combined response of the electronics and recording
media-in other words, of everything but the seismometer. Because
the Oscillomink response (Fig. 9E) is flat to well beyond 100 Hz, the