principles and applications of microearthquake networks

3.3. Event Deteclion 51
selected are processed. Events detected by automatic systems either are
recorded immediately (as in triggered recording systems) or are processed
and located right away (as in on-line location systems). The output from
the triggered system or the on-line system serves as the scan list. Regardless
of how the scan list is prepared, it is important to document the
criteria used for event detection and to include this in the record keeping
described previously.
3.3.1. Vhal Methods
The most direct method of event detection is simply to examine the
paper or film seismograms. In permanent microearthquake networks the
seismograms from slow-speed drum recorders frequently are used for
event detection, while the seismic data recorded on 16-mm microfilms or
on playbacks from magnetic tape are used for event processing. Alternatively,
the microfilms themselves are scanned, as a first pass, to detect the
earthquakes and record their approximate times of occurrence. The earthquakes
are timed and processed in more detail during subsequent passes
at the microfilms or from the magnetic tape playbacks.
For some temporary microearthquake networks, it is common to use a
few drum recorders for event detection and portable tape recorders to
collect seismic data for event processing. If the tape systems are
triggered, then the continuous seismograms from the drum recorders can
be used to verify that the triggered systems are functioning properly.
Some temporary networks have used only magnetic tape recorders, without
a visible recording monitor. In one instance, a 14-channel FM system
was used to record 11 channels of seismic data, 1 channel of tape speed
compensation signal, and 2 channels of time code (e.g., Stevenson, 1976).
The tape was played back at a time compression of 80x and the output
from all tape channels was recorded continuously on a high-speed multichannel
oscillograph. The effective playback speed was 75 mdminfast
enough to read the time code and identify the earthquakes, but slow
enough to conserve paper. The scan list was prepared by examining the
playback visually.
Other temporary networks have used a number of multichannel tape
recorders, each recording seismic data and radio time code at one station
site. In one example, the output of one seismic channel from selected
recorders was played back and recorded on a drum seismograph modified
to record at a high speed (e.g., Eaton et a/., 1970b). By matching the time
compression of the tape playback system to the increased drum speed, a
24-hr seismogram with recording speed of 60 mdmin was obtained. The