principles and applications of microearthquake networks

52 3. Data Processirig Procedures
scan list was prepared by visually examining the standard seismograms
made in this way. Absolute time was determined by recording a few
minutes of radio time code at the beginning and end of each seismogram.
3.3.2. Semiautomated Methods
Several semiautomated methods to detect seismic events have been
tried, but the results have not been satisfactory. Nevertheless, we will
give two examples here.
Seismic data recorded on analog magnetic tapes can be played back fast
enough to be audible. Earthquakes can then be detected by hearing
changes in the level and pitch of the sound. With a little practice, it is not
difficult to detect the onset of earthquakes. For smaller events the changes
in the level and pitch of the sound may be subtle. The detected earthquakes
can be verified by displaying the events on an oscilloscope.
When 16-mm microfilms are scanned, earthquakes are often noted to
pass across the viewing screen as a whited-out blur for the large events, or
as a darkened blur for the smaller events. In either case, a change in the
light intensity transmitted through the microfilm is observed. An experimental
detector has been constructed by E. G. Jensen and W. H. K. Lee
of the USGS to take advantage of this property. The light intensity of a
16-mm microfilm as seen on a viewing screen is measured by a phototransducer
along a narrow, vertical portion of the screen. As the microfilm
moves across the detector, the film transport of the viewer is automatically
stopped if the light intensity deviates substantially from an average
value. The presence of an earthquake can be visually verified, its onset
time recorded, and the scanning process resumed by restarting the film
transport. This type of semiautomated scanning of 16-mm microfilms is
about three times faster than visual scanning and is less tiring for the
scanner. However, high-quality recording of the microfilm is required.
3.3.3.
Automated Methods: Principles of Event Detectors
Except for aftershock sequences and swarm activities, signals containing
seismic events are typically a few percent or less of the total incoming
signals from a microearthquake network. Because of this property, event
detectors have been developed and applied for triggered recording of
seismic signals in order to reduce the volume of recorded data and to
generate simultaneously a scan list of earthquakes. In addition, some
event detectors can produce fairly accurate onset times and have been
applied to on-line data processing and real-time earthquake location.
To detect microearthquakes in the incoming signals, we must exploit