principles and applications of microearthquake networks

3.3. Event Detection 57
Ambuter and Solomon (1974) described a system using analog components
for event detection logic and digital components for the delay-line
memory and tape recording unit. Their cutoff criterion for recording was
to stop recording 30 sec after the ratio of the short-term average to the
long-term average, y(t), dropped below the threshold level Yd. They accomplished
this by adding a feedback amplifier to the circuit that determined
the long-term average. When an event was detected, the feedback
amplifier was switched on and produced a slightly positive gain so that the
long-term average would increase. Thus, y(t) eventually would drop
below the threshold level, and the recorder would be turned off 30 sec
later.
Eterno et al. (1974) suggested an alternative technique to cut off the
recording after an event was detected. A cutoff level yc can be defined by
(3.5) Yc = Y(td + 1)
where r (td + 1) is the value of y(r) at 1 sec after the event is detected at
time td. The recording is terminated at some preset time after y(t) drops
below the cutoff level yc. If y(t) drops below the detection threshold level
Yd within 1 sec after the time t d, then the event is considered to be false
and the detection is canceled.
Johnson (1979) used a modification of the short-term and long-term
averages technique (as described in Section 3.3.3) in an on-line detection
and recording system for a large microearthquake network in southern
California. The incoming analog seismic signals were digitized continuously,
and the detected events were written on digital magnetic tapes for
subsequent off-line processing.
Prothero (1979) has designed an ocean bottom seismograph system in
which an event detector triggered a digital recording unit. The event detector
was programmed in a microprocessor and could be changed readily
if necessary.
Other variations in methods of event detection and recording cutoff are
possible. With rapid advances in microprocessor and related technology,
these methods can become increasingly complex. Techniques for automatic
detection of an event may become indistinguishable from those for
automatic determination of the arrival times of an event.
3.3.5. Automated Methods: Applications of Event Detectors for On-Line
Data Processing
On-line processing of the incoming signals from a microearthquake
network is required if real-time earthquake location is desired for earthquake
prediction purposes. As discussed in the previous subsection, it