principles and applications of microearthquake networks

3.3. Event Detection 55
where T~ is the length of the time window for the long-term average, which
depending on the application, may range from a few seconds to a few
minutes. Because T~ is usually much longer than the predominant period
of the incoming signal, p(r) represents the long-term average of the signal
level as detected by the seismometer. It is useful to denote the ratio of the
short-term average to the long-term average at time t by y(r), i.e.,
(3.3) y(t> = W )/P(t)
An event is said to be detected when
(3.4)
where Yd is the threshold level for event detection. There is a trade-off
between the threshold level Yd and the number of detections. If the
threshold level is set too low, noise bursts will cause too many false
detections. If the threshold level is set too high, the number of false
detections will decrease, and so will the number of detected earthquakes.
There are two major applications of event detectors. They have been
implemented with analog or with digital components. In the following two
subsections, we describe applications for triggered recording and applications
for on-line data processing.
3.3.4. A udomated Methods: Applications of Event Detectors for
Triggered Recording
Triggered recording involves two automated tasks. The first task is to
detect an earthquake event and to initiate recording. The second task is to
determine when to stop recording and to return to the detection mode. In
addition, a delay-line memory is required so that at least a few seconds of
the signal preceding the earthquake will be recorded, and recording directly
on computer-compatible media is preferred so that further data
processing can be easily carried out. The simplest method to stop recording
is to record the seismic data for some preset duration. This will assure
that recording for each detection does not use up the storage medium
quickly. For a given amount of storage medium, the maximum number of
detected events that will be recorded can be determined beforehand.
However, for any given choice of recording duration, some larger earthquakes
will not be completely recorded, and some smaller earthquakes
will use up too much storage medium. A variable cutoff criterion will
avoid these difficulties, but will need some protection against recording
indefinitely after an event is detected.
Until digital electronic components became widely available, event detectors
were based on analog technology and were not very satisfactory.
For example, a continuous loop of analog magnetic tape on a recorder