principles and applications of microearthquake networks

3.3. Event Detection 59
Zk is a recursive approximation to a 250 point moving time average of wk.
Zk is intended to approximate the long-term average P(t) as given in Eq.
(3.2) with T~ = 5 sec. The recursive approximations are used to save
computer time and storage space.
The parameters (k and 7)k in Fig. 16 are used to detect and to confirm the
onset of an earthquake, respectively. These parameters are compared
with the predetermined threshold levels as shown by dashed lines in Fig.
16. When the current value of tk exceeds its threshold level, then the time
0.0
TIME-SECONDS
Fig. 16. Variation of the parameters as a function of time calculated in the real-time
detection process described by Stewart et al. (1971). See text for explanation. (a) Example
with long-period noise preceding the earthquake. (b) Example with a small transient preceding
the earthquake.
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