principles and applications of microearthquake networks

54 3. Ihta Processing Procedures
Seismic signals are generally classified into three types depending on
the distance from the source to the station. Earthquakes occurring more
than 2000 km from a seismic network usually are called teleseismic events.
An example of a teleseismic event as recorded by a microearthquake
network station is shown in Fig. 15e. Depending on the size of the event,
teleseismic amplitudes can range from barely perceptible to those that
saturate the instrument. Their predominant periods are typically a few
seconds. Earthquakes occurring, say, a few hundred kilometers to 2000
km outside the network are called regional events. An example of a regional
earthquake as recorded by a microearthquake network station is
shown in Fig. 15f. As with the teleseismic event, amplitudes of regional
earthquakes can range from barely perceptible to large, but their predominant
periods are less than those of the teleseismic events. Earthquakes
occurring within a few hundred kilometers of a seismic network are called
local events. Local earthquakes often are characterized by impulsive onsets
and high-frequency waves as shown in Fig. 15g and h. The envelope
of local earthquake signals typically has an exponentially decreasing tail
as also shown by the dashed line in Fig. 15g. Another characteristic of all
(teleseismic, regional, or local) earthquake signals is that the predominant
period generally increases with time from the onset of the first arrival. On
the other hand, a given transient signal of instrumental or cultural origin
often has the same predominant period throughout its duration.
In summary, the incoming signals from a microearthquake network can
have a wide range of amplitudes, but local earthquakes are characterized
generally by their impulsive onsets, high-frequency content, exponential
envelope, and decreasing signal frequency with time.
To exploit these characteristics of microearthquakes, the concept of
short-term and long-term time averages of incoming signal amplitude has
been introduced by a number of authors (e.g., Stewartet ul., 1971: Ambuter
and Solomon, 1974). If the amplitude of the incoming signal is denoted
by A( T), where T is time, then the short-term average at time t, a(t), can be
defined by
t
(3.1) a(t) = (Ti)-' lA(d d7
where T~ is the length of the time window for the short-term average,
typically 1 sec or less. This permits a quick response to changes in the
signal level that characterize the onset of an earthquake. Similarly, the
long-term average at time t. @(t), can be defined by
(3.2)