principles and applications of microearthquake networks

3.3. Everit Detectinri 53
their signal characteristics. In Fig. 15, examples of various types of incoming
signals are shown. The background signals typically have a low
amplitude and a low frequency; their appearance can be either quite periodic
(Fig. 15a) or quite random (Fig. 15b). Because of instrumental noise
and cultural activities, transient signals over a broad range of amplitudes
and periods are often present. For example, a short-lived and impulsive
transient noise from a telephone line is shown in Fig. 15c. A more emergent
type of noise is shown in Fig. 1Sd. Its envelope, indicated by the
dashed line, is roughly elliptical in shape.
- - - - - * - - - - 1 - - - - - 1 ~ - , - -
- -.
k-10 SEC~- 10 SEC-4
Fig. 15. Examples of signals recorded on the short-period, vertical-component seismographs
of the USGS Central California Microearthquake Network. The time scale is shown
by the last trace. See text for explanation.