In order to extend seismological studies to the microearthquake
range, it is necessary to have a network of closely spaced and
highly sensitive seismographic stations. Such a network is usually called a
microearthquake network. It may be operated by telemetering seismic
signals to a central recording site or by recording at individual stations.
Depending on the application, a microearthquake network may consist of
several stations to a few hundred stations and may cover an area of a few
square kilometers to lo5 km'. Microearthquake networks became operative
in the 1960s; today, there are about 100 permanent networks all over
the world (see Section 7.1). These networks can generate large quantities
of seismic data because of the high occurrence rate of microearthquakes
and the large number of recording stations.