principles and applications of microearthquake networks

1. Introduction
Earthquakes of magnitudes less than 3 are generally referred to as microearthquakes.
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.
By probing in seismically active areas, microearthquake networks are
powerful tools in studying the nature and state of tectonic processes.
Their applications are numerous: monitoring seismicity for earthquake
prediction purposes, mapping active faults for hazard evaluation, exploring
for geothermal resources, investigating the structure of the crust and
upper mantle, to name a few. Microearthquake studies are an important
component of seismological research. Results from these studies are usually
integrated with other field and theoretical investigations in order to
understand the earthquake-generating process.
In the following sections, we present a brief historical account of the
development of microearthquake studies and give an overview of the
contents and scope of the present work.
1.1. Historical Development
Microearthquake studies were stimulated by the introduction of the
earthquake magnitude scale by C. F. Richter in 1935, and by the discovery
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