principles and applications of microearthquake networks

40 2. Instrumentatiori Systems
mometers, many microearthquake networks in the United States use
Model L-4C seismometers from Mark Products, Inc. (10507 Kinghurst
Dr., Houston, TX 77099) because they are small, light weight, and inexpensive.
The principles of seismometry are not discussed here, but readers
may refer to some standard seismology texts, e.g., Bath (1973, pp.
27-58) and Aki and Richards (1980, pp. 477-524). Operating characteristics
of seismometers are available from the manufacturers.
Many microearthquake networks record seismic data continuously in
analog form. Since earthquake signals constitute only a small part of the
continuous records, a few networks have employed triggered recording of
earthquakes (e.g., Teng el al., 1973: Johnson, 1979). Digital transmission
and recording of seismic data has been utilized, for example, by Harjes
and Seidl (1978) in Germany and by Hayman and Shannon (1979) in
Canada. The advantages of an all digital system are increased dynamic
range and easier data processing by computers. Presently, it is more expensive
and less efficient in data density to transmit and record seismic
data continuously in digital form than in analog form.
So far we have discussed instrumentation for telemetered microearthquake
networks that are intended to operate for an indefinite time. To
reduce operating cost, field instrument packages must not require frequent
maintenance, and efficient data transmission and recording must be
established. Thus months of planning and installation are often necessary.
However, in many applications, rapid deployment of a temporary
microearthquake network is essential and a permanent network is not
required.
For temporary microearthquake networks, the instrument system must
be highly portable and self-contained. Seismic data are usually recorded
at the station site or may be transmitted by cable or radio to a nearby
recording point. And it is seldom possible to use the telephone system for
data transmission. In general, the portable instrument systems do not
differ greatly from those used for the permanent networks. The crucial
difference is that seismic data from a temporary network are recorded in
the field and often at many individual sites or a central field site rather than
in a permanent laboratory. Thus temporary networks require more intensive
labor in both field operation and data processing.
The standard portable seismograph for microearthquake studies usually
has a visual recorder and is completely self-contained in a carrying case,
except perhaps for the seismometer. The unit also has an amplifier, filters,
timing system, and power supply. Record size for the seismogram is
approximately 30 by 60 cm. Recording is made by an ink pen or by a
stylus on smoked paper. Daily change of records is generally required in
order to achieve a timing resolution of better than 0.1 sec. Early versions