Seismographs must be designed so as to take into consideration the fact that small-scale earth movements are constantly taking place. The seismogram produced by a simple seismograph sitting on a laboratory table, for example, would show not a straight line but a fairly constant wiggly line resulting from these regular microearthquakes.
Two methods are commonly used to eliminate this background noise in the detection of earthquakes. The first is to sink the supports for the seismograph as deeply into bedrock as possible. When this is done, movements in the more unstable parts of the Earth's upper layers can be eliminated. A second approach is to lay out a network of seismographs. The data obtained from this network can then be averaged out so as to reduce or eliminate the minor fluctuations detected by any one instrument.
Science EncyclopediaScience & Philosophy: Jean-Paul Sartre Biography to Seminiferous tubulesSeismograph - Type Of Seismometers, Recording Systems, Practical Considerations, The Richter Scale - The modern seismograph