eismographs - Keeping Track of Earthquakes
Throw a rock into a pond or lake and watch the waves rippling out in all directions from the point of impact. Just as this impact sets waves in motion on a quiet pond, so an earthquake generates seismic waves that radiate out through the Earth.
Seismic waves lose much of their energy in traveling over great distances. But sensitive detectors (seismometers) can record theses waves emitted by even the smallest earthquakes. When these detectors are connected to a system that produces a permanent recording, they are called seismographs.
There are many different types seismometers, but they all are based on the fundamental principle - that the differential motion between a free mass (which tends to remain at rest) and a supporting structure anchored in the ground (which moves with the vibrating Earth) can be used to record seismic waves.
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