When an earthquake occurs, sound waves, called seismic waves, are generated which travel outward in all directions in the earth. A seismic sensor, called a seismometer or geophone, usually consists of a coil of wire suspended by a spring in the field of a magnet. The slightest vibration of the ground causes the coil to vibrate with respect to the magnet, thus producing a small fluctuating voltage at the coil leads. This voltage is amplified and used to drive a pen motor. The displacement of the pen turns out to be proportional to the velocity of the coil.
Most seismic recorders write a record on a sheet of paper taped to a drum.
The drum rotates as the pen motor is moved slowly along the drum, so that a spiral trace is written, showing a magnified version of any ground vibrations. Earthquakes are located using the arrival time of the seismic waves recorded at four or more stations. For good locations, the relative arrival times must be accurate to at least 0.1 sec. Therefore, accurate and precise timing and uniform drum rotation is required at each seismic station. There are two types of sound waves generated by earthquakes.