Basically, a heavy weight is attached to a needle/pen, which moves according to vibrations in the earth. These vibrations are magnified by the weight and the length of the oscillation on the recording media (a paper roll, for example) is attributed to the strength of the vibrations. The frequency and the strength of vibrations change in a typical way right before earthquakes occur, but not enough time beforehand to predict earthquakes. Using three or more seismographs, you can pinpoint the location of an earthquake. Typically, seismographs are located away from roads and other ground-disturbing interference :)
source: http://funadvice.com/r/3k79srma9n
