Seismometer or seismograph is an instrument that measures motions of the ground, including those generated by earthquakes, nuclear explosions, and other sources. Records of seismic waves called seismograms allow to map the interior of the Earth, and locate, measure or evaluate the size of those different sources.
See also 
- Richter, C.F. (1958). Elementary Seismology. San Francisco: W.H. Freeman.
Other pages 
- The history of early seismometers
- A Java code applet demonstrating the operation of a damped-mass seismometer
- Keeping track