Seismograph

A number of possible arrangements have been designed for detecting the motion of the Earth's surface in comparison to some immovable standard. Early seismometers, for example, extended Chang's invention by measuring the amount by which a pendulum attached to a fixed support moved. Today, however, most seismometers can be classified as inertial or strain devices.

In an inertial seismometer, a heavy mass is suspended by a spring from a heavy support that is attached to the ground. When the ground begins to move, that motion is taken up by the spring and the mass remains motionless with reference to the frame from which it is suspended. The relative motion of the frame with regard to the mass can then be detected and recorded.

A strain seismometer is also known as a linear extensometer. It consists of two heavy objects sunk into the ground. When earth movement occurs, the two objects change their position relative to each other, a change that can be detected and recorded. Many variations in the extent design of this system have been designed. For example, a beam of light can be aimed between the two objects, and any movement in the ground can be detected by slight changes in the beam's path.

A common variation of the strain seismometer is known as a tiltmeter. As the name suggests, the tiltmeter measures any variation in the horizontal orientation of the measuring device. Tiltmeters often make use of two liquid surfaces as the measuring instrument. When an earth movement occurs, the two surfaces will be displaced from each other by some amount. The amount of displacement, then, is an indication of the magnitude of the earth movement.