The first step in detecting ground motions is finding a place to stand from which to observe the movements. Two approaches are used: in some instruments the relative position of an inertial mass only loosely connected to the moving ground is observed and in others the varying separations (strains) of two points are measured.

Devices whose purpose is primarily to determine that an earthquake has occurred are called seismoscopes. Those that make some sort of recording of the motion, usually accompanied by an indication of the time of occurrence, are called seismographs. The record produced is called a seismogram. An apparatus that converts the ground motion into some sort of signal that is proportional to the ground displacement or strain or one of their derivatives is called a seismometer and is usually a part of a seismograph.

Seismoscopes

Chang Heng in China in 132 a.d. built the earliest known seismoscope (Dewey and Byerly, 1969). It consisted of eight bronze dragons on the surface of a hollow vessel, each dragon holding a ball in its mouth. When an earthquake occurred, one or more of the balls fell into the mouth(s) of one or more toads (Figure 4.1). It is believed that an inertial pendulum in the vessel knocked a ball from a dragon's mouth when a strong enough earthquake occurred.