"instrument for measuring the motions of an earthquake," 1858, from seismo- + -graph. Based on Italian sismografo, coined and invented by Luigi Palmieri (1807-1896), director of meteorological observation on Mount Vesuvius. Related: Seismographic; seismography (1865).
An instrument that detects and records vibrations and movements in the Earth, especially during an earthquake. Most seismographs employ a pendulum mounted within a rigid framework and connected to a mechanical, optical, or electromagnetic recording device. When the Earth vibrates or shakes, inertia keeps the pendulum steady with respect to the movements of the frame, producing a graphic record of the duration and intensity of the Earth's movements. Separate instruments are needed to record the north-south horizontal, east-west horizontal, and vertical components of a tremor. By comparing the records produced by seismographs located in three or more locations across the Earth, the location and strength of an earthquake can be determined.