seismograph

A seismograph is an instrument used to record ground motions produced by earthquakes, volcanic activity, and other Earth movement. In common usage the term is often used interchangeably with seismometer, but technically a seismometer refers to the sensor that detects movement, while a seismograph includes the recording system that converts the motion into a visible trace called a seismogram. Modern seismographs are usually digital, though analog systems with pen-and-paper traces remain in some settings.

Most seismographs employ a mass suspended from a frame that is attached to the ground. When the ground moves, the frame moves with it while the inertial mass tends to stay at rest. The relative motion between frame and mass is translated into an electrical signal by the sensor and then recorded. Early instruments used a stylus marking on rotating paper; contemporary devices store data electronically and may record multiple components. Seismographs are typically installed on solid bedrock and oriented to measure vertical and horizontal motions; multi-component instruments capture motion in at least three directions.

Seismographs are categorized by their frequency response. Short-period instruments detect high-frequency motion from local, shallow events, while long-period instruments are more sensitive to distant earthquakes. Broadband seismographs cover a wide range of frequencies, and strong-motion devices are tuned to large ground motions. Seismograms display the arrival times of P-waves, S-waves, and surface waves, enabling scientists to estimate an event’s location, depth, and magnitude. Magnitude scales include local magnitude and moment magnitude, among others.

Seismographs underpin global networks that monitor seismic activity, provide rapid alerts, and verify nuclear tests under international treaties. Data support engineering design, hazard assessment, and Earth science research, improving understanding of Earth’s interior and informing preparedness efforts.