Seismography

Seismography is the practice of measuring and recording ground motion produced by earthquakes and other sources, using seismometers and seismographs. It enables scientists to study the structure of the Earth and to monitor seismic hazards. An instrument typically consists of a mass suspended from a frame that is anchored to the ground; as the ground moves, inertia keeps the mass relatively still, and the relative motion is converted into a trace on a recording medium. Early systems used mechanical or electromechanical recorders; modern installations use digital sensors that sample at high rates and store data electronically.

Data are displayed as seismograms, showing waves such as primary (P) and secondary (S) waves, surface waves, and noise. By comparing arrival times of P and S waves at many stations, the location and depth of an earthquake can be determined, and the amplitude and frequency content of the waves provide estimates of magnitude and source mechanism. Seismography also supports monitoring of volcanic activity, explosions, and for exploration geophysics.

Seismographs fall into several classes, including short-period, long-period, and broadband devices, and they record vertical as well as horizontal motion. Global and regional networks—such as the Global Seismographic Network—collect and share data in real time to enable rapid analysis and alerting. The technology has evolved from mechanical recorders to digital, high-sensitivity, and broadband systems, continuing to improve our understanding of the Earth and our ability to respond to seismic hazards.