Seismographs

Seismographs are instruments that detect and record ground motion caused by earthquakes, volcanic activity, explosions, and other geophysical processes. A seismograph consists of a sensor connected to a recording system; a mass remains relatively stationary while the ground moves, or vice versa, and the relative motion is converted into an electrical signal. Modern seismographs measure ground velocity or acceleration and are often combined with three sensors oriented to record motion in orthogonal directions, providing horizontal and vertical components. The recording, or seismogram, is a time series that can be analyzed to determine waveform arrival times, magnitudes, and locations of seismic events.

History: The concept traces to ancient seismoscopes such as Zhang Heng's device, but practical seismic recording instruments were developed in the late 19th century by John Milne, James Alfred Ewing, and Thomas Gray, who built the first modern seismograph networks in Japan and Britain. Since then, seismographs have evolved from mechanical instruments to electronic sensors and digital recorders, enabling continuous data streams and networked monitoring.

Types: Seismographs are categorized by frequency response and application. Short-period seismographs target high-frequency waves produced by local earthquakes; long-period and broadband instruments measure lower-frequency waves for distant events. Three-component (three-axis) sensors record vertical and two horizontal motions. There are analog and digital recording systems, portable and permanent installations.

Applications: Seismographs support basic research in seismology, earthquake engineering, and hazard assessment. They underpin earthquake early warning systems and international monitoring under the Comprehensive Nuclear-Test-Ban Treaty. Data are shared through networks such as the Global Seismographic Network (GSN) and regional networks like the USGS and IRIS.