A seismograph is an instrument that records the motion of the ground during an earthquake. Modern seismographs — more accurately called seismometers with digital recorders — are sensitive enough to detect motions of nanometers and form the backbone of global earthquake monitoring networks.
A seismograph is an instrument that detects and records ground motion caused by seismic waves. The term technically refers to the complete system — sensor, signal conditioning, digitizer, and recorder — while the sensor itself is more accurately called a seismometer (or seismometer sensor). The first modern seismograph was developed in the late 19th century, with significant contributions from John Milne in Japan and Emil Wiechert in Germany. The basic principle is a mass suspended on a frame attached to the ground: when the ground moves, the suspended mass tends to remain stationary due to inertia, and the relative motion between the mass and the frame is recorded as the seismic signal.
Modern seismographs are dramatically more sensitive and versatile than their mechanical ancestors. A typical broadband seismometer can detect ground motions from 0.001 Hz to 50 Hz, covering the frequency range of both teleseismic (distant) and local earthquakes. Some instruments — called strong-motion accelerographs — are designed specifically to record the large-amplitude, high-frequency motions near earthquake sources, which are the most relevant for engineering applications. These instruments record acceleration directly and are deployed in dense urban networks to capture the ground motions that matter for design. Modern recorders digitize at 100 to 200 samples per second with 24-bit resolution, providing dynamic range of over 130 dB, and transmit data in real time via internet, satellite, or cellular networks.
Seismographs are the foundation of observational seismology and earthquake engineering. Networks of seismographs locate earthquakes, determine their magnitudes and focal mechanisms, and provide the raw data for tomographic imaging of the Earth's interior. In engineering, dense strong-motion networks — such as those maintained by the USGS, the PEER NGA-West2 project, and national networks in Japan, Taiwan, and Iran — produce the ground motion records used to develop GMPEs, calibrate design spectra, and validate numerical models. Iran operates a national seismic network (Iranian Seismological Center) and a strong-motion network with hundreds of stations. Modern instruments increasingly include MEMS-based accelerometers and low-cost sensors that are being deployed in crowdsourced and structural health monitoring applications, democratizing access to ground motion data.