Seismograph
A seismograph (or seismometer) is an instrument that detects and records ground motion caused by seismic waves, volcanic activity, explosions, or other disturbances. The basic principle relies on inertia: a heavy mass suspended by a spring remains relatively stationary while the instrument frame moves with the ground, and this relative displacement is amplified and recorded as a seismogram. Modern broadband seismographs use electromagnetic sensors or force-balance accelerometers and can detect ground motions as small as 10⁻¹⁰ m over a frequency range of 0.001–50 Hz.
Types and Specifications of Seismographs
| Type | Frequency Range (Hz) | Sensitivity | Primary Use | Example Instrument |
|---|---|---|---|---|
| Short-period seismograph | 1–100 | High (small earthquakes) | Local seismicity | Mark Products L-4C |
| Long-period seismograph | 0.001–0.1 | Very high (distant events) | Teleseismic monitoring | LaCoste-Romberg |
| Broadband seismograph | 0.003–50 | Extremely high | Global seismic network | STS-2 (Streckeisen) |
| Strong-motion accelerograph | 0–50 | Low (large earthquakes) | Engineering seismology | Kinemetrics Episensor |
| Ocean-bottom seismograph | 0.01–100 | High | Submarine earthquakes | OBS (various) |
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Related Terms
Earthquake Epicenter
The earthquake epicenter is the point on Earth's surface directly above the hypocenter (or focus), which is the underground location where an earthquake rupture begins. The epicenter is located using seismic wave arrival time differences recorded at multiple seismograph stations, with the distance to the epicenter calculated from the S-P wave time interval. The epicenter is the reference point used in earthquake reporting, and ground shaking intensity is generally greatest near the epicenter, decreasing with distance according to attenuation relations.
Geological Fault
A geological fault is a planar fracture or discontinuity in rock across which significant displacement has occurred due to tectonic stresses. Faults are classified by the direction of relative motion: normal faults (extension, hanging wall moves down), reverse or thrust faults (compression, hanging wall moves up), and strike-slip faults (horizontal shear motion along the fault plane). The sudden release of accumulated elastic strain energy along a fault produces earthquakes, and repeated fault movements over geological time can build mountain ranges, create rift valleys, and shape landscape topography.
Tectonic Plate
A tectonic plate is a massive, irregularly shaped slab of solid rock composed of oceanic or continental crust together with the underlying upper mantle (lithosphere) that moves atop the semi-fluid asthenosphere. Earth's lithosphere is divided into seven major plates and several minor ones that move relative to one another at rates of 2–15 cm per year, driven primarily by mantle convection, slab pull, and ridge push. The movement of tectonic plates is responsible for earthquakes, volcanic activity, mountain building, and the distribution of continents over geological time.
From Greek "seismos" (earthquake, shaking), derived from "seiein" (to shake), and "graphein" (to write or record). The word "seismoscope" appears as early as 132 AD with Zhang Heng's Chinese invention; the modern term "seismograph" was coined in the 19th century when instruments that produced written records were developed, notably by Italian physicist Luigi Palmieri in 1856.