Earthquake
An earthquake is the shaking of Earth's surface caused by the sudden release of energy stored in rocks under stress, generating seismic waves that radiate outward from the point of rupture called the focus or hypocenter. The point on the surface directly above the focus is the epicenter, which typically experiences the strongest ground shaking. Earthquakes occur most frequently along tectonic plate boundaries, active fault lines, and volcanic zones, and are measured by seismographs using scales such as the Moment Magnitude Scale.
Earthquake Magnitude Classification
| Magnitude Range | Class | Average Annual Frequency | Typical Effects |
|---|---|---|---|
| < 2.0 | Micro | > 8,000 per day | Not felt |
| 2.0 – 3.9 | Minor | ~1,000 per day | Felt slightly, no damage |
| 4.0 – 4.9 | Light | ~6,200 per year | Indoor objects shake, minor damage |
| 5.0 – 5.9 | Moderate | ~800 per year | Building damage in weak structures |
| 6.0 – 6.9 | Strong | ~120 per year | Serious damage in populated areas |
| ≥ 7.0 | Major–Great | ~18 per year | Widespread destruction |
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Related Terms
Seismic Wave
Seismic waves are waves of energy that travel through Earth's layers, generated by earthquakes, volcanic eruptions, or artificial explosions, and recorded by instruments called seismometers. They are classified into body waves, which travel through Earth's interior (P-waves and S-waves), and surface waves, which travel along Earth's outer layers and cause most of the shaking felt during an earthquake. The analysis of seismic waves allows geophysicists to determine the location and magnitude of earthquakes and to probe the internal structure of Earth.
Richter Scale
The Richter scale is a logarithmic scale developed by Charles F. Richter in 1935 to measure the magnitude of earthquakes based on the amplitude of seismic waves recorded by a seismograph at a standard distance of 100 km from the epicenter. Because it is logarithmic, each whole-number increase on the scale represents a tenfold increase in measured wave amplitude and approximately 31.6 times more energy released. While largely superseded by the Moment Magnitude Scale (Mw) for scientific use, the Richter scale remains widely used in public communication about earthquake intensity.
Plate Tectonics
Plate tectonics is the scientific theory describing how Earth's lithosphere is divided into large, rigid segments called tectonic plates that move over the underlying asthenosphere. These plates interact at their boundaries through convergence, divergence, or lateral sliding, driving processes such as mountain building, ocean floor spreading, and volcanic activity. The theory unifies many geological phenomena and explains the distribution of earthquakes, volcanoes, and major landforms across Earth's surface.
From Old English "eorthquacian" — "eorthe" (earth) combined with "cwacian" (to shake or tremble). The term has been used in English since at least the 14th century. The scientific study of earthquakes is called seismology, from Greek "seismos" (earthquake) and "logos" (study).