Why they happen
Earth's outer shell is broken into plates that move a few centimetres a year (plate tectonics). Where plates collide or slide past each other, stress builds in the rock until it exceeds the rock's strength and a fault suddenly slips, releasing energy at once. That energy travels as seismic waves: fast P waves arrive first, slower S waves shake harder, and surface waves roll along the ground. About 80% of the world's earthquakes occur in the Ring of Fire around the Pacific. Harry Reid's elastic-rebound theory, proposed after the 1906 San Francisco earthquake, first explained this mechanism.
Magnitude is not intensity
Magnitude measures the energy released by the earthquake itself, so each quake has one value. Charles Richter devised the first scale in 1935; today the moment magnitude scale (Mw), which stays accurate for great quakes, is standard. Each one-unit increase means about 32 times more energy, and two units about 1,000 times. Intensity describes how strongly shaking is felt and how much damage occurs at a particular place, so the same earthquake has lower intensity farther from its source; the Modified Mercalli scale with 12 levels is widely used.
| Magnitude | Typical effects |
|---|---|
| 2–3 | Rarely felt; recorded by instruments |
| 4–5 | Clearly felt indoors; objects fall |
| 6–7 | Damage to weak buildings; casualties possible |
| 8+ | Widespread destruction; major tsunami possible |
Tsunamis, prediction and safety
Large undersea earthquakes can lift the seafloor and generate tsunamis, as in the 2004 Indian Ocean (Mw 9.1) and 2011 Tōhoku (Mw 9.0) disasters. Even regions away from plate boundaries are not immune: South Korea recorded its largest instrumental quake, M5.8, at Gyeongju in 2016. Scientists cannot yet predict the time, place and size of an earthquake, but early-warning systems detect P waves and give seconds to tens of seconds of notice before S waves arrive. During shaking, drop, cover under a sturdy table and hold on; afterwards shut off gas, use stairs rather than lifts and move to open ground, and on coasts head immediately to high ground.
FAQ
- Can earthquakes be predicted?
- Not yet in terms of exact time, place and size. Early-warning systems instead detect the first P waves and alert people seconds before stronger shaking arrives.
- How much bigger is a magnitude 6 than a magnitude 5?
- About 32 times more energy, which is why a small step up in magnitude can mean much wider damage.
Sources reviewed: 2026-10-26
References
- Richter, C. F. (1935). An instrumental earthquake magnitude scale. Bulletin of the Seismological Society of America, 25(1), 1–32.
- Kanamori, H. (1977). The energy release in great earthquakes. Journal of Geophysical Research, 82(20), 2981–2987.
- U.S. Geological Survey. (2024). Earthquake Hazards Program: Earthquake Facts and Statistics.
- 기상청. (2024). 지진 발생 시 국민행동요령.