Glossary term
Peak ground acceleration (PGA)
Measuring an earthquake · earthquake & volcano glossary
What is Peak ground acceleration?
The strongest ground acceleration recorded at a site during shaking, often given as a fraction of gravity (g). Engineers use PGA, not magnitude, to design buildings for seismic loads.
Related terms
Moment magnitude (Mw)The modern measure of an earthquake's size, based on the area of the fault that slipped and how far it moved. It replaced the original Richter scale for medium and large events because Richter saturates (stops increasing) for the biggest quakes. The scale is logarithmic: each whole number is about 10× more shaking amplitude and ~31× more energy.Richter scale (ML)The original 1935 local-magnitude scale. Still widely cited in the media, but agencies now report moment magnitude for most events because it stays accurate for large earthquakes.Intensity (MMI)How strongly shaking is felt at a given place, on the Modified Mercalli Intensity scale (I–XII). Unlike magnitude — one number per earthquake — intensity varies with distance, depth, and local soil. A single quake produces many intensity values.Hypocenter (focus)The point underground where the fault rupture begins. Its depth strongly affects shaking: shallow quakes concentrate energy near the surface and are more damaging than deep ones of the same magnitude.EpicenterThe point on the Earth's surface directly above the hypocenter — the location quoted in most reports.DepthHow far below the surface the rupture started. Shallow is usually defined as 0–70 km, intermediate 70–300 km, and deep 300–700 km. Most damaging quakes are shallow.
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