Magnitude is a logarithmic measure of the size of an earthquake, quantifying the energy released at its source. Unlike intensity, which varies from place to place, magnitude is a single value assigned to each earthquake, independent of where it is observed.
Magnitude is a logarithmic scale that quantifies the size of an earthquake based on the amplitude of seismic waves recorded by seismographs. The concept was introduced by Charles Richter in 1935 with the local magnitude scale (M_L), originally designed to compare earthquakes in southern California. Since then, numerous magnitude scales have been developed, each based on different aspects of the seismic signal β surface-wave magnitude (M_s), body-wave magnitude (m_b), and the now-standard moment magnitude (M_w).
Moment magnitude (M_w) is the modern standard for earthquake size. It is derived from the seismic moment (M_0 = ΞΌ Β· A Β· D), which combines the shear modulus of the rock (ΞΌ), the rupture area (A), and the average slip (D). Unlike earlier scales, M_w does not saturate for large earthquakes β meaning it can distinguish between a magnitude 8.0 and a magnitude 9.0 event, which older scales could not. The 1960 Valdivia earthquake (M_w 9.5) and the 2011 Tohoku earthquake (M_w 9.1) illustrate the enormous energy range that moment magnitude captures. Each unit increase in M_w corresponds to roughly a 32-fold increase in released energy β so a magnitude 7.0 releases about 1,000 times more energy than a magnitude 5.0.
Magnitude is often confused with intensity, especially in media reports. Magnitude describes the earthquake itself; intensity describes its effects at a specific location. A single earthquake has one magnitude but many intensities, depending on distance from the hypocenter and local site conditions. The two are related but not directly convertible β a shallow magnitude 6.0 earthquake near a populated area can cause more damage than a deeper magnitude 7.0 far offshore. Modern seismological practice reports moment magnitude as the preferred value, though local magnitude is still sometimes used for small events and historical catalogs.