How Earthquakes Are Measured: Richter vs Moment Magnitude

Earthquake measurement has evolved significantly since Charles Richter developed his scale in 1935. What started as a local tool for Southern California has become a global framework for understanding the energy released by seismic events.

The Richter Scale (ML)

Charles Richter and Beno Gutenberg developed the local magnitude scale at Caltech. It measures the maximum amplitude of seismic waves recorded on a Wood-Anderson torsion seismograph. The scale is logarithmic: each whole number increase represents a tenfold increase in wave amplitude, and roughly 31.6 times more energy release.

The Richter scale works well for small to moderate earthquakes near the recording station. However, it saturates around M7.0, meaning it underestimates the true size of great earthquakes. This is because very large earthquakes release energy over a much longer period than the Wood-Anderson instrument can capture.

Moment Magnitude (Mw)

Introduced in 1979 by Hanks and Kanamori, the Moment Magnitude scale measures seismic moment: the total energy released by fault rupture. Seismic moment is calculated from three factors: the area of the fault that slipped (measured in square kilometers), the average distance the fault moved (slip in meters), and the rigidity of the rock (how much force was needed to cause the slip).

For earthquakes below M7, Richter and moment magnitudes give similar values. But for great earthquakes, Mw is far more accurate. The 1960 Chile earthquake measured M9.5 on the moment magnitude scale, making it the largest ever recorded. A Richter measurement would have significantly underestimated it.

Other Magnitude Scales

Seismologists use several other scales for specific purposes. Body-wave magnitude (Mb) measures P-waves and is useful for detecting nuclear tests. Surface-wave magnitude (Ms) measures longer-period waves and works better for distant earthquakes. Duration magnitude (Md) estimates size from how long shaking lasts, useful for local networks.

Intensity vs Magnitude

Magnitude measures the earthquake itself (energy at the source). Intensity measures its effects at specific locations. The Modified Mercalli Intensity scale rates shaking from I (not felt) to XII (total destruction). A single earthquake has one magnitude but many intensity values depending on distance from the epicenter, local geology, and building construction.

Why Accurate Measurement Matters

Precise magnitude calculations are critical for tsunami warning systems, which use earthquake size and location to estimate wave heights. They also drive building codes, insurance models, and our understanding of plate tectonics and fault mechanics. Track recent seismic events on our real-time page.