Moment magnitude, abbreviated as \(M_w\), measure the size of an earthquake based on the total amount of energy released by fault movement within the Earth. This scale was developed to provide a more physicaorlly meaningful measure of earthquake size than older magnitude scales because it is directly related to the actual mechanics of fault rupture. Moment magnitude has become the standard magnitude scale used by seismologists worldwide for reporting moderate, large, and great earthquakes.
Moment Magnitude Formula |
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| \( M_w \;=\; \dfrac{ 2 }{ 3 } \cdot log_{10} \cdot M_o - 6.07 \) (Moment Magnitude) | ||
| Symbol | English | Metric |
| \( M_w \) = Moment Magnitude | \(dimensionless\) | \(dimensionless\) |
| \( M_o \) = Seismic Moment | \( ft \cdot lbf\) | \(N \cdot m\) |
The moment magnitude is calculated using a logarithmic formula designed to be consistent with other scales for moderate earthquakes while remaining reliable across all sizes. Because it is tied directly to the physical parameters of the source rather than just the amplitude of specific seismic waves recorded at a distance, Mw provides a more uniform and accurate estimate of earthquake size, particularly for very large events (M8 and above) where other scales saturate or underestimate. Each whole number increase on the moment magnitude scale corresponds to approximately a 32-fold increase in the energy released. Moment magnitude values are dimensionless and typically reported to one decimal place. For small to moderate earthquakes, Mw values are roughly equivalent to Richter magnitudes, but the scale's foundation in seismic moment makes it the standard for scientific and comparative purposes across the full spectrum of earthquake sizes.
Magnitude Scale |
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| Magnitude | Description | What it Feels Like |
| < 3.0 | Minor | Recorded on local seismographs, but generally not felt. |
| 3.0 - 3.9 | Weak | Often felt, but rarely cause damage. |
| 4.0 - 4.9 | Light | At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. |
| 5.0 - 5.9 | Moderate | Can cause damage to poorly constructed buildings and other structures in areas up to about 100 kilometers across where people live. |
| 6.0 - 6.9 | Strong | Can cause serious damage over larger areas. |
| 7.0 - 7.9 | Major | Can cause serious damage and loss of life in areas several hundred kilometers across. |
| > 8 | Great | Rare great earthquake. Can cause major damage over a large region over 1000 km across. |
