Earthquake magnitude scales quantify the size or strength of an earthquake at its source, primarily by measuring the amount of energy released. The original and best-known scale is the Richter scale, also called the local magnitude scale (ML). It is a logarithmic scale based on the amplitude (height) of the largest seismic wave recorded by a seismograph, with adjustments for the distance from the earthquake. Each whole number increase on the scale represents a tenfold increase in the measured amplitude of ground shaking and roughly 31 times more energy released. The scale was originally designed for moderate earthquakes (around magnitude 3 to 7), and it can produce negative values for very small earthquakes detectable only by sensitive equipment.
The Richter scale has limitations, particularly for larger earthquakes and those at greater distances or in different geological settings, where it tends to saturate and underestimate size. As a result, other scales were developed, including body-wave magnitude (MB) for deeper earthquakes using shorter-period waves and surface-wave magnitude (MS) for larger, shallower events. These scales are valid within specific ranges and were calibrated to align with Richter magnitudes where they overlap.
The moment magnitude scale (MW), is now the preferred and authoritative scale used by organizations like the USGS for most earthquakes, especially those of moderate to large size. It is based on the seismic moment, a physical measure incorporating the fault's rupture area, the average slip (displacement) along the fault, and the rigidity of the rocks involved. This provides a more accurate estimate of the total energy released without the saturation issues of earlier scales. Moment magnitude is also logarithmic and designed to be consistent with Richter magnitudes for events where both apply, but it works reliably across a much broader range, including the largest earthquakes. An earthquake has one magnitude value, though different methods or networks may report slight variations before converging on a final estimate.
Magnitude Scale (MW) |
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| Magnitude | Description |
| < 3.0 | "Minor" earthquake. Recorded on local seismographs, but generally not felt. |
| 3.0 - 3.9 | "Weak" earthquake. Often felt, but rarely cause damage. |
| 4.0 - 4.9 | "Light" earthquake. At most slight damage to well-designed buildings. Can cause major damage to poorly constructed buildings over small regions. |
| 5.0 - 5.9 | "Moderate" earthquake. 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" earthquake. Can cause serious damage over larger areas. |
| 7.0 - 7.9 | "Major" earthquake. Can cause serious damage and loss of life in areas several hundred kilometers across. |
| > 8 | "Great" earthquake. Rare great earthquake. Can cause major damage over a large region over 1000 km across. |
Richter Scale (ML)This is a log base 10 scale, meaning that a 5.0 quake has a shaking amplitude that is 10 times stronger than a 4.0, and a 6.0 is 100 times stronger than a 4.0 quake. |
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| Magnitude | Description | What it Feels Like | Frequency |
| Less than 2.0 | Micro | Notmally only recorded by seismographs. Most people cannot feel them. | Million per Year |
| 2.0 - 2.9 | Minor | A few people feel them. No building damage. | Over 1 Million per Year |
| 3.0 - 3.9 | Minor | Some people feel them. Objects inside can be seen shaking. | Over 100,000 per Year |
| 4.0 - 4.9 | Light | Most people feel it. Indoor objects shake dor fall to floor. | 10,000 to 15,000 per Year |
| 5.0 - 5.9 | Moderate | Can damage or destroy buildings not designed to withstand earthquakes. Everyone feels it. | 1,000 to 1,500 per Year |
| 6.0 - 6.9 | Strong | Wide spread shaking far from epicenter. Damages buildings. | 100 to 150 per Year |
| 7.0 - 7.9 | Major | Wide spread damage in most areas. | 10 to 12 per Year |
| 8.0 - 8.9 | Great | Wide spread damage in large areas. | About 1 per Year |
| 9.0 - 9.9 | Great | Severe damage to most buildings. | 1 per 5 - 50 Years |
| 10.0 or over | Massive | Never Recorded | Never Recorded |
Mercalli Scale (MM) |
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| Intensity | Description | Richter Magnitude |
| I | INSTRUMENTAL - detected by seismography | 3.5 |
| II | FEEBLE - noticed only by sensitive people | 4.2 |
| III | SLIGHT - like the vibration due to a passing train, felt by people at rest, especially by upper floors | 4.3 |
| IV | MODERATE - felt by people while walking, rocking of loose objects, including standing houses | 4.8 |
| V | RATHER STRONG - felt generally, most sleepers are awakened and bells ring | 4.9 - 5.4 |
| VI | STRONG - trees sway and all suspended objects swing, damage by overturning and falling of loose objects | 5.5 - 6.0 |
| VII | VERY STRONG - general alarm, walls crack, plaster falls | 6.1 |
| VIII | DESTRUCTIVE - car drivers seriousely disturbed, masonry fissused, chimneys fall, poorly constructed buildings damaged | 6.2 |
| IX | RUINOUS - some houses collapse where ground begins to crack, and pipes break open | 6.9 |
| X | DISASTROUS - ground cracks badly, many buildings destroyed and railway lines bend, landslides on steep slopes | 7.0 - 7.3 |
| XI | VERY DISASTEROUS - few building remain standing, bridges destroyed, all services (railways, pipes, and cables) out of action, great landslides and floods | 7.4 - 8.1 |
| XII | CATASTROPHIC - total destruction, objects thrown into air, ground rises and falls in waves | > 8.1 |
