In short: Seismologists moved from the Richter scale to moment magnitude because Richter, developed in 1935, was calibrated for Southern California seismographs, works only for M3 to M7 events, and saturates around M7 so it cannot tell large earthquakes apart. Moment magnitude (Mw), introduced by Hanks and Kanamori in 1979, is computed from fault area, slip and crustal rigidity, applies globally and never saturates.
The Richter scale, developed in 1935 by Charles F. Richter, was the first scale to measure the size of earthquakes. The scale is logarithmic, meaning that each whole number increase on the scale represents a tenfold increase in measured amplitude and approximately 31.6 times more energy release. The scale was specifically calibrated for Southern California and used a particular type of seismograph, so it was most accurate for medium-sized earthquakes (M3 to M7) within a certain distance from the seismograph.
However, as our understanding of earthquakes has grown and technology has improved, seismologists have identified limitations with the Richter scale:
- Regional Limitations: The Richter scale was based on California’s geology and the specific seismographs used at the time. It does not scale well for extremely large or small earthquakes, nor does it account for variations in the Earth’s crust in different regions of the world.
- Energy Release: The Richter scale does not accurately estimate the energy released by very large earthquakes. The scale saturates around M7, meaning that it does not distinguish well between the energy released by the largest earthquakes, which can differ significantly.
- Seismograph Limitations: The original scale was based on the recordings from a particular type of seismograph that is not used as widely today. Modern seismographs provide more detailed data, and the Richter scale does not take full advantage of this.
To address these limitations, the Moment Magnitude Scale (Mw) was introduced by Hank and Kanamori (1979). It is based on the seismic moment of an earthquake, which is a measure of the total energy released by the earthquake. The moment magnitude scale is now the most common scale for measuring the size of earthquakes for several reasons:
- Global Applicability: Moment magnitude is calculated based on the physical properties of the earthquake (such as the rigidity of the Earth’s crust, the area of the fault that slipped, and the amount of slip) and can be used globally without regional corrections.
- Accuracy for Large Earthquakes: The moment magnitude scale does not saturate like the Richter scale. It provides an accurate measure of the energy release for very large earthquakes (greater than M7), which is essential for understanding their potential impact.
- Consistency: The scale provides a more uniform and consistent measure of an earthquake’s size, which is useful for both historical comparisons and scientific research.
- Detailed Data Use: Modern seismographs record a full seismic wavefield. Moment magnitude takes advantage of this data to provide a more complete picture of an earthquake’s characteristics.
Because of these advantages, the moment magnitude scale has largely replaced the Richter scale for most seismological applications, especially for earthquakes that are recorded at long distances from the epicenter or that are very large.
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Key Takeaways
- Each whole number on the Richter scale is a tenfold increase in amplitude and about 31.6 times more energy.
- Richter was calibrated for Southern California and a specific seismograph, so it is most accurate for M3 to M7 events.
- The Richter scale saturates near M7 and cannot distinguish the energy of the largest earthquakes.
- Moment magnitude (Mw), introduced in 1979, derives from seismic moment: fault area, slip and crustal rigidity.
- Mw applies globally without regional corrections and uses the full wavefield recorded by modern seismographs.
Frequently Asked Questions
Richter magnitude (1935) is derived from the amplitude recorded on a specific type of seismograph at a set distance, calibrated for Southern California. Moment magnitude (Mw, 1979) is derived from the seismic moment, the product of fault rupture area, average slip and crustal rigidity, which measures the total energy released. Mw works worldwide and remains accurate for very large earthquakes where Richter saturates.
The Richter scale measures the amplitude of a particular seismic wave on a particular instrument, and above roughly magnitude 7 that amplitude stops growing in proportion to the earthquake's true energy release. As a result two earthquakes with very different energies can produce similar Richter values. Moment magnitude avoids this because it is based on physical rupture parameters rather than a single recorded amplitude.
For most seismological applications the moment magnitude scale has replaced Richter, especially for large earthquakes and those recorded far from the epicenter. The term Richter still appears in the media, and local magnitude methods remain useful for small regional events, but agencies report official magnitudes of significant earthquakes as Mw because it is consistent globally and takes advantage of modern broadband recordings.
On the logarithmic magnitude scale, each whole-number increase corresponds to a tenfold increase in measured seismic wave amplitude and approximately 31.6 times more energy release. A magnitude 7 earthquake therefore releases about 31.6 times the energy of a magnitude 6 and roughly 1,000 times the energy of a magnitude 5, which is why magnitude differences of one unit matter so much for damage.
The moment magnitude scale was introduced by Thomas Hanks and Hiroo Kanamori in 1979. It built on the concept of seismic moment, a physical measure of earthquake size that combines the rigidity of the crust, the area of the fault that slipped and the amount of slip. It replaced the 1935 Richter scale of Charles F. Richter as the standard for measuring earthquake size.
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