Earthquake magnitude scales estimate the size of an earthquake, but they do not all use the same waves, periods or physical model. Local magnitude—the original “Richter scale”—was designed for moderate nearby earthquakes in southern California. Body-wave and surface-wave magnitudes use particular parts of distant seismograms. Moment magnitude is based on seismic moment, which relates to rock rigidity, fault area and average slip, and is preferred for comparing small and very large earthquakes. The scales are calibrated to give similar values in some ranges, yet they can disagree because of saturation, station distance and measurement uncertainty. Magnitude is also different from intensity: an earthquake has an overall magnitude, while shaking and damage vary from place to place.
Key takeaways
- Magnitude is logarithmic rather than a simple linear scale.
- Local magnitude is the historical Richter formulation.
- Some scales saturate and underestimate the largest earthquakes.
- Moment magnitude relates most directly to fault size and slip.
- Early automatic values can change as more data arrive.
- Intensity describes local shaking and effects, not source size.
Why several scales exist
Seismometers record waves with different frequencies travelling through different parts of Earth. Early regional networks and global stations did not observe the same signal, so scientists developed measures suited to particular instruments and distances.
A magnitude is calculated from amplitude or moment with corrections. Different scales remain useful because rapid alerts, historical catalogues and specialist events have different available data.
Local magnitude and Richter
Charles Richter’s local magnitude, ML, used the logarithm of maximum amplitude corrected for distance and referenced to a specified instrument. It was developed for southern California at regional distances.
Modern networks adapt the calculation for digital sensors. Calling every magnitude “Richter” is misleading because agencies commonly report moment magnitude or another type.
Body-wave magnitude
Body-wave magnitude, mb, usually uses short-period P waves travelling through Earth’s interior. Their early arrival supports rapid estimates for distant events and helps characterise underground explosions.
For large earthquakes, short-period amplitude stops rising in proportion to total rupture. This saturation can make the greatest events appear too small.
Surface-wave magnitude
Surface-wave magnitude, Ms, traditionally uses waves near a 20-second period travelling along Earth’s surface. It is useful for shallow global earthquakes and historical comparisons.
Deep earthquakes generate weaker surface waves, and Ms also saturates. A body-wave and surface-wave value can differ because they sample different parts of the source.
Moment magnitude
Seismic moment equals rock rigidity multiplied by rupture area and average slip. Moment magnitude, Mw, converts its logarithm to a scale aligned with older magnitudes for moderate events.
Mw avoids severe high-end saturation and is preferred for great earthquakes. It can take longer to calculate because long-period waves and a fuller rupture solution are required.
Understanding the logarithmic scale
One whole magnitude unit corresponds roughly to ten times the characteristic wave amplitude and about 32 times the radiated energy. Two units imply about 1,000 times the energy.
These are source comparisons, not local shaking predictions. Distance, depth, rupture direction and geology determine what a community experiences.
Magnitude, location and depth
Distance corrections depend on source location. Earthquake location uses arrival times and velocity models, so an improved location can alter an early magnitude.
A smaller shallow event near a city can cause more local damage than a larger distant or deep earthquake.
Magnitude versus intensity
Intensity describes observed shaking and effects at a place using reports, damage or instrumental ground motion. One event produces many intensity values across a map.
Magnitude estimates overall source size. Soft sediment, building type and distance explain why equal-magnitude earthquakes can have different consequences.
Why magnitudes are revised
The first automatic solution may use few stations and a rapid magnitude type. Later, analysts review clipped signals, add distant recordings and calculate moment tensors.
A change of a few tenths is normal and does not mean the earthquake itself changed. Reports should state the magnitude symbol and whether the solution is automatic or reviewed.
Tsunamis and large ruptures
A great subduction earthquake may rupture for minutes over hundreds of kilometres. Mw captures total moment better than short-period scales, but equal Mw events can have different slip distributions.
Tsunami generation also depends on shallow seafloor displacement and fault geometry, as how tsunamis form explains.
Tectonic patterns and catalogues
Magnitude-frequency patterns reveal active fault systems linked to earthquake causes and plate tectonics.
Catalogue completeness matters. Better networks detect more small events, so an apparent rise may reflect instruments rather than a true change in seismicity.
Reading reports critically
Check magnitude type, time, depth, coordinates, uncertainty and agency. Mw 6.4 and mb 6.1 are not necessarily contradictory.
For risk, add intensity maps, exposure, building vulnerability and aftershock information. Magnitude is a compact source measure, not a damage forecast.
A worked comparison of two earthquakes
Suppose a shallow regional earthquake is reported ML 5.8 and later Mw 5.7. The close values are expected because the scales are calibrated to align in this range. A much larger event might receive mb 6.4 but Mw 8.0 because short-period body waves saturate while seismic moment continues to reflect the enormous rupture.
The numbers do not mean that an instrument malfunctioned. They describe different parts of the signal. Historical catalogues often convert or homogenise magnitudes, but conversions add uncertainty and should not erase the original magnitude type.
Radiated energy is not seismic moment
Seismic moment describes faulting, while radiated energy is the portion carried away by seismic waves. Two earthquakes with similar moment can radiate different energy because rupture speed, stress drop and friction differ. The common “32 times more energy per magnitude” relationship is a broad scaling, not a complete source description.
Seismologists also examine spectra, duration and focal mechanism. Magnitude compresses complex rupture into one useful number, but hazard models need more information about frequency content and spatial slip.
Magnitude uncertainty and significant figures
A reported value such as 6.3 has uncertainty from calibration, station coverage, Earth structure and source complexity. Quoting 6.327 can imply precision the data do not support. Agencies may publish confidence information or compare several estimates from station subsets.
Uncertainty is often greater for older earthquakes reconstructed from sparse instruments or felt reports. Researchers should not rank two historical events confidently when estimated ranges overlap.
Special source types
Volcanic tremor, landslides, mine collapses and explosions can generate seismic waves whose source processes differ from ordinary fault rupture. Standard magnitude procedures may still provide size indicators, but interpretation of moment and hazard changes.
Event discrimination combines waveform shape, depth, location and multiple magnitude types. A number alone does not establish whether a signal came from tectonic faulting or another source.
How to compare catalogue entries
Download the magnitude type as well as the number. Restrict analysis to a consistent scale or use a documented conversion with uncertainty. Check whether the catalogue’s detection threshold changed through time and whether aftershocks were removed for the intended question. Counts of small earthquakes usually rise when a denser network begins operating.
For a classroom graph, plot events only above a completeness magnitude and label the period and region. A trend in detected events is not automatically a trend in tectonic hazard. Fault exposure, building vulnerability and the probability of rare large events require additional evidence.
Why magnitude alone cannot predict damage
Shaking duration and frequency content affect structures differently. Basin sediments can amplify motion, and rupture directivity can focus energy towards one area. Building codes, construction quality and time of day influence casualties independently of the source magnitude.
Consequently, two events with equal Mw can produce very different losses. Emergency communication should pair magnitude with location, depth, intensity observations and local guidance rather than presenting the number as a complete severity score.
Historical earthquakes before modern instruments
For earthquakes that occurred before seismometer networks, researchers use written damage reports, geological evidence and the area over which shaking was felt. They estimate intensity at multiple locations and infer a likely source and magnitude range. Building practices and population distribution complicate comparison across centuries.
These estimates are valuable but should retain broad uncertainty. A single converted magnitude printed to one decimal place can hide conflicting accounts and missing rural evidence. Historical catalogues are strongest when they preserve the observations and method behind the summary number.
Conclusion
Magnitude scales differ because they use different portions of the seismic signal and were built for different distance ranges and instruments. Local, body-wave and surface-wave magnitudes remain useful but can saturate. Moment magnitude relates to the physical scale of rupture and gives the most consistent comparison for great earthquakes. Every value should be read with its type and uncertainty and kept separate from intensity, which describes the shaking experienced at particular places.
Frequently asked questions
Is every earthquake measured on the Richter scale?
No. Richter refers to local magnitude; large modern events are commonly reported as moment magnitude.
What does magnitude 7 mean compared with 6?
Roughly ten times the characteristic amplitude and about 32 times the radiated energy.
Why do agencies report different values?
They may use different data, scales, models or review stages.
Can one earthquake have many intensities?
Yes. Intensity varies by place, while magnitude estimates source size.
Which scale is best for huge earthquakes?
Moment magnitude is generally preferred because it does not saturate like older amplitude scales.
