Guide · Earthquake science
Earthquake Magnitude Scales Explained
The Richter scale is famous but retired. Here is what scientists actually use, how the scales differ, and why the numbers on PlainQuake mean what they mean.
The short answer
Moment magnitude (Mw) is the modern standard, it never saturates, and each whole step up means about 32× more energy.
- M9.5
- Strongest ever recorded (1960 Valdivia, Chile)
- ~32×
- More energy per whole-magnitude step
- 310,010
- M4+ events catalogued worldwide since 2005
- Mw
- USGS-preferred scale for significant quakes
Key Takeaway
The moment magnitude scale (Mw) is the standard used by USGS for all significant earthquakes. Unlike the Richter scale, it does not saturate at high magnitudes and measures the actual physical energy released. Each whole-number increase represents roughly 31.6 times more energy. PlainQuake data uses the USGS-preferred magnitude for each event.
Why Multiple Scales Exist
Seismology developed different magnitude scales over decades because no single measurement works perfectly for all earthquakes at all distances. Each scale was designed for a specific type of seismic wave, instrument, or distance range. The result is a family of magnitude types that can give slightly different numbers for the same earthquake, which confuses the public but makes scientific sense.
The USGS reports the most appropriate magnitude type for each event. For earthquakes below about M3.5, local magnitude (ML) or duration magnitude (Md) may be used. For moderate regional events, body wave magnitude (mb) or surface wave magnitude (Ms) may appear. For all significant events (roughly M4+), the moment magnitude (Mw) is the standard. PlainQuake shows the USGS-preferred magnitude for each earthquake.
The Four Major Scales
| Scale | Symbol | What It Measures | Best For | Limitation |
|---|---|---|---|---|
| Local (Richter) | ML | Max amplitude on Wood-Anderson seismograph | Small, nearby quakes | Saturates above M~7; regional only |
| Body Wave | mb | Amplitude of P-waves (1 sec period) | Moderate teleseismic events | Saturates above M~6.5 |
| Surface Wave | Ms | Amplitude of surface waves (20 sec) | Shallow quakes at teleseismic distance | Saturates above M~8; poor for deep quakes |
| Moment Magnitude | Mw | Seismic moment (fault area × slip × rigidity) | All earthquakes | Requires waveform modeling (slower) |
The Richter Scale: Famous but Obsolete
Charles Richter developed his local magnitude scale (ML) in 1935 to compare Southern California earthquake sizes. It was elegantly simple: the logarithm of the maximum amplitude recorded on a specific seismograph at a standard distance. For small to moderate California earthquakes, it worked beautifully.
The problem emerged with larger events. Because ML measures only peak wave amplitude at a specific frequency, it cannot capture the full energy release of a major earthquake that ruptures a fault for minutes across hundreds of kilometers. Above M7, ML gives artificially similar values for physically very different events, a phenomenon called saturation. The 1960 Chile earthquake and the 1906 San Francisco earthquake would have nearly identical Richter magnitudes, despite the Chile event releasing over 100 times more energy.
Media outlets still say "Richter scale" as shorthand, but no seismological agency has used it as the primary scale for significant earthquakes since the 1970s.
Moment Magnitude: The Modern Standard
Hiroo Kanamori and Thomas Hanks developed the moment magnitude scale in 1979 to replace the saturating older scales. Instead of measuring wave amplitude, Mw is calculated from the seismic moment - a physical quantity that combines three measurable properties:
- Fault area: The total surface area of the fault that ruptured (length × width).
- Average slip: How far the two sides of the fault moved past each other.
- Rock rigidity: The stiffness of the rock along the fault (a material property).
This produces a number that is deliberately calibrated to match the Richter scale at moderate magnitudes (M3-7) but does not saturate at any magnitude. A M9.5 Mw is physically meaningful where ML cannot distinguish it from M8.
The trade-off is that calculating Mw requires full waveform analysis, which takes longer than reading peak amplitude. That is why initial earthquake reports sometimes use mb or ML before being updated to Mw within minutes or hours. The USGS earthquake data on PlainQuake reflects the final preferred magnitude.
The Logarithmic Energy Relationship
The most commonly misunderstood aspect of all magnitude scales is their logarithmic nature. People intuitively expect a M6 to be "twice as strong" as a M3. The reality is staggeringly different:
| Magnitude Increase | Shaking Amplitude | Energy Increase |
|---|---|---|
| +1 (e.g., M5 to M6) | 10x | ~31.6x |
| +2 (e.g., M5 to M7) | 100x | ~1,000x |
| +3 (e.g., M5 to M8) | 1,000x | ~31,600x |
| +4 (e.g., M5 to M9) | 10,000x | ~1,000,000x |
This means the 2011 Tohoku M9.1 earthquake released roughly one million times more energy than a M5 that rattles dishes. This is why the jump from M7 to M8 or M8 to M9 represents such a profound escalation in destructive potential, and why such events are genuinely rare.
Practical Framework: Reading Magnitudes on PlainQuake
When browsing earthquake data on PlainQuake, whether by country, US state, or year - keep these points in mind:
- The magnitude shown is the USGS-preferred type for that event, usually Mw for M4+ events.
- Small differences (e.g., M5.8 vs M6.0) are within the margin of uncertainty. Do not treat them as meaningfully different.
- Always pair magnitude with depth. A shallow M5.5 is more hazardous locally than a deep M6.5.
- Country comparisons by earthquake count should focus on M5+ to avoid detection bias from uneven seismograph coverage.
- The significant earthquakes catalog uses M6+ as the threshold, events large enough to be reliably detected and potentially damaging worldwide.
Frequently Asked Questions
Why did scientists switch from the Richter scale to moment magnitude?
The Richter scale (ML) was designed in 1935 for Southern California earthquakes using a specific type of seismograph. It saturates above M7, meaning it cannot accurately distinguish between a M7.5 and a M9. Moment magnitude (Mw) measures the total energy released by the earthquake using the seismic moment (fault area × slip distance × rock rigidity), which scales correctly at all magnitudes. Since the 1990s, USGS has used Mw as the primary magnitude for all significant earthquakes.
Can two earthquakes have the same magnitude but different damage?
Absolutely. Magnitude measures energy at the source, not the effect at the surface. A M6.5 at 5 km depth under a city causes far more damage than a M6.5 at 200 km depth in a remote ocean trench. Local soil conditions, building quality, population density, and time of day all determine damage independently of magnitude. The Modified Mercalli Intensity (MMI) scale captures these ground-level effects where magnitude does not.
What is the difference between magnitude and intensity?
Magnitude is a single number describing the total energy released at the earthquake source, one earthquake has one magnitude. Intensity describes how strongly the shaking is felt at a specific location and varies across the affected area. The same M6 earthquake might produce MMI VIII (severe damage) near the epicenter and MMI III (barely felt) 200 km away. Magnitude is measured by instruments; intensity is assessed from observed effects on people, structures, and the environment.
How much more powerful is a magnitude 7 than a magnitude 5?
The moment magnitude scale is logarithmic: each whole number increase represents a 31.6× increase in energy released. A M7 releases 31.6 × 31.6 = approximately 1,000 times more energy than a M5. In terms of shaking amplitude (what a seismograph records), each whole number represents a 10× increase. So a M7 has 100 times the shaking amplitude of a M5.
What is the largest earthquake ever recorded?
The 1960 Valdivia earthquake in Chile measured M9.5 on the moment magnitude scale, the largest instrumentally recorded earthquake in history. It ruptured a fault segment roughly 1,000 km long and generated a transoceanic tsunami that caused fatalities in Hawaii, Japan, and the Philippines. The 2011 Tohoku earthquake (M9.1) and the 2004 Indian Ocean earthquake (M9.1) are the only other events to exceed M9 in the modern seismographic era.
Are there earthquakes too small for humans to feel?
Yes. Earthquakes below about M2.0 are almost never felt by humans. The USGS detects thousands of these microearthquakes daily across well-monitored regions. Some research seismographs can detect events as small as M-2 (negative magnitude). PlainQuake catalogs M4.0+ events worldwide from the USGS Comprehensive Earthquake Catalog (ComCat), the range relevant to human awareness and potential impact.
Sources
- USGS Earthquake Hazards Program, Magnitude Types and Preferred Magnitudes
- Kanamori, H. (1977) - The energy release in great earthquakes, Journal of Geophysical Research
- Hanks, T. C. and Kanamori, H. (1979) - A moment magnitude scale, Journal of Geophysical Research
- USGS, Earthquake Glossary: Magnitude, Moment, Intensity
This content is for educational purposes only. For official earthquake information, real-time alerts, and emergency preparedness guidance, visit earthquake.usgs.gov and ready.gov.