One of the most persistent sources of confusion in earthquake science is the difference between magnitude and intensity. News reports routinely conflate the two, leading to widespread misunderstanding of what a "6.0 earthquake" actually means. The problem is compounded by the fact that most people still refer to "the Richter Scale" — a measurement system that seismologists largely stopped using decades ago.
This article explains every major earthquake measurement scale in use worldwide, how seismographs work, why the scales are logarithmic, and what each level actually looks and feels like in practice. Whether you are trying to understand a news report about a recent earthquake or studying seismology, this is the complete reference.
For the science of what causes earthquakes, see our complete guide to earthquake causes. For how seismic waves travel through the Earth, visit earthquake waves explained.
Magnitude vs. Intensity: The Critical Distinction
Magnitude and intensity answer two different questions:
Magnitude answers: How much energy did this earthquake release at its source? Every earthquake has a single magnitude value. It is measured instrumentally by seismographs and calculated from the seismic waves recorded worldwide. Magnitude is an intrinsic property of the earthquake itself.
Intensity answers: How strong was the shaking at this particular location? A single earthquake produces different intensities at different locations. Intensity depends on distance from the epicenter, depth of the earthquake, local soil and rock conditions, and the type of structures present. Intensity is assessed through observed effects — what people felt, what damage occurred.
To use an analogy: magnitude is like the wattage of a light bulb (a fixed property of the source), while intensity is like the brightness you perceive at a given distance (which depends on where you are standing).
A deep M7.0 earthquake might cause moderate shaking (Intensity V–VI) at the surface. A shallow M5.5 earthquake directly beneath a city might cause severe shaking (Intensity VIII) locally. This is why magnitude alone does not determine how destructive an earthquake is — depth, distance, local geology, and building construction all play critical roles.
The Richter Scale: History and Limitations
Origins
In 1935, seismologist Charles F. Richter, working with Beno Gutenberg at the California Institute of Technology, developed what he called the "local magnitude" scale (ML) to compare the sizes of earthquakes in Southern California. The scale was based on the maximum amplitude of seismic waves recorded on a specific type of seismograph (the Wood-Anderson torsion seismometer) at a standardized distance from the epicenter.
Richter chose a logarithmic scale because earthquake amplitudes vary over an enormous range — the largest earthquakes produce ground motion millions of times greater than the smallest detectable events. A base-10 logarithmic scale compressed this vast range into manageable numbers.
How It Works
The original Richter magnitude was calculated using:
ML = log₁₀(A) − log₁₀(A₀)
Where A is the maximum recorded amplitude and A₀ is a reference amplitude based on distance from the earthquake. Each whole number increase in ML corresponds to a tenfold increase in recorded wave amplitude.
Why Scientists Stopped Using It
The Richter Scale has three fundamental limitations that make it inadequate for modern seismology:
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Magnitude saturation. The Richter Scale "saturates" for large earthquakes — it cannot accurately distinguish between events above approximately M6.5. A M8.0 and a M9.0 earthquake, which differ enormously in energy release and destructive potential, would appear nearly identical on the Richter Scale. This is because the specific wave types and frequencies used in the calculation reach a maximum amplitude for very large events.
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Distance dependency. The Richter Scale was calibrated for a specific instrument at specific distances and was designed for use in Southern California. It does not work well for earthquakes at great distances or in different geological settings.
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Single-frequency measurement. The Richter Scale relies on the peak amplitude of waves at a particular frequency range. Large earthquakes radiate most of their energy at lower frequencies that the original Wood-Anderson instrument did not measure well.
Despite these limitations, the term "Richter Scale" became so embedded in public consciousness that media outlets and the general public continue to use it colloquially, even though the numbers reported in news articles are almost always moment magnitudes (Mw), not Richter local magnitudes.
The Moment Magnitude Scale (Mw)
Development
Seismologists Thomas C. Hanks and Hiroo Kanamori introduced the Moment Magnitude Scale in 1979 to address the limitations of the Richter Scale. Published in the Journal of Geophysical Research, their approach was based on the seismic moment — a physical quantity that directly relates to the mechanical work done by the earthquake.
How It Works
The seismic moment (M₀) is calculated as:
M₀ = μ × A × D
Where:
- μ (mu) = the rigidity (shear modulus) of the rock, typically 30–36 GPa for crustal rock
- A = the area of the fault that ruptured
- D = the average displacement (slip) across the fault
The moment magnitude is then:
Mw = (2/3) × log₁₀(M₀) − 6.07 (when M₀ is in newton-meters)
What Different Magnitudes Look Like Physically
Because moment magnitude is derived from the physical dimensions of a fault rupture, larger magnitudes correspond to dramatically larger fault areas and greater amounts of slip. The following table illustrates typical fault dimensions for each magnitude level, providing physical intuition for the scale.
| Magnitude (Mw) | Typical Fault Rupture Length | Typical Fault Rupture Area | Typical Average Slip | Real Example |
|---|---|---|---|---|
| 5.0 | ~3–5 km | ~25 km² | ~0.1–0.3 m | 2014 Napa, CA (M6.0 — 12 km rupture) |
| 6.0 | ~10–15 km | ~200 km² | ~0.3–1 m | 1994 Northridge (M6.7 — 18 km rupture) |
| 7.0 | ~50–70 km | ~1,500 km² | ~1–3 m | 2010 Haiti (M7.0 — 65 km rupture) |
| 8.0 | ~200–300 km | ~10,000 km² | ~3–8 m | 1906 San Francisco (M7.9 — 477 km rupture) |
| 9.0 | ~500–1,000+ km | ~100,000+ km² | ~10–20 m | 2011 Tōhoku (M9.1 — 500 km × 200 km rupture area) |
Note: These are approximate typical values. Actual dimensions vary with fault geometry, depth, and stress conditions. The Tōhoku rupture dimensions are from USGS finite fault modeling.
This table makes clear why the moment magnitude formula produces such different numbers for different-sized earthquakes: a M9.0 event involves a fault area roughly 4,000 times larger than a M5.0 event, with slip distances 50–100 times greater. For detailed histories of these earthquakes, see our pages on the 1906 San Francisco earthquake and the 1994 Northridge earthquake.
Why It's Better
The Moment Magnitude Scale has several advantages over the Richter Scale:
- No saturation. It accurately measures earthquakes of all sizes, from the smallest to the largest, because it is based on the total energy released rather than a single-frequency amplitude measurement.
- Physically meaningful. It directly relates to the physical process of faulting — the area that broke and how far it slipped.
- Consistent. For small-to-moderate earthquakes, Mw values are nearly identical to Richter ML values, which maintained continuity with the existing catalog and public understanding.
Today, the USGS and all major seismological agencies worldwide report Mw for significant earthquakes. When the news reports a "magnitude 7.1 earthquake," they are almost certainly reporting the moment magnitude USGS — Earthquake Magnitude, Energy Release, and Shaking Intensity.
Understanding Logarithmic Scales
The logarithmic nature of magnitude scales is one of the most misunderstood aspects of earthquake measurement.
Amplitude
Each whole number increase in magnitude represents a 10-fold increase in the amplitude of ground motion recorded by a seismograph. A M7.0 earthquake produces seismograph recordings with 10 times the amplitude of a M6.0, and 100 times the amplitude of a M5.0.
Energy
Each whole number increase represents approximately a 31.6-fold increase in total energy released. This relationship comes from the empirical observation that the logarithm of energy scales at 1.5 times the magnitude: log₁₀E = 1.5M + 4.8 (in joules).
This means:
- A M6.0 releases ~31.6× more energy than a M5.0
- A M7.0 releases ~1,000× more energy than a M5.0 (31.6 × 31.6)
- A M8.0 releases ~31,623× more energy than a M5.0
- A M9.0 releases ~1,000,000× more energy than a M5.0
Energy Equivalents Table
The following table compares earthquake magnitudes to approximate energy equivalents, using the standard seismological energy-magnitude relationship. TNT equivalents are a common reference used by the USGS for public communication.
| Magnitude | Approximate Energy Release (joules) | TNT Equivalent | Real-World Comparison |
|---|---|---|---|
| 1.0 | 2.0 × 10⁶ | 0.5 kg (1 lb) of TNT | Hand grenade |
| 2.0 | 6.3 × 10⁷ | 15 kg of TNT | Construction demolition blast |
| 3.0 | 2.0 × 10⁹ | 480 kg of TNT | Small quarry blast |
| 4.0 | 6.3 × 10¹⁰ | 15 tons of TNT | Large industrial explosion |
| 5.0 | 2.0 × 10¹² | 480 tons of TNT | Early nuclear weapon yield |
| 6.0 | 6.3 × 10¹³ | 15 kilotons of TNT | Hiroshima atomic bomb (~15 kt) |
| 7.0 | 2.0 × 10¹⁵ | 480 kilotons of TNT | Large thermonuclear weapon |
| 8.0 | 6.3 × 10¹⁶ | 15 megatons of TNT | Largest US nuclear test (Castle Bravo, ~15 Mt) |
| 9.0 | 2.0 × 10¹⁸ | 480 megatons of TNT | ~10× the Tsar Bomba (largest nuclear weapon ever detonated, ~50 Mt) |
| 10.0 (theoretical) | 6.3 × 10¹⁹ | 15 gigatons of TNT | No human comparison; ~160× annual global energy consumption |
Energy values calculated using log₁₀E = 1.5M + 4.8 (joules). TNT equivalents use 4.184 × 10⁹ joules per ton of TNT.
[CHART: Logarithmic bar chart — Energy release by earthquake magnitude] Data: Energy in joules from M1 to M9 using the standard formula. Use log scale on y-axis with labeled magnitude examples (Hiroshima at ~M6, Tsar Bomba at ~M8.3).
Line chart — Gutenberg-Richter frequency-magnitude relationship
Data: Worldwide earthquake frequency by magnitude (M2 through M9). Y-axis: log₁₀ of annual count. X-axis: magnitude. Show the approximately linear relationship with slope ~-1 (b-value). Label key data points from USGS statistics.
Modified Mercalli Intensity Scale (MMI)
Origins
The Modified Mercalli Intensity Scale was developed in 1931 by American seismologists Harry Wood and Frank Neumann, based on the original Mercalli scale created by Italian volcanologist Giuseppe Mercalli in 1902. Unlike magnitude, which is determined instrumentally, Mercalli intensity is based on observations of an earthquake's effects on people, structures, and the natural environment at a specific location.
The Complete MMI Scale
The MMI scale uses Roman numerals I through XII. The following table provides the full scale with descriptions, approximate corresponding peak ground acceleration (PGA), and real earthquake examples.
| MMI Level | Description | Perceived Shaking | Potential Damage | Approximate PGA (g) | Example |
|---|---|---|---|---|---|
| I | Not felt | None | None | < 0.0017 | Most M2 earthquakes at distance |
| II | Felt by few people at rest, especially on upper floors | Weak | None | 0.0017–0.014 | Small local earthquake felt indoors |
| III | Felt indoors, especially upper floors; hanging objects swing | Weak | None | 0.0017–0.014 | Moderate distant earthquake |
| IV | Felt by many indoors, few outdoors; dishes and windows rattle; sensation like heavy truck passing | Light | None | 0.014–0.039 | M4.5 at 20 km |
| V | Felt by nearly everyone; some dishes and windows broken; unstable objects overturned | Moderate | Very light | 0.039–0.092 | M5.0 at 15 km |
| VI | Felt by all; many frightened; some heavy furniture moved; slight structural damage | Strong | Light | 0.092–0.18 | 2014 Napa M6.0 at 10 km |
| VII | Considerable damage to poorly built structures; slight to moderate damage in well-built ordinary structures | Very Strong | Moderate | 0.18–0.34 | 1994 Northridge M6.7 at 20 km |
| VIII | Damage to ordinary well-built structures; partial collapse of poorly built structures; fall of chimneys, monuments, walls | Severe | Moderate to Heavy | 0.34–0.65 | 1989 Loma Prieta in San Francisco Marina District |
| IX | Considerable damage to well-built structures; buildings shifted off foundations; ground cracking | Violent | Heavy | 0.65–1.24 | 1994 Northridge near epicenter |
| X | Most masonry and frame structures destroyed; rails bent; landslides | Extreme | Very Heavy | > 1.24 | 1906 San Francisco near fault rupture |
| XI | Few if any masonry structures remain standing; bridges destroyed; broad fissures in ground | Extreme | Very Heavy | > 1.24 | 2011 Tōhoku near coast |
| XII | Total destruction; waves seen on ground surface; objects thrown into the air | Extreme | Total | > 1.24 | Near-fault areas of M9+ events |
PGA values from the USGS ShakeMap system, which relates instrumental measurements to MMI levels.
USGS — Modified Mercalli Intensity Scale
How Intensity Is Determined
The USGS determines Mercalli intensity through two methods:
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Community reports. The "Did You Feel It?" (DYFI) system collects reports from the public via an online questionnaire. Respondents describe what they felt and observed, and these reports are aggregated to produce community intensity maps. Thousands to millions of reports are collected for significant earthquakes.
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ShakeMap. Instrumental data from seismograph networks is combined with site amplification models to produce automated intensity maps within minutes of an earthquake. ShakeMap incorporates both recorded ground motion and statistical predictions for areas without nearby instruments USGS ShakeMap.
Factors That Affect Intensity
The same earthquake produces different intensities at different locations because of:
- Distance from the epicenter. Shaking diminishes with distance, though the pattern is complex.
- Depth. Shallow earthquakes produce stronger but more localized shaking; deep earthquakes produce weaker but more widespread shaking.
- Local geology (site effects). Soft sediments amplify shaking significantly. In the 1989 Loma Prieta earthquake, areas of San Francisco built on landfill (Marina District) experienced Intensity IX, while areas on bedrock just 2 km away experienced only Intensity VI. This amplification effect, known as site response, can increase shaking by factors of 2 to 10 in sedimentary basins.
- Directivity. Seismic energy radiates more strongly in the direction of fault rupture propagation, a phenomenon called forward directivity. Areas ahead of the propagating rupture receive stronger shaking.
- Building construction. The same shaking level causes more damage to unreinforced masonry than to modern steel-frame or reinforced concrete buildings, which is reflected in the intensity assessment.
How Seismographs Work
Basic Principles
A seismograph detects and records ground motion by exploiting the principle of inertia. The fundamental design involves a heavy mass (typically a pendulum or a mass on a spring) that remains relatively stationary due to its inertia while the frame of the instrument, which is attached to the ground, vibrates with the passing seismic waves. The relative motion between the mass and the frame is the seismic signal.
Modern broadband seismometers are far more sophisticated than the original mechanical instruments. They use electronic feedback systems to detect motion across a wide range of frequencies (from 0.001 Hz to 100 Hz or more) and amplitudes (from nanometers to meters of displacement). The output is digitized and transmitted in near real-time to data centers worldwide.
Seismic Networks
Global seismology depends on international networks of seismograph stations. The Global Seismographic Network (GSN), operated jointly by the USGS, the Incorporated Research Institutions for Seismology (IRIS), and the University of California, San Diego, comprises approximately 150 stations distributed across the globe, including on the ocean floor and in boreholes IRIS — Incorporated Research Institutions for Seismology.
Regional networks, such as the Advanced National Seismic System (ANSS) in the United States, provide denser coverage in seismically active areas. California alone has over 3,000 seismic sensors. These dense networks enable rapid detection, location, and magnitude determination — often within minutes for significant events.
From Waveform to Magnitude
When an earthquake occurs, seismic waves travel outward from the source and are recorded by seismographs at varying distances. Seismologists analyze the waveforms — their arrival times, amplitudes, frequencies, and polarities — to determine the earthquake's location (epicenter and depth), magnitude, and focal mechanism (the orientation of the fault and the direction of slip).
For the moment magnitude, seismologists analyze long-period seismic waves to determine the seismic moment. For very large earthquakes, this analysis may use waves that travel all the way around the Earth.
Other Magnitude and Intensity Scales Used Worldwide
Japan Meteorological Agency Seismic Intensity Scale (Shindo)
Japan uses its own intensity scale called Shindo (震度), maintained by the Japan Meteorological Agency (JMA). Unlike the 12-level MMI scale, the Shindo scale has 10 levels: 0 through 7, with levels 5 and 6 each divided into Lower and Upper, yielding: 0, 1, 2, 3, 4, 5 Lower, 5 Upper, 6 Lower, 6 Upper, 7.
The Shindo scale is determined instrumentally using accelerometers, making it more objective and faster than the observation-based MMI scale. Japan's extensive network of intensity meters (over 4,200 stations) can produce intensity maps within 2 minutes of an earthquake.
| Shindo Level | Approximate MMI Equivalent | Description |
|---|---|---|
| 0 | I | Not felt by people; recorded by instruments |
| 1 | II–III | Felt by some people indoors |
| 2 | III–IV | Felt by many; hanging lights swing |
| 3 | IV–V | Felt by most; dishes rattle |
| 4 | V–VI | Many frightened; hanging objects swing considerably |
| 5 Lower | VI | Some furniture moves; objects fall from shelves |
| 5 Upper | VII | Difficult to move; unreinforced walls may crack |
| 6 Lower | VIII | Difficult to stand; heavy furniture may topple |
| 6 Upper | IX | Impossible to stand; some buildings collapse |
| 7 | X–XII | Severe destruction; buildings thrown from foundations |
European Macroseismic Scale (EMS-98)
The European Macroseismic Scale, adopted in 1998, is a 12-level intensity scale (I–XII) used across Europe. It is conceptually similar to the MMI but incorporates a more systematic classification of building vulnerability. EMS-98 classifies buildings into vulnerability classes (A through F) and defines damage grades (1 through 5) for each class, providing a more structured and reproducible assessment than the narrative descriptions of the MMI.
Other Scales
Several other scales are used in specific contexts:
| Scale | Type | Developer/Agency | Use | Notes |
|---|---|---|---|---|
| Local Magnitude (ML) | Magnitude | Richter, 1935 | Small local earthquakes | Original "Richter Scale"; still used for small events |
| Body-Wave Magnitude (mb) | Magnitude | Gutenberg, 1945 | Teleseismic events | Based on P-wave amplitude; saturates above ~6.5 |
| Surface-Wave Magnitude (Ms) | Magnitude | Gutenberg, 1945 | Shallow teleseismic events | Based on surface wave amplitude; saturates above ~8.0 |
| Moment Magnitude (Mw) | Magnitude | Hanks & Kanamori, 1979 | Universal standard | No saturation; based on seismic moment |
| Energy Magnitude (Me) | Magnitude | Choy & Boatwright, 1995 | Energy assessment | Measures radiated seismic energy directly |
| Duration Magnitude (Md) | Magnitude | Various | Local small events | Based on signal duration; used where amplitude is clipped |
| MMI | Intensity | Wood & Neumann, 1931 | Americas, much of the world | 12 levels (I–XII), observation-based |
| EMS-98 | Intensity | European Seismological Commission, 1998 | Europe | 12 levels (I–XII), structured vulnerability/damage |
| Shindo (JMA) | Intensity | JMA | Japan | 10 levels (0–7 with subdivisions), instrumental |
| MSK-64 | Intensity | Medvedev, Sponheuer, Kárník, 1964 | Former Soviet states | 12 levels, predecessor to EMS-98 |
Notable Earthquakes: Magnitude Comparisons
The following table illustrates how the same earthquake can be described differently depending on which scale is used, and demonstrates why moment magnitude has become the universal standard. For detailed accounts of these events, see our earthquake history section, including pages on the 1906 San Francisco earthquake, the 1994 Northridge earthquake, and the 2004 Indian Ocean earthquake.
| Earthquake | Year | ML (Richter) | Ms (Surface Wave) | Mw (Moment) | Max MMI | Key Notes |
|---|---|---|---|---|---|---|
| 1960 Valdivia, Chile | 1960 | — | 8.5 | 9.5 | XII | Largest earthquake ever recorded; Ms severely underestimated size |
| 1964 Great Alaska | 1964 | — | 8.4 | 9.2 | XI | Second largest recorded; Ms underestimate by nearly 1 unit |
| 2004 Indian Ocean | 2004 | — | 8.5 | 9.1 | IX | Initial Ms estimate revised upward significantly by Mw |
| 2011 Tōhoku, Japan | 2011 | 7.9 (JMA) | — | 9.1 | IX (XII near coast) | JMA magnitude initially underestimated |
| 1994 Northridge, CA | 1994 | 6.4 | — | 6.7 | IX | ML and Mw diverge even at moderate magnitudes |
| 1906 San Francisco | 1906 | 7.7 (est.) | — | 7.9 | XI | No instrumental Richter; magnitude estimated retrospectively |
| 2010 Haiti | 2010 | — | 7.0 | 7.0 | X | Shallow depth (13 km) caused extreme local intensity |
| 2023 Turkey (Kahramanmaraş) | 2023 | — | — | 7.8 | XII | Followed by M7.5 aftershock 9 hours later |
This table demonstrates a critical point: for the three largest earthquakes ever recorded (Chile 1960, Alaska 1964, Indian Ocean 2004), the surface-wave magnitude (Ms) significantly underestimated the true size. Only the moment magnitude correctly captured the enormous energy release of these events. This is precisely why Mw replaced earlier scales as the standard.
Practical Applications of Earthquake Measurement
Building Codes and Engineering
Earthquake magnitude and intensity data directly inform building codes. The International Building Code (IBC), used in the United States, defines seismic design categories based on the expected ground motion at a site — which is derived from probabilistic seismic hazard analysis incorporating historical earthquake data.
In the United States, the USGS National Seismic Hazard Model provides mapped spectral acceleration values used by engineers to design structures that can withstand expected shaking levels. Buildings in seismically active areas like California must meet much more stringent seismic design requirements than buildings in low-seismicity areas like the Midwest — though the 2023 update to the hazard model increased design requirements in several eastern U.S. cities.
Earthquake Early Warning
The ShakeAlert system operating in California, Oregon, and Washington uses real-time seismograph data to rapidly estimate an earthquake's magnitude and predict the intensity of shaking at locations away from the epicenter. This is only possible because the relationship between magnitude, distance, and intensity is well-characterized. The system can issue warnings seconds before strong shaking arrives, triggering alerts on smartphones and automated protective actions in schools, hospitals, and industrial facilities ShakeAlert.
Emergency Response
Within minutes of a significant earthquake, the USGS produces a ShakeMap showing estimated intensities across the affected region, and a PAGER (Prompt Assessment of Global Earthquakes for Response) report estimating the population exposed to each intensity level and the likely range of fatalities and economic losses. Emergency managers use these products to direct resources where they are most needed — before any reports arrive from the ground USGS PAGER.
Insurance and Risk Assessment
The insurance industry uses seismic hazard assessments to set earthquake insurance premiums. Both magnitude frequency (how often large earthquakes occur in a region) and intensity distribution (how strong shaking reaches a specific location) factor into risk models. Catastrophe modeling firms such as AIR, RMS, and CoreLogic simulate millions of possible earthquake scenarios to estimate potential losses.
For information on earthquake insurance, see earthquake insurance in California.