In earthquake news, we often hear phrases like “The earthquake had a magnitude of 7.4” or “It was felt with intensity 9”. These expressions are usually mixed together in everyday language, but in seismology they are not the same thing. Magnitude and intensity describe different aspects of an earthquake, and scales like Richter, Mw and Mercalli are used for different purposes.
This article explains the difference between earthquake magnitude and intensity, what the Richter and Moment Magnitude (Mw) scales measure and how the Mercalli intensity scale works.
Magnitude vs. Intensity: Two Different Concepts
The first step is to clearly separate two key concepts:
- Earthquake magnitude:
A measure of the total energy released at the source of the earthquake, along the fault.
It is expressed as a single number and does not change from place to place. - Earthquake intensity:
A measure of how strongly the earthquake is felt and what kind of effects it has at a specific location.
It can vary from one town to another for the same earthquake.
In other words:
- Magnitude answers the question: “How big was the earthquake at its source?”
- Intensity answers: “How strong was it here where I am?”
The same earthquake may cause severe damage near the epicentre (high intensity) but only light shaking far away (low intensity), even though its magnitude is the same everywhere.
The Richter Scale
The Richter scale was developed in the 1930s by American seismologist Charles F. Richter as a magnitude scale. It is based on the maximum amplitude of ground motion recorded by a standard seismograph and is logarithmic in nature.
Key points about the Richter scale:
- It was originally defined for a specific type of instrument and region (for example, southern California).
- As magnitude increases, the recorded amplitude and released energy increase dramatically. For example:
- A magnitude 6.0 earthquake has about ten times the recorded amplitude of a magnitude 5.0 event.
- In terms of energy, the difference is many times greater than ten.
- The term “on the Richter scale” became so popular that people still use it today, even when the official magnitude is given in a different scale.
Modern seismology often uses other magnitude definitions, especially for large earthquakes, but the word “Richter” remains in common media language.
Moment Magnitude (Mw)
Today, for medium and large earthquakes, seismologists usually prefer the Moment Magnitude scale (Mw). This scale is based on the physical properties of the fault rupture:
- The area of the fault that slipped,
- The average amount of slip on the fault,
- The strength (rigidity) of the rocks involved.
These quantities combine into a physical value called the seismic moment. The Mw scale converts the seismic moment into a logarithmic magnitude.
Why Mw is widely used:
- For very large earthquakes (around magnitude 8 and above), some older scales tend to “saturate”, while Mw continues to represent large energies more reliably.
- It allows more consistent comparison between earthquakes in different regions and at different depths.
- Global agencies typically report large earthquake magnitudes in Mw, even if the symbol is not always shown in media headlines.
So when we hear “a magnitude 7.8 earthquake struck”, the underlying measure is often Mw, even if news outlets simply write “magnitude 7.8”.
Other Magnitude Types: ML, mb, Ms
Magnitude is not just one single scale. Different definitions are used for different kinds of seismic waves and recording conditions:
- ML (Local Magnitude):
Richter’s original local magnitude, suitable for nearby events. - mb (body-wave magnitude):
Calculated from body waves that travel through the interior of the Earth. - Ms (surface-wave magnitude):
Based on surface waves that travel along the Earth’s surface.
These scales each have technical advantages in certain size and distance ranges, but for the general public the most important quantity for significant earthquakes is usually the moment magnitude (Mw).
The Mercalli Intensity Scale
While Richter and Mw measure the energy at the source, the Mercalli scale measures how the earthquake is experienced and what it does at specific locations.
Characteristics of the Mercalli intensity scale:
- It was originally developed in Italy and later modified into the Modified Mercalli Intensity (MMI) scale.
- It has 12 degrees, written with Roman numerals from I to XII.
- It is based on observations: how people feel the shaking, how furniture and objects move, the level of damage to buildings and visible changes at the surface.
Examples:
- I–II: Not felt by most people, only detectable by instruments.
- III–IV: Felt indoors by some people; hanging objects may sway slightly.
- V–VI: Felt by many; light objects move; minor damage in weaker buildings.
- VII–IX: Noticeable damage in buildings, falling chimneys, toppling of heavy furniture.
- X–XII: Severe destruction, collapsed structures, visible ground cracks and deformation.
For a single earthquake, Mercalli intensity can be high near the epicentre and much lower far away. There is one magnitude for the entire event but many different intensities.
“Magnitude 7.4, Intensity 9” – What Does That Mean?
In some reports you may see sentences like: “The earthquake had a magnitude of 7.4 and a maximum intensity of IX.” In that case:
- 7.4 refers to the magnitude (for example Mw 7.4), the energy at the source.
- IX refers to the local intensity on the Mercalli scale in a specific area.
So for the same earthquake:
- There is one magnitude value.
- There can be different intensity values in different cities or districts.
This distinction explains why the same earthquake may be almost unnoticeable in one region while causing serious damage in another.
Why Are Magnitude Scales Logarithmic?
Both the Richter scale and the Moment Magnitude scale (Mw) are logarithmic. This means:
- A difference of 1 unit in magnitude corresponds to about 10 times greater recorded amplitude.
- In terms of energy, the difference is even larger; each step of 1 in magnitude corresponds to many times more energy release.
Therefore, between a magnitude 5.0 and a magnitude 7.0 earthquake, the difference is not “just two units”. The larger earthquake releases dramatically more energy and has far greater destructive potential.
Which Scale Is Used for What?
To summarise:
- Richter and related magnitude scales (ML, mb, Ms):
Measure the earthquake’s energy at the source.
They answer: “How big was the earthquake?” - Moment Magnitude (Mw):
The most widely used modern scale for medium and large earthquakes; considered the most reliable for comparing big events. - Mercalli intensity scale:
Describes how strongly the earthquake is felt and what level of damage occurs at specific locations.
It answers: “How strong was it here?”
When we understand the difference between magnitude and intensity, we can interpret earthquake reports more accurately and better understand what the numbers actually mean.