Modified Mercalli Intensity Scale: Measuring Earthquake Effects
When an earthquake strikes, news reports often lead with a single number representing its strength. However, there is a vital distinction between how much energy an earthquake releases and how much shaking is actually felt on the ground. While magnitude measures the inherent force of the seismic event, the Modified Mercalli Intensity (MMI) scale measures the actual effects and impact at specific locations.
Understanding this difference is crucial for scientists, engineers, and emergency responders. While a massive earthquake deep underground might be barely felt, a much smaller quake occurring near the surface can cause devastating destruction. The MMI scale provides a way to categorize these localized experiences of shaking and damage.
Key Facts
- Intensity vs. Magnitude: Magnitude measures energy released; intensity measures observed effects at a specific site.
- Localized Impact: Shaking intensity varies depending on distance from the epicentre, depth of the quake, and local soil conditions.
- Empirical Basis: The scale is based on observed effects reported by people and structural damage to buildings.
- Historical Utility: Because it relies on observations rather than instruments, it is essential for estimating the size of historical earthquakes.
- Scale Range: The scale typically ranges from I (not felt) to XII (total damage).
The Science of Shaking
The intensity of shaking at the surface is driven by the seismic energy released during an earthquake. However, not all energy reaches the surface in the same way. Several factors influence the intensity experienced at any given point:
- Depth: Deeper earthquakes spread their energy through a larger volume, often resulting in less intense surface shaking compared to shallow events.
- Distance: Generally, intensity diminishes as you move further from the epicentre (the point on the surface directly above the earthquake's focus).
- Geology: Certain environments, such as sedimentary basins or unconsolidated soils, can amplify seismic waves, leading to much stronger shaking than in rocky areas.
Evolution of the Scale
The MMI scale has a rich history of refinement by various scientists. It began with Italian volcanologist Giuseppe Mercalli, who formulated an initial intensity scale in 1883. His 1902 version, which expanded the descriptions of each degree, became widely used and served as the foundation for modern iterations.
In 1904, Adolfo Cancani proposed adding two higher categories for extreme events. Later, August Heinrich Sieberg augmented these descriptions and linked them to peak ground acceleration. This evolved into the Mercalli–Cancani–Sieberg scale (MCS), which is still used in Italy today.
The English-language version we recognize today was developed by Harry O. Wood and Frank Neumann in 1931 (the MM31 scale). This was later revised by Charles Francis Richter in 1956 to avoid confusion with his famous Richter magnitude scale, resulting in the "modified Mercalli scale of 1956" (MM56).
Comparing Intensity Levels
The MMI scale uses lower numbers to describe how people perceive the shaking and higher numbers to describe the structural damage caused. Below is a summary of the typical observations associated with different levels of the scale.
| Scale Level | Description | Typical Effects |
|---|---|---|
| I - III | Not Felt to Weak | Hardly felt or felt only by a few people at rest; standing vehicles may rock. |
| IV - V | Light to Moderate | Dishes and windows disturbed; many people awakened; unstable objects overturned. |
| VI - VII | Strong to Very Strong | Furniture moves; slight damage to buildings; chimneys may break. |
| VIII - IX | Severe to Violent | Considerable damage to buildings; partial collapse; liquefaction may occur. |
| X - XII | Extreme | Masonry structures destroyed; large landslides; total damage to the built environment. |
Magnitude vs. Intensity: A Critical Distinction
It is a common misconception that magnitude and intensity are the same. In reality, an earthquake has only one magnitude, but it can have many different intensities depending on where you are standing. To visualize this, scientists use isoseismal maps, which use contour lines to connect areas of equal intensity.
Consider this comparison: a massive magnitude 8.2 earthquake occurring very deep in the earth (over 600 km deep) might only produce a maximum intensity of VI. Conversely, a much smaller magnitude 2.2 earthquake occurring very close to the surface (1 km deep) could produce an intensity of VIII. This demonstrates that depth and local conditions are just as important as the energy released.
Frequently Asked Questions
What is the difference between magnitude and intensity?
Magnitude measures the total energy released at the source of the earthquake. Intensity measures the strength of the shaking and the level of damage at a specific location on the surface.
Why does shaking feel stronger in some areas than others?
Shaking intensity is influenced by the distance from the epicentre, the depth of the earthquake, and local geological factors. For example, soft soils and sedimentary basins can amplify seismic waves, making the shaking feel much more intense.
How is the MMI scale used for historical earthquakes?
Because the MMI scale is based on observed effects (like damage to buildings or descriptions from witnesses) rather than electronic sensors, it allows historians and scientists to estimate the magnitude and location of earthquakes that occurred before modern instruments were invented.
Does the MMI scale measure how much a building is damaged?
Yes. While the lower levels of the scale focus on human perception, the higher levels are specifically categorized based on the degree of structural damage observed in various types of buildings.
What are the highest levels on the scale?
The highest levels (X, XI, and XII) describe extreme destruction, including the collapse of most masonry structures, the creation of large ground fissures, and total damage to the built environment.