Modified Mercalli intensity scale facts for kids
The Modified Mercalli Intensity Scale (often called MM or MMI) helps us measure how strong an earthquake feels in a specific place. It's different from the magnitude of an earthquake, which tells us how much energy the earthquake released.
Think of it this way: Magnitude is like the power of a light bulb (how bright it *can* be). Intensity is how bright the light actually *looks* to you, depending on how far away you are and if anything is blocking the light.
The MM scale measures how much shaking happens on the surface. It was created from an earlier scale by Giuseppe Mercalli in 1902. Even though an earthquake's energy causes shaking, how much you feel it depends on many things. For example, deeper earthquakes might feel weaker on the surface because their energy spreads out more. Also, the type of ground can make the shaking feel stronger or weaker.
This scale is useful because it doesn't need special instruments. People just report what they felt or saw. This helps scientists understand old earthquakes that happened before we had modern measuring tools. The strongest shaking usually happens closest to where the earthquake started.
Contents
How the Mercalli Scale Started
The first version of this scale was made by an Italian scientist named Giuseppe Mercalli in 1883. It had six levels. Later, in 1902, Mercalli made a new version with 10 levels. This one became very popular.
In 1904, another scientist, Adolfo Cancani, added two more levels for very strong earthquakes. Then, from 1912 to 1923, August Heinrich Sieberg improved the descriptions for each level. This updated scale became known as the "Mercalli–Cancani–Sieberg scale" or "MCS". Many places in Europe still use it today.
The Scale Comes to America
In 1931, Harry O. Wood and Frank Neumann translated the MCS scale into English. They also made some changes and called it the "Modified Mercalli Intensity Scale of 1931" (MM31). Some people call it the "Wood–Neumann scale."
Later, in 1956, Charles Francis Richter (who created the Richter magnitude scale) updated the Wood–Neumann scale. He called his version the "Modified Mercalli Scale of 1956" (MM56).
Today, organizations like the U.S. Geological Survey (USGS) use a version of the 1931 scale. They have made small changes over time because buildings are stronger now. Also, some of the highest levels of the scale (like X and above) are now grouped into one category called "Extreme" (X+). This is because those very strong effects, like bent train tracks, are rare.
Understanding the Modified Mercalli Scale
The lower numbers on the Modified Mercalli Intensity scale describe how people feel an earthquake. The higher numbers describe the damage caused to buildings and other structures.
The table below shows the typical Modified Mercalli intensity levels you might see near where an earthquake starts.
| I. Not felt | Not felt by humans but technology is capable of sensing it. |
|---|---|
| II. Weak | Felt only by a few persons during sleep, especially on upper floors of buildings. |
| III. Weak | Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibrations similar to the passing of a truck. Duration estimated. |
| IV. Light | Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. |
| V. Moderate | Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop. |
| VI. Strong | Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight. |
| VII. Very Strong | Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. |
| VIII. Severe | Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. |
| IX. Violent | Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. |
| X. Extreme | Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent. |
| XI. Extremely Dangerous | Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. |
| XII. Catastrophic | Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air. |
How Intensity Relates to Magnitude
There is a connection between an earthquake's magnitude and its intensity. However, it's not always a simple link. Many things can affect this, like how deep the earthquake is, the type of ground, and how many people live in the area.
For example, a small earthquake (magnitude 0.7) in Central California was felt as intensity III over 100 miles (160 km) away. But a stronger earthquake (magnitude 4.5) in Salta, Argentina, which was very deep, was only felt as intensity I.
| Magnitude | Typical Maximum Modified Mercalli Intensity |
|---|---|
| Under 2.0 | I |
| 2.0 – 2.9 | II – III |
| 3.0 – 3.9 | III – IV |
| 4.0 – 4.9 | IV – V |
| 5.0 – 5.9 | V – VI |
| 6.0 – 6.9 | VI – VII |
| 7.0 – 7.9 | VII – VIII |
| 8.0 or higher | VIII or higher |
Intensity vs. Magnitude
The effects of an earthquake can be very different from one place to another. This means that one earthquake can have many different MMI values. Scientists often show these values on a map with lines connecting areas of equal intensity. This is called an isoseismal map.
However, each earthquake only has one magnitude. Magnitude tells us the total energy released, while intensity tells us how that energy was felt or what damage it caused in specific locations.
See also
In Spanish: Escala sismológica de Mercalli para niños
- European macroseismic scale
- Japan Meteorological Agency seismic intensity scale (Shindo scale)
- Rohn emergency scale
- Seismic intensity scales
- Seismic magnitude scales
- Spectral acceleration
- Strong ground motion
