The Mercalli Scale measures earthquake intensity by examining how shaking affects people, buildings, and the ground at a particular location. In contrast, magnitude scales measure the size of an earthquake at its source, while the Mercalli Scale describes how strongly people experience the earthquake in a specific area.
Additionally, the scale uses 12 intensity levels, represented by Roman numerals from I to XII. At the lower levels, the scale describes how people perceive earthquake shaking, whereas at the higher levels, it describes increasingly severe effects and damage to structures and the ground.
What Is the Mercalli Scale?
The Mercalli Scale describes earthquake intensity by showing the effects of ground shaking at a particular location. Unlike magnitude scales, it does not directly measure the energy an earthquake releases. Instead, scientists use observations of how people experience the shaking, how objects respond, and how the earthquake affects buildings and the ground.
The scale ranges from I to XII:
- I represents an earthquake that people generally do not feel.
- VI indicates strong shaking that can cause minor damage.
- XII represents extreme effects and near-total destruction.
In the United States, scientists commonly call this scale the Modified Mercalli Intensity (MMI) Scale. Importantly, one earthquake can produce different intensity values at different locations. For example, an earthquake may cause strong shaking and damage near its source, while people farther away may experience much weaker effects.
In addition, local ground conditions and building design and construction can influence the intensity people experience at a particular location.
History of the Scale: From Rossi-Forel to Modern MMI
The concept of measuring earthquake intensity developed through more than a century of refinement. Initially, the Rossi-Forel Scale (1880s) became the first widely accepted intensity scale. Italian scientist Michele Stefano de Rossi and Swiss researcher François-Alphonse Forel developed the scale jointly. They used a 10-point ranking system based on human perception and structural damage.
Later, in 1902, Italian volcanologist and priest Giuseppe Mercalli recognized that the Rossi-Forel scale lacked precision for severe earthquakes. As a result, he adapted the scale into a 10-degree system. In 1904, Italian physicist Adolfo Cancani expanded the system to 12 degrees.
Building on these earlier developments, American seismologists Harry O. Wood and Frank Neumann revised the scale in 1931 to reflect modern construction practices, including steel-frame skyscrapers, reinforced concrete, and motor vehicles. As a result, they created what became known as the Modified Mercalli Intensity (MMI) Scale.
Later, in 1956, seismologist Charles Richter made additional refinements, including changes related to California building codes and masonry quality. Together, these developments helped shape the modern approach to describing earthquake intensity.
Modified Mercalli scale:
I. Not felt, except by a very few, under especially favorable circumstances.
II. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
III. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing automobiles may rock slightly. Vibrations like a passing truck.
IV. During the day, felt indoors by many, outdoors by few. At night, some awakened. Dishes, windows, and doors were disturbed; cracking sounds from the walls. A sensation like a heavy truck striking a building. Standing automobiles rock noticeably.
V. Felt by nearly everyone; many awakened. Some dishes, windows, etc., are broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles, and other tall objects is sometimes noticed. Pendulum clocks may stop.
VI. Felt by all; many frightened and ran outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Structural damage is slight.
VII. Everybody runs outdoors. Damage is negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly-built or badly-designed structures; some chimneys are broken. Noticed by people driving.
VIII. Damage is slight in specially-engineered structures; considerable in ordinary structures. Many buildings have experienced partial collapse. Panel walls are thrown out of some frame structures. Fall of chimneys, factory stacks, columns, monuments, and masonry walls. Heavy furniture overturned. Sand and mud are ejected in small amounts. Changes in well water.
IX. Damage is considerable in all structures. Well-designed frame structures thrown out of plumb; partial collapse in many substantial buildings. Buildings shifted off their foundations. Ground conspicuously cracked. Underground pipes are broken.
X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed, including foundations. The ground is badly cracked. Rails bent. Landslides occur on river banks and other steep slopes. Shifted sand and mud.
XI. Few, if any, masonry structures remain standing. Bridges destroyed. Broad fissures in the ground. Underground pipelines are completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
XII. Damage total. Waves are seen on the ground surface. Lines of sight distorted. Objects thrown upward into the air.
How Is the Mercalli Scale Determined?
Scientists determine earthquake intensity by examining the effects of an earthquake at a particular location. Unlike magnitude, which relies heavily on instrumental measurements, scientists determine Mercalli intensity from observations and reports. For example, they may consider:
- Reports from people who experienced the shaking
- Movement of furniture and other objects
- Damage to buildings and structures
- Damage to roads, bridges, and other infrastructure
- Ground cracks and landslides
- Changes in water wells
- Effects on trees, poles, and other objects
- Engineering and field observations
In addition, modern earthquake reporting systems collect information directly from people who felt an earthquake. As a result, scientists can use these observations to create earthquake intensity maps that show how strongly different areas experienced the event.
Furthermore, the quality and type of construction also affect earthquake intensity observations. A well-designed building may withstand strong shaking with limited damage, whereas a poorly constructed building may suffer serious damage from the same earthquake.
Therefore, Mercalli intensity describes the observed effects at a location rather than simply the strength of the earthquake itself.
How Is the Mercalli Scale Used Today?
The Modified Mercalli Scale continues to be useful in modern earthquake studies. Today, scientists use intensity information to understand the effects of earthquakes on communities, buildings, and the natural environment.
Some important applications include:
- Creating earthquake intensity maps: Intensity observations can show where shaking was strongest. Therefore, these maps help researchers understand how earthquake effects varied across different areas.
- Assessing earthquake effects: For example, scientists can study how buildings, infrastructure, and the ground responded. In this way, they can better evaluate the overall impact of an earthquake.
- Studying historical earthquakes: In addition, researchers can use historical reports and damage records to estimate the intensity of earthquakes that occurred before modern instruments existed. As a result, they can learn more about earthquakes from the past.
- Understanding earthquake hazards: Furthermore, intensity information helps researchers identify patterns of earthquake damage. Consequently, this information can support better understanding of earthquake hazards.
- Collecting public observations: Moreover, reports from people who experienced shaking can provide valuable information about earthquake effects. These observations can also help researchers compare reported experiences across different locations.
Modern instruments provide detailed information about an earthquake's magnitude, location, depth, and seismic waves. However, the Mercalli Scale provides a different type of information by describing what people and structures actually experienced at specific locations. In contrast, instrumental measurements focus on measurable characteristics of the earthquake itself. Therefore, the Mercalli Scale remains an important tool for understanding the real-world impact of earthquakes. Overall, it complements instrumental data by showing how earthquake shaking affects people, buildings, and the surrounding environment.
Frequently Asked Questions About Mercalli Scale:
The Mercalli Scale is an earthquake intensity scale that describes the observed effects of ground shaking at a particular location. It ranges from I to XII using Roman numerals.
XII is the highest level of the Modified Mercalli Intensity Scale. It represents extreme effects and widespread destruction.
The Mercalli Scale describes the intensity and effects of earthquake shaking at a specific location, while the Richter scale is a magnitude scale developed to quantify earthquake size using seismic-wave measurements. Today, modern seismology commonly uses other magnitude scales, especially moment magnitude (Mw) for large earthquakes.
Yes. A single earthquake can produce different Mercalli intensity levels at different locations. Distance from the earthquake source, local ground conditions, and building construction can affect the intensity experienced at a location.
Yes. The Modified Mercalli Intensity Scale is still used to describe earthquake effects and to produce intensity information and maps.
Conclusion:
The Mercalli Scale measures earthquake intensity by describing how ground shaking affects people, buildings, objects, and the ground at a particular location. The Modified Mercalli Intensity Scale ranges from I to XII, with levels ranging from earthquakes that people generally do not feel to those that cause extreme and widespread destruction.
Unlike magnitude, however, intensity can vary from one location to another during the same earthquake. Distance from the earthquake source, local ground conditions, and building construction can all influence the effects people observe.
Although modern instruments can precisely measure earthquake magnitude and seismic activity, the Mercalli Scale remains useful because it shows how an earthquake actually affects people and communities.
