Earthquake Intensity: Decoding the Mercalli Scale

RCC Blog August 20, 2025 9 min read

Earthquake Intensity: Decoding the Mercalli Scale - RCC Blog

Imagine waking up to a gentle swaying, thinking it's just a passing truck. But then the swaying intensifies, pictures fall off the walls, and the ground beneath you seems to be alive. The realization hits: it's an earthquake. But how strong is it? How much damage will it cause? Understanding the earthquake's impact goes beyond just knowing its magnitude; it's about understanding its intensity—how it's felt and what damage it inflicts. This understanding is crucial for safety, emergency response, and rebuilding efforts.

What is Earthquake Intensity?

Earthquake intensity provides a qualitative measure of the effects of an earthquake at a specific location. Unlike earthquake magnitude, which quantifies the energy released at the earthquake's source (the epicenter), intensity describes the degree to which an earthquake is felt by people, and the extent of damage it causes to structures. It's a subjective assessment, influenced by factors like distance from the epicenter, local geology, building construction, and even the observer's perception. Therefore, an earthquake will often have a single magnitude, but varying intensities at different locations. The concept of intensity has been around for centuries, predating the development of precise instruments like the seismograph.

Intensity is not a scientific measure in the same way that magnitude is. It relies on observations – what did people feel? What kind of damage occurred? This makes intensity scales invaluable for understanding the historical impact of earthquakes, particularly those that occurred before the advent of modern seismology.

The Modified Mercalli Intensity Scale (MMI) Explained

The modified mercalli intensity scale (MMI) is the most widely used scale for measuring earthquake intensity. Developed by Italian volcanologist and seismologist Giuseppe Mercalli in 1902 and later modified by Harry O. Wood and Frank Neumann in 1931, the MMI assigns Roman numerals from I to XII to describe the severity of an earthquake's effects. Each level corresponds to a specific set of observations, ranging from imperceptible shaking to catastrophic destruction. The scale is based on observed effects, not instrumental measurements, which means that reports from people, descriptions of damage, and analysis of the environment are all used to determine the intensity at a given location. This is especially important when studying historical earthquakes for which no instrumental data exists.

Understanding the MMI Levels

The MMI provides a spectrum of descriptions, from barely noticeable shaking to utter devastation. Here's a breakdown of some key levels:

• I (Not Felt): Not felt except by a very few under especially favorable conditions.
• II (Weak): Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.
• III (Slight): 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. Vibration similar to the passing of a truck. Duration estimated.
• IV (Moderate): 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 (Moderately Strong): 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 in poorly built or badly designed structures; some chimneys broken.
• VIII (Severe): Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Disturbs drivers of motor cars.
• IX (Violent): Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken.
• X (Extreme): Most masonry and frame structures destroyed with their foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks.
• XI (Catastrophic): Few, if any (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.
• XII (Total Destruction): Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown upward into the air.

Factors Affecting Intensity

Several factors influence the intensity experienced at a given location. These include:

• Distance from the Epicenter: Generally, intensity decreases with increasing distance from the epicenter.
• Local Geology: Soft soils, like those found in river valleys or reclaimed land, tend to amplify seismic waves, leading to higher intensities compared to areas with bedrock. This phenomenon is known as site amplification.
• Building Construction: The type of construction plays a critical role in determining the level of damage. Buildings designed to withstand seismic forces will experience lower intensities than those that are not. According to a 2023 report by the Earthquake Engineering Research Institute, "Structures built according to modern seismic codes consistently outperform older, non-retrofitted buildings during earthquakes."

MMI vs. Richter Scale

It's essential to distinguish between the modified mercalli intensity scale and the Richter scale. The Richter scale, developed by Charles F. Richter in 1935, measures earthquake magnitude using a logarithmic scale based on the amplitude of seismic waves recorded by a seismograph. While the Richter scale is useful for comparing the sizes of different earthquakes, it doesn't directly reflect the impact of an earthquake on people and structures. MMI, on the other hand, provides a more nuanced understanding of the effects of earthquakes in specific locations.

Using the Mercalli Scale in Earthquake Damage Assessment

The modified mercalli intensity scale is a crucial tool in earthquake damage assessment. By collecting data on the observed effects of an earthquake, such as the types and extent of damage to buildings, infrastructure, and the environment, seismologists and engineers can estimate the intensity at various locations. This information is then used to create intensity maps, which provide a visual representation of the earthquake's impact. These maps are invaluable for:

• Emergency Response: Identifying the areas most severely affected, allowing for targeted allocation of resources and personnel.
• Risk Assessment: Understanding the vulnerability of different regions to future earthquakes.
• Building Codes: Informing the development and revision of building codes to ensure that structures are designed to withstand expected levels of shaking.
• Historical Studies: Reconstructing the impacts of past earthquakes, even those that occurred before instrumental recordings were available.

The USGS Earthquake Hazards Program uses the modified mercalli intensity scale extensively in its earthquake damage assessment and hazard mapping efforts. The agency collects data from a variety of sources, including citizen reports, field surveys, and remote sensing imagery, to generate detailed intensity maps following significant earthquakes.

Intensity map showing varying levels of shaking and damage after an earthquake.

Earthquake Intensity and Ground Motion

While the modified mercalli intensity scale relies on observed effects, there is a relationship between intensity and ground motion. Higher intensities generally correspond to stronger ground motion, as measured by instruments. However, the relationship is not always straightforward, as factors like local geology and building construction can significantly influence the observed intensity for a given level of ground shaking. Scientists use instruments to record the peak ground acceleration (PGA) and peak ground velocity (PGV) which measure the maximum acceleration and velocity of the ground during an earthquake. These values are then correlated with the MMI scale to better understand the relationship between ground motion and observed effects.

Assessing Building Vulnerability

Understanding the relationship between intensity and ground motion is critical for assessing the vulnerability of buildings to earthquake damage. Engineers use this information to develop fragility curves, which estimate the probability of different levels of damage for a given intensity. These curves are essential for:

• Seismic Retrofitting: Prioritizing buildings for seismic retrofitting based on their vulnerability and the expected level of shaking.
• Insurance Risk Assessment: Estimating the potential losses from future earthquakes for insurance purposes.
• Urban Planning: Developing strategies to reduce earthquake risk in urban areas, such as land-use planning and building code enforcement.

Table: MMI Scale and Expected Building Performance

The following table provides a general overview of the expected performance of different types of buildings at various levels of modified mercalli intensity scale. Note that these are generalized estimates, and actual performance may vary depending on specific factors such as building design, construction quality, and soil conditions.

Earthquake Preparedness and Intensity Awareness

Understanding earthquake intensity is a crucial aspect of earthquake preparedness. Knowing what to expect at different intensity levels can help individuals and communities take appropriate actions to protect themselves and their property. For example, at lower intensity levels (I-IV), it's important to be aware of your surroundings and take precautions such as securing loose objects. At higher intensity levels (V-XII), it's crucial to drop, cover, and hold on, and to evacuate damaged buildings as soon as it is safe to do so.

Community Resilience and Preparedness

Building community resilience to earthquakes requires a multi-faceted approach that includes:

• Public Education: Raising awareness about earthquake hazards and promoting earthquake preparedness measures.
• Building Code Enforcement: Ensuring that buildings are designed and constructed to withstand expected levels of shaking.
• Emergency Planning: Developing and practicing emergency response plans, including evacuation routes and communication protocols.

The Role of Citizen Science

Citizen science initiatives can play a valuable role in earthquake damage assessment. By collecting and reporting observations of earthquake effects through online platforms or mobile apps, citizens can contribute to a more comprehensive understanding of the earthquake's impact. The USGS Earthquake Hazards Program actively encourages citizen participation in its earthquake reporting and data collection efforts.

People participating in an earthquake drill.

Table: Key Earthquake Preparedness Actions by Intensity Level

Here is a simple table outlining actions that should be considered based on the modified mercalli intensity scale:

FAQ

Here are some frequently asked questions about earthquake intensity and the modified mercalli intensity scale:

• Q: What is the difference between magnitude and intensity? A: Magnitude measures the energy released at the earthquake's source, while intensity measures the effects of the earthquake at a specific location.
• Q: Is the modified mercalli intensity scale subjective? A: Yes, to some extent. It relies on observations of earthquake effects, which can be influenced by factors such as building construction and local geology. However, the scale provides a standardized framework for assessing intensity based on these observations.
• Q: How is the MMI scale used in earthquake preparedness? A: The MMI scale helps individuals and communities understand the potential impacts of earthquakes at different intensity levels, allowing them to take appropriate preparedness measures.

Understanding earthquake intensity through the modified mercalli intensity scale is critical for effective earthquake preparedness, earthquake damage assessment, and building community resilience. By recognizing the factors that influence intensity and the potential impacts at different levels, we can better protect ourselves and our communities from the devastating effects of earthquakes. What other questions do you have about the Mercalli Scale? Share your thoughts and experiences in the comments below!