The Modified Mercalli Intensity (MMI) scale is a tool used by seismologists to describe the severity of an earthquake based on its observable effects at a specific location. It functions as a descriptive measure, detailing how the shaking impacts people, buildings, and the natural environment. This system translates the physical experience of an earthquake into a standardized rating, allowing seismologists to gauge how widely the effects are felt across a region.
Intensity Versus Magnitude
The Modified Mercalli Intensity scale measures the effects of an earthquake, which is fundamentally different from the Moment Magnitude Scale, which measures the energy released. Magnitude, often reported as a single number for the entire event, is a measure of the total energy released at the earthquake’s source. This value is determined using scientific instruments called seismographs and is consistent regardless of where the measurement is taken.
Intensity, on the other hand, describes the localized shaking severity and its consequences, which can vary widely across the affected area. A single earthquake will have only one magnitude value, but it can generate many different intensity values across the region it affects. The intensity rating at a specific place depends on factors like the distance from the epicenter, the depth of the earthquake, and the local ground conditions.
The Twelve Levels of the Mercalli Scale
The MMI scale is structured using twelve increasing levels of intensity, which are designated by Roman numerals from I to XII. The lower end of the scale describes how the earthquake is perceived by people, while the higher numbers focus on the extent of structural damage.
Intensity I describes shaking that is not felt by anyone except under the most favorable conditions. Low intensities, such as III or IV, are felt noticeably indoors, causing delicately suspended objects to swing. People often mistake the vibration for that of a passing truck, and some light sleepers might be awakened.
Moving up the scale, moderate intensities like VI and VII describe shaking that is felt by all people, with many becoming frightened and running outdoors. At these levels, there is slight damage to well-built structures, and objects like dishes and windows may be broken or unstable items overturned. High intensities, such as X through XII, indicate widespread destruction and catastrophic effects.
At Intensity X, some well-built wooden structures are destroyed, and most masonry and frame structures are destroyed with their foundations, often cracking the ground. The highest level, Intensity XII, describes total damage, with waves visible on the ground surface and objects potentially being thrown into the air. Structural engineers typically contribute information for assigning the highest intensity values (VIII and above) due to the detailed structural assessment required.
How Intensity Ratings Are Determined
The assignment of an MMI rating to a location is a process of post-earthquake data collection and evaluation. Seismologists rely heavily on observable effects, making the scale based on physical evidence rather than a mathematical formula. This evidence includes detailed reports of structural damage to different types of buildings and infrastructure.
A significant modern tool for determining intensity is the use of citizen science data, such as online “Did You Feel It?” surveys. These surveys collect thousands of eyewitness accounts of shaking and effects, providing qualitative observations. Seismologists analyze this information to assign a single MMI number to a specific geographic point. The final step is often the creation of an isoseismal map, which uses contour lines to show zones of equal intensity across the affected region.