The Modified Mercalli Intensity (MMI) Scale is a fundamental tool used by seismologists to describe the severity of an earthquake in terms of its observable impacts on humans, structures, and the environment. This scale provides a descriptive ranking of an earthquake’s effects at a particular location, characterizing the experience of ground shaking. Unlike other measures that quantify the energy released, the MMI scale focuses entirely on the consequences felt on the Earth’s surface. It communicates the real-world impact of a seismic event to the public and to engineers.
Defining Earthquake Intensity
Earthquake intensity, as measured by the MMI scale, represents the localized effects of ground shaking, distinct from the earthquake’s inherent size. It is a qualitative measure based on observations of effects at a specific site rather than instrumental readings. Consequently, an earthquake does not have a single intensity value; it generates many different intensity values across the affected region.
Intensity varies significantly based on factors like distance from the source and local geology. Areas with soft soils often experience higher intensity shaking than areas on solid bedrock, a phenomenon known as site amplification. The original concept was developed in 1902 by Italian volcanologist Giuseppe Mercalli, and later adapted in 1931 by American seismologists Harry O. Wood and Frank Neumann to reflect contemporary building practices.
The Twelve Levels of Intensity
The Modified Mercalli Intensity scale uses twelve increasing levels of severity, designated by Roman numerals I to XII. Lower levels describe how shaking is perceived by people, while higher levels focus on structural damage.
Levels I through III represent low intensity shaking, often not felt by most people or only noticed by a few individuals at rest, sometimes causing suspended objects to swing slightly.
Moderate shaking (Levels IV through VI) is felt by most people indoors and outdoors. At Intensity IV, windows and doors may rattle. Intensity V can cause small, unstable objects to fall.
Intensity VI marks the transition to minor damage, where plaster may crack and chimneys sustain slight damage, but structures generally remain sound.
Intensities VII through XII describe increasingly severe effects. Intensity VII involves considerable damage to poorly built structures, but minimal damage to modern, well-designed buildings. At Intensity IX, well-built structures are seriously damaged, the ground may crack, and underground pipes may break. The highest levels (X through XII) describe near-total destruction, where even well-constructed wooden structures are destroyed, railroad rails are bent, and large ground fissures may appear.
MMI vs Magnitude Scales
Differentiating the MMI scale from magnitude scales, such as the Moment Magnitude Scale, is important. Magnitude quantifies the energy released at the earthquake’s source, providing a single value for the entire event, regardless of location. This measurement is calculated mathematically using seismic instruments, reflecting the physical size of the earthquake.
Intensity, conversely, describes the effects of the earthquake at a particular point on the surface, meaning a single earthquake will have multiple intensity values. Magnitude is like the wattage of a light bulb—a fixed measure of power output. Intensity is like the brightness of the light in various rooms, which varies depending on distance and obstructions. Therefore, a large magnitude earthquake occurring far away or deep underground may result in a low MMI value, while a smaller, shallow earthquake nearby can cause a much higher MMI.
Practical Applications and Data Collection
The distribution of MMI values across a region is mapped to create an isoseismal map, which shows contours of equal shaking intensity. This visual representation is invaluable for immediate post-disaster response, allowing emergency management teams to quickly identify the areas that experienced the most severe shaking and damage. The resulting data helps seismologists understand how energy attenuates through different geological structures, which is essential for updating seismic hazard models and building codes.
Modern data collection often involves citizen science through systems like the U.S. Geological Survey’s “Did You Feel It?” program. This platform gathers thousands of online reports from people who experienced the earthquake, providing a rapid and widespread source of qualitative data that can be quickly processed into an MMI map. This information is used for immediate response and for long-term planning, including land-use zoning, insurance risk assessment, and guiding efforts to retrofit vulnerable structures against future shaking.