The Modified Mercalli Intensity (MMI) Scale is a qualitative tool used globally to measure the local effects of an earthquake on people, buildings, and the Earth’s surface. Unlike magnitude scales, MMI focuses on the observable results of the shaking at a specific location, not the energy released at the source. The scale uses Roman numerals, ranging from I (not felt) to XII (catastrophic destruction), to assign a value to the severity of the shaking and assess the practical impact on infrastructure.
The Observational Basis of Intensity Measurement
The determination of an MMI value is primarily observational, relying on surveying physical evidence and collecting personal accounts after an earthquake. This qualitative method uses a ranking based on observed effects and resulting damage to structures, rather than a strict mathematical basis. The assigned intensity value reflects the severity of the ground shaking experienced in that specific area.
The data used includes eyewitness reports describing how the shaking felt, whether objects moved, and if people were awakened. Seismologists and engineers also conduct damage surveys, looking for cracked plaster, fallen chimneys, and structural failure in buildings. This reliance on observable effects means intensity is localized; it generally decreases as the distance from the earthquake’s epicenter increases.
The scale is termed “Modified” because it evolved from the original 1902 Mercalli scale, improved by American seismologists Harry Wood and Frank Neumann in 1931. These modifications allowed the scale to account for modern building construction methods and more accurately describe higher levels of destruction. The MMI scale became the standard for assessing ground shaking severity in the United States and is widely used internationally.
How Intensity Differs from Earthquake Magnitude
The distinction between earthquake intensity and magnitude is important, as they measure entirely different characteristics. Magnitude, measured by scales like the Moment Magnitude Scale, quantifies the total energy released at the earthquake’s source, deep beneath the surface. An earthquake has only one magnitude value, determined using instrumental measurements from seismographs.
Intensity measures the strength of the shaking produced by that energy at a specific point on the surface. This measurement is determined by the effects on people and structures rather than by instruments. Since shaking varies widely depending on local geology, distance from the epicenter, and building quality, a single earthquake produces many different intensity values across the affected region.
Intensity is expressed using Roman numerals (I–XII), while magnitude uses a single Arabic number (e.g., 6.5). A magnitude 7.0 earthquake, for instance, will have its highest intensity (VIII or IX) near the epicenter, while distant areas might only register III or IV.
Understanding the Twelve Intensity Levels
The Modified Mercalli Intensity scale describes a progression of effects from imperceptible movement to total devastation. Levels I through III primarily describe how the earthquake is perceived by people. Intensity I is not felt by anyone, while II is only felt by a few people at rest, particularly on upper floors. At intensity III, many people indoors notice the movement, which may feel like a slight vibration or the passing of a light truck.
Levels IV and V represent increasing awareness and movement of small objects. Intensity IV is felt by many indoors, and the shaking is strong enough to cause dishes and windows to rattle, sometimes waking people at night. At level V, the shaking is felt by almost everyone, and small, unstable objects are overturned, with some dishes and windows potentially breaking.
The middle levels, VI and VII, mark the transition to minor structural damage. Intensity VI is felt by everyone, with people having trouble walking, objects falling from shelves, and heavy furniture moving. Intensity VII causes slight to moderate damage in well-built structures and considerable damage to poorly constructed buildings, making it difficult for people to stand.
The highest levels, VIII through XII, indicate severe to catastrophic damage. At intensity VIII, ordinary buildings suffer considerable damage, and some walls may collapse, while specially built structures sustain only minor damage. Intensity IX causes considerable damage even to specially designed structures, shifting buildings off their foundations and cracking the ground noticeably. Intensity X results in the destruction of most masonry and frame structures and their foundations, with the ground badly cracked. Levels XI and XII represent total damage, where few structures remain standing, bridges are destroyed, and massive ground failures occur.