The Modified Mercalli Intensity (MMI) Scale describes an earthquake’s effects at specific locations. It helps understand how seismic events are experienced by people and impact structures and the natural environment. The scale focuses on observed phenomena, not the energy released by the earthquake itself.
Understanding Earthquake Intensity
Earthquake intensity measures ground shaking and damage observed at a specific location during an earthquake. This assessment relies on human perception and visible effects on buildings and the landscape. Intensity values vary significantly across different locations for the same earthquake due to factors like distance from the epicenter, earthquake depth, and local geological conditions.
Local geological conditions, such as soil type, can amplify seismic waves in softer soils compared to hard bedrock, leading to more intense shaking. Building construction quality also affects observed damage, with older or poorly constructed buildings often experiencing greater impact.
The Modified Mercalli Scale: Levels of Impact
The Modified Mercalli Intensity Scale uses Roman numerals I to XII to denote increasing levels of ground shaking and damage. Lower numbers describe how an earthquake is felt by people, while higher numbers indicate observed structural damage. This systematic ranking helps to characterize the local severity of an earthquake.
At levels I to III, shaking is subtle. Intensity I is not felt except by a few individuals under favorable conditions. Intensity II is noticed by a few people at rest, particularly on upper floors, and delicately suspended objects may swing. Intensity III is felt indoors by several people, often described as vibrations similar to a passing truck, though many may not recognize it as an earthquake.
Levels IV through VI show more apparent effects. Intensity IV is felt indoors by many and outdoors by a few, with rattling dishes and windows, and standing vehicles rocking. At Intensity V, nearly everyone feels the shaking, many are awakened, and unstable objects may overturn, with some dishes and windows breaking. Intensity VI is felt by all, causing many to be frightened and run outdoors, with some heavy furniture moving and minor plaster damage.
Levels VII to IX signify increasing structural damage. Intensity VII results in negligible damage to well-designed buildings but slight to moderate damage in ordinary structures, with some chimneys breaking. At Intensity VIII, specially designed structures may have slight damage, but ordinary buildings experience considerable damage, potentially with partial collapse, and heavy furniture overturns. Intensity IX involves considerable damage to specially designed structures, significant damage to substantial buildings with partial collapse, and buildings potentially shifting off their foundations, with ground cracking.
Levels X to XII describe catastrophic destruction. Intensity X leads to the destruction of most masonry and frame structures and their foundations, significant ground cracking, and bent rails. Intensity XI sees few masonry structures remaining standing, with bridges destroyed, broad fissures in the ground, and underground pipelines completely out of service. At Intensity XII, damage is total, with waves seen on the ground surface, lines of sight and level distorted, and objects thrown into the air.
How Intensity Differs from Magnitude
The distinction between earthquake intensity and magnitude is often confused. Magnitude is a quantitative measure of the energy released at the earthquake’s source, calculated from seismograph readings. An earthquake has only one magnitude value, constant regardless of location. This measurement reflects the overall size or strength of the seismic event.
Intensity describes the strength of shaking produced by the earthquake at a particular location. It is determined by observed effects on people, structures, and the natural environment, varying greatly from place to place. A single earthquake has one magnitude but can result in different Modified Mercalli Intensity values across various areas, decreasing with distance from the epicenter. Magnitude is an objective instrumental measurement, while intensity is a qualitative assessment based on observed effects.
Practical Applications of Intensity Data
Data from the Modified Mercalli Intensity Scale has several important practical applications beyond simply describing an earthquake’s effects. It is valuable in emergency response planning, helping authorities quickly identify areas with significant shaking and damage. This allows for effective resource allocation and prioritization of aid to communities needing immediate assistance.
Intensity data also informs building codes and seismic design. Understanding how structures perform under different shaking levels helps engineers and urban planners develop more resilient construction standards. This mitigates future earthquake damage and improves public safety. Intensity data also provides historical context for past earthquakes, especially those before instrumental monitoring. Without seismograph records, historical earthquake magnitudes can be estimated from reported intensity and damage, crucial for long-term seismic hazard assessment.