Modified Mercalli intensity scale

Modified Mercalli intensity scale. The Modified Mercalli intensity scale is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It describes the effects of an earthquake on the Earth's surface, humans, objects of nature, and man-made structures, providing a qualitative measure distinct from instrumental magnitude scales. Developed in the early 20th century, it refines earlier European intensity scales for use in North America, particularly the United States.

Description and development

The scale originated from revisions of earlier European intensity scales, namely the Rossi–Forel scale and the Mercalli–Cancani–Sieberg scale. The primary adaptation was undertaken in 1931 by American seismologists Harry O. Wood and Frank Neumann of the Seismological Laboratory at the California Institute of Technology. Their work, published by the Seismological Society of America, aimed to create a scale more applicable to the building types and construction practices common in North America, such as wood-frame houses. Further refinements were made throughout the 20th century, with a version known as the Abridged Modified Mercalli Scale becoming widely adopted for use in New Zealand and other regions.

Comparison with other scales

Unlike magnitude scales, such as the Richter magnitude scale or moment magnitude scale, which quantify the energy released at the earthquake's source, the Modified Mercalli intensity scale measures the observable effects and perceived strength of shaking at specific locations. A single earthquake will produce only one magnitude value but a range of intensity values that typically decrease with distance from the epicenter. It is one of several intensity scales used globally, with others including the European Macroseismic Scale used primarily in Europe and the Japan Meteorological Agency seismic intensity scale used in Japan. These scales are all conceptually similar but calibrated for local architectural and cultural conditions.

Scale degrees

The scale is composed of twelve increasing levels of intensity, denoted by Roman numerals from I to XII. Intensity I represents shaking not felt except by a very few under especially favorable conditions. Intensity III is often felt noticeably indoors, particularly on upper floors of buildings. Intensity V is felt by nearly everyone, with some dishes and windows broken. At Intensity VII, considerable damage occurs in poorly built or designed structures, with slight to moderate damage in ordinary substantial buildings. Intensity IX sees considerable damage in well-designed structures, with buildings shifted off foundations. Intensity XII, the maximum, results in total destruction, with waves seen on ground surfaces and objects thrown upward into the air. The descriptions encompass effects on people, furniture, vehicles, and various building types from adobe to reinforced masonry.

Use and application

The scale is primarily used to create isoseismal maps, which are contour maps delineating areas of equal seismic intensity following an earthquake. These maps are crucial for emergency management and disaster response, helping agencies like the United States Geological Survey and the Federal Emergency Management Agency assess damage distribution and allocate resources. Intensity data is gathered through online "Did You Feel It?" questionnaires from the public, field surveys by seismologists, and analysis of damage reports from local authorities. The data is also used in engineering and land-use planning, informing the development of building codes in seismic regions like California and for revising historical earthquake catalogs.

Limitations and criticisms

A primary limitation of the scale is its qualitative and subjective nature, as assessments depend on human observation and the density of population and structures in the affected area. An earthquake in a remote region like the Aleutian Islands may have a high magnitude but a low maximum intensity due to a lack of observers or infrastructure. Intensity evaluations can also be inconsistent between different assessors. Furthermore, the scale's descriptors, tied to specific building types common in early 20th-century America, may not translate perfectly to other cultures and modern construction methods. Despite these issues, it remains a vital tool for communicating earthquake effects to the public and for historical and engineering analyses where instrumental data is absent.

Category:Earthquake scales Category:Seismology