PropertyRisk Glossary - Modified Mercalli Intensity Scale

An earthquake intensity scale ranks the strength
of earthquake shaking by its effects on
people and buildings.

The Modified Mercalli Intensity Scale

Unlike earthquake "magnitude," which is a measure of total energy released at the source of the earthquake, earthquake "intensity" is a measure of shaking strength at different locations in the region where a quake is felt. Earthquake intensities are characterised in terms of how the shaking affected people and buildings. There are several "shaking scales" in use internationally, however, in this website we use the Modified Mercalli Intensity (MMI) Scale.

Modified Mercalli Intensities

Originally developed in Italy nearly a century ago, the Mercalli Intensity Scale includes 12 degrees of shaking. It was modified for use in the United States in 1931. The degrees of shaking, as published in this modified scale, are presented in Table 1 below. The reader should note that the damage and sensations listed for each intensity level generally include the entire range of effects described for the lower intensity levels -- but in magnified scope and severity.

In recent years, the highest intensity levels (X, XI and XII) -- originally defined by spectacular damage to building foundations, underground pipelines, etc. -- have been recognized by many scientists to be the effects of shifting ground caused by surface fault rupture and slope failure or soil liquefaction, rather than by increasing levels of shaking as intended in the original Mercalli scale.

The MMI scale was recently "modernized" by including the effects of shaking on architectural and engineering works common in today's urban communities. To supplement the official, but now dated, descriptions of damage in the 1931 scale, we include in Table 2 descriptions of the damaging intensity levels as updated on the basis on this modernized version. (These descriptions are from a research report published by the California Division of Mines and Geology, however, they do not necessarily represent an official re-definition of the MMI Scale.)

TABLE 1. Modified Mercalli Intensity Scale of 1931
Intensity	Observable Effects
I	Not felt. Marginal and long-period effects of large earthquakes.
II	Felt by persons at rest, on upper floors, or favorably placed.
III	Felt indoors. Hanging objects swing. Vibration like passing of light trucks. Duration estimated. May not be recognized as an earthquake.
IV	Hanging objects swing. Vibration like passing of heavy trucks; or sensation of a jolt like a ball striking the walls. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of IV wooden walls and frames creak.
V	Felt outdoors; direction estimated. Sleepers wakened. Liquids disturbed, some spilled. Small unstable objects displaced or upset. Doors swing, close, open. Shutters, pictures move. Pendulum clocks stop, start, change rate.
VI	Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken, knickknacks, books, etc., off shelves. Pictures off walls. Furniture moved or overturned. Weak plaster and masonry D cracked. Small bells ring (church, school). Trees, bushes shaken (visible, or heard to rustle).
VII	Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage to masonry D, including cracks. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices (also unbraced parapets and architectural ornaments). Some cracks in masonry C. Waves on ponds; water turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.
VIII	Steering of motor cars affected. Damage to masonry C; partial collapse. Some damage to masonry B; none to masonry A. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.
IX	General panic. Masonry D destroyed; masonry B seriously damaged. (General damage to foundations.) Frame structures, if not bolted, shifted off foundations. Frames racked. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluviated areas sand and mud ejected, earthquake fountains, sand craters.
X	Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks to canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Rails bent slightly.
XI	Rails bent greatly. Underground pipelines completely out of service.
XII	Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into the air.

Note: To avoid ambiguity, the quality of masonry, brick, or other material is specified by the following lettering system. (This has no connection with the conventional classes A, B, and C construction.):

Masonry A. Good workmanship, mortar, and design; reinforced, especially laterally, and bound together by using steel, concrete, etc.; designed to resist lateral forces.
Masonry B. Good workmanship and mortar; reinforced, but not designed to resist lateral forces.
Masonry C. Ordinary workmanship and mortar; no extreme weaknesses, like failing to tie in at corners, but neither reinforced nor designed to resist horizontal forces.
Masonry D. Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally.

Source: Richter, C.F. Elementary Seismology. San Francisco CA: W. H. Freeman Co., 1957.

"Modernized" Modified Mercalli Intensities

After the 1994 Northridge earthquake, the descriptions of the damaging levels of the MMI scale were "modernized" in a version of the scale published by Dewey, J.W., et al. (1995). As a supplement the original MMI Scale described above, we present updated descriptions of the damaging intensity levels updated on the basis on the 1995 modernized version.

TABLE 2. "Modernized" Descriptions of MMI Levels (1994)
Intensity	Observable Effects
VI	Worst effects include some windows broken out; a few instances of fallen plaster or damaged old masonry chimneys on single-family houses; large cracks in interior walls; many small objects overturned and fallen; many items thrown from store shelves; many glassware items or dishes broken; light furniture overturned and moderately heavy furniture displaced. Effects on people not used to define intensities of VI or above.
VII	Worst effects include significant damage to unreinforced masonry buildings, including cracks in bearing walls and 'out-of-plane' movement or fall of upper walls and parapets; many old masonry chimneys fallen or broken at the roofline on single-family homes; some masonry fences fallen or destroyed; heavy furniture overturned.
VIII	Worst effects include considerable damage to old, unreinforced masonry buildings, with partial collapse; many cases where wood-frame houses are moved on their foundation if not anchored and braced; damage to wood-frame apartment buildings having open first-stories, with some cases of apartments being destroyed; significant damage to reinforced, lined, masonry chimneys on single-family homes, and widespread damage to old masonry chimneys; structural damage to some reinforced-concrete structures built when a seismic code was in effect; very heavy furniture moved conspicuously or overturned.
IX	Worst effects include multiple cases of structural damage to reinforced-concrete buildings and parking structures built when a seismic code was in effect, with some cases of partial or complete collapse; collapse of elevated freeway sections; widespread damage to unreinforced masonry buildings (e.g., old brick buildings), with total collapse; widespread incidence of wood-frame houses shifted off foundations where not securely anchored and braced; widespread destruction of wood-frame apartment buildings having large open areas in their first stories; widespread collapse of masonry (brick, block or stone) chimneys, whether reinforced or not, on single-family homes; furniture and building contents generally overturned and thrown across room.
X-XII	As originally defined and as modified in 1931, these intensity levels described earthquake effects that involve permanent changes in the shape of the ground (fault rupture, landsliding, liquefaction, etc.). Nowadays, however, Intensities X, XI and XII are increasingly regarded as approximately the same level of shaking as Intensity IX. The many phenomena originally associated with intensities X and above are apparently related less to the level of ground shaking than to the presence of ground conditions susceptible to spectacular failure, or to the ease with which seismic faulting of different style and depth can propagate to the ground surface.

Source: Dewey, J.W., Dengler, L., Reagor, B.G., and Moley, K., 1995, Spatial variations of intensity in the Northidge earthquake, in The Northridge, California, Earthquake of 17 January 1994, Calif. Div. Mines & Geology Special Publication 116, p. 39-46.

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