Modified Mercalli Shaking Intensity Scale

 Modified Mercalli Shaking Intensity Scale

In the early days of earthquake investigations, seismologist used earthquake intensities as the applicable yardstick to estimate the size of an earthquake. Intensities are measured by means of the degree of damage to structures, the amount of disturbance to the ground surface, and the extent of human and animal reactions to the shaking.

A scale developed by the Italian seismologist and volcanologist G. Mercalli in 1902 has 12 values for describing an earthquake. The scale was later modified to better fit modern conditions.

Intensity Description

I Not felt except by a very few under especially favorable circumstances

II Felt only by a few persons at rest, especially on upper floors of buildings. Suspended objects may swing.

III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing automobiles may rock. Vibrations like a passing truck.

IV During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors rattle; wall may make creaking sound. Hanging objects swing noticeably. Sensation like heavy truck striking building. Standing automobiles rocked noticeably.

V Felt by nearly everyone, many awakened a direction can be estimated. Some dishes, windows and so on broken; cracked plaster in a few places; unstable objects overturned. Disturbances of trees, poles and other tall objects sometimes noticed. Pendulum clocks may stop.

VI Felt by all, many frightened and run outdoors. Persons walk unsteadily. Windows, dishes and glassware broken; knickknacks, books, etc., off shelves; pictures knocked off walls. Church bells may ring. Some heavy furniture moved; a few instances of fallen plaster and damaged chimneys. Damage slight.

VII Difficult to stand. Everyone runs outdoors. Hanging objects quiver. Waves on ponds, water turbid with mud. Small landslides. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken at roof line. Noticed by persons driving cars.

VIII Damage slight in specially designed structures; considerable in ordinary, substantial buildings with partial collapse; great in poorly built structures. Framed houses moved on foundations if not bolted down. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls, towers and elevated tanks. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Steering of cars effected.

IX General panic. Damage is considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken. Serious damage to reservoirs.

X Some well-built wooden structures destroyed; most masonry and framed structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed, slopped over banks.

XI Few, if any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly.

XII Damage nearly total. Large rock masses displaced. Waves seen on the ground surface. Lines of sight and level distorted. Objects thrown into the air.