What do those earthquake numbers mean, anyway?

By Eric Niiler

Scientists measure earthquakes by magnitude and by intensity. The magnitude reflects how much energy is released at the source of the earthquake and is determined by seismographic readings.

Intensity measures the strength of the shaking at a specific location and is determined by the effects on people, buildings and the environment.

Each quake is assigned to a class:

• Great: Magnitude of 8.0 or higher; capable of tremendous damage.

• Major: Magnitude 7.0 to 7.9; capable of widespread heavy damage.

• Strong: Magnitude 6.0 to 6.9; can cause severe damage.

• Moderate: Magnitude 5.0 to 5.9; can cause considerable damage.

• Light: Magnitude 4.0 to 4.9; capable of moderate damage.

• Minor: Magnitude in the range of 3.0 to 3.9.

• Micro: Magnitude less than 3.0; these are the smallest quakes generally felt by people.

A quake that measures 6.5 is 10 times as great in magnitude as one that measures 5.5.

The 12-point Mercalli intensity scale gives an indication of how the quake affected an area:

I. Not felt except by a very few under especially favorable conditions.

II. Felt by only a few people at rest, especially on upper floors of buildings.

III. Felt quite noticeably indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Stopped cars may rock slightly. Vibrations are similar to that of a passing truck.

IV. Felt indoors by many, outdoors by few during the day. At night, some people are awakened. Dishes, windows, doors disturbed; walls make cracking sounds. Autos rock noticeably.

V. Felt by nearly everyone; many awakened. Some dishes and windows broken. Unstable objects overturned.

VI. Felt by all; many are frightened. Some heavy furniture moved; damage slight.

VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.

VIII. Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned.

IX. Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.

X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rails bent.

XI. Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly.

XII. Damage total. Lines of sight and level are distorted. Objects thrown into the air.


Source: LiveScience and U.S. Geological Survey