Earthquake Scales

People have created different scales to measure how violent earthquakes are. In America, the Modified Mercalli Scale is commonly used and it is based on the effects that a seism causes in human facilities. This is called intensity. Another scale used is the Richter Scale, which measures an earthquake's magnitude.

The Modified Mercalli Intensity Scale

Levels Common Characteristics

I Only detectable by seismographs.
II Felt particularly by people who are on the upper floors of buildings.
III Not many people indoors feel the earthquake. A rapid vibration is felt, like that of a passing heavy vehicle.
IV Not very noticeable outside of buildings or other constructions. In buildings and houses, a slight swing of lamps and the rattle of tableware are detected. Some pieces of furniture are overturned. It awakens some people.
V It is also felt outside of buildings. Grass and leaves are moved by nonexistent wind. Lamps, dishes and furniture also sway dangerously and some of them fall down and break. Doors and windows are swung as well. Window glass breaks and almost everyone awakes.
VI Earthquake is felt by everyone. A considerable amount of glassware and dishes breaks, Pictures fall from walls and books and
other objects fall from shelves. It cracks plaster walls and ceilings.
VII Church bells ring. Water in lakes and rivers becomes turbid. Hanging objects on walls fall. Slight damage in built structures, particularly in towers and chimneys. Some architectural ornaments fall.
VIII Trees sway and some of them are uprooted and fall down. Some pieces of furniture are noticeably moved or overturned. Towers
and chimneys crack, break and fall. Buildings are considerably damaged. On steep slopes, ground is cracked, and mud-charged water is ejected in small amounts from wet ground.
IX Even those structures built to withstand earthquakes are damaged. Buildings in general are severely damaged: wood-framed structures are shifted off their foundations and masonry is destroyed.
X Most non-earthquake-proof buildings are destroyed including their frame structures. Railroad rails bend slightly. Cracks and folds
are found on street pavement. Water violently strikes lake and river banks, among others, causing them to suddenly collapse.
XI All masonry structures and almost all wooden constructions are destroyed. Railroad rails are considerably bent. Landslides and
earth slumps occur; ground shows serious cracks and fissures. In soft or wet ground mud and water eject.
XII All construction works are destroyed. Faults are detected in stony areas. River and lake banks slump; waterfalls spring forth, river courses change and entire lakes are moved from original locations, etc.


The Richter Magnitude Scale

It measures earthquakes in terms of their size, depending on the amount of energy released. The difference between the Modified
Mercalli and the Richter scales is that the former reads the intensity with which a quake damages human facilities. On the other hand,
the Richter scale measures the amount of energy released by a seism. A seismologist quantifies an earthquake's magnitude through
the lines recorded by a seismogram. It is said that Richter is the most widely-used scale, but it is also the standard most incorrectly
used by the population at large.

It is worth mentioning that the Richter Magnitude Scale has a logarithmic basis. What does this mean? Each whole number step
represents a ten-fold increase in measured amplitude. The following example illustrates the above: if tremor A -whose wave recorded
by a seismogram allows a specialist to determine that its magnitude is 4.0-, is compared with earthquake B, -whose magnitude is 8.0-, it would be wrong to state that B doubles A in magnitude. What would be correct to say is that B has released 100,000 times more
energy than A.

According to Richter, the magnitude logarithmic expression is conceived as follows:

0 - X
1 - 10
2 - 100
3 - 1000
4 - 10.000
5 - 100.000
6 - 1.000.000
7 - 10.000.000
8 - 100.000.000
9 -

This scale is more objective, given that it seeks safer and more reliable elements when measuring an earthquake. According to this
scale, A magnitude 2.0 quake is the lowest event that human beings are able to feel. The highest earthquake registered to date had a magnitude of 8.9.

Bibliographical references:

"Guía para preparar información sismológica dirigida a los medios de información". Editado electrónicamente por el PED/OPS y DIRDN 1992.

The Fujita Scale

There are many scales used to measure a tornado, but the most accepted standard in the world is the Fujita Scale, developed in 1957 by Theodore Fujita from the University of Chicago. This scale is based on the destruction that tornadoes cause to man-made structures, and not on their size, diameter or speed. Their intensity, therefore, cannot be measured or determined by just watching them; damage must be assessed first.

There are 6 different levels (from 0 to 5) and an "F" is placed in front of each level, in honor of its creator:

Scale Level


Wind Speed



Gale Tornado

60-100 km/h

40-72 mph

DChimneys are damaged and branches and small trees are broken. Sign boards are also damaged.


Moderate Tornado

100-180 km/h

73-112 mph

The lower limit is the beginning of hurricane wind speed. Tornado peels surface off roots, overturns cars and mobile homes. Some small trees are uprooted.


Considerable Tornado

180-250 km/h

113-157 mph

Considerable damage. Roofs are torn off from frame houses. Poor-built houses are destroyed. Large trees are uprooted and light objects are thrown out at high speeds.


Severe Tornado

250-320 km/h

158-206 mph

Tornado damages well-constructed structures and overturns trains. Most trees are uprooted.


Devastating Tornado

320-420 km/h

207-260 mph

Well-built structures are severely damaged and those with weak foundations are blown off or overturned. Cars and other heavy objects are also overturned.


Incredible Tornado

420-550 km/h

261-318 mph

Large buildings are seriously damaged or destroyed. Cars fly through the air 100 yards or
more. Steel structures are damaged.


Unconceivable Tornado

319-379 mph

Total destruction of all man-made structures

In theory, an F6 tornado could occur, with a Mach 1 wind speeds up to mach 1. Their occurrence, however, has not been proved.

Light Tornadoes: F0 and F1. These represent 69% of the total and only cause approximately 5% of casualties. Light or weak tornadoes last between 1 and 10 minutes.

Strong Tornadoes: F2 and F3. They correspond to 29% of tornadoes, causing 30% of deaths. They may last more than 20 minutes.

Violent Tornadoes: F4 and F5. They represent only 2% of the total and cause 70% of casualties. These tornadoes last more than one hour.

Bibliographical references: