How Are Earthquakes Measured?
The Richter Scale
The magnitude of most earthquakes is measured on the Richter
scale, invented by Charles F. Richter in 1934. The Richter
magnitude is calculated from the amplitude of the largest seismic
wave recorded for the earthquake, no matter what type of wave
was the strongest.
The Richter magnitudes are based on a logarithmic scale (base
10). What this means is that for each whole number you go up on
the Richter scale, the amplitude of the ground motion recorded by
a seismograph goes up ten times. Using this scale, a magnitude 5
earthquake would result in ten times the level of ground shaking
CHARLES RICHTER STUDYING as a magnitude 4 earthquake (and 32 times as much energy would
A SEISMOGRAM. be released). To give you an idea how these numbers can add up,
think of it in terms of the energy released by explosives: a magnitude 1 seismic wave releases as much
energy as blowing up 6 ounces of TNT. A magnitude 8 earthquake releases as much energy as
detonating 6 million tons of TNT. Pretty impressive, huh? Fortunately, most of the earthquakes that
occur each year are magnitude 2.5 or less, too small to be felt by most people.
The Richter magnitude scale can be used to describe earthquakes so small Class Magnitude
that they are expressed in negative numbers. The scale also has no upper
limit, so it can describe earthquakes of unimaginable and (so far) Great 8 or more
unexperienced intensity, such as magnitude 10.0 and beyond. Major 7 - 7.9
Although Richter originally proposed this way of measuring an earthquake's Strong 6 - 6.9
"size," he only used a certain type of seismograph and measured shallow Moderate 5 - 5.9
earthquakes in Southern California. Scientists have now made other
Light 4 - 4.9
"magnitude" scales, all calibrated to Richter's original method, to use a
variety of seismographs and measure the depths of earthquakes of all Minor 3 -3.9
Earthquakes are also classified
in categories ranging from
minor to great, depending on
Earthquake Magnitude Scale their magnitude.
Magnitude Earthquake Effects Estimated Number
2.5 or less Usually not felt, but can be recorded by seismograph. 900,000
2.5 to 5.4 Often felt, but only causes minor damage. 30,000
5.5 to 6.0 Slight damage to buildings and other structures. 500
6.1 to 6.9 May cause a lot of damage in very populated areas. 100
7.0 to 7.9 Major earthquake. Serious damage. 20
8.0 or Great earthquake. Can totally destroy communities near the
One every 5 to 10 years
The Mercalli Scale
Another way to measure the strength of an earthquake is to use the
Mercalli scale. Invented by Giuseppe Mercalli in 1902, this scale uses
the observations of the people who experienced the earthquake to
estimate its intensity.
The Mercalli scale isn't considered as scientific as the Richter scale,
though. Some witnesses of the earthquake might exaggerate just how
bad things were during the earthquake and you may not find two
witnesses who agree on what happened; everybody will say something
different. The amount of damage caused by the earthquake may not
accurately record how strong it was either. Some things that affect the
amount of damage that occurs are: the building designs, the distance
from the epicenter, and the type of surface material (rock or dirt) the
buildings rest on.
Different building designs hold up differently in an earthquake and the further you are from the
earthquake, the less damage you'll usually see. Whether a building is built on solid rock or sand
makes a big difference in how much damage it takes. Solid rock usually shakes less than sand,
so a building built on top of solid rock shouldn't be as damaged as it might if it was sitting on a
Modified Mercalli Intensity Scale
Mercalli Eq. Richter Witness Observations
I 1.0 to 2.0 Felt by very few people; barely noticeable.
II 2.0 to 3.0 Felt by a few people, especially on upper floors.
III 3.0 to 4.0 Noticeable indoors, especially on upper floors, but may not be recognized as an earthquake.
IV 4.0 Felt by many indoors, few outdoors. May feel like heavy truck passing by.
Felt by almost everyone, some people awakened. Small objects moved. trees and poles may
V 4.0 to 5.0
Felt by everyone. Difficult to stand. Some heavy furniture moved, some plaster falls.
VI 5.0 to 6.0
Chimneys may be slightly damaged.
Slight to moderate damage in well built, ordinary structures. Considerable damage to poorly
built structures. Some walls may fall.
Little damage in specially built structures. Considerable damage to ordinary buildings, severe
VIII 6.0 to 7.0
damage to poorly built structures. Some walls collapse.
Considerable damage to specially built structures, buildings shifted off foundations. Ground
cracked noticeably. Wholesale destruction. Landslides.
Most masonry and frame structures and their foundations destroyed. Ground badly cracked.
X 7.0 to 8.0
Landslides. Wholesale destruction.
Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in ground.
Waves seen on ground.
XII 8.0 or greater Total damage. Waves seen on ground. Objects thrown up into air.
http://www.geo.mtu.edu/UPSeis/intensity.html http://earthquake.usgs.gov/eqcenter/dyfi/ (Did you feel it?)