DEFINITION OF EARTHQUAKE MAGNITUDE

Earthquakes vary greatly in intensity, from undetectable up

to ones that kill millions of people and wreak total destruction.

For instance, one severe earthquake in December of

2003 killed approximately 50,000 people in Iran, yet several

thousand earthquakes that do no damage occur every day

throughout the world. The energy released in large earthquakes

is enormous; up to hundreds of times more powerful

than large atomic blasts. Strong earthquakes may produce

ground accelerations greater than the force of gravity, enough

to uproot trees, or send projectiles right through buildings,

trees, or anything else in their path. Earthquake magnitudes

are most commonly measured using the Richter scale.

The Richter scale gives an idea of the amount of energy

released during an earthquake and is based on the amplitudes

(half the height from wave-base to wave-crest) of seismic

waves at a distance of 61 miles (100 km) from the

epicenter. The Richter scale magnitude of an earthquake is

calculated using the zigzag trace produced on a seismograph,

once the epicenter has been located by comparing signals

from several different, widely separated seismographs. The

Richter scale is logarithmic, where each step of one corresponds

to a tenfold increase in amplitude. This is necessary

because the energy of earthquakes changes by factors of

more than a hundred million.

The energy released during earthquakes changes even

more rapidly with each increase in the Richter scale, because

the number of high amplitude waves increases with bigger

earthquakes and also because the energy released is according

to the square of the amplitude. Thus, it turns out in the end

that an increase of one on the Richter scale corresponds to a

30 times increase in energy released. The largest earthquakes

so far recorded are the 9.2 Alaskan earthquake of 1964, the

9.5 Chilean earthquake of 1960, and the 9.0–9.2 Sumatra

earthquake of 2004, each of which released the energy equivalent

to approximately 10,000 nuclear bombs the size of the

one dropped on Hiroshima.

Before the development of modern inertial seismographs,

earthquake intensity was commonly measured using the modified

Mercalli intensity scale. This scale, named after Father

Giuseppe Mercalli, was developed in the late 1800s and measures

the amount of vibration people remember feeling for

low-magnitude earthquakes and the amount of damage to

buildings in high-magnitude events. The table compares the

Richter and modified Mercalli scales. One of the disadvantages

of the Mercalli scale is that it is not corrected for distance

from the epicenter. Therefore, people near the source of

the earthquake may measure the earthquake as a IX or X,

whereas people further from the epicenter might only record

a I or II event. However, the modified Mercalli scale has

See also PLATE TECTONICS.

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