DEFINITION OF RICHTER SCALE

The Richter scale is a widely used openended

scale that measures the strength of earthquakes. The

scale was devised in 1935 by Dr. Charles Richter, a prominent

seismologist who was based at the California Institute of

Technology. The Richter scale gives an idea of the amount of

energy released during an earthquake and is based on the

amplitudes of seismic waves (equal to half the height from

wave-base to wave-crest) at a distance of 62 miles (100 km)

from the epicenter. The Richter-scale magnitude of an earthquake

is calculated using the trace produced on a seismograph

by an earthquake, once the epicenter has been located

by comparing signals from several different, widely separated

seismographs. The Richter scale is logarithmic, where each

step 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 actual energy released in earthquakes changes even

more rapidly with each increase in the Richter scale, because

the number of high amplitude waves increases with bigger

earthquakes and 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 may correspond to as

much as 30 times the energy released at the number below it.

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 2004 Sumatra earthquake, each of which released

the energy equivalent to approximately 1,000–10,000 nuclear

bombs the size of the one dropped on Hiroshima.

Before the development of modern inertial seismographs

and the Richter scale, earthquake intensity was commonly

measured using the modified Mercalli intensity scale. This

scale, developed in the late 1800s and named after Father

Giuseppe Mercalli, measures the amount of vibration people

remember feeling for low-magnitude earthquakes, and the

amount of damage to buildings in high-magnitude events.

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 farther from the epicenter

might only record a I or II event. However, the modified

Mercalli scale has proven very useful for estimating the magnitudes

of historical earthquakes that occurred before the

development of modern seismographs, since the Mercalli

magnitude can be estimated from historical records.

Many seismologists now use a different method of estimating

the strength and energy released in an earthquake.

The seismic moment accounts better for the low-frequency

wave motions produced during an earthquake, but it is more

difficult to calculate than the Richter magnitude so it is not

commonly used outside the seismological community.

See also EARTHQUAKES.

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