Earthquake Magnitude

Earthquakes vary greatly in intensity, from unde-tectable ones up to ones that kill millions of people and wreak total destruction. For example, an earthquake in 2008 killed at least 90,000 people in China, 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 through buildings, trees, or anything else in their path. Earthquake magnitudes are most commonly measured by the Richter scale.

The Richter scale gives an idea of the amount of energy released during an earthquake. It 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 by 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, whereby each step of 1 corresponds to a 10-fold increase in amplitude. This is necessary because the energy of earthquakes changes by factors of more than a hundred million.

The 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 also because the energy released is according to the square of the amplitude. Thus it turns out in the end that an

Schematic diagram of an inertial seismograph showing a large inertial mass suspended from a spring. The mass remains stationary as the ground and paper wrapped around a rotating drum move back and forth during an earthquake, creating the seismogram.

Platform vibrates in response to ground shaking

Suspended inertial mass -Paper wrapped around rotating drum

-Plate anchored in ground

Suspended inertial mass -Paper wrapped around rotating drum

-Plate anchored in ground

Rayleigh wave Love waves

Quiet period normal background

P-waves arrival

Quiet period normal background

P-waves arrival

Low amplitude

Higher amplitude

Irregular spacing

Low amplitude

Higher amplitude

Irregular spacing

Regular spacing

Very high amplitude

O Infobase Publishing increase of 1 on the Richter scale corresponds to a 30-fold increase in energy released. The largest earthquakes so far recorded are the 9.2 Alaskan earthquake (1964), the 9.5 Chilean earthquake (1960), and the 9.0 Sumatra earthquake (2004), each of which released the energy equivalent to more than 10,000 nuclear bombs the size of the one dropped on Hiroshima.

Before the development of modern inertial seismographs, earthquake intensity was commonly measured by the modified Mercalli intensity scale. This scale, named after Giuseppe Mercalli, was developed in the late 1800s; and it measures the amount of vibration people remember feeling for low-magnitude earthquakes, and measures the amount of damage to buildings in high-magnitude events (see table on

MODiFiED MERCALLi iNTENSiTY SCALE COMPARED WiTH RiCHTER MAGNiTUDE

Mercalli intensity

Richter magnitude

Description

I-II

< 2

Not felt by most people

III

3

Felt by some people indoors, especially on high floors

IV-V

4

Noticed by most people. Hanging objects swing and dishes rattle

VI-VII

5

Everyone feels. Some building damage (esp. to masonry), waves on ponds

VII-VIII

6

Difficult to stand and people scared or panicked. Difficult to steer cars. Moderate damage to buildings

IX-X

7

Major damage, general panic of public. Most masonry and frame structures destroyed. Underground pipes broken. Large landslides

XI-XII

8 and higher

Near total destruction

Note: This table can be used to compare the relative magnitudes of earthquakes from the historical record for which only information on the Mercalli intensity may be known, with the intensity on the modern Richter magnitude scale.

Note: This table can be used to compare the relative magnitudes of earthquakes from the historical record for which only information on the Mercalli intensity may be known, with the intensity on the modern Richter magnitude scale.

page 232). one of the disadvantages of the Mercalli scale is that it is not corrected for distance from the epicenter. People near the source of the earthquake therefore may measure the earthquake as an IX or X, whereas those farther from the epicenter might record only a i or ii event. The modified mercalli scale, however, has proven 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.

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