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Magnitude and Intensity

How s trong an e arthquake f eels to an o bserver d epends o n d istance to the q uake , subsurface geology, whether one is in a building or not, and what type of building one is in, and the observer. Scientists rely on a recording instrument known as a seismograph to determine the magnitude, defined as a measure of the strength of an earthquake or strain energy released by it . A simple seismograph consists of a pen or pencil, suspended on a spring or other moveable item, which makes a mark on a piece of paper.

A Chinese philosopher, Chang Heng, invented one of the first mechanical means of detecting earthquakes, in 132 AD. It was a cast bronze vessel with a domed lid, resembling a wine jar, around which sat eight dragons, each a holding a ball in its mouth. When an earthquake hit, one or more of the dragons dropped a ball into the mouth of a toad sitting below the dragon.

Chang Heng's seismoscope, as visualized by Wang Chen-To (1936)

English scientists working in Japan developed the first modern seismograph between 1880 and 1890. More complicated seismographs use sensors called seismometers , which can detect ground motions caused by seismic waves from both near and distant earthquakes. Some seismometers are capable of detecting ground motion as small as 0.1 nanometer. One nanometer is 1 billionth of a meter or about 39 billionths of an inch. Seismometers measure vertical motion and horizontal motion in north-south and east-west direction.

Shaking causes the recording instrument to move, which produces a mark on paper, film, or recording tape. This is known as a seismogram. The bigger the quake the larger the size or amplitude of the waves recorded.

Probably the best-known gauge of earthquake intensity is the local Richter magnitude scale, developed in 1935 by United States seismologist Charles F. Richter. This scale, commonly known as the Richter scale, measures the energy released by an earthquake. An increase of one unit of magnitude (for example, from 4.5 to 5.5) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released . For example, an earthquake of magnitude 5.5 releases about 32 times as much energy as an earthquake measuring 4.5. Another way to look at this is that it takes about 900 magnitude 4.5 earthquakes to equal the energy released in a single 6.5 earthquake. According to long-term records (since about 1900), seismologists expect about 18 major earthquakes (7.0 - 7.9) and one great earthquake (8.0 or above) in any given year. (Also described in Panel text, pg. 4)

In general, a magnitude 3 earthquake is about the smallest that one can feel. A magnitude 1 quake produces the same amount of energy as a small blast at a construction site (6 oz. TNT), whereas a magnitude 3 equals about 400 pounds of TNT.

Magnitude Energy released Energy equivalence

(millions of ergs)

-2 600 100-watt light bulb left on for a week

-1 20000 Smallest earthquake detected at Parkfield, CA

0 600000 Seismic waves from one pound of explosives

1 20000000 A two-ton truck traveling 75 miles per hour

2 600000000

3 20000000000 Smallest earthquakes commonly felt

4 600000000000 Seismic waves from 1,000 tons of explosives

5 20000000000000

6 600000000000000

7 20000000000000000 1989 Loma Prieta ,CA earthquake (magnitude 7.1)

8 600000000000000000 1906 San Francisco earthquake (magnitude 8.3)

9 20000000000000000000 Largest recorded earthquake (magnitude 9.5)

Although large earthquakes are customarily reported on the Richter scale, scientists also measure earthquakes on the moment magnitude scale. The moment magnitude scale measures more of the ground movements produced by an earthquake and is especially useful for large scale earthquakes.

Modified Mercalli Intensity Scale

The effect of an earthquake on human structures is called the intensity . The intensity scale consists of a series of certain key responses such as people awakening, movement of furniture, damage to chimneys, and finally - total destruction. Although numerous intensity scale s have been developed over the last several hundred years to evaluate the effects of earthquakes, the one currently used in the United States is the Modified Mercalli (MM) Intensity Scale . It was developed in 1931 by the American seismologists Harry Wood and Frank Neumann. This scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

The Modified Mercalli Intensity value assigned to a specific site after an earthquake has a more meaningful measure of severity to the nonscientist than the magnitude because intensity refers to the effects actually experienced at that place. After the occurrence of widely-felt earthquakes, the Geological Survey mails questionnaires to postmasters in the disturbed area requesting the information so that intensity values can be assigned. The results of this postal canvass and information furnished by other sources are used to assign an intensity within the felt area. The maximum observed intensity generally occurs near the epicenter.

Abridged from The Severity of an Earthquake , a U. S. Geological Survey General Interest Publication. U.S. GOVERNMENT PRINTING OFFICE: 1989-288-913
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