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Earthquake intensity varies. Some are so minor that they can only be felt at the epicenter. Other earthquakes are so strong that it shakes hundreds of thousands of square miles.
Those who live in earthquake regions know that it is better to have several small earthquakes than to have no earth activity for a long time. This is because the frequent, small earthquakes release energy in little bursts, instead of building up the pressure to release in one huge tremor. If pressure between two plates is allowed to constantly build, one of the plates will tear loose more violently, causing a huge shock and a big earthquake.
Earthquake shock waves radiate out from the epicenter faster than a bullet! There are slow and powerful surface waves that cause the majority of earthquake damage - Love waves, which move horizontally, and Rayleigh waves, which move vertically. There are also two types of body waves that travel underground - P waves and S waves. P-waves, or primary waves, that travel at 3 miles (5 km) per second, stretching and squeezing rocks laterally. S-waves, or secondary waves, move from side to side at about 2 miles (3 km) a second.
Scientists measure the strength of earthquakes using machines known as seismographs, which record the trembling of the ground. This instrument is simply a pen that traces a straight line on a turning drum. When the earth moves, the pen is jolted out of its normal course, and traces a wiggle on the paper beneath it. A very sensitive seismograph can magnify the tiniest tremors one million times. The stronger the earth’s movement, the larger the wiggle is drawn.
Using the information from the seismograph, scientists determine its strength using a magnitude scale known as the Richter Scale. The official definition of the magnitude of an earthquake is: the logarithm to base ten of the maximum seismic-wave amplitude (in thousandths of a millimeter) recorded on a standard seismograph at a distance of 100 kilometers from the earthquake epicenter.
Basically, this scale is exponential, so that each number is many times the strength of the number below it. An earthquake with a magnitude of 2.0 is hardly felt, while a 5.0 earthquake releases the same amount of energy as that of the Hiroshima atomic bomb. An 8.0-level earthquake is so devastating that few buildings, roads, and people will escape its effects. Originally, Charles Richter took the smallest-size earthquake he could record at the time and gave it a magnitude of 0. Because today’s instruments are so much more sensitive, scientists can detect even smaller earthquakes that are given negative magnitudes. The highest Richter earthquakes were measured at 9.0.
||This magnitude earthquake is widely felt and is strong enough to crack plaster.|
||A strong vibration shakes the earth, damaging chimneys and weak buildings.|
||This earthquake is strong enough to badly damage average buildings.|
||This earthquake is strong enough to destroy even well-built structures.|
||Even special, earthquake-resistant buildings will be badly damaged.|
||There is widespread destruction.|
Another scale that is sometimes used is known as the Modified Mercalli Intensity Scale, created by Giuseppe Mercalli in 1902 to measure earthquake intensity. The damage that the Mercalli Scale measures is descriptive and can’t be machine-recorded. The intensity of an earthquake, then, is measured by inspectors who look at the damage and ask survivors to answer questionnaires. An earthquake can have many intensities, which decrease as a person moves away from the epicenter. A tremor that makes lights swing from ceilings is ranked as a 2, while a quake that heaves the ground into waves, smashing everything around, ranks as a 12.
||Although people cannot feel the tremor, machines are able to record it.|
||Resting people can detect shaking, especially if they are on a building’s upper floors. Suspended objects may swing back and forth.|
||When indoors, people can feel a small vibration, as if a light truck has passed by. Hanging objects will swing. It is difficult for people to realize that it is an earthquake.|
||People feel a vibration, like a heavy truck hitting the building. Wooden walls will creak, dishes rattle, and cars will rock.|
||This level earthquake can be felt outdoors and is strong enough to splash liquids out of containers. Small objects will fall over and doors swing open and close.|
||Everyone feels this earthquake, and some people run outside in fright. People find difficulty walking in straight lines. Windows and dishes break, pictures fall off walls, and small bells ring.|
||People find it difficult to stand. Pond water begins to ripple and form waves, furniture breaks, bricks fall, plaster cracks, and large bells ring.|
||It is difficult to drive, and wet ground will crack. Buildings, walls, chimneys, and monuments collapse. Tree branches break and the flow of water in wells and springs is changed.|
||This type of earthquake will cause general panic, causing people and animals to run in confusion. Building foundations are damaged. Frame buildings will shift in place, and underground pipes crack.|
||Brick, wooden, and frame buildings will collapse. The earthquake causes notable landslides, and water is splashed out of rivers and waterways.|
||The ground shifts so much that railroad lines are distorted. Underground pipes are destroyed and highways cut. Fissures appear in the ground, and there will be large landslides and rock falls.|
||There is extensive damage. Almost all structures above and below ground are reduced to rubble. Shock waves extend out from the earthquake, causing the ground to roll in waves. Objects are made airborne, rivers are altered, and large fissures appear.|
The worst damage is usually done near the epicenter, which is the place above the earthquake’s underground center, or focus. Scientists are able to locate the epicenter using information from seismographs at different locations. There are currently hundreds of these instruments continuously at work around the world.
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