What causes earthquakes？ The earth is formed of layers. The surface of the earth, about 20

0 kilometers thick, is made of large pieces. When they move against each other, an earthquake happens. A large movement causes a violent earthquake, but a small movement causes a mild one.

Earthquakes last only a few seconds. The rolling movements are called seismic waves. The seismic waves start in one place, called the epicenter, and roll outward. A seismic wave travels a round the earth in about twenty minutes. Usually, an earthquake is strong enough to cause dam age only near its epicenter.

However, epicenters at the bottom of the ocean create huge sea waves as tall as 15 meters. These waves cross the ocean in several hours. Rushing toward land, they destroy small islands and ships in their path. When they hit land, they flood coastal areas far from the epicenter of the earthquake. In 1868, a wave reached 4.5 kilometers inland in Peru. In 1896, a wave in Japan killed 27,000 people.

After an earthquake happens, people can die from lack of food, water, and medical supplies. The amount of destruction caused by an earthquake depends on where it happens, what time it happens, and how strong it is. It also depends on types of buildings, soil conditions, and population. Of the 6000 earthquakes in the world each year, only about fifteen cause great damage and many deaths.

In 1556, an earthquake in northern China killed 830,000 people—the most in history. There was no way to measure its strength. In 1935, scientists started using the Richter Scale to measure seismic waves. A seriously destructive earthquake measures 6.5 or higher on the Richter Scale.

How can scientists predict earthquakes？ Earthquakes are not just scattered anywhere but hap pen in certain areas, places where pieces of the earth's surface meet. This pattern causes them to shake the same places many times. For example, earthquakes often occur on the west coasts of North and South America, around the Mediterranean Sea, and along the Pacific coast of Asia.

Another way to predict earthquakes is to look for changes in the earth's surface, like a sudden drop of water level in the ground. Some people say animals can predict earthquakes. Before earth quakes, people have seen chickens sitting in trees, fish jumping out of the water, snakes leaving their holes, and other animals acting strangely.

On February 4, 1975, scientists predicted an earthquake in northeastern China and told people in the earthquake zone to leave the cities. More than a million people moved into the surrounding countryside, into safe, open fields away from buildings. That afternoon, the ground rolled and shook beneath the people's feet. In seconds, 90 percent of the buildings in the city of Haicheng were destroyed. The decision to tell the people to leave the cities saved 10,000 lives.

However, more than a year later, on July 28, 1976, the scientists were not so lucky. East of Beijing, Chinese scientists were discussing a possible earthquake. During their meeting, the worst earthquake in modern times hit. Estimates of deaths ranged from 250,000 to 695,000. The earth quake measured 7.9 on the Richter Scale.

Earthquakes often come together with volcanic eruptions. In late 1984, strong earthquakes began shaking the Nevado Del Ruiz volcano in Colombia every day. On November 14, 1985, it erupted. A nearby fiver became a sea of mud that buried four towns. This disaster killed more than 2100 people.

Mexico City has frequent earthquakes. An earthquake there on September 19, 1985, measured 8.1 on the Richter Scale and killed 7000 people. Most victims died when buildings fell on them.

San Francisco, California, also has frequent earthquakes. However, newer buildings there are built to be safe in earthquakes. Therefore, when an earthquake measuring 7.1 on the Ri

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