单项选择题

What causes earthquakes The earth is formed of layers. The surface of the earth, about 100 kilometers thick, is made of large pieces. When they move against each other, an earthquake hap pens. 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 around the earth is a about twenty minutes. Usually, an. earthquake is strong enough to cause damage 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 6.5or higher on the Richter Scale. How can scientists predict earthquakes Earthquakes are not just scattered anywhere but happen 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 earthquakes, 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 tel1 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 earthquake 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 river 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 all earthquake measuring 7.1 on the Richter Scale hit northern California on October 17, 1989, only 67 people were killed. The earthquake hit in the afternoon, when thousands of people were driving home from work. Freeways and bridges broke and fell. Buried under the layers of the Oakland Freeway, people were crushed in their flattened cars. Explosions sounded like thunder as older buildings seemed to burst apart along with the freeways. As the electric power lines broke from the falling bridges and buildings, the sky, covered with huge clouds of black dust, appeared to be filled with lightning. Water rushed into the streets from broken pipes and mixed with gas from broken gas lines, causing more explosions. Emergency workers had to cope with medical problems. Everyone worked together to save survivors and comfort victims. The next day, the disaster sites looked terrible. Victims couldn’’t find their houses, their cars, or even their streets. Boats were destroyed, and debris covered the surface of the sea. There was no water, no electricity, no telephone, only the smell of garbage floating in melted ice in refrigerators open to the sun, Losses and property. damage from the earthquake amounted to millions of dollars. Seismology is the study of earthquakes, and a seismologist is a scientist who observes earthquakes. Seismologists have given us valuable knowledge about earthquakes. Their equipment measures the smallest vibration on the surface of the earth. They are trying to find ways to use knowledge about earthquakes to save lives and to help solve the world’’s energy shortage. The earth’’s natural activity underground creates energy in the form of heat. Geothermal means earth heat. This geothermal energy could be useful. However, if we take natural hot water out of the earth in earthquake zones, we might cause earthquakes. People live in earthquake zones because of natural beauty, productive soil, and large existing centers of population. However, people who live there should expect earthquakes. They should be prepared to protect their lives and property. They must build safer buildings and roads. Hospitals and electric power stations must be built as far as possible from probable earthquake sites. When an earthquake starts, people must run to open ground or stay in protected areas like doorways or even bathtubs. If seismologists could predict earthquakes, we could save about 20,000 human lives each year. Humans can control many things about nature, but we cannot control earthquakes. Seismologists can measure the size of sea waves.