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Text 3 (Questions 11-15)

How Earthquakes Happen

    Earthquake is one of the most destroying natural disasters. Unluckily it often happens in several regions. Recently a horrible earthquake has shaken West Sumatra. It has brought great damages. Why did it occur? Do you know how an earthquake happens?

    Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves. It make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs.

    During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus is called the epicenter of the earthquake.

What is earthquake usually caused by ?