EARTHQUAKES

Earthquakes are tremors in the ground, created by the sudden movement of tectonic plates-huge slabs of rock that make up the Earth's crust. Tectonic plates usually slide past each other, but sometimes get stuck together. The stress on the rocks accumulates until they crack. The tectonic plates then jolt pass each other, sending shock waves through the ground. These vibrations are known as seismic waves, thus causing the earth to quake. The majority of earthquakes are so gentle that no one notices them, but some are very violent that they can destroy a whole city or a town. One of the famous earthquakes in Europe took place in Lisbon, Portugal, in 1755. The city was destroyed and at least thirty thousand people were killed. In 1908, in Calabria and Sicily, a quake killed more than seventy-five thousand people. Two great earthquakes caused enormous damage took place in Tokyo, Japan, and in the Kansu Province in China.

• Epicenter of the earthquake

The point at which the earthquake occurs is known as the focus, and above the focus is the epicenter. The epicenter is the point at which the effects of the earthquake is more devastating in the earth's surface. The focus may be more than the 700 km(approximately 185 miles) below the epicenter.

• Earthquake zones

Earthquakes can occur anywhere, but they are more frequent in the earthquake zones. The earthquake zones such as California and Japan lies near the margins of the tectonic plates called the fault lines.

• Seismometer

These show seismic waves and measure an earthquake's location and intensity on the Richter scale. The height of each line shows the wave's force.