Earthquakes and Waves

            We cannot obtain information about the deep structure of the Earth by mining or drilling. The world’s deepest mine is about 3.5 km in depth, while the radius of the Earth is over 6000 km. We can only make guesses of the Earth’s structure by studying earthquakes.

            The study of earthquakes is called seismology. An earthquake occurs when forces inside the Earth become strong enough to fracture large masses of rock and make them move. The place where an earthquake occurs in the rocks of the crust is called the focus. The place directly above this on the surface of the Earth is called the epicenter.

The energy which is released travels through the rock as a series of shock waves. They can be detected by a seismometer. An earthquake will send out three different types of waves:

·        P waves (Primary waves) are longitudinal waves and therefore travel as a series of compressions in the same way as a sound wave. These waves can move through both solids and liquids and can therefore travel through both the mantle and the core to reach the opposite of the Earth. They travel very quickly, 30 times as fast as sound in air. The changing density of the mantle may cause the path of P waves to bend due to refraction. If the density of the rock changes gradually, the waves are bent in curves. If the density changes suddenly, the waves are bent sharply.

·        S waves (Secondary waves) are transverse waves and they can travel through the solid of the mantle but not the liquid of the outer core. Some S waves may be reflected from the surface of the liquid core and a shadow is left at the other side of the Earth that the S waves cannot reach. This proves that the core does exist and helps scientists to work out its size.

·        L waves (Long waves) are the waves that travel round the crust on the surface. They will arrive last but do the most damage.

The strength of earthquakes is measured on two scales:

1.      The Richter scale measures the energy of the earthquake and going from one point to the next (i.e. from 3 to 4) is an increase of ten times (so from 4 to 6 is 100 times). This is useful for large powerful earthquakes.

2.      The Mercalli scale is more help when describing earth tremors or smaller earthquakes and describes the effects that would be noticed. Here are some levels of the scale:

2)                 Hardly felt. Noticed by people asleep.

4)                 Hanging objects swing. Doors and windows rattle.

8)                 Cracks open in ground. Some buildings collapse.

12)      Large areas of ground moved up and down. Total damage.

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