What is an Earthquake?
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Earth is made up of an inner and outer core, the mantle and crust. The crust and upper mantle form a cold, strong layer known as the lithosphere. The lithosphere is broken up into plates. The downward pull of gravity on the plates as they cool, and to a lesser extent the push of new lithospheric material from ocean ridges, causes them to move. Most of the earthquakes occur at plate boundaries. Where plates converge, one plate is drawn slowly beneath the other. This takes place over thousands of years. Where plates collide, rock layers are forced upwards creating mountains. Where plates diverge, lava emerges from the mantle and cools to form new sections of crust. Diverging plate boundaries are often found underwater. Other plates move very slowly alongside each other. Faults are found at the edges of the plates where the crust is moving in different directions. In some places the plates become locked together. Potential energy builds up in the locked plates. When the plates give, the stored energy is released in the form of an earthquake. The point of the earthquake’s origin beneath the surface is called the hypocenter.
An earthquake emits its power as three waves of energy. Primary, or P-waves are felt as a sudden jolt. Secondary, or S-waves arrive a few seconds later and are felt as a more sustained side-to-side shaking. Surface waves radiate outwards from the epicenter – the point on the surface directly above the hypocenter – and arrive after the main P and S waves.
There are two types of wave with different movements. Rayleigh waves create a rolling, up and down motion. Love waves (named after mathematician A. E. H. Love) cause the ground to twist from side to side. These two types of surface wave cause great damage to buildings.