What does the behaviour of P and S waves tell geologists about the structure of the Earth's interior?

What does the behaviour of P and S waves tell geologists about the structure of the Earth’s interior?

Vibrations are produced in the Earth’s crust when rocks in which elastic strain has been building up suddenly rupture, and then rebound. The vibrations can vary from barely noticeable to catastrophically destructive. Earthquakes can release energy thousands of times greater than the world’s first atom bomb.

Six types of shock waves are generated in the process. Two are classified as body waves-that is, they travel through the Earth’s interior-and the other four are surface waves. The waves are further differentiated by the kinds of motions they impart to rock particles. Primary or compressional waves (P waves) send particles oscillating back and forth in the same direction as the waves are travelling, whereas secondary or transverse shear waves (S waves) impart vibrations perpendicular to their direction of travel. P waves always travel at higher velocities than S waves, so whenever an earthquake occurs, P waves are the first to arrive and be recorded at geophysical research stations throughout the world.

An earthquake will occur when rocks, (usually at plate boundaries) are put under immense strain or pressure. This pressure will eventually fracture the rock, similar to when bending a brittle object such as a wafer, or a ruler. This fracture can occur in several different directions: Apart from each other, together or past each other-called a shearing movement. When the rocks eventually do fracture, a large amount of energy is released, most notably in the form of P and S waves, sound waves and the movement of the rocks releases movement (kinetic) energy.

After an earthquake, P waves are always detected at seismic stations well in advance of the S waves. From this we can make the assumption that P waves travel faster than S waves. The properties of the waves must therefore be different for the waves to be reaching one point at two different times.

The difference is in the wave type: P waves are longitudinal, their energy travels in a straight line.

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