Earthquake

The ground beneath us feels solid, but it is not. An earthquake is, in the simplest terms, the shaking of the earth — a natural event caused by the sudden release of energy that sends waves travelling in all directions through the planet. That release of energy happens along a fault, which is a sharp break in the crustal rocks. Here is the full mechanism, step by step.

Rocks on either side of a fault tend to move in opposite directions. But the weight of the overlying rock strata presses down on them, and the resulting friction locks the blocks together. For a while, the rocks hold still, even though they want to move. Eventually, their tendency to move apart overcomes the friction. The blocks get deformed under the strain, and then they suddenly slide past one another. That abrupt sliding releases a burst of energy, and the energy travels outward as waves in every direction.

The point inside the earth where the energy is released is called the focus, also known as the hypocentre. The waves travel outward from the focus and eventually reach the surface. The point on the surface that lies directly above the focus — the nearest surface point to it — is called the epicentre. The epicentre is the first place to experience the waves.

Earthquake Waves

All natural earthquakes take place in the lithosphere, which for this discussion means the portion of the earth up to about 200 km from the surface. An instrument called a seismograph records the waves as they reach the surface, producing a curve with three distinct sections, each representing a different type of wave pattern.

Earthquake waves are basically of two types: body waves and surface waves.

• Body waves are generated by the release of energy at the focus and move in all directions, travelling through the body of the earth — hence the name.
• Surface waves are a new set of waves generated when body waves interact with the surface rocks. These waves move along the surface.

There are two types of body waves, called P-waves and S-waves.

• P-waves (primary waves) move faster and are the first to arrive at the surface. They are similar to sound waves and can travel through gaseous, liquid, and solid materials.
• S-waves (secondary waves) arrive at the surface with some time lag. The crucial fact about them is that they can travel only through solid materials. This single characteristic has helped scientists understand the structure of the earth's interior — wherever S-waves fail to pass through, scientists know the material is not solid.

The velocity of waves changes as they travel through materials of different densities: the denser the material, the higher the velocity. Their direction also changes, as they reflect or refract when they come across materials of different densities. Reflection causes waves to rebound, while refraction makes them move in different directions. These variations in direction are inferred from their records on the seismograph.

The surface waves are the last to report on the seismograph. They are the most destructive of all the waves, because they cause displacement of rocks, and hence the collapse of structures.

Propagation of Earthquake Waves

Different types of earthquake waves travel in different manners, and as they propagate they cause vibration in the body of the rocks through which they pass.

• P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of propagation, creating density differences in the material that lead to stretching and squeezing.
• The other three waves — the S-waves and the two surface waves — vibrate perpendicular to the direction of propagation. The direction of vibration of S-waves is perpendicular to the wave direction in the vertical plane, so they create troughs and crests in the material through which they pass.

The Shadow Zone

Earthquake waves get recorded on seismographs located at far-off locations. But there exist specific areas where the waves are not reported at all. Such a zone is called the shadow zone. The study of different events reveals that for each earthquake, there exists an altogether different shadow zone.

The observations are precise:

• Seismographs located at any distance within 105° from the epicentre record the arrival of both P and S-waves.
• Seismographs located beyond 145° from the epicentre record the arrival of P-waves but not S-waves.
• The zone between 105° and 145° from the epicentre is the shadow zone for both types of waves.
• The entire zone beyond 105° does not receive S-waves.

So the shadow zone of S-waves is much larger than that of P-waves. The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre. The shadow zone of S-waves is not only larger in extent, but it also covers a little over 40 per cent of the earth's surface. If you know the location of the epicentre, you can draw the shadow zone for any earthquake.

Types of Earthquakes

The book classifies earthquakes into these types: …