Every earthquake that happens is recorded by seismographs installed around the world. The records are analyzed to study the seismic waves. This understanding of waves is used to create an image of the structure of the earth’s interior even before drilling.
All data collected by seismographs are stored in a computer at the central repository. This helps us to understand the layers, depth and location of earthquakes.
The seismograph consists of different types of instruments capable of taking records without human intervention.
A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. (as in case of indian and Eurasia plate happen during Nepal earth quack)
As the overlying rock strata press them, the friction locks them together.
However, their tendency to move apart at some point of time overcomes the friction.
As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions.
Earthquake Waves
The vibrations produced by an earthquake are of two types, body waves and surface waves. The body waves can pass through any substance but they travel at different speeds. Surface waves travel up and down the surface on which they occur.
All natural earthquakes take place in the lithosphere (depth up to 200 km from the surface of theearth.) An instrument called ‘seismograph’ records the waves reaching the surface.
The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface.
The velocity of waves changes as they travel through materials with different densities. The denser the material, the higher is the velocity.
Their direction also changes as they reflect or refract when coming across materials with different densities.
Body Waves.
Body waves are of two types: compressional or primary (P) waves and shear or secondary (S) waves. P- and S- waves are called “body waves” because they can travel through the interior of a body such as the Earth’s inner layers, from the focus of an earthquake to distant points on the surface.
1. P-waves: move faster and are the first to arrive at the surface. These are also called ‘primary waves’.
- The P-waves are similar to sound waves.
- They travel through gaseous, liquid and solid materials. (as sound)
- P-waves vibrate parallel to the direction of the wave. (P for parallel)
2. S-waves: arrive at the surface with some time lag. These are called secondary waves.
- An important fact about S-waves is that they can travel only through solid materials.
- It has helped scientists to understand the structure of the interior of the earth.
3. Shadow Zone
- Earthquake waves get recorded in seismographs located at far off locations. However, there
- exist some specific areas where the waves are not reported. Such a zone is called the ‘shadow
- zone’.
- The entire zone beyond 105° does not receive S-waves. The shadow zone of S-wave is much larger
- than that of the P-waves.
- The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre.
Types of Earthquakes
The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.
A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.
In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
Measuring Earthquakes
• The earthquake events are scaled either according to the magnitude or intensity of the shock.
• The magnitude scale is known as the Richter scale.
• The magnitude relates to the energy released during the quake.
• The magnitude is expressed in absolute numbers, 0-10.
• The intensity scale is named after Mercalli, an Italian seismologist.
Effects of Earthquake
• Ground Shaking
• Differential ground settlement
• Land and mudslides
• Soil liquefaction
• Ground displacement
• Floods from dam and levee failures
• Structural collapse
• Tsunami
