Direct Sources

• Surface rock or the rocks we get from mining areas.
• Example– Gold mines in South Africa are as deep as 3 – 4 km.
• Example- Scientists world over are working on two major projects such as “Deep Ocean Drilling Project” and “Integrated Ocean Drilling Project”.
• The deepest drill at Kola, in Arctic Ocean, has so far reached a depth of 12 km
• Volcanic eruption forms another source of obtaining direct information.
• As and when the molten material (magma) is thrown onto the surface of the earth, during volcanic eruption it becomes available for laboratory analysis.
• This and many deep drilling projects have provided large volume of information through the analysis of materials collected at different depths.

Indirect Sources

• Through the mining activity that temperature and pressure increase with the increasing distance from the surface towards the interior in deeper depths.
• Moreover, it is also known that the density of the material also increases with depth.
• Another source of information are the meteors that at times reach the earth.
• The material and the structure observed in the meteors are similar to that of the earth.
• They are solid bodies developed out of materials same as, or similar to, our planet.
• The other indirect sources include gravitation, magnetic field, and seismic activity.
• The gravitation force (g) is not the same at different latitudes on the surface.
• It is greater near the poles and less at the equator.
• This is because of the distance from the centre at the equator being greater than that at the poles. The gravity values also differ according to the mass of material.
• The reading of the gravity at different places is influenced by many other factors. These readings differ from the expected values. Such a difference is called gravity anomaly.
• Gravity anomalies give us information about the distribution of mass of the material in the crust of the earth.
• Magnetic surveys also provide information about the distribution of magnetic materials in the crustal portion, and thus, provide information about the distribution of materials in this part.
• Seismic activity is one of the most important sources of information about the interior of the earth.

Earthquake

• The study of seismic waves provides a complete picture of the layered interior.
• An earthquake in simple words is shaking of the earth.
• It is caused due to release of energy, which generates waves that travel in all directions.
• The release of energy occurs along a fault.
• 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.
• The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre. ( somewhere inside the earth )
• The point on the surface, nearest to the focus, is called epicentre.
• It is the first one to experience the waves.
• It is a point directly above the focus. (Always above the hypocenter and on the surface of earth)

Earthquake Waves

• An instrument called ‘seismograph’ records the waves reaching the surface.
• All natural earthquakes take place in the lithosphere (depth up to 200 km from the surface of the earth.)
• Two types of waves — body waves and surface waves.
• Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence, the name body waves.
• 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

They are called P and S-waves.

P-waves-

• Moves 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)
• This exerts pressure on the material in the direction of the propagation.
• As a result, it creates density differences in the material leading to stretching and squeezing of the material

S-waves-

• Arrives 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.
• Reflection causes waves to rebound whereas refraction makes waves move in different directions.
• These waves are more destructive.
• They cause displacement of rocks, and hence, the collapse of structures occurs.
• The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane.
• Hence, they create troughs and crests in the material through which they pass.

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’.
• A zone between 105° and 145° from epicentre was identified as the shadow zone for both the types of waves.
• 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

(i) The most common ones are the tectonic earthquakes. These are generated due to sliding of rocks along a fault plane.

(ii) A special class of tectonic earthquake is sometimes recognised as volcanic earthquake. However, these are confined to areas of active volcanoes.

(iii) In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.

(iv) Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.

(v) The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced 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.
• The intensity scale takes into account the visible damage caused by the event.
• The range of intensity scale is from 1-12.

Effects Of Earthquake

1. Ground Shaking
2. Differential ground settlement
3. Land and mud slides
4. Soil liquefaction
5. Ground lurching
6. Avalanches
7. Ground displacement
8. Floods from dam and levee failures
9. Fires
10. Structural collapse
11. Falling objects
12. Tsunami

The first six listed above have some bearings upon landforms, while others may be considered the effects causing immediate concern to the life and properties of people in the region.

The effect of tsunami would occur only if the epicentre of the tremor is below oceanic waters and the magnitude is sufficiently high.

Tsunamis are waves generated by the tremors and not an earthquake in itself.

Though the actual quake activity lasts for a few seconds, its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale.

[Title: Shadow zone of P waves and S waves]