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INTERIOR OF THE EARTH 19 Indirect Sources information about the interior of the earth. Hence, we shall discuss it in some detail. Analysis of properties of matter indirectly provides information about the interior. We know through the mining activity that Earthquake temperature and pressure increase with the Thestudyofseismicwavesprovidesacomplete increasing distance from the surface towards picture of the layered interior. An earthquake the interior in deeper depths. Moreover, it is in simple words is shaking of the earth. It is a alsoknownthatthedensityofthematerialalso natural event. It is caused due to release of increases with depth. It is possible to find the energy, which generates waves that travel in rateofchangeofthesecharacteristics.Knowing all directions. the total thickness of the earth, scientists have estimated the values of temperature, pressure Why does the earth shake? andthedensityofmaterialsatdifferentdepths. The details of these characteristics with The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. reference to each layer of the interior are Rocks along a fault tend to move in opposite discussed later in this chapter. directions. As the overlying rock strata press Another source of information are the them,thefrictionlocksthemtogether.However, meteorsthatattimesreachtheearth.However, it may be noted that the material that becomes their tendency to move apart at some point of time overcomes the friction. As a result, the available for analysis from meteors, is not from blocks get deformed and eventually, they slide the interior of the earth. The material and the past one another abruptly. This causes a structure observed in the meteors are similar release of energy, and the energy waves travel to that of the earth. They are solid bodies developed out of materials same as, or similar in all directions. The point where the energy is released is called the focus of an earthquake, to, our planet. Hence, this becomes yet another alternatively, it is called the hypocentre. The source of information about the interior of the energy waves travelling in different directions earth. reach the surface. The point on the surface, The other indirect sources include gravitation,magneticfield,andseismicactivity. nearest to the focus, is called epicentre. It is thefirstonetoexperiencethewaves.Itisapoint The gravitation force (g) is not the same at directly above the focus. different latitudes on the surface. It is greater near the poles and less at the equator. This is Earthquake Waves because of the distance from the centre at the equator being greater than that at the poles. All natural earthquakes take place in the The gravity values also differ according to the lithosphere. You will learn about different mass of material. The uneven distribution of layers of the earth later in this chapter. It is mass of material within the earth influences sufficienttonoteherethatthelithosphererefers thisvalue.Thereadingofthegravityatdifferent to the portion of depth up to 200 km from the places is influenced by many other factors. surface of the earth. An instrument called These readings differ from the expected values. ‘seismograph’ records the waves reaching the Such a difference is called gravity anomaly. surface. A curve of earthquake waves recorded Gravity anomalies give us information about on the seismograph is given in Figure 3.1. Note the distribution of mass of the material in the that the curve shows three distinct sections crust of the earth. Magnetic surveys also each representing different types of wave patterns.Earthquakewavesarebasicallyoftwo provide information about the distribution of magnetic materials in the crustal portion, and types — body waves and surface waves. Body thus, provide information about the wavesaregeneratedduetothereleaseofenergy distribution of materials in this part. Seismic atthefocusandmoveinalldirectionstravelling activity is one of the most important sources of throughthebodyoftheearth.Hence,thename Reprint 2026-27