Understanding the interior of the earth, its compositions, its mechanical & chemical properties and the structural arrangements hold a key to understand the numerous features and landforms we have on the earth surfaces which are manifestation of the internal instability and dynamism.The study of behavior of EQ waves such as ‘P’ and ‘S’ waves using sensitive instrument offer us conducive evidences about earths interiors. 

The various sources which provides information about the earth's interior includes artificial sources, evidences from the theories of origin of the earth and natural sources. Among the natural sources volcanicity and seismological evidences reveals the information about the structure of interior of the earth.During an earthquake three types of siesmic waves are generated which includes P waves, S waves and L waves. where L wave remain confine to the surface layer only. the P and S waves traverses and travel inside the earth.

Velocity of these three waves varies inside the earth and thisproves that earth is not made up of homogeneous material rather it is made upof different materials SIAL,SIMA and NIFE.

Along with that seismology also provide measurement of the depthof different layers of earth Crust, Mantle and Core along with the density ofthese layers e.g. Mohorovicic discontinuity(Lower crust and upper mantleboundary), Wichert-Gutenberg Discontinuity(Mantle-Core Boundary) and ConardDiscontinuity.

It also tells about the physical and chemical properties of the interior of earthbecause all type of waves are not found in every layer however they arespecific layers.

The behavior of sesmic waves in different layers of the earth provides the most authentic evidence about the composition & structure of the earth. The different types of waves generated during the occurrence of earthquake , are generally divided into 3 broad categories;-a)Primary waves, b)Secondary waves ,c)surface waves. Seismic Waves- both P-waves & S-waves travel faster through rigid material than through soft or plastic material.The velocities of these waves travelling through specific part of earth gives an indication of type of rocks there.Abrupt changes in seismic wave velocities indicate significant changes in earths interior.

Seismic waves passing through the earth are refracted in ways that show distinct discontinuities within earth's interior & provide basis for the belief that earth has

1)a solid inner core, 2)a liquid outer core, 3)a soft aesthenosphere, 4) a rigid lithosphere.Seismic vibrations follow property of light waves being reflected & refracted with change in medium.interference:-

a)NON HOMOGENEOUS- If earth were homogeneous solid, seismic waves waves would travel in straight line with a constant speed. But seismic waves arriving sooner at distant stations & later at closer stations indicate that earth is not a homogeneous , uniform mass but has physical properties that change with depth.

b)SHADOW ZONE- Refraction at the boundary between the core & the mantle thus causes a p-wave shadow zone over part of earth's surface i.e. zone where no P-waves are received between 103 & 143 deg. S- waves do not pass through the core . there is a huge shadow zone therefore outer core is liquid (as S-waves do not pass through liquid)

c)SEISMIC TOMOGRAPHY - Here seismic waves are used as X-rays to analyze the velocity of seismic waves as they pass through earth in different directions with different temperatures. The tomography thus can be interpreted as "temperature maps". Hot parts of the mantle being less dense than their surroundings will rise, whereas cool mantle will sink. Thus tomography can be used to outline the pattern flow in mantle

Thus seismic waves act as probes of earth's Earth's Interior.

Seismic waves, the waves of energy that travel through the Earth as a result of an earthquake can tell us a lot about the internal structure of the Earth because these waves travel at different speeds in different materials. There are two types of waves that travel through the Earth: p-waves and s- waves. P- waves are faster and they can travel through both solids and liquids. S-waves are slower and cannot travel through liquids. For both kinds of waves, the speed at which the wave travels also depends on the properties of the material through which it is traveling.Scientists are able to learn about Earth’s internal structure by measuring the arrival of seismic waves at stations around the world. For example, we know that Earth’s outer core is liquid because s-waves are not able to pass through it; when an earthquake occurs there is a “shadow zone” on the opposite side of the Earth where no s-waves arrive. Similarly, we know that the Earth has a solid inner core because some p-waves are reflected off the boundary between the inner core and the outer core. By measuring the time it takes for seismic waves to travel along many different paths through the Earth, we can figure out the velocity structure of the Earth. Abrupt changes in velocity with depth correspond to boundaries between differentlayers of the Earth composed of different materials.

(The diagram of Interior of the Earth with all the annotations, p waves, s waves etc.)The interior of the Earth was demonstrated in a scientific way for the first time using the seismology. 1) P Waves: (Diagram of P waves)The study of P waves was relevant in determining that the interior of the Earth experiences change in the medium from Solid mantle to liquid core after 2900 km. 2) S waves: (Diagram of S waves) S waves confirmed the liquid state of the Core as it does not pass through liquids.3) Seismic study was also used to determine the layers of the mantle and core. This was proved by the fact that the velocity of P waves increase with the increase in density

increases. 4) Seismology is still used to determine the nature of the volcanic activity in the interior of the Earth.By using the Seismic studies, we can take adequate steps to survive the effects due to the changes in the interior of Earth felt on the surface.