INTERNAL STRUCTURE OF THE EARTH IN REFERENCE TO SEISMOLOGY
Seismologists study shock, or seismic, waves as they travel through the Earth’s interior. These waves originate from natural sources like earthquakes, and from artificial sources like man-made explosions. Knowing how the waves behave as they move through different materials enables us to learn about the layers that make up the Earth. Seismic waves tell us that the Earth’s interior consists of a series of concentric shells, with a thin outer crust, a mantle, a liquid outer core, and a solid inner core.
P waves, meaning primary waves, travel fastest and thus arrive first at seismic stations. The S, or secondary, waves arrive after the P waves.
Seismic Waves:
The slipping of land generates seismic waves and these waves travel in all directions.
Earthquake is caused by vibrations in rocks. And the vibrations in rocks are produced by seismic waves.
Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and slip.
Seismic waves are basically of two types – body waves and surface waves.
Body 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.
There are two types of body waves. They are called P and S-waves.
Primary waves (P waves) are also called as the longitudinal or compressional waves. They are analogous to sound waves. P-waves move faster and are the first to arrive at the surface.
Secondary Waves (S waves) are also called as transverse or distortional waves. They are analogous to water ripples or light waves. S-waves arrive at the surface with some time lag. These waves are of high frequency waves and travel at varying velocities through the solid part of the Earth’s crust, mantle.
Surface 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 elasticity or stiffness. The more elastic the material is, the higher is the velocity. Their direction also changes as they reflect or refract when coming across materials with different densities.
Surface waves are of two types viz. Rayleigh Waves and Love waves.
Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects.
L waves are also called as long period waves. They are low frequency, long wave length, and transverse vibration. Generally affect the surface of the Earth only and die out at smaller depth. They cause displacement of rocks, and hence, the collapse of structures occurs. These waves are the most destructive and are recorded last on the seismograph.
Seismic Waves and Structure of Earth:
Seismic waves can tell us a lot about the internal structure of the Earth because these waves travel at different speeds in different materials.
Reflection causes P and S waves to rebound whereas refraction makes waves move in different directions.
The variations in the direction of these waves are inferred with the help of their record on seismograph.
Change in densities greatly varies the wave velocity. By observing the changes in velocity, the density of the earth as a whole can be estimated. By the observing the changes in direction of the waves (emergence of shadow zones), different layers can be identified.
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 different layers of the Earth composed of different materials.
1. CRUST:
It is the outermost solid part of the earth, normally about 8-40 kms thick. It is brittle in nature. Nearly 1% of the earth’s volume and 0.5% of earth’s mass are made of the crust. The thickness of the crust under the oceanic and continental areas are different. Oceanic crust is thinner (about 5kms) as compared to the continental crust (about 30kms). Major constituent elements of crust are Silica (Si) and Aluminium (Al) and thus, it is often termed as SIAL (Sometimes SIAL is used to refer Lithosphere, which is the region comprising the crust and uppermost solid mantle, also). The mean density of the materials in the crust is 3g/cm3. The discontinuity between the hydrosphere and crust is termed as the Conrad Discontinuity.
Study of seismic waves reveals following details about thickness of the crust:
i. Mountainous Areas:
Under the Himalayas, the crust is believed to be 70-75 km thick; under the Hindukush Mountains it is 60 km thick and under the Andes 75 km thick.
ii. Continental Areas:
Thickness of the crust in continents varies from 30 to 40 km along the continental slopes, thickness of the crust shows considerable variation.
iii. Oceanic Areas:
The crustal cover below the oceanic water varies in thickness from a maximum of 19 km to low value of 5 km in deep oceans.
The Continental Crust is further distinguished into three layers- A, B and C.
The A or the Upper Layer is between 2-10 km thick and is of low density (2.2 g/cc). It is mostly made up of sedimentary rocks. In this layer, the P wave velocities range from 1.8 to 5.0 km/sec.
The B or the Middle layer of the continental crust is relatively dense (2.4 to 2.6 g/cc) Seismic waves attain velocities of 5 to 6.2 km/sec. This layer is also sometimes called the Granite Layer and is made up mostly of granites, gneisses and other related igneous and metamorphic rocks. At places, it acquires thickness of 20 km or more.
In fact at many places in the world, it is the B layer of the crust which is exposed on the surface because the overlying A layer has already been removed due to prolonged erosion by weathering agents. Since granite layer is mostly made up of silicates of aluminium and potassium, it is also sometimes referred as SIAL (Si = silica, AL = alumina) layer while discussing internal structure of the earth.
The C layer is the lowermost layer of the continental crust and has a density of 2.8 to 3.3 g/cc in which P waves attain as high velocity as 6 to 7.6 km/sec. This layer is also referred as Basaltic Layer of the crust and acquires a thickness of 25 to 40 km under the continents. It is made predominantly of basic minerals (rich in magnesium silicates) and hence is sometimes named as SIMA (Si for silica and Ma for magnesium).
The Oceanic Crust:
It is generally the extension of C layer of the continental crust that makes the top layer of the oceans in most cases; A and B layers being practically absent from there. The oceanic crust is estimated to have a volume of 2.54 × 109 cc with an average density of 3.00g/cc.
2. The Mantle:
It is the second concentric shell of the Earth that lies beneath the crust everywhere. This zone starting from the lower boundary of the crust continues up to a depth of 2,900 km. The exact nature of the mantle is as yet incompletely understood. It has been sub-divided into an upper and lower mantle, the boundary between the two layers being placed at 900-1,000 km below the earth. The upper mantle is further divided into two layers of 400 and 600 km thickness respectively.
Enough seismic data is available to suggest that density in the mantle rises from 3.3 g/cc from just below the crust to about 5.7g/cc at the base of the mantle. Recent studies indicate that a part of the upper mantle, from 100 km to 500 km depth, is in a plastic rather than solid state.
This zone has been named as asthenosphere (Greek “asthenes” – without strength). It is believed to be the source of much volcanic activity of the Earth and many other processes. The asthenosphere is believed to be located entirely in the upper mantle and supports the slowly moving tectonic plates.
3. The Core:
It is the innermost concentric shell of the Earth as concluded from the record of seismic waves. Its existence was suggested by R.D. Oldham in 1906 and subsequently confirmed by other seismologists. The core boundary begins at depth of 2,900 km from the surface and it extends to the center of the earth at 6,371 km.
Further studies of seismic waves with special reference to core indicate that the core itself can be distinguished into two distinct zones- the outer core and the inner core.
The outer core comprises the region from a depth of 2,900 km to 4,580 km below the earth surface and behaves more like a liquid because the S-waves from the earthquake shocks reaching this zone are not transmitted through this zone at all. (It is characteristic of S waves, also called shear waves that these are unable to travel through liquids).
The inner core, with a thickness of around 1,790 km is believed to be a solid metallic body. Much variation in composition is also suggested tor the material lying between the outer core and the inner core but nothing can be said conclusively.
Very significant variations in the density of material immediately outside and inside of the core are suggested by seismic observations. At the base of the mantle, density is inferred as 5.7 g/cc that jumps to 9.9 g/cc at the top of the core. This value reaches a figure of 12.7 g/cc at the boundary of the inner core and becomes 13.0 g/cc at the center of the earth.
As regards, the chemical composition of the inner core, the hypothesis that it is made up chiefly of iron and nickel has found support from many accounts. Seismologically, velocities of P waves recorded in the core bear close resemblance to those recorded for nickel iron alloys.
The structure of Earth’s deep interior cannot be studied directly. But geologists use seismic (earthquake) waves to determine the depths of layers of molten and semi-molten material within Earth. Geologists are now using these records to establish the structure of Earth’s interior.
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