DYNAMIC GEOLOGYENGINEERING GEOLOGY

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The Lithosphere

o Litho = stone

o Hence, lithosphere stands for the

stony part of the Earth!

o It refers to the uppermost shell of the

Earth, i.e. the crust, and a portion of

the second layer, the mantle, up to

which the material exists in a definite

solid state

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Introduction to Seismology

o Seismology is the study of the

elastic/seismic waves generated within

the earth, during earthquakes

o The study of seismic waves forms the

single most important source of

information for the interior of the Earth

o Every earthquake generates two types of

waves: body (P and S) and surface (L)

waves

4P Wave S Wave

Thompson and Turk

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Propagation of Body and Surface waves

Thompson and Turk

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Properties of Seismic Waves

o In a uniform, homogeneous medium, a

wave radiates outward in concentric

spheres, at constant velocity

o The velocity of a seismic wave depends on

the nature of the material that it travels

through

o Boundaries between the Earth’s layers

refract and reflect seismic waves

o P waves are compressional waves and

travel through all gases, liquids, and

solids, whereas S waves travel only

through solids

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The Path of a Seismic Wave

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Internal Structure of the Earth

The Earth is divided into three well

defined shells or zones:

o The Crust

o The Mantle

o The Core

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Internal Structure of the Earth

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Internal Structure of the Earth

http://en.wikipedia.org

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1. The Crust

o The uppermost solid shell of the earth

o It has a varying thickness:

o Under the oceans : 5-6 km

o Under the continents : 30-35

km

o Under the mountains : 60-70

km

o The Continental Crust is divided into

three layers: A, B and C

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Continental and Oceanic Crust

http://www.physicalgeography.net/fundamentals/10h.html

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The A Layer

o Also known as the upper layer

o Thickness is in between 2-10 km

o Mostly made up of sedimentary rocks

o This layer is of low density, 2.2 g/cc

o The P wave velocities range from 1.8

to 5.0 km/sec

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The B Layer

o Relatively denser, 2.4 to 2.6 g/cc

o Comprised chiefly of granites and

gneisses, hence called the Granite

Layer

o At places, the layer cures thickness >

20 km

o Seismic waves attain velocities from 5

to 6.2 km/s

o Referred to, as the SIAL layer

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The C Layer

o The lowermost layer of the Continental

crust

o Thickness is in between 25-40 km

o Mostly made up of basic minerals,

hence called SIMA layer

o This layer is of densities, 2.8 to 3.3

g/cc

o The P wave velocities range from 6 to

7.6 km/sec

o The Oceanic Crust is generally an

extension of the C layer

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SIAL and SIMA layers

P Singh

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2. The Mantle

o Refers to the second concentric shell of the

earth, that lies beneath the crust

o Starting from the lower boundary of the crust,

the mantle continues to a depth of 2900 km

o The mantle is divided into outer and inner

mantles, at about 900-1000 km below the earth

o A portion of the upper mantle, is plastic, rather

than solid, and is called the Asthenosphere.

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The Asthenosphere

o It is the highly viscous, mechanically weak,

ductilely deforming region of the upper

mantle

o It is thought to extend from 100km to

500km beneath the surface

o The temperature of the materials that

make up the Asthenosphere tend to be

just below their melting point

o The upper part of the Asthenosphere is

believed to be the zone upon which the

lithospheric plates of the earths crust

move about

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Internal Structure of the Earth

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3. The Core

o The core begins from a depth of 2900 km, and

extends to the center of the Earth

o It is divided into the outer and inner cores

o The outer core extends from a depth of 2900 km

to 4580 km below the Earth’s surface and

behaves like a liquid

o The inner core is believed to be soft metallic,

made up of nickel and iron alloys

o The density at the centre of the Earth is 13.0

g/cc

21P Singh

Internal Structure of the Earth

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Seismic Discontinuities

o The P and S waves reach the surface

seismographic stations, after being reflected

and refracted at various depths

o A major change in the velocity of these

waves at some specific depths indicate a

change in nature of the material at that

particular depth

o Such major changes are called Seismic

Discontinuities, they demarcate the internal

zones of the earth

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1. Mohorovicic Discontinuity

o Also known as the Crust-Mantle

discontinuity, or the Moho

o Occurs at a depth of 30-40 km below

continents and 5-6 km below oceans

o At these depths, both P and S waves

undergo a sharp increase in their

velocity

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P wave velocity in the Crust and upper Mantle

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2. Guttenberg Discontinuity

o Known as the Mantle-Core

discontinuity

o At a depth of 2900 km, the P waves

become sluggish, and S waves get

stopped

o This discontinuity between the Mantle

and the Core, is the Guttenberg

discontinuity

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Major Discontinuities

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3. Lehmann Discontinuity

o Refers to an abrupt

increase in P and S

wave velocities at

depths of 220-230

km

o It appears beneath

continents, but not

below oceans

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Paths of Seismic Waves

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Reference

Parbin Singh, Engineering and General

Geology, S K Kataria & Sons

Chenna Kesavulu, N, Textbook of Engineering

Geology, MacMillan India

Reddy, D V, Engineering Geology, Vikas

Publishing House

Garg, S K, Physical and Engineering Geology,

Khanna Publishers

Thompson, G R and J Turk, Introduction to

Physical Geology, Thomson Brooks/Cole