Lecture3 Plate Tectonics Part 2

8/12/2019 Lecture3 Plate Tectonics Part 2

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PLATE TECTONICS

Part -II

Lecture-3

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A fracture (crack) in the earth, where the two sides move past each other and the

relative motion is parallel to the fracture.

90 dip = vertical fault plane

0 strike = north parallel fault plane

Fault

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Source: wikipedia


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Surface Trace of a fault

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Source: USGS public domain


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Different Fault Types

shear)

n)

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A Normal dip sl ip fault

hanging wall moves down

Normal Dip-slip fault

5Source: google images


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A reverse dip-slip fault

Hanging wall moves upThis is also called a Thrust Fault.

Reverse Dip-slip fault

6Source: google images


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A strike-slip fault

Displacement in horizontal direction

Strike-slip fault

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Source: google images


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Strike-Slip Fault Left Lateral

8Source: USGS public domain


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Strike-Slip Fault Right Lateral



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Displacement in both vertical and

horizontal directions

An oblique-slip fault

Oblique-slip fault

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Blind/Hidden faults



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Faults and Plate Boundaries

Normal faults are associated with divergent plate boundaries

Animation of divergent boundary

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Reverse faults are associated with convergent plate boundaries

Animation of convergent boundary

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Strike-slip faults are associated with transform plate boundaries

Animation of transform boundary

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Elastic Rebound Theory

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After the great 1906 San Francisco

earthquake, Harry Fielding Reid

examined the displacement of the

ground surface around the San

Andreas Fault. From his observations

he concluded that the earthquakemust have been the result of the

elastic rebound of previously stored

elastic strain energy in the rocks on

either side of the fault. In an

interseismic period, the Earth's

plates move relative to each other

except at most plate boundaries

where they are locked.


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After the great 1906 San Francisco

earthquake, Harry Fielding Reid

examined the displacement of the

ground surface around the San

Andreas Fault. From his observations

he concluded that the earthquakemust have been the result of the

elastic rebound of previously stored

elastic strain energy in the rocks on

either side of the fault. In an

interseismic period, the Earth's

plates move relative to each other

except at most plate boundaries

where they are locked.


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The elastic rebound theory explains how energy is spread during

earthquakes. As plates on opposite sides of a fault are subjectedto force and shift, they accumulate energy and slowly deform.

When the stresses exceed the internal strength of the rock, a

sudden movement occurs along the fault, releasing the

accumulated energy, and the rocks snap back to their originalundeformed shape.

This theory was discovered by making measurements at a

number of points across a fault. Prior to an earthquake it was

noted that the rocks adjacent to the fault were bending. Thesebends disappeared after an earthquake suggesting that the

energy stored in bending the rocks was suddenly released during

the earthquake.

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Original alignment

of points

Alignment of pointsafter accumulation of

elastic strain

Final alignment of

points

Fault

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Elastic Rebound

The animated picture shows a road, a fence, and a line of trees crossing a

fault. As the rocks adjacent to the fault are deformed, stresses build up in

rock, rupture occurs when the shearing stresses induced in the rocks

exceed the shear strength of the rock, followed by sudden slip, releasing

energy that causes destruction. 20


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Sequence of elastic rebound: Stresses

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Sequence of elastic rebound: Bending

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Sequence of elastic rebound: Rupture

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Sequence of elastic rebound: Rebound

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Sequence of Elastic Rebound

Tectonic plates move relative to each other

Elastic strain energy builds up in the rocks along fault planes

Since fault planes are not usually smooth, great amounts ofenergy can be stored (if the rock is strong enough) as

movement is restricted due to interlock along the fault. Stresses (force/area) are applied to a fault.

Strain (deformation) accumulates in the vicinity of friction-locked faults.

When the shearing stresses induced in the rocks on the faultplanes exceed the shear strength of the rock, rupture occurs.

Rupture continues over some portion of the fault. Slip is thedistance of displacement along a fault.

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Rock Deformation and Earthquakes

Earthquakes result from motion along faults

Earthquakes represent the brittle failure of rock and hencethey occur in upper crust, where the temperature andpressure are relatively low.

Not all motions on faults produce earthquakes. Rocks mayalso creep if the faults are too weak to store up the energy ofprolonged stress.

Elastic rebound theory explains deformation before andduring earthquakes as brittle failure following theaccumulation of elastic strain.

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Kramer (1996) Geotechnical Earthquake Engineering, Prentice Hall.

Udias, A. (1999): Principles of Seismology, Cambridge University Press,

Cambridge.

Shearer, P. M. (1999): Introduction to Seismology, Cambridge University

Press, Cambridge.

Ben Menahem, A. and Singh, S. J. (1980): Seismic Waves and Sources,

Springer-Verlag, New York.

Cox, A. and Hart, R.B. (1986): Plate Tectonics - How it Works, Palo Alto,

California, Blackwell Scientific Publications, 392 p.

http://pubs.usgs.gov/gip/dynamic/dynamic.html(Accessed on 25 September

2012)

References

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http://pubs.usgs.gov/gip/dynamic/dynamic.htmlhttp://pubs.usgs.gov/gip/dynamic/dynamic.html

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Lecture3 Plate Tectonics Part 2