lecture3 plate tectonics part 2
TRANSCRIPT
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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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Source: USGS public domain
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A Normal dip sl ip fault
hanging wall moves down
Normal Dip-slip fault
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A reverse dip-slip fault
Hanging wall moves upThis is also called a Thrust Fault.
Reverse Dip-slip fault
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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
9Source: USGS public domain
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Displacement in both vertical and
horizontal directions
An oblique-slip fault
Oblique-slip fault
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Source: google images
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Blind/Hidden faults
11Source: USGS public domain
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Faults and Plate Boundaries
Normal faults are associated with divergent plate boundaries
Animation of divergent boundary
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Source: USGS public domain
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Faults and Plate Boundaries
Reverse faults are associated with convergent plate boundaries
Animation of convergent boundary
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Source: USGS public domain
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Faults and Plate Boundaries
Strike-slip faults are associated with transform plate boundaries
Animation of transform boundary
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Source: USGS public domain
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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.
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.
Elastic Rebound Theory
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Source: google images
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Elastic Rebound Theory
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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Elastic Rebound Theory
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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Source: USGS public domain
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Sequence of elastic rebound: Bending
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Source: USGS public domain
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Sequence of elastic rebound: Rupture
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Source: USGS public domain
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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