Transcript Introduction to Earthquakes EASA

Lecture #06Plate Motions
Lecture-6
1
Tectonic Plates are Rigid
“Caps” not Flat, Planar Sheets
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Plate Motions on a Sphere
 Mathematically
speaking, the most general
way to describe the movement of a rigid
plate (cap) on a sphere is through rotation
about a fixed pole
is known as Euler’s Fixed Point
Theorem
 The
Lecture-6 3
Plate Motions on a Sphere
The pole of rotation for
plate motion is an
abstract mathematical
concept, it is not a
physical feature. The
point representing the
pole is known as an
Euler pole.
Lecture-6 4
Euler Poles
Euler poles for plate
motions are different than
the geographic North and
South Poles.
The Euler pole for the
motion of a plate is often
“located” on another
plate
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Example of Euler pole
describing the relative
motion of Africa from a
fixed, North American
reference frame.
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Euler Poles and Ridges
Lecture-6 7
Euler Poles and Ridges/Transforms
Ridge Segments “point”
towards the Euler pole
Corresponding transform
faults are arcs of circles
having the Euler pole as a
center.
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GPS Derived Plate Motions
Where is the Euler Pole ?
Lecture-6 9
Relative Plate Motions
 It
is important to remember that plate
motions are generally given relative to a
reference frame.
 Example:
The half-spreading rate vs. full spreading
rate at a MOR.
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Relative Plate Motions
Hypothetical three
plate example: We
know the relative
velocity of B and C;
we also know the
relative velocity of A
and B. What is the
relative velocity of A
and C???
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Relative Plate Motions
 To
solve this problem we note the basic law
relating relative plate velocities:
aVc
= aVb + bVc
 The
velocity of plate C relative to plate A
equals the velocity of plate B relative to
plate A plus the velocity of plate C relative
to plate B.
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Remember That Velocities
are Vector Quantities
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Remember That Velocities
are Vector Quantities
(This is an example of adding two vectors graphically.)
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Solution to Problem
The velocity of plate C relative to plate A has a direction
that is perfectly horizontal, and a magnitude of
(3)(1.414) cm/yr.
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Plate Motion Summary




Tectonic plates are spherical caps
The motion of tectonic plates is best
described as rotation about an Euler pole
Plate motion is generally generally given
with respect to a fixed reference frame
The velocities of plate are vectors, they
have a magnitude and direction
Lecture-6 16