Early Tectonic Ideas
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Herodotus of Halicarnassos(484
BC-ca. 425 BC), now part of
Turkey, observed seashells in the
hills of Egypt and concluded that
they had lain beneath the sea.
Predecessors Anaximander and
Xenophanes are quoted as
making similar statements.
Thus the first aspect of tectonics
to be noted was vertical motion.
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Wegener, in
1924, proposed a
reconstruction
of Pangaea,
based primarily
on geometric fit.
From USGS “This Dynamic Earth, Kious and Tilling,1996
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Smith and Hallam, 1970, generated
fits which were consistent with
structural and paleontological data.
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Sequence of key ideas
1924 Wegener: Continents drift
 1962-1963 Harry Hess (Princeton) and Bob Dietz
(San Diego): Seafloor spreads and carries
continents with it.
 1965 J. Tuzo Wilson: Transform fault idea solves
mystery of ridge offsets and fracture zones. Uses
name “plates”.
 1967 Lynn Sykes: Fault mechanisms support
transform fault idea.
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Current Plate Geography
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Velocities from the Global Positioning
System confirm that movement continues
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The signs left on continental
cratons by plate tectonic
activity are not as clear as we
see when we have todays
maps of the ocean basins
before us.
The remains of unsubducted
fragments of oceanic
lithosphere are stuck onto the
edges of continents and
represent many cycles of
basin openings and closing
(Wilson cycles). These were
not so easy to interpret.
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Key Theoretical Advance:
Transform faults
1965- J. Tuzo Wilson, A new class of faults and their
bearing on Continental Drift
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This explained: Segmentation of Mid-Atlantic Ridge,
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Long linear features (Fracture Zones) in Pacific described by Menard
Seismic
Proof:
1967 Lynn Sykes,
Mechanism of
Earthquakes and
Nature of Faulting
on the Mid-Oceanic
Ridges
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Provided real-time
evidence for Plate
Tectonics in
addition to historic
evidence from
continent shapes
and magnetic
anomaly paterns.
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Earthquakes give us
information about
their sources
through the waves
they radiate.
Surface waves (L,R)
are bound to the
surface, while body
waves travel through
the interior.
Body waves can be
polarized.
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Elastic rebound
mechanism
(a) Initial state
(b) During strain
(c)After earthquake
Figure from Stein and Wysession, 2003
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Many faults are not as simple as the strike-slip example in the
previous slide. The geometric variables which define fault
plane geometry and relative motion are defined below.
Figure from Stein and Wysession, 2003
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Other simple geometries and slip angles.
Figure from Stein and Wysession, 2003
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Up motion occurs in the compression quadrant
Down motion occurs in the dilation quadrant
The little doughnuts show where the rays whose seismograms
are shown exit from the box.
The following several slides are from Stein and Wysession, 2003
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For a small fault, the fault plane and the auxiliary plane are
indistinguishable
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The amplitude depends on
the angle of the vector to the
receiver as seen from the
fault.
For P waves this radiation
pattern can be represented as
a beach ball, which represents
a FOCAL SPHERE around
the source.
For S waves it is more
complicated because of the
polarization.
The lower panel shows the
direction of first motion for S
waves on a sphere.
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This slide shows the radiation
pattern from a source which
is called a double couple,
since it is composed of two
vector pairs instead of one.
It conserves angular
momentum, which is usually
a good thing.
Here the right panels show
the direction of first motion
for both P and S waves.
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In the real earth, we make observations on the surface, which
we can map back to the focal sphere by ray tracing, since we
know the earth's velocity structure (of which more later).
This means that the fault and auxiliary planes will map onto the
earth's surface in a diagnostic pattern, as these rays are
mapped.
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One of Sykes plots from MAR
transform faults
Black dots indicate
compressional first
motion
 Open circles indicate
dilatational motion
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Projections are ways of displaying all or part of the surface of
the spherical (or more complicated) earth on a flat surface,
such as a piece of paper.
Useful properties of projections are that they be
(1) conformal, that is, that for small areas, angles are preserved.
So a right angle on the earth's surface is represented as a right
angle on the map.
(2) equal-area. So areas are conserved.
Unfortunately, we cannot have both simultaneously.
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A more fanciful description
From
Six Books of Optics,
useful
for
philosophers
and
mathematicians alike
François
d'Aiguillon,
1613
Figure by Peter Paul
Rubens
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To visualize the
fault and
auxiliary planes
onto a flat
piece of paper,
we use a
stereographic
net.
The azimuth
coordinate is
used to plot
strike and,
naturally, dip is
for the dip.
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Here are three
fault planes
mapped onto the
lower
hemisphere.
All NS striking
planes will be
meridians.
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To draw planes with different strikes,
just rotate the line.
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To plot a plane
normal
(perpendicular) to
plane A, rotate the
net until A is on a
meridian.
The pole for plane A
will be 90 degrees
from its “equator”.
All planes through
this pole will be
normal to plane A.
Then any meridian
in a rotated net, if it
passes through the
pole of A, will be
normal to A.
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Plotting Points
The direction a ray leaves the focus is
defined by its azimuth from north, and its
dip (measured from horizontal) or its
takeoff angle, measured from vertical.
To plot a ray on the focal sphere, use its dip
to plot it on the equator of the net. This
would be correct if the azimuth were east
(090), then rotate the point as shown here.
This not the same as following the
meridian.
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The compressional area of the P
radiation pattern is, by
convention, shown as black.
The direction of motion on the
fault depends on which plane is
the fault plane.
Choose a fault plane, then
imagine standing in the white
area. Then follow the directions
of Darth Vader and “come to the
dark side”*. You will then be
following the motion of the earth
on that side of the fault.
* Noted by Vince Cronin
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Here are beachball representations
of some faults with combinations of
dip-slip and strike-slip behavior.
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The fault geometry also controls
the radiation of surface waves,
since they are composed of
interference patterns of body
waves.
Here are radiation patterns of
Love waves (made of SH body
waves) and Rayleigh waves (made
of P and SV waves).
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The classical method for finding focal
mechanisms is to assemble
seismograms which observe a quake
and trace their rays back to the focal
sphere, plotting the points as filled
circles if the first motion is dilational
(down) and solid if up (compressional).
By examination, two planes are fit
dividing the compressional and
dilational quadrants. Needless to say,
the results of this process are do not
always define planes uniquely.
Now mechanisms are usually derived
using moment tensor inversion, about
which more later.
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We have looked at focal
mechanisms from the point of
view of earth motions. These
motions have clear interpretations
in the case of the strike-slip fault.
These motions are, however, not
immediately indicative of the
forces which produce them.
So we introduce the pressure and
tension axes.
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