The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.
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Transcript of The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.
![Page 1: The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.](https://reader035.fdocuments.us/reader035/viewer/2022081514/56649f0d5503460f94c20f40/html5/thumbnails/1.jpg)
The deformation in the Plate Boundary zones
Shear Zone : San Andreas
-
Frédéric Flerit
![Page 2: The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.](https://reader035.fdocuments.us/reader035/viewer/2022081514/56649f0d5503460f94c20f40/html5/thumbnails/2.jpg)
The geometry of the Plates, and their position
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Three kind of ocean plate boundary
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Tectonics
The Pa-NA Boundary Zone
TOPOGRAPHYFault SYSTEM
Motion of
Pa/NA
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The NAF offsets the geology features : 200km
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100 km
The NAF Offsets mountain ranges
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The NAF Offsets rivers : 100m – 1km
200 m200 m
200 m
200 m
… m
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Long term deformation : lateral offsets at all scales
PA
NA
![Page 9: The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.](https://reader035.fdocuments.us/reader035/viewer/2022081514/56649f0d5503460f94c20f40/html5/thumbnails/9.jpg)
The plates are rigid and they are separated by plastic faults
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Two historical major Earthquakes
1906 San Andreas 1857 Big bend
average slip of 6 m.
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1906 San Andreas Eq 6m of lateral offset
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The NAF Offset orange trees rows : 6 m
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The profile of the co seismic displacement measured on the triangulation network of California
(Reid 1910)
100 km
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SouthCalifornia
San Andreas Big Bend San Andreas south
Los AngelesLos Angeles
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The velocity profile assoiated with the Plate motion
PA
NA
100 km
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How to reconcile the Long term deformation with the deformation produced by an earthquake ?
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The concept of Seismic Cycle
Two phases
1) interseismic loading
2) Coseismic relaxation
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The seismic cycle
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Consequence : The crust is elastic (load / relaxation)And the faults in the crust are plastic(permanent deformation)
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The concept of Locking depth
Seismic
-
Brittle
(10-20 km)
Aseismic
-
Ductile
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The velocity profile
Associated with the
Locking depth : d
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Problems
Draw three profiles of the velocity vectors across SAF?
How can we invert the locking depth of the SAF (depth of the transition brittle-ductile) ?
Draw a profile of the velocity vectors parallel to the SAF.
Define the Azimut of the Pa/NA plate motion in Los Angeles Region, What is the Azimut of The SAF ?
What can you tell about the components of GPS velocity vectors perpendicular to the Pa/NA direction ?
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Can you predict when and where the next one should occur
NB : the San Andreas Fault takes only 35 mm/yr
out of 50 mm/yr imposed by the plate motion
You will make the hypothese of the seismic cycle
Exercice on the Seismic risk on the San Andreas Fault
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The shear strain rates
Dx
v1 v2
yy = Shear strain rates
= Dvy/Dx
= (V1 –V2)/Dx
The deformation in the direction perpendicular to V
y
Draw a profile of the Shear Strain rate and conclude?
x
. .
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Question
Define the maximun strain on the SAF assuming that 200 years separate two successive earthquakes.
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Geometric interpretation ofshear strain rate
=change in angle
Dx
V1 v2
yy = Shear strain rates = tan()
y
x
. . .
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Geometric interpretation ofnormal strain
?
(shortening or lengthening)
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Geometric interpretation of normal strain
=Change in length
Dx
l1
l2
yy = normal strain = Dly /Dy
y
x
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xx yx
xy yy
The Matrix notation
( )
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NOTE
yx = xy
xx yy
( )
The strain matrix is symetric
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Deformation of the vector P ?
xx
yy Matrix:
P = Px
Py P
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The matrix product allow to resolve the components of the strain For a given direction P
xx
xx
That is Deformation of the vector P :
pxx Px + Py
Px + yy Py
Px
Py.P =
Along x
Along y
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Exercices :deform the above square and
circle using the following strainssupposed uniform
xx = 0.5
yy = 2
xy = 0.5
yx = 0.5
xx = -0.5 and yy = 0.5 and
xy = -0.5 and yx = -0.5
xy = 2 and yx = 2
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Displacement -
Velocities
Strain
-
Strain rates
To measure the rigid motion of the plates or
of individual points we use the concepts of :
To measure the deformation of the crust or of the lithosphere
we use the concepts of :
The math object associated is a vector
The math object associated is a matrix
REMEMBER
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SouthCalifornia
San Andreas Big Bend San Andreas South
Parallel and
tangeantcomponents
of the plate
motion resolved on the 1) SAFand on the2) Big bend
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The perpendicular component is associated with the creation of topography with inverse faults
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IN MAP view
Lateral motion :
Strike-Slip Fault
Perpendicular motion :
Inverse Fault
Perpendicular motion :
Normal Fault
V
n
![Page 50: The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.](https://reader035.fdocuments.us/reader035/viewer/2022081514/56649f0d5503460f94c20f40/html5/thumbnails/50.jpg)
Resoling the tangeant and perpendicular component
• nXV=O
•Tangential slip rate t = n.V
Strike-Slip Fault
Perpendicular motion :
• n.V=0•perpendicular slip : e =p.V
Vn
n
np
![Page 51: The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit.](https://reader035.fdocuments.us/reader035/viewer/2022081514/56649f0d5503460f94c20f40/html5/thumbnails/51.jpg)
The concept of Azimut to define the orientation of the plate
boundary in the horizontal plane
nNorth
Azimut
Horizontal plane
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GPS velocities
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Their Kinematic - motion
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Divergent plate boundary condition in Atlantic
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The segmentation of the mid oceanic ridge (MOR)
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