The deformation in the Plate Boundary zones Shear Zone : San Andreas - Frédéric Flerit

The geometry of the Plates, and their position

Three kind of ocean plate boundary

Tectonics The Pa-NA Boundary Zone TOPOGRAPHYFault SYSTEM Motion of Pa/NA

The NAF offsets the geology features : 200km

100 km The NAF Offsets mountain ranges

The NAF Offsets rivers : 100m – 1km 200 m … m

Long term deformation : lateral offsets at all scales PA NA

The plates are rigid and they are separated by plastic faults

Two historical major Earthquakes 1906 San Andreas 1857 Big bend average slip of 6 m.

1906 San Andreas Eq 6m of lateral offset

The NAF Offset orange trees rows : 6 m

The profile of the co seismic displacement measured on the triangulation network of California (Reid 1910) 100 km

South California San AndreasBig BendSan Andreas south

The velocity profile assoiated with the Plate motion PA NA 100 km

How to reconcile the Long term deformation with the deformation produced by an earthquake ?

The concept of Seismic Cycle Two phases 1)interseismic loading 2)Coseismic relaxation

The seismic cycle

Consequence : The crust is elastic (load / relaxation) And the faults in the crust are plastic (permanent deformation)

The concept of Locking depth Seismic - Brittle (10-20 km) Aseismic - Ductile

The velocity profile Associated with the Locking depth : d

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 ?

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

The shear strain rates Dx v1 v2    yy = Shear strain rates = Dv y /Dx = (V1 –V2)/Dx The deformation in the direction perpendicular to V y Draw a profile of the Shear Strain rate and conclude? x..

Question Define the maximun strain on the SAF assuming that 200 years separate two successive earthquakes.

Geometric interpretation of shear strain rate = change in angle Dx V1v2    yy = Shear strain rates = tan(  ) y x ...

Geometric interpretation of normal strain ? (shortening or lengthening)

Geometric interpretation of normal strain = Change in length Dx l1 l2  yy = normal strain = Dl y /Dy y x

 xx  yx  xy  yy The Matrix notation   ( )

NOTE    yx =  xy  xx    yy   ( ) The strain matrix is symetric

Deformation of the vector P ?  xx    yy Matrix:   P = Px Py P

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 :  p  xx P x +  P y  P x +  yy P y P x P y.   P = Along x Along y

Exercices : deform the above square and circle using the following strains supposed 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

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

South California San AndreasBig BendSan Andreas South Parallel and tangeant components of the plate motion resolved on the 1) SAF and on the 2) Big bend

The perpendicular component is associated with the creation of topography with inverse faults

IN MAP view Lateral motion : Strike-Slip Fault Perpendicular motion : Inverse Fault Perpendicular motion : Normal Fault V n

Resoling the tangeant and perpendicular component n X V=O Tangential slip rate t = n.V Strike-Slip Fault Perpendicular motion : n.V=0 perpendicular slip : e =p.V V n n n p

The concept of Azimut to define the orientation of the plate boundary in the horizontal plane n North Azimut Horizontal plane

(1)coordinates of rotation pole for pair of plates (2)angular velocity

GPS velocities

Their Kinematic - motion

Divergent plate boundary condition in Atlantic

The segmentation of the mid oceanic ridge (MOR)