GEOL 240Lg 1st Edition Lecture 7 Outline of Last Lecture I Polar Wander Curves Paleo North Poles for Europe and North America II Review of what is an Earthquake III What are fault strikes IV Stress in Earthquakes Outline Current Lecture I The three principle stresses II Definition of total stress III Maxium Compressive Stress IV Normal Fault V Reverse Faults VI Strike Slip Faults VII Principal stress planes VIII Thrust Planes Current Lecture s 1 s 2 s 3 s 1 maximum compressive stress axis s 2 intermediate stress axis s 3 minimum compressive stress axis Mutually perpendicular stress is a measure of the internal forces acting within a deformable body Quantitatively it is a measure of the average force per unit area of a surface within the body on which internal forces act At every point in a stressed body there are at least three planes called principal planes with normal vectors called principal directions where the corresponding stress vector is perpendicular to the plane i e parallel or in the same direction as the normal vector and where there are no normal shear stresses The three stresses normal to these principal planes are called principal stresses Define the total stress at a point We can resolve that in 3 perpendicular stresses the maximum compressive stress always pushing inwards from all directions all stresses inside the earth are always compressive because it has rock above it 1 the direction into where rocks are being pushed 2 minimum intermediate what happens in a normal fault Reverse faults A geologic fault in which the hanging wall has moved upward relative to the footwall Reverse faults occur where two blocks of rock are forced together by compression Strike slip faults motion is horizontal ll strike of the fault Strike slip faults are classified according to the direction of motion of the blocks on either side of the fault If the block on the opposite side of a strike slip fault has moved to the left it is a left lateral strike slip fault If it has moved to the right it is a right lateral strike slip fault The relative motion left or right is the same regardless on which block an observer stands The famous San Andreas Fault in California is a right lateral strike slip fault Anderson s theory The basis of the theory is the assumption that the surface of the Earth must be a principal plane of stress containing two of the principal stress directions Hence the third principal stress direction must be oriented normal to the Earth s surface Anderson combined this idea with the Coulomb law of failure to explain the orientations of conjugate sets of faults within the Earth In each case s2 is always parallel to the line of intersection of the two faults principal stress perpendicular a principal plane principal plane a plane with no shear stress acting along it The earth s surface is a principal plane it means that by definition one of them is vertical and the other two have to be perpendicular to that but we don t know there orientation What the direction of horizontal extension a trust fault Is a special form of reverse fault that deeps less than 45 degrees all trust faults are reverse fault the nearest fault to us is right beneath us 3km down and it is a trust fault Nature is lazy at a divergent plate are moving apart one of the stresses is vertical which one are you going to push away from sigma 3