GEOL 240Lg 1st Edition Lecture 7Outline of Last Lecture I. Polar Wander Curves/ Paleo-North Poles for Europe and North AmericaII. Review of what is an Earthquake?III. What are fault strikes?IV. Stress in Earthquakes Outline Current Lecture I. The three principle stressesII. Definition of total stressIII. Maxium Compressive StressIV. Normal Fault V. Reverse FaultsVI. Strike- Slip FaultsVII. Principal stress planes VIII. Thrust PlanesCurrent 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 pointWe 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 pushed2) 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 faultStrike-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 planeprincipal 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 andthe 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 faultthe 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 apartone of the stresses is vertical which one are you going to push away from… sigma