1. The last of the CIRCULAR GRAPHS which will HAUNT YOU ALL QUARTER January 13, 2005 [Many thanks to H. Bob]

2. Stress - Real Time

6. Map of global seismicity

7. Definitions: FRICTION Same as sliding blocks from physics class (kinetic friction) Determines slope of Coulomb failure envelope For unconsolidated materials,  = angle of internal friction = angle of repose. For rocks,  = coefficient of friction of sliding surfaces

8. Definitions: COHESION For whole rocks: C = breaking strength. For broken surfaces: C = static friction. For unconsolidated sediment: C = ZERO. Like “static friction” from physics class

9. No shear stress on a FREE SURFACE

10. If stresses were everywhere aligned with the earth’s surface…

11. But if stresses are angled at depth… Then they rotate to alignment near the surface!

12. Andersonian Theory of Faulting: The optimum shear fracture plane is always oriented at 60° to  1, 30° to  3,  2 is in the plane. Simple idea but VERY USEFUL - Provides link between faults and stresses - present and past.

13. Rules of Thumb “Thinning Direction” parallel to  1 “Thickening Direction” parallel to  3 Simple model - no motion along  2 SIMPLIFICATION! For now anyway. Remember the sandbox model from last week’s lab…

14.  1 horizontal,  3 vertical

16.  1 horizontal,  3 horizontal

18.  1 vertical,  3 horizontal

20. The last of the CIRCULAR GRAPHS which will HAUNT YOU ALL QUARTER And Now, As Promised…

21. Plane intersects sphere = Arc!

22. Convention: use the “Southern” Hemisphere of an imaginary sphere

23. Let’s start with a vertical plane cutting the hemisphere. It intersects on an ARC.

24. Now let’s look down from the North Star…

25. Assign a coordinate system to get us oriented… N S W E

26. Reduce the clutter to a circle. N S W E Now let’s graph a plane onto it! Vertical plane - straight line through the pole Strike=45

27. How about a horizontal plane? N S W E Strike=?? It sits through the equator - all lines are horizontal so there’s no unique strike line

28. How about plane 0/25W? N S W E Strike=0 Or plane 0/55W?

29. How about plane 45/25NW? N S W E Strike=45 How about plane 330/25NE ? Strike=330 PLANES INTERSECT AT A POINT

30. stereonet Template on which we can draw our circles: stereonet