The last of the CIRCULAR GRAPHS which will HAUNT YOU ALL QUARTER January 13, 2005 [Many thanks to H. Bob]
Stress - Real Time
Map of global seismicity
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
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
No shear stress on a FREE SURFACE
If stresses were everywhere aligned with the earth’s surface…
But if stresses are angled at depth… Then they rotate to alignment near the surface!
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.
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…
1 horizontal, 3 vertical
1 horizontal, 3 horizontal
1 vertical, 3 horizontal
The last of the CIRCULAR GRAPHS which will HAUNT YOU ALL QUARTER And Now, As Promised…
Plane intersects sphere = Arc!
Convention: use the “Southern” Hemisphere of an imaginary sphere
Let’s start with a vertical plane cutting the hemisphere. It intersects on an ARC.
Now let’s look down from the North Star…
Assign a coordinate system to get us oriented… N S W E
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
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
How about plane 0/25W? N S W E Strike=0 Or plane 0/55W?
How about plane 45/25NW? N S W E Strike=45 How about plane 330/25NE ? Strike=330 PLANES INTERSECT AT A POINT
stereonet Template on which we can draw our circles: stereonet