Fault types

A fault is a surface across which there has been motion.

The forces at any point can be resolved into three orthogonal principal stresses (σ), and there will be no shear stresses (τ) in that orientation.  While the forces will be 3D in nature, we can generally look at them in the plane with the minimum and maximum principal stresses, and the intermediate stress will come in/out of the page.  You must be careful to understand if you are looking at a plan (map) view, or a cross section.

In the earth, there is no tension below a very shallow depth in the crust.  This is fortunate, because like concrete, rocks are extremely weak in tension but very strong in compresssion.  To a rock exposed to a very large  σmax the much smaller  σmin will provide the easy way out and the rock will extend in that direction even though it is actually being squeezed infrom that direction.

Fault type

Wikipedia Figure

σmax

σmin

Plate setting

Deformation

Earthquakes Beach ball diagram (nodal planes)

Strike slip

horizontal

horizontal

Conservative plate boundary like San Andreas,

Ridge transforms

 

Major, for example along the San Andreas Fault

Normal

Cross section view

vertical

horizontal

Mid Ocean Ridges, Basin and Range

Extend and thin crust

Younger on older rocks

Omits section

Generally minor, especially along ridges with hot, young (and weak) crust, and nothing to be damaged

Reverse/thrust

Cross section view

horizontal

vertical

Subduction zones, fold and thrust belts

Shorten and thicken crust

Older on younger rocks

Duplicates section

Major (include tsunamis)
Megathrusts are major hazard
Oblique slip

Block diagram view

      Combines strike slip with either normal or reverse motion   

The pattern of first arrivals of the earthquake waves (compressional or extensional) allow creation of two potential focal mechanisms ("beach balls"). These divide a sphere into 4 quadrants, with black showing compression and white extension. Two focal planes divide the quadrants, and one of these will be the fault plane. The seismic waves do not differentiate among the two, but examination of the local geology can often reveal which is the fault plane.  For pure normal and thrust faults, the two focal planes will have the same strike.

 

For a strike slip fault, the beach ball will indicate whether the fault is right or left lateral.  The motion will be toward the black back (compressional) quadrant. 

This will be a left lateral fault (stand at the fault plane facing the other side, and you would have to go to the left to find the offset features).  It is left lateral for either focal plane, and the earthquake data alone does not allow selection of the fault plane, which requires additional information.  For a strike slip fault, the distribution of foreshocks and aftershocks will usually align with the fault plane, and the mapped orientation of faults in the region will also often help to pick the correct plane.

For a thrust fault, particularly in a subduction zone, the gently dipping plane is generally the fault surface.

Last revision 10/4/2018