Faulting Geometry

Faulting is a complex process and the variety of faults that exists is large. We will consider a simplified but general fault classification based on the geometry of faulting, which we describe by specifying three angular measurements: dip, strike, and slip.

Dip:

  • In Earth, faults take on a range of orientations from vertical to horizontal.
  • Dip is the angle that describes the steepness of the fault surface. T
  • his angle is measured from Earth’s surface, or a plane parallel to Earth’s surface.
  • The dip of a horizontal fault is zero (usually specified in degrees: 0°), and the dip of a vertical fault is 90°.
  • Some old mining terms are used to label the rock “blocks” above and below a fault. If you were tunneling through a fault, the material beneath the fault would be by your feet, the other material would be hanging above you head. The material resting on the fault is called the hanging wall, the material beneath the fault is called the foot wall.
dip

Strike:

  • The strike is an angle used to specify the orientation of the fault and measured clockwise from north.
  • For example, a strike of 0° or 180° indicates a fault that is oriented in a north-south direction, 90° or 270° indicates east-west oriented structure.
  • To remove the ambiguity, always specify the strike such that when you “look” in the strike direction, the fault dips to you right.
  • If the fault is perfectly vertical you have to describe the situation as a special case.
  • If a fault curves, the strike varies along the fault, but this is seldom causes a communication problem if you are careful to specify the location (such as latitude and longitude) of the measurement.
strike

Slip:

  • Dip and strike describe the orientation of the fault, we also have to describe the direction of motion across the fault.
  • That is, which way did one side of the fault move with respect to the other.
  • The parameter that describes this motion is called the slip.
  • The slip has two components, a “magnitude” which tells us how far the rocks moved, and a direction (it’s a vector).
  • We usually specify the magnitude and direction separately.
  • The magnitude of slip is simply how far the two sides of the fault moved relative to one another; it’s a distance usually a few centimeters for small earthquakes and meters for large events.
  • The direction of slip is measured on the fault surface, and like the strike and dip, it is specified as an angle.
  • Specifically the slip direction is the direction that the hanging wall moved relative to the footwall.
  • If the hanging wall moves to the right, the slip direction is 0°; if it moves up, the slip angle is 90°, if it moves to the left, the slip angle is 180°, and if it moves down, the slip angle is 270° or -90°.
strike