Fault Scarps

A fault scarp is a slope created by motion along a fault.  The largest scarps will form along range front normal faults, where the footwall forms the mountain and hanging wall forms an adjacent valley.

The best scarps will form along normal faults, because:

In unconsolidated sediments or weak rocks, the maximum angle will be the angle of repose, which is typically around 30.  While these sediments can support very small vertical slopes, these will occur only in active erosional areas such as stream channels, or the most recent active scarps, which will rapidly erode back to the angle of repose.

Often hills will be asymmetrical in cross section, with one side steeper than the other.  In this case the steeper side will generally be the candidate for the fault scarp.

Fault traces cannot continue forever as straight lines on a curved earth, and fault ruptures rarely affect more than a few tens of km.  Fault traces will curve, and displacement will die out laterally, perhaps to be replaced by motion on another fault.  Fault blocks are complex three dimensional geometric features, and must obey the law of conservation of matter/mass.  Nature also abhors a vacuum, and faults cannot open up a hole underground.  Thus our notion of dip and strike for a fault, implying a planar fault surface, will probably be a useful local simplification of reality.

The direction of lighting in a reflectance view of a DEM determines which faults will be emphasized.  You can adjust the lighting to best showcase the faults; faults of different orientations may require different lighting.

The slope shown by digital elevation models depends in part on the data spacing.  The larger the DEM grid spacing, the more generalized the topography becomes and the gentler the recorded slopes.

The west (left) side of this mountain block is steeper than the right, and is the more likely to be the fault scarp.

This range front scarp in hard bedrock has several distinct segments.

  • The upper segment has gentler slopes.  Because the faulting event with each earthquake probably moves the earth less than 10 m, this range front would have required 40 or 50 earthquakes.  The oldest parts of the scarp have had the longest to be eroded on the earth's surface.
  • A middle stretch, which was measured here with a 50 slope, might more closely approximate the fault plane because it has had less time for erosion.
  • The talus slope at the base of the range reflects the buildup of material eroded from higher up the slope.
  • This pattern leads to what is known as a wine glass valley, which can show up very clearly in a reflectance view map.

Measure slope from  Topographic Profile

Insure that you mentally consider vertical exaggeration in evaluating the profiles.

Fault scarp example

last revised 10/11/2016