Beach Ball and Fault Scarp

Basic techniques are covered here.   This example shows an earthquake along the Wasatch Front in Utah, north of Salt Lake City. 

Location of  earthquake focal mechanism.  The location is reported to the nearest 0.01, which is about 1 km.  This is the precision, which means that even if the location were perfectly accurate, the best we could say is that it lies within a circle with a diameter of about 1 km.  If the earthquake was small, or there are not a large number of stations nearby, or the earthquake is very old, the location uncertainty could be substantially greater.  The depth is listed as 7.9 km, which should probably only be taken to mean that it is very shallow; the trace would project to the surface displaced from the epicenter (greater displacement for shallow fault dips and deeper focus).

The location in a valley adjacent to a mountain front strongly suggests a range front scarp. It would also suggest that the west dipping plane is the fault plane, and the east dipping plane is the auxiliary plane.

Slopes over 20 degrees, which occur mostly along the range front any aspect.

"Calculate" menu, Slopes, Excessive slope.  This exaggerates the number of points but rapidly find points that exceed a particular slope.  This can help to pick slopes to show on the Focal plane analysis.

Note here that the range front is steeper than the regions to the east.

Slopes over 35 degrees, any aspect.  Only the steepest part of the range is this steep.

Locations for cross sections shown below.

  •  Topographic Profile  (below)  shown by the red line.
  • Multiple parallel profiles.  The purple/cyan profile was drawn, and then 5 parallel profiles were drawn on each side.

You always want to draw the cross sections perpendicular to the range front.

Topographic Profile with a protractor shown.  The range front here dips about 30 to the WNW.
Multiple parallel profiles.  The green profile was selected, and then 5 parallel profiles were drawn on each side.  The red line shows the average profile


Last Update: 11/28/2016