Reading Fault Plane Solutions

Interpreting fault plane solutions can be a little tricky. There are several things to keep in mind when converting a symbol (like the one at lower left) to a sense of slip and fault plane orientation.

First, remember that the fault plane solution is generally given as a two-dimensional projection of the lower hemisphere of a focal mechanism sphere, not just an overhead view of the outside of that sphere. (Sometimes the outer surface of the sphere is used, but in such cases, that will be noted.) Also keep in mind that the lines crossing the circle represent the intersection of two perpendicular planes with a sphere. If you have information about the orientation of the fault plane, then there is only one possible interpretation of the symbol. Otherwise, either plane may represent the fault plane, and both possibilities should be noted. And finally, regardless of the colors used in the fault plane solution, dark ("filled") areas always represent compression (first motion toward the observer) and white ("blank") areas show dilatation (first motion away from the observer).

If you can determine which nodal plane on a fault plane solution corresponds to the orientation of the geologic fault plane, you know that the other plane must be the auxiliary plane. Because this plane is oriented perpendicular to the direction of slip, its point of intersection with the fault plane (and the lower surface of the sphere) provides you with information about the relative proportions of dip slip and strike slip involved in the fault rupture. The closer the point of intersection to an endpoint of the line representing the fault plane, the greater the amount of dip slip. If the line of the auxiliary plane bisects the fault plane's line, this represents pure strike slip.

The viewer at left will take you through the entire slip-sense "spectrum", showing a representative focal mechanism for each of sixteen combinations of dip slip and strike slip. Note how the fault plane in these examples varies somewhat in dip, while its strike is always oriented exactly north-south. These symbols show only a small number of the possible combinations of slip senses and fault-plane orientations, but they should cover a wide enough range to help you interpret fault plane solutions in general.