What is a focal mechanism? A focal mechanism is a model of the exact orientation and sense of slip of the fault rupture that generates an earthquake. This model can be described very simply with a sphere, cut by two perpendicular nodal planes that intersect in the center, dividing the sphere into four equal quadrants. Consequently, focal mechanisms look something like beach balls (see image at right). One of the two nodal planes represents the orientation of the fault plane that generated the earthquake. The other is the auxiliary plane, a plane perpendicular to the line describing the direction of slip in the earthquake (this line must lie on the fault plane). On a two-dimensional surface, the sphere of a focal mechanism is usually represented by a circle, the projection of one half of that sphere. The center of the sphere represents the earthquake's hypocenter; the center of a circular projection represents its epicenter.
Focal mechanisms really only describe the motion involved
at the start of a rupture -- the hypocenter (also called the
focus, hence their name) -- because they are calculated
using the very first wave arrivals from an earthquake. In an
earthquake large enough to involve several kilometers of fault
rupture, slip will sometimes "evolve", changing in sense and/or
orientation as the rupture propagates. This can happen in response
to changes in fault geometry or rupture boundary conditions.
In such a case, the entire fault rupture may not exactly match the model
supplied by the focal mechanism, but the focal mechanism will still
provide insight into the initial rupture behavior at the hypocenter.
Since focal mechanisms are constructed using first motions, the direction of the first deflection recorded by a seismometer as that instrument experiences the initial arrival of seismic waves, it takes a large number of seismometers in the area surrounding the hypocenter to produce a reliable focal mechanism. Gaps in coverage will increase the uncertainty in the model. The next page will explain how focal mechanisms are assembled from seismic data.