How are focal mechanism models created? The actual work and calculations are not simple, but the general process is relatively easy to explain. This page will explain the basic steps; the animation at right provides a visual demonstration.
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First, though focal mechanisms represent spheres, in this walk-through, we'll be making a two-dimensional, lower-hemisphere projection, like the kind you might see plotted on a seismicity map. So imagine cutting the focal mechanism sphere in half, removing the top half, and shining a light on the interior from directly above, so that the lower hemisphere projects as a circle onto a flat surface below. This projection is typically called a fault plane solution.
Now we need to plot the first motions recorded from our earthquake. To do this, we check the waveforms (not shown here) recorded by various seismic stations in the area, and mark the first arrivals as "up" (compressional) or "down" (dilatational). Each station's location relative to the hypocenter is then projected onto our circular diagram with a symbol representing the type of motion, up or down, first recorded there. Stations that fall within the "missing" upper hemisphere (above the horizontal) are translated appropriately onto our lower-hemisphere projection.
Once the first motions are correctly plotted, it is time to solve for the two nodal planes. The sphere is divided into quadrants using two perpendicular planes that best fit the set of first motions. Again, since the two-dimensional plot shows only the projection of the inner surface of the lower hemisphere, those planes will look like two intersecting lines within a circle. Though the two nodal planes must intersect at the hypocenter -- the center of the sphere -- the intersection of these lines, since it occurs on the sphere's surface, need not be in the center of our circular projection.
After the nodal planes have been identified, the symbol is complete,
but of somewhat limited use, because either of the two nodal planes could be
the fault plane. Other types of data can be useful in determining
which plane is the fault plane, and consequently, what type of slip
occurred in the earthquake. Knowing the geology of the area will
help you identify the most likely fault geometry. Aftershocks can
be of great assistance, too; as seen in
Activity #5 of Section 1, aftershock sequences
can provide valuable hints about fault plane orientation at depth.
Though not always feasible, the use of these insights can lead
to a final determination of the fault plane solution.
