This page explains the concept of focal mechanisms, commonly referred to as "beachballs".
A focal mechanism, or "beachball", is a graphic symbol that indicates the type of slip that occurs during an earthquake: strike-slip, normal, thrust (reverse), or some combination. It also shows the orientation of the fault that slipped. The 2-dimensional focal mechanism circle is really the projection of the fault orientation and slip on the lower half of a sphere surrounding the hypocenter, the location of the earthquake in the crust.
Let's look at the simplest example, a strike-slip fault. A pure strike-slip fault would have a vertical plane, and would look like this on a map (2D) view and on a focal mechanism.
The darker-shaded quadrants of the focal mechanism indicate motion toward those quadrants; the lighter-shaded quadrants indicate motion away from those quadrants. The sense of slip on the fault and on the focal mechanism are shown with arrows. The fault on this example focal mechanism could be either of the red lines shown in the two focal mechanism. Additional information, such as aftershock locations or a previously-mapped fault, are needed to determine which of the two possible solutions is the correct one.
Now let's look at a normal fault and a thrust (reverse) fault below. These are harder because the fault is not vertical, but instead dips at an angle beneath the surface.
Of course, most earthquakes are not pure strike-slip, normal, or thrust motion, but some combination, so focal mechanisms tend to look more like the example at the bottom. This one is for an earthquake with mostly strike-slip motion and a little component of normal motion.
A focal mechanism is determined by either:
- looking at the initial vertical deflection (up or down ) of the waveform recording at each seismogram that indicates what direction the first motion was at each instrument,
- fitting the entire waveform to a model.
Check out the focal mechanism animation/short video with more details from the Incorporated Research Institutes for Seismology (IRIS) at Focal Mechanisms Explained.