Focal Mechanisms

Seismologists refer to the direction of slip in an earthquake and the orientation of the fault on which it occurs as the focal mechanism. They calculate it using information from seismograms, and typically display it on maps with a "beach ball" symbol.

The beach ball is created by identifying the orientation of the first wiggle from an earthquake's P wave on a seismogram. Where the first wiggle, or "first motion," is up, the ground is initially being pushed towards that seismometer (compression). Where the first motion is down, the ground is initially pulled away from the seismometer (dilation). This is illustrated in the following animation:

An animation detailing the process by which an earthquake's focal mechanism is identified. *Animation from IRIS/EarthScope*.

Upward (compressional) first motions are shaded dark on the focal mechanism, and downward (dilational) first motions are left unshaded. By mapping first motions from a number of stations around the earthquake's hypocenter (the point underground where the earthquake begins), patterns emerge that indicate the orientation of the fault the earthquake occurred on and its faulting mechanism. Certain types of faulting result in distinct focal mechanisms, as the diagram below shows.

A diagram showing above/bird's-eye views of the focal mechanisms of five different fault orientations, including reverse, normal, strike-slip, low-angle reverse, and oblique faults. *Diagram from Charles Ammon, Penn State*.

With focal mechanisms calculated from first motion directions, as well as from some methods that model waveforms, there is an ambiguity in identifying the fault plane on which slip occurred. In each of the focal mechanisms above, there are two lines that cross the circles. Each line represents a plane in three dimensions, and either could be the fault that produced the earthquake. One line is the actual fault, and the other is called the "auxiliary plane," which is oriented perpendicular to the fault plane and has no physical significance.

In order to determine which plane is the actual fault plane, geologic context is needed. For well-known faults like the San Andreas, the true fault plane is the one that aligns with the known fault. For less-well-resolved faults, using focal mechanisms from multiple earthquakes can reveal the true orientation of the fault. The following diagram provides examples of how two different fault orientations can produce the same focal mechanism.

A schematic diagram showing A) the side and above/bird's-eye views of focal mechanisms, and B) 3D fault diagrams alongside bird's-eye diagrams of focal mechanisms for strike-slip, normal, reverse, and oblique reverse faults. *Diagram from U.S. Geological Survey*.