Focal Mechanisms - Pacific Northwest Seismic Network

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 animated diagram demonstrating how earthquake focal mechanisms are constructed from many seismograms of the same earthquake. Parts of a wave spread out from the epicenter leading with a compressional signal, and other parts lead with a dilational signal. — 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.

Five overhead views of the focal mechanisms for different types of faults. Each diagram consists of a circle with the cardinal directions labeled around its circumference, and two curved lines that represent the fault plane and the auxiliary plane. The orientation for these two planes changes for each type of fault, and between them the two sides of the planes are shaded to differentiate them. The first is a reverse fault, in which the fault plane and the auxiliary plane curve towards each other, creating a football-shaped shaded section in the center. The second is a normal fault, which has the same orientation of the fault plane and auxiliary plane as the reverse fault, making a football shape, but the shading is now outside of the football. This indicates that for normal faults, the hanging wall is moving downwards instead of upwards as in the reverse fault. The third is a strike-slip fault, where the planes are straight and perpendicular to each other, forming an 'X' across the center of the circle and creating four equal sections with alternating colors. The fourth and fifth low-angle reverse and oblique focal mechanisms each have curved planes that are slightly offset from the center. The oblique fault's planes are shifted downwards, indicating the bidirectional movement they undergo. — 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.

Titled "A schematic diagram of a focal mechanism." The first two diagrams detail the side and bird's-eye views of focal mechanism diagrams. The side view consists of a semicircle that is sliced by the auxiliary plane and the fault plane at perpendicular angles to each other. The bird's-eye view consists of a circle also sliced by the auxiliary plane and the fault plane, now curved towards each other, forming a 'football' shape in the center of the circle. The other part of the diagram shows two 3D diagrams of each type of fault (strike-slip, normal, reverse, and oblique reverse) alongside their bird's-eye views. — 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*.