Online material: Software source code; examples; sample plots.

The graphical representation of a focal mechanism as a plot of the radiation pattern on the focal sphere (sometimes referred to as “beachball diagram”) is well established. It consists of a spherical projection on a unit sphere centered at the source point of a seismic event. The sign of the P-wave radiation pattern is shown as colored areas on the sphere (as described, e.g., by Udías 1999; Stein and Wysession 2003; and Crotin 2004).

For pure shear motion on a plane, the focal sphere may be fully...

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