A description of earthquake source geometry, often strike, dip and rake for a double-couple shear fault. First-motion polarities constrain the radiation sign pattern, with a fundamental interchange ambiguity between fault and auxiliary planes.
The negative and positive principal directions of a double-couple moment tensor. They bisect the nodal-plane quadrants, but are not automatically the principal axes of regional stress. A stress inversion needs additional faulting assumptions and multiple mechanisms.
Interchanging the slip direction and fault normal leaves a double-couple moment tensor unchanged. First-motion polarities therefore cannot identify the physical fault plane without information such as mapped rupture, aftershock geometry or geodetic displacement.
An imagined sphere around a seismic point source on which source-to-station ray takeoff directions are plotted. A lower-hemisphere equal-area projection commonly displays first-motion polarities and nodal planes.
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Focal mechanism, also known as a fault mechanism or "beachball" diagram in seismology, refers to the orientation and movement along fault planes that cause an earthquake. It is a graphical representation that simplifies the complex three-dimensional motion of seismic waves generated by the rupture during an earthquake. The focal mechanism depicts the types of faulting that can occur, such as: 1. **Normal Faulting**: This occurs when the Earth's crust is extended.