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 dependence of radiated P wave or S wave amplitude and polarity on direction from an earthquake source. For an ideal double couple it has nodal directions, so amplitude differences between stations do not measure distance or directivity alone.
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.