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The United States Geological Survey explains lower-hemisphere focal mechanisms, compression shading, and the ambiguity between the two candidate fault planes.

Extension accommodated by a hanging wall moving down relative to the footwall. Its focal mechanism has an extensional dip-slip pattern, with the centre unshaded under the convention that shaded quadrants denote compression.

Fault slip with both dip-slip and strike-slip components. The focal mechanism can have an off-centre nodal intersection, with the central polarity indicating its dip-slip sense under a specified shading convention. A reverse component with horizontal shear is compatible with transpression.

For a double-couple seismic source, measure P-wave first-motion polarity signs and use source-to-station takeoff angles from the location and velocity model to plot them on a focal sphere. Fit the alternating radiation-sign quadrants, optionally adding amplitude ratios and waveform information. The two nodal planes determine a focal mechanism, conventionally expressed by fault strike, fault dip and fault rake.

A more general waveform inversion fits a seismic moment tensor to several components and stations. Its double-couple part supplies candidate slip and normal vectors, but a general moment tensor should not be forced to be double-couple without justification. Polarity calibration, near-nodal observations, path effects and takeoff-angle errors limit the result.

Point-source radiation alone interchanges the fault plane and auxiliary plane without changing the double couple. Resolve this fault-plane ambiguity using mapped rupture, aftershock geometry, geodesy or finite-rupture directivity. A dense set of polarity picks does not by itself remove that fundamental ambiguity.

Shortening accommodated by a hanging wall moving up relative to the footwall. The centre of its dip-slip focal mechanism is compressional under the usual lower-hemisphere convention. Low-angle reverse faulting is commonly called thrust faulting.

Predominantly horizontal relative motion along a fault. A pure strike-slip double couple has two vertical nodal planes; a focal mechanism gives both possible planes, and independent evidence selects the actual one.