Focal mechanism

A representation of an earthquake’s fault-plane orientation and slip direction, inferred from the seismic waves it generates. Its beach-ball diagram shows the possible fault planes and the compressional and dilatational regions of first-motion radiation.

First-motion focal mechanism: A focal mechanism inferred from whether initial seismic waves arrive as compressions or dilatations at seismic stations. It uses the polarity observations that divide a beach-ball diagram into compressional and dilatational regions.
Earthquake: A sudden release of energy in Earth's crust or mantle that generates seismic waves. The mechanism describes the rupture source that generates the waves.
Beach ball (seismology): A circular diagram of an earthquake focal mechanism, with shaded and unshaded regions marking opposite first-motion polarities. It is the common visual form used to display a focal mechanism.
Strike-slip fault: A fault on which the dominant motion is horizontal and parallel to the fault’s strike. Its predominantly horizontal slip contrasts with dip-slip mechanisms.
Moment tensor: A mathematical representation of a seismic source’s strength, orientation, and deformation pattern. Its double-couple form encodes the fault orientation and slip represented by a focal mechanism.
Fault (geology): A fracture or zone of fractures along which rocks have moved relative to one another. The mechanism depicts the orientation and motion of a fault during rupture.
Focal-mechanism solution: An estimate of an earthquake’s source orientation and slip derived from seismic observations. It is the interpreted result produced from polarities or waveform data.
Normal fault: A fault on which the hanging wall moves downward relative to the footwall. Its extension-related motion produces a different mechanism pattern from reverse faulting.
P-wave: A compressional seismic body wave that travels through solids and fluids. Its first-motion polarity is a principal observation used to infer the mechanism.
Strike and dip: The orientation of a planar geological surface, specified by its horizontal direction and inclination. These angles describe each candidate fault plane in a focal mechanism.

Linked from 3 pages

Volcano seismologyRelated: It helps distinguish volcanic fracture patterns from other sources of seismicity.