Focal mechanism

The focal mechanism of an earthquake describes the deformation in the source region that generates the seismic waves.

Focal mechanisms are derived from a solution of the moment tensor for the earthquake, which itself is estimated by an analysis of observed seismic waveforms.

The focal mechanism can be derived from observing the pattern of "first motions", whether the first arriving P waves break up or down.

The pattern of energy radiated during an earthquake with a single direction of motion on a single fault plane may be modelled as a double couple, which is described mathematically as a special case of a second order tensor (similar to those for stress and strain) known as the moment tensor.

This is an essential part of monitoring to distinguish between earthquakes and explosions for the Comprehensive Test Ban Treaty.

A simple example of a successful test of the hypothesis of sea floor spreading was the demonstration that the sense of motion along oceanic transform faults[4] is opposite to what would be expected in classical geologic interpretation of the offset oceanic ridges.

[5] Fault plane solutions also played a crucial role in discovering that the deep earthquake zones in some subducting slabs are under compression while others are under tension.