Focal Mechanism

The focal mechanism represents the geometry of fault rupture during an earthquake (see Fault Plane).

The focal mechanism represents the geometry of fault rupture during an earthquake (see <a href="#Fault Plane"><i>Fault Plane</i></a>).

It is based on the polarity of the first arrivals of P and S waves recorded by a network of far-field seismic stations.

3 basic types of focal mechanisms are distinguished:

• strike-slip fault: vertical fault plane and relative horizontal displacement of the two blocks divided by the fault;
• reverse or thrust fault: the Earth’s crust is in extension along a dipping fault plane, with the hanging wall moving downwards relative to the footwall;
• normal fault: the Earth’s crust is in compression along a dipping fault plane, with the hanging wall moving upwards relative to the footwall.