Definition
Focal mechanisms are geometrical or mathematical representations of the faulting during an earthquake. In very simple terms, the latter consists of the relative motions of two blocks of Earth called walls along a planar surface called fault. Figure 1 shows that the description of an earthquake rupture needs three angles. The strike angle ϕ is the azimuth (with respect to North) of the trace of the fault on a horizontal plane such as the Earth’s surface; the dip angle δ characterizes the steepness of the fault, and the rake or slip angle λ, the direction of motion, within the fault plane and relative to the horizontal, of the hanging wall relative to the foot wall. A full description of the earthquake rupture requires an additional scalar, related to the length of slip of the fault walls, and thus to the size or magnitude of the event.
(a, b) Before and after sketches of the displacement of fault blocks during an earthquake. The fault...
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Okal, E.A. (2021). Earthquake, Focal Mechanism. In: Gupta, H.K. (eds) Encyclopedia of Solid Earth Geophysics. Encyclopedia of Earth Sciences Series. Springer, Cham. https://doi.org/10.1007/978-3-030-58631-7_158
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