Focal Sphere
Focal Sphere : Hypothetically thought (brain-constructed) sphere around a seismic point source (i.e., a source with linear dimensions smaller than the diameter of the focal sphere). It is used for calculating the take-off angles under which the seismic rays leave the source through the surface of this sphere towards the seismic stations and to reconstruct the fault-plane solution (i.e., the type and spatial orientation of the earthquake rupture) from the stereographic projection of these ray "penetration points" through the focal sphere and their associated first-motion polarities, polarisation and amplitude ratios
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