The nature of force systems at the foci of earthquakes can be studied by analyzing initial longitudinal (P) and transverse (S) displacements produced by them on the surface of the earth. The force system described in this paper results from a superposition of three mutually orthogonal double forces which act at a point focus. A family of equations is derived which depends only on S polarization angles and not on initial P displacements to determine the orientation of this generalized force system. An IBM 1620 computer has been programmed to solve the family of equations for two particular focal mechanisms—the single couple (Honda's Type I) and the double dipole (Honda's Type II). Two possible force systems are thus calculated for each earthquake using only S angles. The appropriate mechanism for each earthquake is selected by comparing the distribution of initial P displacements actually recorded, with that predicted from the solutions of the mechanism equations making use of S data.

Computer solutions are presented for 32 earthquakes for which data are available in the literature. The orientation of the force system for each earthquake calculated from S data alone is in general agreement with that determined from P data.

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