The determination of focal mechanisms using P- and S- Wave data
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The determination of focal mechanisms using P- and S- Wave data

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    The National Oceanic and Atmospheric Administration computes focal plane solutions using P-wave first motion, S-wave polarization angles, and combinations of the two. These calculations have been programmed for a CDC-6600 computer. A new method is used, based on a maximum-likelihood argument implemented by the generation of a rigorously derived score surface on a discrete grid.The combined score is of the form a(P-score) + b(S-score) where a and b and the score values are derived by a maximum—likelihood argument.For the P-solution and S-solution, the computer system also produces contoured fiducial regions about the poles of the focal planes on an equal-area projection of the lower focal hemisphere. This permits an evaluation of the quality of the solutions and makes possible the comparison of solutions for different earthquakes. These contoured limits have a probabilistic interpretation as 95-percent fiducial regions, a term explained in the context of the general problem of the construction of confidence regions. A new graphical method for P-wave solutions makes possible the construction of exact boundaries of the regions of possible variation. The Aleutian earthquake of May 14,19 69, is analyzed, and the solution shows good agreement between the observed P-wave first motions and the S-polarization angles. This analysis gives practical support to the validity of combined solutions.
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