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There has been considerable discussion in recent years of the kind of stress changes that occur at the focus of an earthquake as fracture takes place. Two mechanisms consisting of stress patterns in the form of a single couple with moment (nucleus I) and a double couple without moment (nucleus II) were first proposed by Nakano 1923. (Here we use the notation of Honda 1957.) Nakano showed that the distributions of first motions of earthquake wave arrivals can be used to investigate the mechanism concerned, since the P-wave distributions for the nuclei are identical but the S-wave distributions are different.
Michael A. Chinnery (Tue,) studied this question.