In a recent paper Dahlen (1974) has used a simple model of shallow-focus earthquake faulting to study the ratio of S to P corner frequencies. The model predicts higher S than P corner frequencies, but, as he points out, observational studies have indicated the reverse.

It has been shown by Burridge (1973) and Burridge and Levy (1974) that rupture speeds up to the P-wave speed are to be expected on a purely frictional fault lacking cohesion but in Dahlen's calculations the rupture speed never exceeds the speed of S.

In this paper, we consider far-field radiation when the initial phase of rupture is a self-similar circular rupture zone spreading at the speed of P. For this model, we find that on 70 per cent of the focal sphere the P corner frequency is higher than that of S in accordance with the observations.

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