An instrumental method is described for the determination of the extent of faulting in a given earthquake which depends upon the early arrival of S. If the faulting velocity is known, the exact extent is determined; otherwise, only upper and lower limits are given. Conversely, if the extent of faulting is known from field or other evidence, the faulting velocity is determined.

The development given refers to a vertical fault exhibiting horizontal displacement, or, more generally, to a fault in which the displacement is parallel to the direction of extension. When the displacement is perpendicular to the direction of extension, as in a simple vertical fault, the faulting impulse is propagated by shear waves. For this condition the shear waves from the end point can never arrive earlier than the wave from the focus. On the basis of these considerations an observed early arrival of S indicates that the source is a fault movement in which the displacement is parallel to the direction of extension. In general this would refer to a horizontal extension of fault slip, although for observers in the vicinity of the epicenter it could refer to a vertical fault movement originating in the depths and propagating upward.

In applying this method to the Long Beach earthquake, the available evidence indicates that faulting extended from the focus approximately to Signal Hill, a distance of 27 km., with a faulting speed of approximately 4.2 km/sec.

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