David King, Donald V. Helmberger; Time functions appropriate for some aftershocks of the point Mugu, California earthquake of February 21, 1973. Bulletin of the Seismological Society of America ; 65 (1): 127–132. doi:
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Broad-band recordings of aftershocks of the Pt. Mugu earthquake at small epicentral distances provided an excellent opportunity to test source models for small earthquakes. Simple events recorded at nearly vertical incidence produced a single P-wave pulse of a duration of about 0.07 sec and a somewhat more complicated S wave with a slightly longer duration. Such events are consistent with a point dislocation source for which Qβ = 100 or for which there is directivity with the fault breaking downward. We attribute the more usual complexities of small earthquake records to multiple events, some of which we observed, layering effects combined with greater epicentral distances, and scattering.