The Triggering of Aftershocks: Part II: Normal Mode Periodicities in the 1989 Loma Prieta (California) Aftershock Sequence
In Part I of this paper it was shown that the spectrum of the aftershock sequence of the 1989 Loma Prieta (California) earthquake did not show any peaks at diurnal and semidiurnal tidal periods. In Part II we report on the discovery of 55.4, 43.2 and 27.6 minute periodicities in the same aftershock sequence. The 55.4 minute period is close to the period of the spheroidal mode, 0S2 and half of this period is 27.6 minute. The 43.2 minutes corresponds to the toroidal mode, 0T2. This implies that the free oscillations, excited by the main shock, are triggering some of the aftershocks. Speculations on the possible trigger mechanisms are presented.
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