Transition from aftershock to normal activity: The 1965 Rat Islands earthquake aftershock sequence

A transition from aftershock to normal activity is identified 2200 days after the occurrence of the 1965 Rat Islands earthquake of M_w = 8.7. After this time, we observe a significant departure of the seismicity in the focal region from the extrapolated trend of the aftershock activity predicted by the modified Omori formula proposed by Utsu in 1961. The maximum likelihood estimate of the parameters for the formula and the selection of a statistical model based on the Akaike Information Criterion show that the whole aftershock sequence is best modeled by the combination of an ordinary aftershock sequence and secondary aftershock activity associated with the largest aftershock. Fitting of the activity after the transition period to the modified Omori formula results in an unusually low value of the decay constant. We interpret that this activity represents normal or background seismicity and not another secondary aftershock sequence. Defining background activity in a focal region of a large earthquake is essential for detection of seismic quiescence prior to the next major event.