Record-Breaking Earthquake Intervals in a Global Catalog and an Aftershock Sequence

Inter-occurrence times are the time intervals between successive earthquakes with magnitudes greater than a specified value in a specified region. A record-breaking inter-occurrence time interval is defined to be larger (or smaller) than all previous intervals; a starting date must be specified. In this paper, we show that the succession of record-breaking intervals for random background seismicity is very different from the succession for aftershocks. Specifically, we consider the number of record-breaking interval times as a function of the number of events that have occurred. We first consider earthquakes on a world-wide basis with moment magnitudes greater than 5.5 for the years 1977 to 2006. We determine the number of record-breaking earthquakes, nrb, during 10,592 successive periods of 1024 events and average the results for sub-intervals. For a random (iid) process, the prediction is nrb ~ ln(n) where n is the number of events. Our results are in excellent agreement with this prediction. We next consider the sequence of record-breaking time intervals in the aftershock sequences following the 2004 Parkfield earthquake. As the aftershock sequence decays in time, it is expected that there will be a strong increase in both the numbers and durations of record-breaking intervals. For the example considered, we find that nrb ~ n is a good approximation. Thus, there is a strong statistical difference between the behavior of aftershocks and background random seismicity.