Increased probability of large earthquakes near aftershock regions with relative quiescence

Abstract

It is shown that after a large earthquake of M6 class or over has taken place, another event of similar size or larger is relatively more likely (in the sense of rate per unit area) to occur in the near field than in the far field. In particular, we show that the aftershock activity of the first event provides useful information for assessing the probability of a following large event in the neighborhood (within a distance of a few degrees). Namely, if the aftershock activity from the first event becomes relatively quiet compared to the normal decay, the occurrence rate of larger events in the neighborhood is several times higher during the first decade after the main shock than would be the case for normal aftershock activity. For measuring such phenomena precisely, we need to model the normal aftershock activity. In fact, many aftershock sequences are more complex than the simple inverse power decay represented by the modified Omori formula. The epidemic type aftershock sequences (ETAS) model is a generalized version of the modified Omori formula, which fits well with various aftershock sequences, including nonvolcanic type swarms. Using this model, aftershock sequences are investigated for the 76 main shocks that occurred in and around Japan during the last three quarters of a century. The focus is placed on objective examination of whether or not, in an aftershock sequence, there exists a significant change point followed by relative quiescence, that is, a significant lowering of the seismicity from that predicted by the ETAS model. Relative quiescence can take place regardless of the seismicity level. The extent of such quiescence, if it exists, is seen from the diagrams of cumulative numbers versus the transformed occurrence times based on the predicted occurrence rate by the estimated ETAS model. It is thus demonstrated that relative quiescence can be a helpful factor in detecting anomalous aftershock activity. This can then be used to forecast whether or not a large event is more likely to follow shortly (within ∼6 years) in the neighborhood (within a distance of 3°) of the initial event.