Aftershock Sequences of the Recent Major Earthquakes in New Zealand

Abstract

The three clusters of the epicenters of the nine recent (1993–2018) earthquakes of magnitude 7.0 or larger in New Zealand are located in three different tectonic environments of the Australia–Pacific Plate boundary, including the southern part of the Kermadec Trench (showing rapid westward subduction), the oblique collision zone between the Pacific Plate and Indo-Australian Plate with the dominant Alpine Fault (showing right-lateral strike-slip movement), and the Puysegur Trench (showing eastward oblique subduction). From the viewpoint of the unified scaling law for earthquakes (USLE), these regions are characterized by different levels of seismic rate (A), earthquake magnitude exponent (B), and fractal dimension of epicenter loci (C). The recent major earthquakes exemplify different scenarios of aftershock sequences in terms of either the dynamics of interevent time (τ) or the USLE control parameter (η = τ × 10B×(5−M) × LC), where τ is the time interval between two successive earthquakes, M is the magnitude of the second one, and L is the distance between them. We find the existence, in the long term, of different, intermittent levels of rather steady seismic activity characterized by near-constant values of mean η (〈η〉), which, in the mid-term, switch between one another at times of critical transitions, including those associated with all but one magnitude 7.0 or larger earthquake. At such a transition, seismic activity may follow different scenarios with interevent time scaling of different kinds. Evidently, although these results based on analysis of an individual series do not support the presence of universality in seismic energy release, they provide constraints on modeling realistic seismic sequences for earthquake physicists and supply decision-makers with information for improving local seismic hazard assessments.