Long-Delayed Aftershocks in New Zealand and the 2016 M7.8 Kaikoura Earthquake P. SHEBALIN 1 and S. BARANOV 2 Abstract—We study aftershock sequences of six major earth- quakes in New Zealand, including the 2016 M7.8 Kaikaoura and 2016 M7.1 North Island earthquakes. For Kaikaoura earthquake, we assess the expected number of long-delayed large aftershocks of M5? and M5.5? in two periods, 0.5 and 3 years after the main shocks, using 75 days of available data. We compare results with obtained for other sequences using same 75-days period. We esti- mate the errors by considering a set of magnitude thresholds and corresponding periods of data completeness and consistency. To avoid overestimation of the expected rates of large aftershocks, we presume a break of slope of the magnitude–frequency relation in the aftershock sequences, and compare two models, with and without the break of slope. Comparing estimations to the actual number of long-delayed large aftershocks, we observe, in general, a significant underestimation of their expected number. We can suppose that the long-delayed aftershocks may reflect larger-scale processes, including interaction of faults, that complement an iso- lated relaxation process. In the spirit of this hypothesis, we search for symptoms of the capacity of the aftershock zone to generate large events months after the major earthquake. We adapt an algorithm EAST, studying statistics of early aftershocks, to the case of secondary aftershocks within aftershock sequences of major earthquakes. In retrospective application to the considered cases, the algorithm demonstrates an ability to detect in advance long- delayed aftershocks both in time and space domains. Application of the EAST algorithm to the 2016 M7.8 Kaikoura earthquake zone indicates that the most likely area for a delayed aftershock of M5.5? or M6? is at the northern end of the zone in Cook Strait. 1. Introduction A major earthquake of M7.8 occurred near the coast of New Zealand on 13 November 2016. The earthquake has initiated a significant tsunami with amplitude more than 4 m. The earthquake fault was located in about 100 km from the fault zone of the Canterbury earthquake sequence. The Canterbury sequence started on 3 September 2010 (Mw7.2) near Darfield. An earthquake of Mw6.2 occurred in the Eastern part of its fault zone about 6 months later, on 21 February 2011. The epicenter was located in Christchurch, the second most populous city in New Zealand, and the earthquake caused huge damage including about 200 casualties. Further large events with magnitude 5.5 and higher occurred in the area on 13 June 2011, on 22 December 2011, and on 14 February 2016. The proximity of the 2016 M7.8 and 2010 M7.1 fault zones raises a question: should we expect a similar scenario, with a set of successive large long-delayed aftershocks? Various methods were developed recently aimed to an operational forecasting of aftershocks (Ger- stenberger et al. 2005; Omi et al. 2013, 2016; Steacy et al. 2014; Cattania et al. 2014). Some aftershock forecasting models are being tested in real time in the New Zealand earthquake forecast testing center (Gerstenberger and Rhoades 2010). Most of those models are based on the idea of independent com- bining of well-known Gutenberg–Richter and Omori–Utsu relations, the idea first proposed by Reasenberg and Jones (1989). All those models are designed to assess the expected rates of seismic events in specified space–time–magnitude volumes. An important direction in development of the earth- quake rate models is the ETAS model (Ogata 1983) and its modifications, including applications in New Zealand (Harte 2014). Another important new ten- dency is combining different models, including hybrid statistical and physics-based models (Rhoades 2013; Rhoades et al. 2014, 2016; Shebalin et al. 2014; Cattania et al. 2014). The Canterbury earthquake sequence has strongly reactivated the interest to the problem of forecasting aftershocks. Recently, a retrospective analysis of 1 Institute of Earthquake Prediction Theory and Mathemati- cal Geophysics, Russian Academy of Sciences, Moscow, Russia. E-mail: p.n.shebalin@gmail.com 2 Kola Branch of the Geophysical Survey of Russian Acad- emy of Sciences, Apatity, Russia. Pure Appl. Geophys. Ó 2017 Springer International Publishing AG DOI 10.1007/s00024-017-1608-9 Pure and Applied Geophysics