Fig. 21

Cross recurrence plots and joint recurrence plot for CNST_e (top frame) and KAIK_e (bottom frame: with downward trend removed) signals, (a) 772 days before the Kaikōura event. b 72 days before the Kaikōura event. c 2 days before the Kaikōura event (d) 1 day before the Kaikōura event, (e) 3 days after the Kaikōura event. The ratios of the characteristic frequencies for CNST_e and KAIK_e signals are large so that the LOSs are straight vertical lines. Figures b, c and d appear almost identical but show the gradual increase in the presence of a low recurrence region on the right hand side of the plot as the Kaikōura event is approached. f Joint recurrence plot