caption: Frequency--magnitude distributions for event clusters associated with fracturing stages in Wells 1 to 4. Each line corresponds to one cluster. The resulting b-values are clearly bimodal, with most clusters having b > 1.5 (coloured blue/green), and a smaller number of clusters having b < 1.5 (coloured yellow/red). The four low b-value clusters are those shown in Fig. 3, which make up the activity in reactivation periods 1 and 2.