Fig. 6

Houston plots with two bins, positive and negative, for each tidal index in each region. See Fig. 3b for how to read the figure. For each region, the number of earthquakes \({N}_{{\text{EQ}}}\) meeting the threshold magnitude \({M}_{{\text{th}}}\) of 1.3 is shown along with \({N}_{{\text{BG}}}\) which is the number sampled at 15-min intervals during four days before and four days after each earthquake, \({N}_{{\text{BG}}}=769 \times {N}_{{\text{EQ}}}\). The first value in each panel is the absolute value of the difference between the \({N}_{{\text{obs}}}/{N}_{{\text{exp}}}\) ratios in the two bins (corresponding to the slope of the blue line); the slope becomes greater as the event occurrence distribution deviates from the background distribution. The second value in each panel is the Chi-square value. In panels showing a Chi-square value of 3.8415 or higher (framed in red), the null hypothesis is rejected at a significance level of 5%