Fig. 3

Analysis of seasonal trends in seismicity around the New Madrid Seismic Zone (NMSZ). a Histogram (in 2-month bins) for the number of earthquakes in the complete catalogue between 1 January 2000 and 31 December 2015 for the entire NMSZ region, as shown in Fig. 1a by the red box. Colours are indicative of the magnitude cut-off used in each case. b Ratio of the number of earthquakes occurring in the 4-month period encompassing January, February, March, April, to those occurring in July, August, September, October as a function of cut-off magnitude. Grey shaded areas indicate the magnitude of completeness. Dashed and solid black lines indicate the 99 and 95% confidence limits, respectively. Black points are those where the ratio exceeds the 95% confidence limit. c Results of a Jack-knife analysis of the seasonal trend. Lines indicate the residual at each magnitude band between the calculated ratio and the 95% confidence limit. Red line is for the full catalogue, as shown in b. Black lines are for the same catalogue with successive years of data removed. Confidence limits are estimated independently for each test. Grey area again indicates the magnitude of the completeness of the catalogue. d–f are as a–c, but for the seismicity catalogue after declustering