Are megaquakes clustered

We study statistical properties of the number of large earthquakes over the past century. We analyze the cumula-tive distribution of the number of earthquakes with magni-tude larger than threshold M in time interval T, and quan-tify the statistical significance of these results by simulating a large number of synthetic random catalogs. We find that in general, the earthquake record cannot be distinguished from a process that is random in time. This conclusion holds whether aftershocks are removed or not, except at magnitudes below M = 7.3. At long time intervals (T = 2-5 years), we find that statistically significant clustering is present in the catalog for lower magnitude thresholds (M = 7-7.2). However, this clustering is due to a large number of earthquakes on record in the early part of the 20th century, when magnitudes are less certain. 1