Have we seen the largest earthquakes in eastern North America?

The assumed magnitude of the largest future earthquakes, Mmax, is crucial in assessing seismic hazard, especially for critical facilities like nuclear power plants. Estimates are made using various methods and often prove too low, as for the 2011 Tohoku, Japan, earthquake. Estimating Mmax is particularly challenging within tectonic plates, where large earthquakes are infrequent, vary in location and time, and often occur on previously unrecognized faults. For example, it is unclear whether the short historical record includes the largest possible earthquakes along the eastern continental margin of North America. We explore this issue by generating synthetic earthquake histories and sampling them over a few hundred years. Due to the short histories, the maximum magnitudes appearing most often in a sub-catalog, Mmaxa, are often smaller than the maximum magnitude in the parent catalog, Mmaxp, that can occur. Future earthquakes along the continental margin may thus be significantly larger than those observed to date. More generally, these simulations demonstrate that the largest earthquake in a catalog likely reflects a combination of catalog length, a region's earthquake productivity, and relative proportion of small to large events. For regions with low seismicity, small variations in b value, the ratio of large to small events, due to sampling has a significant impact on the expected recurrence times of large magnitude earthquakes. Although the precise likelihood of observing Mmaxp depends on the distribution of recurrence times, a catalog shorter than an earthquake's mean recurrence time will likely not contain an event of that size. As a result, Mmax cannot always be reliably estimated from earthquake catalogs.