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 strain accumulation is hard to measure geodetically, large earthquakes are infrequent, vary in location and time, and often occur on previously unrecognized faults. The spatial and temporal seismicity patterns reflect long-term tectonic stress/strain fields, which depend on factors including topography, lithospheric structure, coupling to sub-lithospheric mantle flow, and inherited structural and compositional features. With a variety of tectonic processes influencing the stress/strain field, 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, are smaller than the maximum magnitude in the parent catalog, , 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 even if larger earthquakes occur, 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 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.