HAVE WE SEEN THE LARGEST EARTHQUAKES IN EASTERN NORTH AMERICA?

The assumed magnitude of the largest future earthquakes, M_max, is crucial in assessing seismic hazard, especially for critical facilities like nuclear power plants. Absent any theoretical basis, estimates are made using various methods and often prove far too low, as for the 2011 Tohoku, Japan, earthquake. Estimating M_max is particularly challenging within tectonic plates, where large earthquakes are infrequent compared to the length of the available earthquake history, vary in space and time, and often occur on previously unrecognized faults. For example, it is unclear whether the largest possible earthquakes along the eastern continental margin of North America have occurred, or whether the apparent differences between the largest events alog the US and Canadian east coasts should be viewed as real. We explore this issue by generating synthetic earthquake histories and sampling them over a few hundred years. The maximum magnitudes appearing most often in the simulations are essentially those observed, and smaller than the simulation maxima. 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 the length of the catalog, even if larger earthquakes occur. Although the precise fraction depends on the distribution of recurrence times, a catalog shorter than an earthquake’s mean recurrence time is likely to not contain an event of that size. As a result, M_max cannot be reliably estimated from earthquake catalogs. Similar simulations show that the commonly-used different Mmax values in different continental regions could just reflect samples from the same parent distribution due to the short time series represented by earthquake catalogs. If so, seismic hazard analyses could be done using a single large Mmax value, perhaps 7.9. This would have the benefit of simplicity, and would not necessarily increase the hazard to unrealistic values because such large earthquakes would be very rare.