Limitations of the short earthquake record for seismicity and seismic hazard studies

Attempts to study earthquake recurrence in space and time are limited by the short history of instrumental seismology compared to the long and variable recurrence time of large earthquakes. As a result, apparent concentrations and gaps in seismicity and hence seismic hazard within a seismic zone, especially where deformation rates are slow (<10 mm/yr), are likely to simply reflect the short earthquake record. Simple numerical simulations indicate that if seismicity were uniform within a tectonically similar seismic zone, such as the Atlantic coast of Canada, St. Lawrence valley, or the coast of North Africa, thousands of years of record would be needed before apparent concentrations and gaps of seismicity and hazard did not arise. Hence, treating sites of recent seismicity as more hazardous for future large earthquakes is likely to be inappropriate, and it would be preferable to regard the hazard as comparable throughout the seismic zone.