Paleoseismicity: Seismicity Evidence for Past Large Earthquakes

Clusters of earthquakes in continental intraplate regions are used to estimate the times and magnitudes of past earthquakes in a model we call “paleoseismicity.” The time of a past earthquake is estimated from an Omori-law decay of the aftershocks with time, while the magnitude of the earthquake is inferred from the length of the current zone of seismic activity. The observed aftershocks of several intraplate earthquakes are used to find the parameters describing the Omorilaw aftershock decay, and these parameters are found to fall in the same range as those for aftershock sequences from California. The paleoseismicity model is shown to be approximately consistent with the current seismicity rates at the New Madrid, Missouri; Charleston, South Carolina; and Charlevoix, Quebec seismic zones. Near Basel in Switzerland, the paleoseismicity model is consistent with the occurrence of the large earthquake there in 1356. However, at Ardennes-Hautes Fagnes, Belgium; Hainaut, Belgium; and Bree, Belgium the paleoseismicity model either underestimates the rate of current seismicity or suggests earthquakes not known in the historic record. The paleoseismicity model may be most applicable to low-strain-rate, intraplate regions where aftershock rates above the background seismicity level can persist for very long periods of time. If many current small earthquakes are aftershocks of past large events, then it may not be appropriate to use earthquake rates from recent seismicity to calculate the seismic hazard in probabilistic analyses.