The magnitude 7.3 Landers earthquake of 28 June 1992 triggered a remarkably sudden and widespread increase in earthquake activity across much of the western United States. The triggered earthquakes, which occurred at distances up to 1250 kilometers (17 source dimensions) from the Landers mainshock, were confined to areas of persistent seismicity and strike-slip to normal faulting. Many of the triggered areas also are sites of geothermal and recent volcanic activity. Static stress changes calculated for elastic models of the earthquake appear to be too small to have caused the triggering. The most promising explanations involve nonlinear interactions between large dynamic strains accompanying seismic waves from the mainshock and crustal fluids (perhaps including crustal magma).

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An updated frequency‐magnitude relation for the New Madrid seismic zone is used to derive conditional probabilities for future, large New Madrid earthquakes. We estimate that there is a 40–63% probability of an mb ≥ 6.0(Ms ≥ 6.3) event occurring by the year 2000 and an 86–97% probability by the year 2035. The estimates for a great 1812‐type event (Ms ≥ 8.3) are less than 1% probability by 2000 A.D. and less than 4% by 2035 A.D. These probabilities are contingent on many factors, a number of which remain assumptions because of the lack of a geological or paleoseismological chronology of past New Madrid activity. A conditional probability requires knowledge of a mean recurrence time, the type of distribution, and the standard deviation of actual repeat times about this mean. Four assumed distribution functions (Gaussian, lognormal, Weibull, and Poisson) were fit to recurrence estimates based on a combination of historical and instrumental seismicity data. Standard deviation was allowed to vary between one third and one half of the mean recurrence time, and a range of conditional probabilities was generated for time intervals of 15 and 50 years from the year 1985. The largest uncertainty in this procedure was the size of the seismic source area to use for recurrence estimation. Calculations were done for both a large and a small source zone which led to variation in estimated recurrence intervals by a factor of 2. The large source zone was favored for the final probability estimates because of the large crustal volume required to store elastically the strain energy for great New Madrid earthquakes.

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