ABSTRACT

The Eastern Tennessee Seismic Zone is roughly ten times more active than the average central and eastern United States, second only to the New Madrid Seismic Zone. Yet unlike New Madrid, no large earthquake is documented historically or paleoseismologically. Nonetheless, some of the ongoing seismicity could represent aftershocks of an unknown prehistoric event rather than as the result of focused long‐term strain accrual. We compare modern seismicity rates with those expected after a large earthquake. We first adopt an extreme scenario in which the entire 250‐km zone ruptured in an M 7.9 event on the eve of European settlement of the region 500 yrs ago, and we model its aftershocks with deterministic, probabilistic, and stochastic approaches. Each method shows that modern rates are significantly higher (to at least 97.3% confidence) than this upper bound on aftershock rates. Using a more reasonable set of assumptions, far less than 10% of modern seismicity could be ascribed to an ongoing aftershock sequence. We thus conclude that at least 90% of seismicity reflects time‐independent rates most readily interpreted in terms of localized release of long‐term strain in eastern Tennessee. The current distinction in estimated seismic hazard—based on historical and instrumental earthquake rates—between eastern Tennessee and its surroundings is warranted, but what localizes modern strain release in eastern Tennessee is still an open question.

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