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A fundamental question about continental intraplate earthquakes is why they are where they are. For example, why are the New Madrid seismic zone earthquakes concentrated on the Reelfoot rift when the continent contains many fossil structures that would seem equally likely candidates for concentrated seismicity? A key to answering this question is understanding of the thermal-mechanical structure of the seismic zone. If it is hotter and thus weaker than surrounding regions, it is likely to be a long-lived weak zone on which intraplate strain release concentrates. Alternatively, if it is not sig-nificantly hotter and weaker than its surroundings, the seismicity is likely to be a transient phenomenon that migrates among many similar fossil weak zones. These different models have important implications for the mechanics of the seismic zone, stress evolution after and between large earthquakes, and seismic hazard assessment.

The sparse heat-flow data in the New Madrid area can be interpreted as supporting either hypothesis. There is a possible small elevation of heat flow in the area compared to its surroundings, depending on the New Madrid and regional averages chosen. The inferred high heat flow has been interpreted as indicating that the crust and upper mantle are significantly hotter and thus significantly weaker than surrounding areas of the central and eastern United States. In this model, the weak lower crust and mantle concentrate stress and seismicity in the upper crust. However, reanalysis of the heat flow indicates that the anomaly is either absent or much smaller (3 ± 23 rather than mW m−2) than assumed in the previous analyses, leading to much smaller (∼90%) temperature anomalies and essentially the same lithospheric strength. Moreover, if a small heat-flow anomaly exists, it may result from ground-water flow in the rift's fractured upper crust, rather than higher temperatures. The latter interpretation seems more consistent with studies that find low seismic velocities only in parts of the seismic zone and at shallow depths. Hence, although the question cannot be resolved without additional heat-flow data, we find no compelling case for assuming that the New Madrid seismic zone is significantly hotter and weaker than its surroundings. This result is consistent with migrating seismicity and the further possibility that the New Madrid seismic zone is shutting down, which is suggested by the small or zero motion observed geodetically. If so, the present seismicity are aftershocks of the large earthquakes of 1811 and 1812, and such large earthquakes will not recur there for a very long time.

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