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...