Is the New Madrid seismic zone hotter and weaker than its surroundings?

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 signifi cantly 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-fl ow data in the New Madrid area can be interpreted as supporting either hypothesis. There is a possible small elevation of heat fl ow in the area compared to its surroundings, depending on the New Madrid and regional averages chosen. The inferred high heat fl ow has been interpreted as indicating that the crust and upper mantle are signifi cantly hotter and thus signifi cantly 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 fl ow indicates that the anomaly is either absent or much smaller (3 ± 15 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-fl ow anomaly exists, it may result from groundwater fl ow in the rift’s fractured upper crust, rather than higher temperatures. The latter interpretation seems more consistent with studies that fi nd low seismic velocities only in parts of the seismic zone and at shallow depths. Hence, although the question E-mail: [email protected]. 168 McKenna et al. spe425-12 page 168