ABSTRACT

The spatial variation of seismic b‐value is mapped for the New Madrid seismic zone using local earthquakes occurring between 1995 and 2013. A region of high b‐value at about 1.8 is found in the northern part of the Reelfoot fault. By performing probability tests and following different procedures to obtain the result, we show the anomalously high b‐value is not a processing artifact and is statistically significant. We attribute the b‐value anomaly to creep behavior on the northern segment of the Reelfoot fault. Creep behavior is suggested by the recently discovered possible tremor and by the presence of quartz‐rich rock as indicated by low VP/VS ratios detected in a local earthquake tomography study. Because quartz is a weak mineral, ductile, creeping behavior could be facilitated at depth resulting in generation of earthquakes in the shallower, brittle crust. To the south of the Reelfoot fault, the b‐value is about 1.2. The character of seismic activity along the Reelfoot fault clearly changes from north to south indicating a change in the physical state of the fault zone.

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