Abstract

The first-order seismicity alignment of the New Madrid Seismic Zone (NMSZ) was recognized after just a few years of network monitoring in the Upper Mississippi Embayment and adjacent region. However, two decades of monitoring are now resolving secondary alignments. The most distinct of the second-order alignments is along the southeastern flank of Reelfoot rift. This rift margin zone includes the 32 km long Crittenden County fault zone, just 25 km from Memphis, Tennessee. Among those earthquakes recorded and located by the regional seismic network in the NMSZ between 1974 and 1994, 75 earthquakes with magnitude ∼0.0–4.0 are spatially associated with the southeastern rift margin. The opposing northwest flank is essentially aseismic except where crossed by active fault segments of the northern main NMSZ. Data are adequate to recover P-wave first-motion focal mechanisms for nine southeastern flank earthquakes. They indicate a mixture of right-lateral strike-slip and high-angle reverse faulting. The faulting style, while still poorly constrained by the sparse seismicity, is consistent with ancient rift-flank normal faults reactivated in a modern compressional stress regime oriented approximately east-west. The reason(s) for the enhanced seismicity of the southeastern relative to the northwestern flank remain unknown, but the question is of more than academic interest given the proximity of the southeastern flank to the city of Memphis.

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