Seismic-reflection profiles across part of the New Madrid seismic zone in northeastern Arkansas and southeastern Missouri show a faulted and structurally complex zone, originally known as Charlie’s ridge but herein renamed Blytheville arch, which is about 10 to 15 km wide and about 110 km long. Several exploratory drill holes in the arch penetrate Upper and possibly Middle Cambrian formations directly below Cretaceous rocks, whereas drill holes off the arch penetrate the Cambrian and Ordovician Knox and Arbuckle Groups equivalents and possibly younger Paleozoic rocks below the Cretaceous; therefore, the pre-Cretaceous rocks in the arch are structurally high. Most earthquakes in the northeast-trending segment of the New Madrid seismic zone along the axis of the Reelfoot rift occur along the arch. Focal mechanisms of earthquakes in the trend show right-lateral, strike-slip movement. Epicenters in the northeastern part of the seismic trend between Caruthersville, MO, and Blytheville, AR, are spatially less dispersed than those to the southwest between Blytheville and Marked Tree, AR. Most of the hypocenters to the northeast cluster between 6 to 12 km deep, whereas those to the southwest range from near the surface to about 15 km deep. Earthquakes in the northeast part of the seismic trend are concentrated along a fault zone under the arch, whereas those to the southwest are more dispersed under the arch. A seismic trend that extends south-southwest from near Charleston, MO, projects to the northwest side of the ridge near Blytheville, where the seismicity changes character and the southeast boundary of the arch trends more easterly. The relationship between the structural boundaries of the arch and the seismicity may establish the extent of part of New Madrid seismicity and improve the basis for seismic hazard assessment.

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