New Madrid seismic zone fault geometry

The New Madrid seismic zone of the central Mississippi River valley has been interpreted to be a right-lateral strike-slip fault zone with a left stepover restraining bend (Reelfoot reverse fault). This model is overly simplistic because New Madrid seismicity continues 30 km southeast of the stepover. In this study we have analyzed 1704 earthquake hypocenters obtained between 1995 and 2006 in three-dimensional (3-D) space to more accurately map fault geometry in the New Madrid seismic zone. Most of the earthquakes appear to align along fault planes. The faults identifi ed include the New Madrid North (29°, 72° SE), Risco (92°, 82° N), Axial (46°, 90°), Reelfoot North (167°, 30° SW), and Reelfoot South (150°, 44° SW) faults. A diffuse zone of earthquakes exists where the Axial fault divides the Reelfoot fault into the Reelfoot North and Reelfoot South faults. Regional mapping of the top of the Precambrian crystalline basement indicates that the Reelfoot North fault has an average of 500 m and the Reelfoot South fault 1200 m of down-to-the-southwest normal displacement. Since previously published seismic refl ection profi les reveal reverse displacement on top of the Paleozoic and younger strata, the Reelfoot North and South faults are herein interpreted to be inverted basement normal faults. The Reelfoot North and Reelfoot South faults differ in strike, dip, depth, and displacement, and only the Reelfoot North fault has a surface scarp (monocline). Thus, the Reelfoot fault is actually composed of two left-stepping restraining bends and two faults that together extend across the entire width of the Reelfoot rift.