REPEATED LARGE EARTHQUAKES 4,800 TO 38,000 YEARS AGO TOWARDS THE SOUTHWESTERN END OF THE REELFOOT RIFT FAULT SYSTEM

Paleoearthquakes about 4.8, 5.5, 6.8, 9.9 ka, and sometime between 9.9-38 ka, produced very large sand blows, up to 3 m thick, 150 m wide, and 600 m long, near Marianna, Arkansas, located towards the southern end of the Reelfoot Rift fault system and north of the Arkansas transform fault. The Marianna sand blows are as large as, but older and more weathered than, sand blows in the New Madrid seismic zone. None of the Marianna sand blows formed during the New Madrid earthquakes in C.E.1811-1812, 1450, or 900. Therefore, the Marianna liquefaction features do not represent distant effects of earthquakes generated by the New Madrid seismic zone. The Marianna sand blows are indicative of large magnitude (greater than 6.5) earthquakes generated by local sources, most likely the northeast-trending Eastern Reelfoot Rift Margin fault zone (EMFZ) and the northwest-trending White River fault zone (WRFZ). A compound sand blow that formed during a sequence of large earthquakes about 6.8 ka is located in the Marianna Gap of Crowley’s Ridge and close to the intersection of the two fault zones. Sand blows that formed during the other paleoearthquakes occur along the Daytona Beach lineament that parallels the nearby WRFZ. Geophysical surveys (ground-penetrating radar and resistivity) at thirteen sites and trenching of sand blows at three sites confirm severe liquefaction-related ground failure along the lineament. At one site, a fault with a similar strike to the lineament displaces layering within one of the feeder dikes. The lineament is at least 17 km long, fairly straight, and probably related to faulting rather than to lateral spreading. This hypothesis should be tested by borehole and/or geophysical studies of geologic units below liquefiable Quaternary deposits. If found to be related to a northwest-oriented fault at depth, the Daytona Beach lineament is likely to be the surface expression of a western member of the WRFZ. Significant uncertainties remain regarding the completeness of the paleoseismic record as well as sources, magnitudes, and recurrence times of large earthquakes in the Marianna area. Additional study is needed to reduce these uncertainties, to better understand the long-term behavior of the EMFZ and WRFZ, and to assess the potential threat of these fault zones to Memphis, Little Rock, and other communities in the region.