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Very Large Earthquakes Centered Southwest of the New Madrid Seismic Zone 5,000–7,000 Years Ago

Earthquake-induced liquefaction features, including large sand blows, occur near Marianna, Arkansas, about 75 km southwest of Memphis, Tennessee, and 80 km south of the southwestern end of the New Madrid seismic zone. The Marianna sand blows formed between 5,000 and 7,000 years ago and predate paleoearthquakes attributed to the New Madrid seismic zone. The Marianna sand blows are similar in size to New Madrid sand blows, suggesting that they too formed as a result of very large earthquakes but were centered near Marianna and outside the New Madrid seismic zone. A large sand blow that formed about 3500 B.C. (5,500 years B.P.) may correlate with smaller sand blows to the northeast and southwest up to 175 km away. A compound sand blow that formed about 4800 B.C. (6,800 years B.P.) may have formed as the result of several very large, closely timed earthquakes. A fault zone associated with the eastern Reelfoot Rift margin seems the most likely source of large Middle Holocene earthquakes because of its great length (300 km), history of seismic activity, Late Wisconsin–Early Holocene fault movement in western Tennessee, and structural relationship to the New Madrid fault system. Additional study is needed to verify our initial findings, to identify the earthquake source, and to further define the earthquake potential of the Marianna area. If the eastern Reelfoot Rift margin were confirmed to be the source of Middle Holocene earthquakes near Marianna, seismicity would appear to vary in space and time within the Reelfoot Rift system. This would have important hazard implications for currently aseismic faults of the rift system and possibly of other aulacogens embedded in intraplate regions.