VERY LARGE EARTHQUAKES CENTERED SOUTHWEST OF THE NEW MADRID SEISMIC ZONE 5,000-7,000 YEARS AG

TUTTLE, M.P., M. Tuttle & Associates, 128 Tibbetts Lane, Georgetown, ME 04548, MAHDI, H. and AL-SHUKRI, H., Department of Applied Science, University of Arkansas, 2801 South University Ave., Little Rock, AR 72204, mptuttle@earthlink.net.

Earthquake-related sand blows at two sites near Marianna, Arkansas, about 180 km southwest of the heart of the New Madrid seismic zone, suggest that very large earthquakes originated in this area 5,000 to 7,000 years ago. An exceptionally large sand blow at the Daytona Beach site in the Western Lowlands formed about 3500 B.C.E. (5450 years B.P.) and may correlate with smaller sand blows near Blytheville to the northeast and Kelso and Montrose to the southwest. A compound sand blow at the St. Francis River site in the St. Francis Basin formed about 4850 B.C.E. (6800 years B.P.) and is buried by backswamp deposits. The compound sand blow pre-dates previously recognized paleo earthquakes in the region and, like compound sand blows in the New Madrid seismic zone, may have formed as the result of an earthquake sequence. Several fault zones cross the study area, including the eastern margin of the Reelfoot Rift, the White River fault zone, and the Big Creek fault zone. The eastern Reelfoot Rift margin seems the most likely source of large Middle Holocene earthquakes due to its great length (~300 km), history of seismic activity northeast of Marianna, Late Wisconsin-Early Holocene fault movement in western Tennessee, and relationship to the New Madrid fault system that produced the 1811-1812 New Madrid earthquakes. Additional geophysical and geological studies are necessary to confirm our initial findings, to identify the earthquake source, and to further define its earthquake potential. If the eastern Reelfoot Rift margin were found 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.