INVESTIGATING THE EARTHQUAKE POTENTIAL OF THE MARIANNA SOURCE, EAST-CENTRAL ARKANSAS

Paleoliquefaction features, including large and weathered sand blows and related sand dikes, indicate repeated large-magnitude earthquakes in the Marianna area during the Late Pleistocene to Middle Holocene. Based on investigations of liquefaction features concentrated in the Marianna area, as well as those discovered along the St. Francis Ditch in Arkansas and the Coldwater River in Mississippi, paleoearthquakes occurred about 4.8, 5.5, 6.8, and 9.9 thousand years ago (ka) and at least two times between 11 ka and 41 ka. No liquefaction features have been found in the Marianna area that formed during the past 4.8 thousand years (kyr), when the New Madrid seismic zone, located 80-280 km to the northeast, produced repeated large-magnitude earthquakes. Many large Marianna sand blows define a northwest-oriented lineament that likely represents the surface expression of an active fault zone. The long axes of the sand blows, the main feeder dikes, and soft-sediment faults that crosscut the dikes are all northwest-oriented and subparallel to the lineament. The lineament is relatively straight, and though discontinuous, can be traced for about 12 km. Seismic reflection surveys imaged faults below the lineament and found increasing fault displacement with depth, suggesting long-term recurrent faulting. The northwest-oriented fault zone probably was the source of several of the paleoearthquakes. Liquefaction potential analysis indicates that an earthquake of M 6-6.5 could induce liquefaction in a thick section of sandy sediment identified more than 13 m below the surface. In addition, preliminary analysis suggests that an earthquake of M 6.9 or greater would be required to induce liquefaction along the Coldwater River in Mississippi, where liquefaction features similar in age to the Marianna sand blows have been found. The Marianna source may have been seismically active for at least 36 kyr between 4.8 ka and 41 ka before entering its current inactive period. This finding may have implications for the New Madrid seismic zone which began a seismically active period about 4.3 ka. Paleoliquefaction studies are ongoing in the Marianna area and will help to improve our understanding of the periodicity and migration of seismicity, the long-term behavior of faults in the region, and the seismic hazard they pose to society.