FOUR MAJOR HOLOCENE EARTHQUAKES ON THE REELFOOT FAULT, NEW MADRID SEISMIC ZONE (Invited Presentation)

Developing long-term earthquake records in intraplate and low-strain regions is challenging because earthquakes occur infrequently and the surficial geologic record of those earthquakes is often destroyed through a combination of natural and anthropogenic processes. In the Central and Eastern United States, records of earthquakes rarely extend back further than a few thousand years. One of the best studied faults in this region is the Reelfoot reverse fault, a major structure within the New Madrid Seismic Zone which was involved in the historic 1811-1812 CE earthquake sequence. The paleoearthquake record on the Reelfoot fault extends back to the mid-Holocene, through a combination of paleoseismic trenching, paleoliquefaction investigations, and fluvial geomorphic analyses. In this investigation, we extend the paleoearthquake record for the Reelfoot fault via investigation of ridge-top gravitational failure features, interpreted as sackungen. These sackungen occur along the bluffs above the eastern margin of the Mississippi River floodplain and are concentrated near (<15 km) the southeast-dipping Reelfoot reverse fault. A paleoseismic trench excavated across sackungen at the Paw Paw site exposed four packages of colluvial sediment that postdate 30-11 ka Peoria loess. We interpret the colluvial packages to have been deposited following episodic failure of the sackungen as a result of strong ground motions. Radiocarbon and luminescence dating, along with ²¹⁰Pb and ¹³⁷Cs concentrations, constrain the ages of the colluvial packages and indicate the following sequence of earthquakes: event 4, 1640 ± 1730 BCE; event 3, 340 ± 670 CE; event 2, 1430 ± 380 CE; and event 1, 1810 ± 50 CE (2-sigma). If the sackungen have recorded all major earthquakes associated with the Reelfoot fault, our new record represents the longest archive of paleoearthquakes along the Reelfoot fault. Importantly, these new results in combination with prior investigations, indicate that only a single earthquake (or earthquake sequence) occurred during the early-to-middle Holocene. In contrast, the most recent few thousand years have been punctuated by an episode of three major seismic events. This non-periodic earthquake recurrence has important implications for seismic-hazard and geodynamic modeling in the New Madrid Seismic Zone.