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Large Mid-Continent Earthquakes are a Thing of the Past: The Facies Proxy Record of Holocene Deformation Events in the New Madrid Seismic Zone Courtesy of the Mississippi River


Dr. John Holbrook
Professor
University of Texas at Arlington


The earthquake cycles that characterize continental interiors have proven highly cryptic and difficult to resolve. We used a novel paleoseismic proxy to address this issue. Namely, we reconstructed Holocene Mississippi River channels from maps of floodplain strata in order to identify channel perturbations reflective of major displacement events on the high-hazard and mid-plate Reelfoot thrust fault, New Madrid seismic zone, U.S.A. Only three discrete slip events are currently documented for the Reelfoot fault (~AD900, ~AD1450, AD1812). This study extends this record and, thus, illustrates the utility of stratigraphic proxies as paleoseismic tools. We concurrently offer here some of the first quantified response times for tectonically induced channel pattern changes in large alluvial rivers.

We identified at least two cycles of pervasive meandering that were interrupted by channel-straightening responses occurring upstream of the Reelfoot fault scarp. These straightening responses occurred at 2244BC+/-269 to 1620BC+/-220 and ~AD900, respectively, and each records initiation of a period of Reelfoot fault slip after millennia of relative tectonic quiescence. The second (or New Madrid) straightening response was triggered by the previously known ~AD900 fault slip event, and this initial low sinuosity has been protracted until the modern day by the latter ~AD1450 and AD1812 events. The first (or Bondurant) straightening response began a period of several hundred to ~1400 years of low river sinuosity which evidences a similar period of multiple recurrent displacement events on the Reelfoot fault. These Bondurant events predate the existing paleoseismic record for the Reelfoot fault.

These data offer initial evidence that deformation on the Reelfoot fault was temporally clustered on millennial scales and, thus, offers the first direct evidence of millennial-scale clustering on a continental-interior fault. This carries additional ramifications. Namely, faults that have been quiescent and non-hazardous for millennia could re-enter an enduring period of recurrent hazardous earthquakes with little warning. Likewise, the Reelfoot fault reveals evidence of temporal clustering of earthquakes on short-term cycles (months), as well as evidence for longer-term reactivation cycles (104-106 years). This introduces the possibility that temporal clustering could be hierarchical on some continental-interior faults.