TECTONIC DEFORMATION IN THE NEW MADRID SEISMIC ZONE - INFERENCES FROM MAP VIEW AND CROSS-SECTIONAL BOUNDARY ELEMENT MODELS

The lack of instrumental recordings and obvious fault scarps associated with the 1811-1812 New Madrid earthquakes necessitates examination of more subtle indicators of the geometry and type of faulting associated with these events. Morphologic and geologic features and the distribution of modern seismicity are used to infer the slip distribution and type of faulting (strike- or dip-slip), number, strike, length, and width of the major faults in the New Madrid seismic zone (NMSZ). This is accomplished through two-dimensional boundary element modeling of the strain field arising from slip on hypothetical faults that is driven by either coseismic or uniform regional strains. Tectonic deformation is reflected in (1) the seismicity, (2) the Lake County uplift, (3) Reelfoot Lake, (4) the fractured rocks of the Blytheville arch, and (5) the St. Francis Sunk Lands. Many of these features can be qualitatively explained as resulting from tectonic deformation due to slip on two left-stepping fight-lateral strike-slip faults that are coincident with the northeast trending zones of seismicity and the Blytheville arch. The morphology appears to be, at least in part, a consequence of major earthquakes that rupture these faults. The locations of the 1811-1812 and largest post-1812 earthquakes and the models are consistent with a process in which the 1811-1812 earthquakes relieved accumulated regional shear strain causing the greatest post-1812 shear strains to exist at the ends of the fault zone. Modeling results also suggest that the numerous small earthquakes in the NMSZ are not aftershocks of the 1811-1812 earthquakes but instead represent continuous localized adjustments to a uniform regional strain field. The Bootheel lineament does not appear to be significant in shaping the morphology, geologic structure, and pattern of seismicity in the NMSZ. The absence of fault scarps and coherence of shallow reflectors overlying the Blytheville arch suggests that the major NMSZ faults may not have ruptured to the surface during the 1811-1812 earthquakes. The inferred lengths of the 1811-1812 earthquake ruptures also suggest that their sizes may have been overestimated. Model-predicted subsidence within the St. Francis Sunk Lands indicates that tectonic deformation also may have influenced alluvial processes in the NMSZ.