A Two-Dimensional Numerical Model for Current Seismicity in the New Madrid Seismic Zone

Abstract

In order to understand the mechanics of intraplate earthquakes better, a simple 2D numerical model was developed to try to explain current seismicity in the New Madrid seismic zone, using a distinct element method. The model comprises a block geometry representing the structural framework of the New Madrid seismic zone, consisting of intersecting faults with elastic properties corresponding to the known geology. The blocks were subjected to tectonic loading for four days along the direction of the maximum horizontal stress field and the resulting patterns of stress and strain distributions were studied. The results of the modeling showed that shear stresses were higher within the Reelfoot Rift than outside it. In this 2D model the shear stresses on the horizontal plane gave a sense of rotation of the modeled blocks, and an implied sense of movement on the faults. They duplicated the right-lateral strike-slip movement along the Blytheville Fault zone and New Madrid North Fault, and left-lateral strike-slip movement along the Reelfoot Fault. Due to the two-dimensional nature, however, results of modeling do not show the observed reverse motion along the Reelfoot Fault. The observed seismicity pattern was consistent with the amplitudes and signs of the maximum shear stresses along the major faults located within the Reelfoot Rift. A linear extrapolation of model results gave an annual strain rate consistent with geodetic observations. The results of modeling support the idea that in a localized volume of pre-existing weak crust, fault intersections act as stress concentrators and cause anomalous stress build-up in their vicinity, resulting in observed seismicity.