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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.

[1] Loud, shallow microearthquakes (M < 3.0), occurring in the vicinity of granitic plutons represent a different category of seismicity compared to other recognized seismic sources in South Carolina. We demonstrate this difference by comparing the locations of microearthquakes in the vicinity of three granitic plutons in South Carolina, with the results of two-dimensional numerical modeling and analytical studies. The less rigid plutons, embedded in more rigid country rock, were loaded by applying ambient tectonic plate stresses along the direction of maximum horizontal compression. The results of modeling showed that regions of computed high stresses lie on the periphery of the plutons, and coincide with both the observed locations of seismicity and with lobes of elevated stresses obtained by analytical calculations for a weak pluton subjected to a homogenous stress field. The amplitude of the modeled stresses appears to be a function of the shape and size of the pluton. Citation:

S U M M A R YWe describe a waveform modelling technique and demonstrate its application to determine the crust-and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S, Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200-800 km), moderateto-large magnitude (5.5-7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P-and S-wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust-and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P-and S-wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21-46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P-(4.7-8 km s −1 ) and S-wave velocities (2.5-4.7 km s −1 ) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa.