PRELIMINARY 3-D NUMERICAL MODELS FOR NEW MADRID AND MIDDLETON PLACE SUMMERVILLE SEISMIC ZONES

GANGOPADHYAY, A., and TALWANI, P., Department of Geological Sciences, University of South Carolina, Columbia, SC 29208, abhijit@seis.sc.edu.

Synthesis of multidisciplinary data from 20 intraplate regions worldwide that had hosted 39 earthquakes (M >= 5), showed that intersecting faults are spatially associated with the observed seismicity in majority of the regions. Simple two-dimensional mechanical models of New Madrid Seismic Zone (NMSZ) and Middleton Place Summerville Seismic Zone (MPSSZ) supported a causal association. However, limitations of modeling in two-dimensions prevented us from adequately demonstrating vertical tectonics. Our current research is directed towards developing three-dimensional mechanical models of NMSZ and MPSSZ so as to better demonstrate the causal associations of intersecting faults with intraplate earthquakes. Numerical modeling using a Distinct Element Method was performed and the program used for the purpose is called "3DEC". The models comprise of the structural framework of the concerned region represented by a set of blocks with the faults within them treated as joints or discontinuities. The blocks and the faults are assigned elastic properties conforming to the known geology, and subjected to tectonic loading along the direction of maximum regional compression (S_Hmax ) at a rate similar to the ambient plate velocity measured from GPS studies. The stresses and displacements of the blocks and along the faults are then observed at the end of the loading period. The results of the modeling indicate that locations of stress build-up in three-dimensions correlate well with those of the instrumental seismicity in the regions. The results of the models also replicate uplift along the Reelfoot fault in NMSZ and Ashley River fault - Sawmill Branch fault system in MPSSZ. The model results further support our hypothesis that "Intraplate earthquakes occur due to a localized stress build-up in response to plate tectonic forces in the vicinity of intersecting faults which act as stress concentrators within a pre-existing zone of weakness".