| To better understand the mechanisms of continental intraplate earthquakes, a multistep approach was used. The first step involved analysis and synthesis of multidisciplinary data from 39 intraplate earthquakes spanning 20 continental intraplate regions, to identify their characteristic and diagnostic features. This led to the following testable hypothesis: Intraplate earthquakes occur within pre-existing zones of weakness (most commonly failed rifts), in the vicinity of stress concentrators, such as, intersecting faults, buried plutons, and/or rift pillows in the presence of the ambient stress field. The next step involved testing this hypothesis---first with 2-D mechanical models and then with 3-D models. Since two-thirds of the examined intraplate regions had intersecting faults as a stress concentrator, its role was first evaluated. A Distinct Element Method was used wherein the models comprised of the structural framework of the concerned region represented by a set of rock blocks that 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. The 2-D modeling was performed for two major intraplate regions in eastern U.S., viz., New Madrid and Middleton Place Summerville seismic zones, using a commercially available code called UDEC. These models adequately explain the spatial distribution of current seismicity in the regions. However, the absence of the third dimension limited the observation of tectonics in the depth dimension. Thus, 3-D models were developed for these two regions using the 3-D version of UDEC, called 3DEC. The preliminary results of these models adequately demonstrate correlation of locations of current seismicity with fault intersections in 3-D space, and also duplicate vertical movements. Although, the mechanical models demonstrated a causal association of seismicity with intersecting faults, their abundance in nature questions their uniqueness in causing seismicity, as many such faults are aseismic. Hence, a 2-D parametric study using UDEC, and a block model consisting of two and three intersecting faults, was performed to investigate whether preferred orientations of intersecting faults were responsible for causing seismicity. In the model, the orientation of the main fault with respect to SHmax was alpha, and the interior angle between the main and intersecting faults was beta. The results of this study showed that the optimum orientation of alpha for causing seismicity was 45° +/- 15°, whereas 65° ≤ beta ≤ 125° and 145° ≤ beta ≤ 170° were the optimum angles of beta. A similar 2-D parametric study using UDEC was performed of plutons of different shapes, sizes, and density contrasts, to evaluate their role in concentrating stresses to cause seismicity. The results showed that plutons of larger area, ellipticity, and density contrasts concentrated greater shear stresses around their peripheries. Additionally, plutons that are weaker than the surrounding country rocks concentrated larger shear stresses than those that are stronger than them. Cumulatively, the results of this study support the hypothesis of localized stress concentration in response to plate tectonic forces. |
