Abhijit Gangopadhyay and Pradeep Talwani Institute for Geophysics University of Texas at Austin

1. Localized Stress Concentration: A PossibleCause of Current Seismicity in New Madrid and Charleston Seismic Zones Abhijit Gangopadhyay and Pradeep Talwani Institute for Geophysics University of Texas at Austin Department of Geological Sciences University of South Carolina

2. STRATEGY Models wherein stress perturbation occurs in upper crust • Multi-Step • Analyze and synthesize global data • Develop simple mechanical models

3. GLOBAL SURVEY (Gangopadhyay and Talwani, 2003) (3) (1) (1) (1) (2) (3) (1) (1) (1) (3) (3) (4) (1) (1) (1) (1) (1) (2) (5) (3) • 39 Earthquakes • 20 Continental Intraplate Regions • 12 Rifted, 8 Non-Rifted Johnston (1994)

4. Spatial Association with Stress Concentrators • Intersecting faults and bends • 8 out of 12 cases in rifts • 5 out of 8 cases in non-rifted regions • Buried plutons • 6 out of 8 cases in rifts • 5 out of 8 cases in non-rifted regions • Rift pillows • 4 cases

5. Testable Hypothesis Observed spatial association Causal association Intraplate earthquakes occur due to a localized stress build-up in response to plate tectonic forces, in the vicinity of stress concentrator/s, such as intersecting faults, buried plutons, rift pillows located in a pre-existing zone of weakness

6. SIMPLE MECHANICAL MODELS • Distinct Element Method : UDEC & 3DEC • Structural Framework in a Block Model (Deformable) • Faults treated as Discontinuities • Constant Strain Triangular Zones • Elastic Properties based on Known Geology (Densities and Elastic properties of blocks, Stiffnesses, Cohesion, and Friction for faults) • Tectonic Loading along SHmax • Resultant patterns of stresses, strains, and displacements

7. Summary of 2-D Model for NMSZ(Gangopadhyay et al., 2004) Y Q N B M A P

8. Need for 3-D Models • 2-D Models do not show uplift • 3-D Models are more realistic with respect to Fault Geometry

9. 3-D Model for NMSZ (using 3DEC)[Gangopadhyay and Talwani, 2006 (In Revision, JGR)]

10. Max. Shear Stress along BFZ

11. Max. Shear Stress along RF

12. Max. Shear Stress along BL & NMNF

13. Movement along BFZ, BL, NMNF

14. Vertical Movement along RF

15. Max. Shear Stress Vs. Seismicity in Depth

16. Seismogenic Intersecting Faults (Gangopadhyay and Talwani, 2007)

17. SUMMARY • Spatial Association of Continental Intraplate Seismicity with Stress Concentrators such as: • Intersecting Faults • Based on 2-D and 3-D Mechanical Models: • Stress Concentration due to Intersecting Faults explains current seismicity and tectonic features in NMSZ

18. THE FINAL ANSWER! A Cause of Continental Intraplate Seismicity may be Localized Stress Concentration due to Stress Concentrators such as Intersecting Faults (favorably oriented) in response to Plate Tectonic Forces, and simple models involving these stress concentrators can explain the seismicity in NMSZ

19. RESERVE SLIDES

20. UDEC/3DEC Computation Cycle

21. Rounding Concept – Avoiding Singularities

22. Elastic Properties (NMSZ)

23. Computational Sequence • Calculations done at each grid point üi = (Fi)/m Fi = FZ + FC + FL + FG Force due to gravity Contribution of internal stresses in zones adjacent to grid point External applied loads Contact forces for grid points along block boundary

24. Computational Sequence (contd.) • Acceleration at each grid point • Finite difference form of Newton’s second law of motion m[Vi(t + Δt/2) - Vi(t – Δt/2)]/t =  Fi(t) • For each time step • Strains and rotations computed ij = ½ (Vi,j + Vj,i) ij = ½ (Vi,j - Vj,i)

25. Computational sequence (contd.) • Constitutive equations for blocks applied ij = 2ij + kkij where,  = k – (2/3) • Failure criteria for faults applied S C + ntan where, n = - knun S = - kSuS

26. 3-D Model for MPSSZ (using 3DEC)[Gangopadhyay and Talwani, 2006 (In Revision, JGR)]

27. Shear Stress along WF(N)

28. Shear Stress along SBF

29. Shear Stress along WF(S)

30. Movement along WF(N) and WF(S)

31. Vertical Movement along SBF

32. Shear Stress Vs. Seismicity in Depth