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Localized Stress Concentration: A Possible Cause 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. STRATEGY.
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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
STRATEGY Models wherein stress perturbation occurs in upper crust • Multi-Step • Analyze and synthesize global data • Develop simple mechanical models
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)
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
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
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
Summary of 2-D Model for NMSZ(Gangopadhyay et al., 2004) Y Q N B M A P
Need for 3-D Models • 2-D Models do not show uplift • 3-D Models are more realistic with respect to Fault Geometry
3-D Model for NMSZ (using 3DEC)[Gangopadhyay and Talwani, 2006 (In Revision, JGR)]
Seismogenic Intersecting Faults (Gangopadhyay and Talwani, 2007)
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
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
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
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)
Computational sequence (contd.) • Constitutive equations for blocks applied ij = 2ij + kkij where, = k – (2/3) • Failure criteria for faults applied S C + ntan where, n = - knun S = - kSuS
3-D Model for MPSSZ (using 3DEC)[Gangopadhyay and Talwani, 2006 (In Revision, JGR)]