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 University of Texas at Austin Department of Geological Sciences University of South Carolina

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

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

Spatial Association with Stress Concentrators Intersecting faults and bends Intersecting faults and bends 8 out of 12 cases in rifts8 out of 12 cases in rifts 5 out of 8 cases in non-rifted regions5 out of 8 cases in non-rifted regions Buried plutons Buried plutons 6 out of 8 cases in rifts6 out of 8 cases in rifts 5 out of 8 cases in non-rifted regions5 out of 8 cases in non-rifted regions Rift pillows Rift pillows 4 cases4 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 S Hmax  Resultant patterns of stresses, strains, and displacements

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

Need for 3-D Models o 2-D Models do not show uplift o 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)]

Max. Shear Stress along BFZ

Max. Shear Stress along RF

Max. Shear Stress along BL & NMNF

Movement along BFZ, BL, NMNF

Vertical Movement along RF

Max. Shear Stress Vs. Seismicity in Depth

Seismogenic Intersecting Faults (Gangopadhyay and Talwani, 2007)

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

RESERVE SLIDES

UDEC/3DEC Computation Cycle

Rounding Concept – Avoiding Singularities

Elastic Properties (NMSZ) Blocks pertainin g to Bulk Modulus (GPa) Shear Modulus (GPa) Density (kg/m 3 ) Reelfoot rift Missouri Batholith Outside of rift JointsFriction Angle (deg) Normal Stiffness (GPa/m) Shear Stiffness (GPa/m) Cohesion (MPa) BFZ, RF, NMNF, and BL Margins of the Missouri Batholith Rift boundary faults

Computational Sequence Calculations done at each grid point Calculations done at each grid point ü i = (F i )/m F i = F Z + F C + F L + F G Contribution of internal stresses in zones adjacent to grid point Contact forces for grid points along block boundary External applied loads Force due to gravity

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

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

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

Shear Stress along WF(N)

Shear Stress along SBF

Shear Stress along WF(S)

Movement along WF(N) and WF(S)

Vertical Movement along SBF

Shear Stress Vs. Seismicity in Depth