Localized Stress Concentration: A Possible Cause of Current Seismicity in New Madrid and Charleston...
-
date post
20-Jan-2016 -
Category
Documents
-
view
214 -
download
0
Transcript of Localized Stress Concentration: A Possible Cause of Current Seismicity in New Madrid and Charleston...
Localized Stress Concentration: Localized Stress Concentration: A PossibleA Possible
Cause of Current Seismicity in Cause of Current Seismicity in New Madrid and Charleston New Madrid and Charleston
Seismic ZonesSeismic Zones
Abhijit Gangopadhyay and Pradeep TalwaniAbhijit Gangopadhyay and Pradeep TalwaniInstitute for GeophysicsInstitute for Geophysics
University of Texas at AustinUniversity of Texas at AustinDepartment of Geological SciencesDepartment of Geological Sciences
University of South CarolinaUniversity of South Carolina
STRATEGYSTRATEGY
Multi-Step Analyze and synthesize global data
Develop simple mechanical models
Models wherein stress perturbation occurs in upper crust
GLOBAL SURVEY GLOBAL SURVEY (Gangopadhyay and Talwani, (Gangopadhyay and Talwani, 2003)2003)
Johnston (1994)
(1)(3)
(2)(1)(1)
(1)
(1)
(3)
(2)
(1)(1) (1)
(3)
(1) (1)
(4)
(5)(3)
(3)
(1)
•39 Earthquakes•20 Continental Intraplate Regions
•12 Rifted, 8 Non-Rifted
Spatial Association with Stress Spatial Association with Stress ConcentratorsConcentrators
Intersecting faults and bendsIntersecting 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 plutonsBuried 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 pillowsRift pillows• 4 cases4 cases
Testable HypothesisTestable 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 SIMPLE MECHANICAL MODELSMODELS
Distinct Element Method : UDEC & 3DECDistinct Element Method : UDEC & 3DEC Structural Framework in a Block Model Structural Framework in a Block Model
(Deformable)(Deformable) Faults treated as DiscontinuitiesFaults treated as Discontinuities Constant Strain Triangular ZonesConstant Strain Triangular Zones Elastic Properties based on Known Elastic Properties based on Known
Geology (Densities and Elastic properties Geology (Densities and Elastic properties of blocks, Stiffnesses, Cohesion, and of blocks, Stiffnesses, Cohesion, and Friction for faults)Friction for faults)
Tectonic Loading along STectonic Loading along SHmaxHmax
Resultant patterns of stresses, strains, Resultant patterns of stresses, strains, and displacementsand displacements
Summary of 2-D Model for Summary of 2-D Model for NMSZNMSZ
(Gangopadhyay et al., 2004)(Gangopadhyay et al., 2004)
A
B
P
Q N
Y
M
Need for 3-D ModelsNeed 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)3-D Model for NMSZ (using 3DEC)[Gangopadhyay and Talwani, 2006 (In [Gangopadhyay and Talwani, 2006 (In
Revision, JGR)]Revision, JGR)]
Max. Shear Stress along Max. Shear Stress along BFZBFZ
Max. Shear Stress along Max. Shear Stress along RFRF
Max. Shear Stress along BL & Max. Shear Stress along BL & NMNFNMNF
Movement along BFZ, BL, Movement along BFZ, BL, NMNFNMNF
Vertical Movement along Vertical Movement along RFRF
Max. Shear Stress Vs. Max. Shear Stress Vs. Seismicity in DepthSeismicity in Depth
Seismogenic Intersecting FaultsSeismogenic Intersecting Faults(Gangopadhyay and Talwani, 2007)(Gangopadhyay and Talwani, 2007)
SUMMARYSUMMARY
Spatial Association of Continental Spatial Association of Continental Intraplate Seismicity with Stress Intraplate Seismicity with Stress Concentrators such as:Concentrators such as:• Intersecting FaultsIntersecting Faults
Based on 2-D and 3-D Mechanical Based on 2-D and 3-D Mechanical Models:Models:• Stress Concentration due to Stress Concentration due to
Intersecting Faults explains current Intersecting Faults explains current seismicity and tectonic features in seismicity and tectonic features in NMSZNMSZ
THE FINAL ANSWER!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 SLIDESRESERVE SLIDES
UDEC/3DEC Computation UDEC/3DEC Computation CycleCycle
Rounding Concept – Avoiding Rounding Concept – Avoiding SingularitiesSingularities
Elastic Properties Elastic Properties (NMSZ)(NMSZ)
Blocks pertainin
g to
Bulk Modulus
(GPa)
Shear Modulus
(GPa)
Density(kg/m3)
Reelfoot rift
47.28 28.48 2690
Missouri Batholith
57.66 34.74 2705
Outside of rift
58.61 35.32 2750
Joints Friction Angle(deg)
Normal Stiffness(GPa/m)
Shear Stiffness(GPa/m)
Cohesion(MPa)
BFZ, RF, NMNF, and BL
27 101 76 0
Margins of the Missouri Batholith
33 133 100 0.5
Rift boundary faults
27 101 76 0.5
Computational SequenceComputational Sequence
Calculations done at each grid pointCalculations done at each grid pointüi = (Fi)/m
Fi = FZ + FC + FL + FG
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 Computational Sequence (contd.)(contd.)
Acceleration at each grid pointAcceleration at each grid point• Finite difference form of Newton’s Finite difference form of Newton’s
second law of motionsecond law of motionm[Vi(t + Δt/2) - Vi(t – Δt/2)]/t = Fi(t)
For each time stepFor each time step• Strains and rotations computedStrains and rotations computed
ij = ½ (Vi,j + Vj,i)ij = ½ (Vi,j - Vj,i)
Computational sequence Computational sequence (contd.)(contd.)
Constitutive equations for blocks appliedConstitutive equations for blocks appliedij = 2ij + kkij
where, = k – (2/3) Failure criteria for faults appliedFailure criteria for faults applied
S C + ntan
where, n = - knun
S = - kSuS
3-D Model for MPSSZ (using 3-D Model for MPSSZ (using 3DEC)3DEC)
[Gangopadhyay and Talwani, 2006 (In [Gangopadhyay and Talwani, 2006 (In Revision, JGR)]Revision, JGR)]
Shear Stress along Shear Stress along WF(N)WF(N)
Shear Stress along SBFShear Stress along SBF
Shear Stress along WF(S)Shear Stress along WF(S)
Movement along WF(N) and Movement along WF(N) and WF(S)WF(S)
Vertical Movement along Vertical Movement along SBFSBF
Shear Stress Vs. Seismicity Shear Stress Vs. Seismicity in Depthin Depth