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