Transcript Mechanisms of Continental Intraplate Earthquakes

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
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)
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•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
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:
• 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
RESERVE SLIDES
UDEC/3DEC Computation Cycle
Rounding Concept – Avoiding Singularities
Elastic Properties (NMSZ)
Joints
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
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 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 = 2ij + kkij
where,  = k – (2/3)

Failure criteria for faults applied
S  C + ntan
where, n = - knun
S = - kSuS
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