Title: Localized Stress Concentration: A Possible Cause of Current Seismicity in New Madrid and Charleston Seismic Zones
1Localized 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
2STRATEGY
Models wherein stress perturbation occurs in
upper crust
- Multi-Step
- Analyze and synthesize global data
- Develop simple mechanical models
3GLOBAL SURVEY (Gangopadhyay and Talwani, 2003)
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- 39 Earthquakes
- 20 Continental Intraplate Regions
- 12 Rifted, 8 Non-Rifted
Johnston (1994)
4Spatial 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
5Testable 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
6SIMPLE 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
7Summary of 2-D Model for NMSZ(Gangopadhyay et
al., 2004)
Y
Q
N
B
M
A
P
8Need for 3-D Models
- 2-D Models do not show uplift
- 3-D Models are more realistic with respect to
Fault Geometry
93-D Model for NMSZ (using 3DEC)Gangopadhyay and
Talwani, 2006 (In Revision, JGR)
10Max. Shear Stress along BFZ
11Max. Shear Stress along RF
12Max. Shear Stress along BL NMNF
13Movement along BFZ, BL, NMNF
14Vertical Movement along RF
15Max. Shear Stress Vs. Seismicity in Depth
16Seismogenic Intersecting Faults (Gangopadhyay and
Talwani, 2007)
17SUMMARY
- 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
18THE 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
19RESERVE SLIDES
20UDEC/3DEC Computation Cycle
21Rounding Concept Avoiding Singularities
22Elastic Properties (NMSZ)
Blocks pertaining 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
23Computational 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
24Computational Sequence (contd.)
- Acceleration at each grid point
- Finite difference form of Newtons second law of
motion - mVi(t ?t/2) - Vi(t ?t/2)/?t ? Fi(t)
- For each time step
- Strains and rotations computed
- ?ij ½ (Vi,j Vj,i)
- ?ij ½ (Vi,j - Vj,i)
25Computational 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
-
263-D Model for MPSSZ (using 3DEC)Gangopadhyay
and Talwani, 2006 (In Revision, JGR)
27Shear Stress along WF(N)
28Shear Stress along SBF
29Shear Stress along WF(S)
30Movement along WF(N) and WF(S)
31Vertical Movement along SBF
32Shear Stress Vs. Seismicity in Depth