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I. Introduction
II. Methods in Morphotectonics
III. Methods in Geodesy an Remote sensing
IV. Relating strain, surface displacement and stress, based on elasticity
V. Fault slip vs time
VI. Learnings from Rock Mechanics
VII. Case studies
IV. Relating strain, surface displacement and stress
- Some elastostatic solutions (for elastodynamic solutions see a Seismology textbook, Aki& Richard for example)
- Inversion techniques
- The circular crack model
Some classical elasticity solutions of use in tectonics
• Elastic dislocations in an elastic half-space (Steketee, 1958, Cohen, Advances of geophysics, 1999; Segall)– Surface displacements due to a rectangular dislocation: Okada, 1985:– Displacements at depth due to a rectangular dislocation: Okada,1992:– The infinitely long Strike-slip fault (Segall, 2009)– 2-D model for a dip-slip fault: Manshina and Smylie, 1971, Rani and Singh,
1992; Singh and Rani, 1993, Cohen, 1996.
• The Boussinesq pb (normal point load at the surface of an elastic half-space); (Jaeger, Rock mechanics and Engineering)
• The Cerruti pb (shear point load at the surface of an elastic half-space); (Jaeger, Rock mechanics and Engineering)
• Point source of pressure: the ‘Mogi source’(Segall, 2010)
• The circular crack model (Scholz, 2002)
ReferencesSegall, P, Earthquake and Volcano Deformation, Princeton University Press, 2010.Scholz, C. (1990), The Mechanics of Earthquakes and Faulting, 439 pp.,
Cambridge University Press, New York.Cohen, S. C., Convenient formulas for determining dip-slip fault parameters from
geophysical observables., Bulletin of seismological society of America, 86, 1642-1644, 1996.
Cohen, S. C., Numerical models of crustal deformation in seismic zones, Adv. Geophys., 41, 134-231, 1999.
Okada, Y., Surface deformation to shear and tensile faults in a half space, Bull. Seism. Soc. Am., 75, 1135-1154, 1985.
Okada, Y., Internal Deformation Due To Shear And Tensile Faults In A Half-Space, Bulletin Of The Seismological Society Of America, 82, 1018-1040, 1992.
Kositsky, A. P., and J. P. Avouac (2010), Inverting geodetic time series with a principal component analysis-based inversion method, Journal of Geophysical Research-Solid Earth, 115.
Chanard, K., J. P. Avouac, G. Ramillien, and J. Genrich (2014), Modeling deformation induced by seasonal variations of continental water in the Himalaya region: Sensitivity to Earth elastic structure, Journal of Geophysical Research-Solid Earth, 119(6), 5097-5113.
In crack mechanics, 3 modes are distinguished
Mode I= Tensile or opening mode: displacement is normal to the crack walls
Mode II= Longitudinal shear mode: displacement is in the plane of the crack
and normal to the crack edge (edge dislocation)
Mode III= Transverse shear mode: displacement is in the plane of the crack
and parallel to the crack edge (screw dislocation)
I II III
Infinite Strike-Slip fault
Let’s consider a fault parallel to Oy, with infinite length, and surface deformation due to uniform slip, equal to Sy, extending from the surface to a depth h. (Slip vector is (0,Sy,0) // Oy)
h
Co-seismic displacementparallel to Oy
y
x
Infinite Strike-Slip fault
00 yCo-seismic slip ( ,for a left-lateral fault)0y
Co-seismic strain
NB: far-field displacements and strain decay with x- 1 and x-2 respectively
Infinite Thrust fault
2)(
)(
)(tan
cos.)(
221
xsignDxx
Dxx
D
xxSxx
D
PD
2
))(1(
)(tan
sin.)(
221 xsign
Dxx
Dx
xx
DSxz
DD
Surface displacements due to slip S on a fault dipping by θ
(Manshina and Smylie, 1971; Cohen, 1996)
where
sin .cosP
Dx
tanD
Dx
Infinite Thrust fault
2)(
)(
)(tan
cos.)(
221
xsignDxx
Dxx
D
xxSxx
P
PD
Surface displacements
Horizontal strain
22 2
2 ( )( )1 .cos( )
2 ( )
P Dxx
P
D x x x xx Sx
x x x D
2
))(1(
)(tan
sin.)(
221 xsign
Dxx
Dx
xx
DSxz
DD
Displacements are proportional to fault slip (lineraity)Note that the far-field displacements and strains decay with x- 1 and x-2
Infinite Thrust fault
(see Cohen, 1996)
Convention in Okada (1985, 1992)Function [ux,uy,uz] = calc_okada(U,x,y,nu,delta,d,len,W,fault_type,strike)
This function computes the displacement field [ux,uy,uz] on the grid [x,y] assuming uniform slip, on a rectangular fault withU: slip on the faultnu: Poisson Coefficient delta: dip angled: depth of bottom edgelen=2L: fault lengthW: fault widthfault_type: 1=strike,2=dip,3=tensile,4=inflation
NB:C is the middle point of bottom edge
• Surface displacements (Okada, 1985)
useful for inverting geodetic data
• Displacement and strain at depth (Okada, 1992)
useful for Coulomb stress change (ΔCFF) calculations
It is with images of ERS acquired before and after the Landers 1992 earthquake that the first interferogram of an earthquake was produced.
Observed phase
Massonnet et al., 1993
after USGS
Setting of the 1992 Mw 7.3 Landers and1999 Mw 7.1 Hector Mine Earthquakes
Co-seismic displacement field due to the 1992, Landers EQ
G. Peltzer
Here the measured SAR interferogram is compared with a theoretical interferogram computed based on the field measurements of co-seismic slip using the elastic dislocation theory
This is a validation that coseismic deformation can be modelled acurately based on the elastic dislocation theory
(based on Massonnet et al, Nature, 1993)
Co-seismic deformation during the Hector Mine earthquake
Grey areas are zones of low phase coherence
Courtesy of G. Peltzer, UCLA
Line Of Sight component of displacement
• The crack model works approximately in this example, In general the slip distribution is more complex than perdicted from this theory either due to the combined effects of non uniform prestress, non uniform stress drop and fault geometry.
• The theory of elastic dislocations can always be used to model surface deformation predicted for any slip distribution at depth,
IV. 2-Inverting Surface Displacement
• Principle:– Source is gridded– Linear (slip, imposed geometry) vs Nonlinear
inversions (slip+geometry)– Regularisation (generally inversion is ill-
posed)
• Inversion of times series– ENIF (Paul Segall, Jeff McGuire)– PCAIM
Linear inversion• Using Green functions G calculated
with Okada solve:
where X: geodetic displacement
S: slip on the gridded source
• Laplacian Regularisation:
• Weighting: - data uncertainties e.g., - λ?
Inversion of Time series
the PCAIM technique
(Kositsky and Avouac, JGR, 2010)
PCAIM available on-line at:
http://www.tectonics.caltech.edu/resources/pcaim
• Divide time series as principal components ordered amount of data variance explained
• PCA and Okada Formulation are linear and associative and thus you can switch their ordering
Elas
tic
Dis
loca
tion
Forw
ard
Mod
el
Elas
tic
Dis
loca
tion
Forw
ard
Mod
el
Inversion of Time series
the PCAIM technique
Principal Components
• Each component has several aspects:- Mode, time variation associated with the PC
(v)- Surface Displacement, left singular value associated with the PC (u)- Singular Value, a measure of the variance of
the data explained by this PC (s)- Slip Distribution, a slip map associated with
the PC (l)
Singular Value Decomposition
• First component explains maximal data variance• nth component maximal given n-1th component
Linear Inversion
• Using Green functions G calculatedwith Okada solve:
Slip Decomposition of X
Singular Value Decomposition
+
Slip Decomposition
Okada Formulation
=
Long Valley Caldara 1997-1998 Inflation Episode
• multiple inflation events since 1980's
•~10 cm uplift near the resurgent dome during 1997-98 episode
•8 EDM time series
•24 ERS scene and 65 interferograms
Original Data
Electronic Distance Measurements
SBAS Time series
PCA Decomposition
Spatial functionTime function
PCA Reconstruction
InSAR SBAS Time Series
PCA Reconstruction
Electronic Distance Measurements
Joint Inversion* 1st comp only
Summary
• PCAIM allows the joined analysis of multiple datasets with very different temporal and spatial resolutions
• The approach allows to filter out tropospheric effects in the InSAR data.
http://www.tectonics.caltech.edu/resources/pcaim
The Elastic crack model
See Pollard et Segall, 1987 or Scholz, 1990 for more details
A planar circular crack of radius a with uniform stress drop,Δσ, in a perfectly elastic body (Eshelbee, 1957)
NB: This model produces infinite stress at crack tips, which is not realistic
Slip on the crack
Stress on the crack
2 24 (1 )
(2 )u a x
See Pollard et Segall, 1987 or Scholz, 1990 for more details
A planar circular crack of radius a with uniform stress drop, Δσ, in a perfectly elastic body (Eshelbee, 1957)
NB: This model produces infinite stress at crack tips, which is not realistic
i. The predicted slip distribution is elliptical
ii. Dmean and Dmax increase linearly with fault length (if stress drop is constant).
Slip on the crack
Stress on the crack
2 24 (1 )
(2 )u a x
8 (1 )
3 (2 )meanD u a
max max
4 (1 )
(2 )D u a
The Elastic crack model
The 1999, Mw 7.1Hector Mine Earthquake
(Leprince et al, 2007)
Coseismic deformation due the 1999 Mw 7.1 Hector Mine earthquake measured from 10m GSD SPOT images.
N-S Component
Measurements of NS and EW displacement fields from the correlation of SPOT panchromatic images (pixel size 10m) taken before and after the EQ. Displacements as low of 1/10th of the pixel size (1m) can be measured from this technique
of the order of 5 MPa
(Treiman et al, 2002)
(Leprince et al, 2007)
Coseismic deformation due the 1999 Mw 7.1 Hector Mine earthquake measured from 10m GSD SPOT images.
N-S Component
Localized and off-fault distributed anelastic deformation add to form a smooth slip distribution
The 1999, Mw 7.1Hector Mine Earthquake