Download CHAPTER 6 COULOMB STRESS CHANGES AND SEISMOGENSIS

CHAPTER 6
COULOMB STRESS CHANGES
BENEATH KACHCHH REGION
AND
SEISMOGENSIS
6.1 Introduction
An earthquake alters the shear and normal stress in the crust and mantle
surrounding the seismic sources as well as surrounding faults and can lead to trigger
earthquakes and aftershocks. The stress field of an earthquake is in general timedependent. It is believed that when the stress exceeds the strength of rock material of
corresponding seismic source or fault, an earthquake occurs. It has been also observed
that an earthquake produces a net reduction of regional stress then it increase stress in
the nearby region. Such region of increased stress with further tectonic loading would
be the focus of future earthquake. This phenomenon is called as ‘earthquake
triggering’. In spite of this simplest mechanical process, the earthquake triggering has
not been observed as widely as might be expected. The earth’s lithosphere behaves as
inhomogeneous (at a scale of microns to hundreds kilometers), porous and commonly
anisotropic. A small part of this lithosphere is brittle-elastic which can generate
earthquakes due to faulting in it, but a large part of the lithosphere is non-seismogenic
which is not able to generate earthquakes. It has been observed that only the upper
10–20 km of the continental lithosphere (the upper crust) is seismogenic and the
lower crust is viscous with no long term strength (Scholz, 1990; Deng and Sykes,
1996; Papadimitriou and Sykes, 2001).
Thus, the lower crust and mantle behave not simply as an elastic medium but
as a viscoelastically, particular over longer period (Roeloffs, 1996). The crust will
first deform as an elastic body, but later deformation will continue because of stress
relaxation (Thatcher and Rundle, 1984). Since inelastic stress relaxation also leads to
redistribution of the stress, it is necessary to include inelastic behavior when
analyzing stress transfer and earthquake interaction. Nevertheless, an important
limitation of the elastic/co-seismic stress change approach is that it cannot explain
time delays of the triggered events.
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A number of researchers have modelled Coulomb stress changes by
considering some critically stressed crustal faults in the KRB region (Mandal et al.
2007; Chanderasekher et al. 2009). They have successfully explained the causes of
occurrences of moderate size events in terms of increase in positive coulomb stress
changes. However, these studies did not account the process of postseismic relaxation
by using viscoelastic processes at the lower crust and upper mantle, which following
major earthquakes, can transfer significant stress upwards to the seismogenic shallow
crust (Pollitz 1997; Freed and Lin 1998). A number of studies have shown that such
stress transfer can have a broad impact on the evolution of the stress field (Stein et al.,
1992; Pollitz et al. 2003).
In this chapter, we estimated 3-D coseismic and post-seismic stress in the
Coulomb Failure Stress (∆CFS) change at different depths due to 2001 Bhuj
earthquake to understand the viscoelastic stress relaxation in the lower crust. This
study is also useful to understand the plausible cause of continuing of aftershock
activities and the triggering of later events at the time scale of years in the KRB
region. This is virtually no study is made in active Kachchh region to understand the
role of elastostatic in seismogenesis of the region except few studies by Toa et al.,
2004; Mandal et a., 2007) that dealt limited area in around the Allah bund and GEDI
faults, respectively using the concept of ∆CFS. The present study is comprehensive in
comparison to the earlier study made by Toa et al., (2004) Mandal et al., (2007)
Chandrasekher et al. (2009) in fallowing ways:
(i) Entire Kachchh region is caused to address the nature of Coulomb stress changes
while earlier studies.
(ii) 3-D Viscoelastic process is considered while others used either GPS
measurements or receiver faults to assess the Coulomb stress changes.
(iii) 3-D concept is used to estimate 3-D Coulomb Failure Stress changes
(iv) We have used recently developed concepts using Wang et al. (2006) for
estimating Coulomb Failure Stress changes, while used Okada (1992) and Pollitz
(1997) etc.
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6. 2 Concept of coulomb stress modeling
It is assumed that the faults are very narrow planes across which some
components of displacement are discontinuous while the stress components across the
plane are continuous. This system is controlled by the boundary conditions which
determine the nature of faulting, strike slip, normal or reverse slip. The frictional
behavior of faults determines the stress levels at which they will slip and generates the
earthquakes. After all, these assumptions become failed but allow a simple theoretical
and computational framework which is to be developed on the basis of dislocation
theory. In this theory, the static displacements, stains and stresses can be computed by
solving the elastostatic equation for a dislocation on an extended fault in an elastic,
isotropic and homogeneous medium.
The static displacement can be computed with the knowledge of fault
geometry, the slip distribution and the strain nuclei (i.e. static green functions). The
analytical expressions for the static displacement, strain, and stress field caused by a
finite rectangular fault either at the earth surface or at the depth can be derived with
the help of Hooke’s law (Okada, 1992).
6.2.1 Coulomb Failure Criteria
Earthquake interaction is a fundamental feature of seismicity, leading to
earthquake sequences, clustering, and aftershocks. One interaction criterion that
promises a deeper understanding of earthquake occurrence, and a better description of
probabilistic hazard, is Coulomb stress transfer (King et al., 1994). A number of
methods have been used worldwide to explain the conditions which are responsible
for the failure in rocks. The coulomb failure criterion is one of the methods which are
used widely to explain the conditions of failure. It is based on the fact that the normal
and shear stress on a prevailing fault plane satisfy conditions corresponding to those
of friction on a prevailing surface.
The earthquakes occur when the stress exceeds the strength of the rocks of the
fault. The failure starts and closeness of failure is computed using the changes in
Coulomb stress sometime referred as Coulomb Failure Function (ΔCFF). It depends
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on the changes in both shear stress Δτ and normal stress Δσ, which can be expressed
as,
ΔCFF = Δτ - μ' (Δσ- ΔP)
(6.1)
where, Δτ is the changes in shear stress (positive in the direction of fault slip),
Δσ is the changes in normal stress (positive for the compression), ΔP is the changes in
pore fluid pressure of the fault (positive for the compression) and µ' is the effective
coefficient of friction which can be expressed as,
µ' = µ (1 - B)
(6.2)
Where B is the Skempton’s coefficient whose value lies in the range of 0 to 1.
B indicates the presence of fluids in the rock, with B = 0 for completely drained rocks
and B = 1 for maximum possible presence of fluids. The pore fluid pressure and their
changes with time can be very important for active fault zones. The low value of µ' is
related with either weak fault material or high fluid pressure within active fault zone.
With time, pore fluid diffusion allows ΔP return to zero and hence μ' rises to actual
coefficient of friction µ. If fluid diffusion is, a low value of µ is predicted for the
instantaneous behavior following a sudden change in normal stress and a large value
would be appropriate once fluid diffusion is nearly complete. The value of Δτ in
equation (6.1) always should be positive; however it may be positive or negative
depending upon the potential for slip whether it is right or left lateral. A positive value
of ΔCFF for a particular fault denotes movement of that fault towards failure i.e. the
likelihood that it will rupture in an earthquake is increased. The direct measurement of
stress is not possible; hence changes in stress value can be calculated fairly readily
from information about the geometry and slip of an earthquake rupture.
6.3 Methodology
During the last decade, Coulomb stress modelling has become a popular and
accepted tool to analyses the conditions under which earthquakes occur (King et al.,
1994; Stein et al., 1994; Zeng, 2001; Muller et al., 2003; Lin and Stein, 2004). In this
study, we computed stress field produced by dislocation sources embedded in a mixed
elastic/inelastic layered half-space (Wang et al., 2006). We use the elastic-viscoelastic
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correspondence principle (Christensen, 1982), which states that a linear viscoelastic
boundary-value problem can be solved by adopting the associated elastic solutions
where the elastic moduli are replaced by the Laplace of Fourier transferred complex
moduli. The time domain solutions are then obtained using inverse Fast Fourier
Transform. A comparison between the method used here and the one published by
Fernadez et al., (1996) can be found in Wang (2005). Most significantly, Wang’s
approach corrects an inconsistency in the former formulation when including gravity
effects to calculate the deformation.
We computed surface and subsurface deformation due to the common
geophysical sources in a multi-layered viscoelastic-gravitational half space caused by
the mainshock using the software PSGRN/PSCMP (Wang et al., 2006). The first
program, PSGRN, calculates the time-dependent Green functions of a given layered
viscoelastic-gravitational half space for four fundamental dislocation sources (the
strike-slip double-couple, the dip-slip double couple, the compensated linear vertical
dipole (CLVD) and the point inflation). The result provide a data base for the second
program, PSCMP, which automatically discretizes the earthquake’s extended rupture
area into a number of discrete point dislocations and calculates the co- and postseismic deformation by linear superposition.
6.3.1 Model parameters
In this study, the layered model used for our calculations is described by the rock
parameters summarized in Figure 6.1 (Antolik and Dreger, 2003; Negishi et al., 2002;
Mandal and Pujol, 2006; Singh et al., 2011). We considered a Poisson’s ratio of 0.25
for co-seismic Coulomb stress changes either as of 0.26 for post seismic stress
changes determined from 3-D tomography studies (Singh et al., 2011; Singh et al.,
2012a). Concerning the rheological properties, we first considered two layer earth
model with mantle viscosity ηm = 2 X 1019 pa.s (herein after Model 1), second
considered three layer model with elastic viscoelastic lower crust viscosity η lc = 1 X
1020 pa.s and viscoelastic mantle viscosity ηm = 2 X 1019 pa.s a (herein after Model 2)
(Figure 6.2). For the sensitivity studies, we considered these two types of rheological
properties Model 1 and Model 2. With these parameters, we carried out calculations
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for a grid of 59X101 points with spacing of approximately 2 km, covering the study
region.
Figure 6.1:
Parameters of the horizontally stratified medium. Vp is the velocity of P wave, Vs
is velocity of S –wave, μ is the shear modulus, ρ is the rock density, η is visocity
(modified Mandal and Pujol, 2006; Singh et al., 2011).
Coulomb stress change depends upon the geometry and slip of the earthquake,
the geometry and sense of slip of the fault or of surface of interest (optimally-oriented
plane), regional stress and effective coefficient of friction (Yagi and Kikuchi, 2001).
The stress changes on the optimally oriented planes are sensitive to the orientation of
the regional stress but insensitive to its magnitude (King et al., 1994). The earthquake
stress changes plus the regional stress control the orientation of the optimum failure
planes. Since the regional stress driving plate motion is larger than the stress changes
caused by the earthquakes or fault slip, the regional stress dominates the orientation of
the failure planes, except very close to the fault (Stein et al., 1992). King et al. (1994)
shows that as long as the regional stress is larger than the earthquake stress drop, the
stress change on optimally-oriented planes is little affected, however, the regional
stress magnitude less than the earthquake stress drop is highly unlikely. Completely
incorrect focal mechanism clearly results incorrect results. King and Cocco (2000)
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observed that the error in dip is much less serious and they showed that except at
distances comparable to the source dimensions, the distributions of Coulomb stress
changes are identical for about 500 dipping normal fault and a vertical dike with the
same opening displacement (Singh et al., 2011). This clearly indicates that, except
close to a dip slip fault, the fault dip is not important. It is observed that large errors in
the strike of the focal planes are serious while small errors can only affect the rotation
of the Coulomb stress distribution.
The fault rupture area and average slip are two important parameters necessary
for the model application, which is considered from Yagi and Kikuchi, 2001 in this
study. Yagi and Kikuchi (2001) have modeled deformation along a reverse fault
matching with the inferred fault along a 75 km X 35 km rupture plane. The maximum
dislocation is 8.5 m at 25 km depth, reducing to about a meter near surface. The
source duration is 25 sec. The viscosity of this under laying half space is set to
2X1019 Pas (Model 1). The distribution of slip is not actually uniform along a fault,
but one is interested in its average value as well as fault area expressing the main
rupture. For this reason, we used the scaling laws suggested by Wells and
Coppersmith (1994) in order to calculate both these values as a function of magnitude
Mw of the particular earthquake. The value of µ' is taken 0.2 to 0.8 (King et al., 1994)
throughout in this study. A high value of µ' (about 0.8) has been considered for thrust
faults and strike slip faults with little cumulative slip (Parsons et al., 2000; Toda and
Stein, 2002) whereas low value of friction (µ' < 0.4) is favors for strike slip faults with
significant cumulative slip (Harris et al., 1998; Parsons et al., 2000; Toda and Stein,
2002). For the subduction zones, the value of µ' will likely to be intermediate value
because of large cumulative slip and likelihood of high pore pressure. Lin and Stein
(2004) adopted µ' = 0.4 for the calculation of Coulomb stress changes in the Kachch
rift Basin.
The outputs from PSGRN are green functions covering 13 observables (3
displacement components, 6 stress components, 2 tilt components, and 2 geopotential
components i.e. the gravity and geoid changes) induced by the 4 different dislocation
sources. Since the CLVD and inflation source are axis symmetric, the associated
observables are reduced by their tangential components.
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(a) Earth Model 1
Figure 6.2:
(b) Earth Model 2
Earth Model for Bhuj region (a) Two layer earth model with mantle viscosity ηm =
2 X 1019 pa.s (b) Three layer model with Elastic Viscoelastic Lower Crust ηlc = 1X
1020 pa.s and Viscoelastic mantle ηm = 2 X 1019 pa.s
As an input of the PSCMP, an earthquake is represented by an arbitrary
number of rectangular fault planes with different locations and orientations (strike, dip
and rake). PSCMP discretizes these fault planes automatically to a set of point
dislocations using the same spatial resolution as used for Green function, and carries
out the convection integration.
6.3.2 Viscoelasticity and model geometry
Prior investigations have suggested that postseismic viscous flow in the lower
crust or upper mantle can cause significant stress and strain increases in the
seismogenic upper crust, causing it eventually to become the main layer to store the
strains caused by an earthquake (Pollitz and Sacks, 1995, 1997; Freed and Lin, 1998;
Deng et al., 1999). The 2001 Bhuj earthquake in Kachchh provide a unique
opportunity to investigate the detailed processes of viscoelastic stress transfer and
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earthquake triggering because relatively abundant seismological, geodetic, and
geological constraints are available.
We estimated a three-dimensional (3D) coulomb stress model, taking
viscoelastic relaxation into account for a wide region of the KRB and utilized
PSGRN/PSCMP computer code developed by (Wang et al., 2006). The lithosphere in
the study region is modeled with a Young’s modulus E that increases with depth
according to the relationship;
E = (2/3) (1+n) r Vp
(5.3)
Where n is Poisson’s ratio, r is density, and Vp is P-wave velocity.
Several researchers observed that relatively fast regional changes in surface
deformation have been observed following the 1992 Landers earthquake through
space geodetic measurements (Shen et al., 1994; Massonnet et al. 1996; Savage and
Svarc, 1997; Jackson et al., 1997; Bock et al., 1997). While a number of mechanisms
may have contributed to postseismic surface deformation, e.g., fault zone collapse
(Massonnet et al., 1996), afterslip (Shen et al., 1994; Savage and Svarc, 1997), and
poroelastic rebound (Peltzer et al., 1998; Masterlark and Wang, 2002), it appears that
the observed post-Bhuj horizontal surface deformation at the time scale of months to
years is best explained by viscous flow in either the lower crust (Ivins, 1996; Deng et
al., 1999) or upper mantle (Pollitz et al., 2000). Petrological considerations argue that
a felsic lower crust is weaker than a mafic upper mantle (Kirby and Kronenberg,
1987), thus favoring viscous flow in the lower crust. On the other hand, the limited
observations of post-Bhuj vertical surface deformation appear to be better explained
by models of viscous flow in the upper mantle (Pollitz et al., 2000; Mandal et al.,
2007; Chanderasekhar et al., 2009). We thus consider viscous flow in either the lower
crust or upper mantle is shown in Figure 6.2 (Model 1 and Model 2). In the following
we first illustrate the coseismic stress patterns of the 2001 Bhuj earthquake, to be
followed by a description of the role of postseismic viscoelastic stress transfer. In
most cases, a range of values for the apparent friction coefficient, from low (m¢=0.2)
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to intermediate (m¢=0.5) to high (m¢=0.8), were examined to test the sensitivity of
model results.
6.4 Evolution of Coulomb stress for 2001 bhuj earthquake
6.4.1 Co-seismic Coulomb stress
We estimated co-seismic deformation, taking viscoelastic relaxation into
account fallowed by Wang et al. (2006) code at depth of 5 km interval beneath 2001
Bhuj source region (Figures 6.3 and 6,4). The rheological model used in this study is
shown in Figures 6.1 and 6.2. Density, bulk modulus, and shear modulus are
consistent with seismic velocity and density layering used from other studies (Antolik
and Dreger, 2003; Negishi et al., 2002; Mandal and Pujol, 2006). We used Model 2 as
described earlier, with µ’ = 0.4 and lower crust and upper mantle viscosities of 1X1020
Pa.s and 2X1019 Pa.s, respectively. Though, we experimented with different values of
apparent frictional coefficient and rheological models, which did not show significant
changes in the results. The calculated co-seismic Coulomb stress increase spread out
in a butterfly–shaped pattern beneath 2001 Bhuj mainshock at all depths. Positive
changes in ΔCFS at the depth of 5-15 km indicate the increase in likelihood failure
due to mainshock. However, suddenly at the depth of 20-25 km ∆CFS shows
variation of ∆CFS between -0.10.0 and -300.0 bars, close to the hypocentral depth of
mainshock. Since 2001 Bhuj earthquake occurred have a hypocentral depth of 20-25
km (Kayal et al., 2002; Mishra and Zhao, 2003; Rastogi et al., 2004; Mishra et al.,
2008; Singh et al., 2011; Singh et al., 2012).
The total ∆CFS from co-seismic deformations are 0.17, 0.22 and 0.24 bars at
the depth of 30 km, 10km, and 0km respectively. It is found that the ∆CFS show some
significant differences at different depth. All these differences are contributed
aftershocks sequences till date in KRB region particularly towards NW and east of the
2001 Bhuj earthquake rupture. Numerous studies have shown a correlation between
calculated positive coseismic stress changes and the location of aftershocks as well as
triggering of moderate to large earthquakes (Harris, 1998). It is observed that the
Coulomb stress changes of ≥0.1 bar near Bhuj mainshock have significantly impact
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on seismicity patterns in the KRB (Reasenberg and Simpson, 1992; Harris, 1998;
Stein, 1999).
5 km
At Surface
15 km
10 km
Figure: 6.3:
Co-seismic Coulomb stress changes, taking viscoelastic relaxation into account at
different depth at surface, 5, 10, and 15 km due to Bhuj 26, 2001. Star shows
epicenter of Bhuj 2001 earthquake. The focal mechanism solutions are also shown.
Yagi and Kikuch (2001) variable slip model used in this study. The vertical scale
values are shown on the right side of each color scale bar. Solid lines repent active
faults and lineaments with the abbreviation namely the Kachchh mainland fault,
KMF; Katrol Hill fault, KHF; Gedi fault, GF; North Kathiawar fault, NKF;
North Wagad fault, NWF.
It is also observed that the total cumulative stress filed is towards NW and SE
direction at each depth (Figures 6.3 & 6.4) and aftershocks sequences are migrating
towards the same direction, which is briefly described under chapter-4 (ISR report,
2007, 2008, 2009 and 2010). Furthermore, it is interesting to note that variation of
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∆CFS in and around the mainshock in the direction where aftershocks sequences
migrating of NE and SE at all depths beneath hypocenter.
20 km
25 km
30 km
Figure: 6.4: Same as figure 6.3 but at depths of 20, 25, and 30 km.
6.4.2 Postseismic Coulomb stress
The evaluation of the postseismic coulomb stress changes beneath 2001 Bhuj
mainshock after 10 years at different depths is shown in Figures 6.5 & 6.6. The
estimated post-seismic coulomb stress is as butterfly–shaped pattern beneath 2001
Bhuj earthquake source region at all depths, which is similar shape as coseismic stress
changes. The ∆CFS show positive ∆CFS in the direction of NW and SE of the
mainshock. The continuing of seismicity M≥0.8 to till date between 15 and 25 km
depth strong correlates with the calculated postseismic coulomb stress changes.
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At Surface
5 km
10 km
15 km
Figure 6.5:
Postseismic Coulomb stress changes (after 10 yrs), taking viscoelastic relaxation
into account at different depth at surface, 5, 10, and 15 km due to Bhuj 26, 2001.
Star shows epicenter of Bhuj 2001 earthquake. The focal mechanism solutions are
also shown. Yagi and Kikuch (2001) variable slip model used in this study.The
vertical scale values are shown on the right side of each color scale bar. Solid lines
repent active faults and lineaments with the abbreviation namely the Kachchh
mainland fault, KMF; Katrol Hill fault, KHF; Gedi fault, GF; North Kathiawar
fault, NKF; North Wagad fault, NWF.
It is found that vicinity of GF is associated with stress chnages between -0.001
and -300.00. It could be due to an Mw = 5.6 event (hypocentre: 23.840N, 70.720E, 3.0
km depth) occurred along the neighbouring GF fault (GF, 30 km north of NWF) after
more than five years of the aftershock activity of 2001 Bhuj earthquake. This Mw =
5.6 GEDI mainshock occurred on 2006 March 7 and it was followed by another Mw =
5.6 event on 2006 April 6 that located on the NWF (hypocentre: 23.35◦N, 70.32◦E,
28.2 km depth), which was the second largest aftershock of the 2001 Bhuj sequence
(Mandal, 2007). The 2001 Bhuj mainshock and its second largest aftershock were
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more complex than the GEDI event, rupturing about 50 × 40 km area between
Kachchh mainland fault and NWF, with a mixture of reverse and right-lateral motion
generation of these kind of intraplate earthquakes could be controlled by the variation
in the brittle–ductile transition in the rift zone, which will be influenced by the crustal
thermal conditions perhaps controlled by the mafic intrusives as revealed by the
detailed velocity tomography and modeling of gravity as well as magnetic data
(Manglik and Singh, 2002; Chandrasekhar and Mishra, 2002; Mishra and Zhao,
2003; Mandal and Pujol 2006; Singh et al., 2011; Singh et al., 2012).
25 km
20 km
30 km
Figure 6.6:
Same as figure 6.5 but at the depths of 20, 25, and 30 km.
We observed positive changes in Coluomb stress (∆CFS) from the year of
occurrence of the mainshock till the year 2011 to the NW and SE of the Bhuj
earthquake rupture. We also considered the fault locations in the Rann of Kachchh
region, postseismic relaxation from the 2001 event enhances the stress along the
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Kachchh Mainland Fault (KMF) and faults in the Wagad highlands. The ∆CFS is
slightly negative on the KHF. The postseismic coulomb stress changes after 10 years
are more prominent in NW and SE zones of the mainshock due to viscoelastic stress
relaxation processes, which may be a site for occurrence of future aftershocks in these
regions. It is observed that the viscoelastic ∆CFS perturbs the stress level up to
eastern Saurashtra within shallow depth (0-10 km) that triggered seismic activity in
recent time in the region (Rastogi et al., 2011).
6.5 Results and discussion
The magnitude and pattern of co- and post-seismic deformation depend
strongly on the rheological layering of the crust and upper mantle, which in turn
depends on composition and ambient temperature and pressure (Wang et al., 2006).
At least six processes may be involved in changing the stress distributions namely
propagation of seismic wave, changes of static stress caused by major co-seismic slip,
cascading seismicity, fluctuations in hydrological conditions, ductile flow of
subcrustal rocks, and inter-seismic strain accumulation. Seismic data of KRB region
shows a Moho depth of 35-42 km (Kumar et al, 2001; Reddy et al. 2001; Mandal,
2007). Since most earthquakes in the study region occurred at depths 20-25 km, and
all slip models of the Bhuj mainshock (e.g., Yagi and Kikuchi, 2001) suggested that
the rupture extend down to a depth of 20-25km.We therefore set the thickness of
elastic crust and the locking depth to be 25 km. We assumed that viscoelastic
processes occur below the depth of 25 km below the depth, coseismic stress changes
within the viscous lower crust and upper mantle cannot be sustained and lead to
viscoelastic flow, which induces stress changes in the seismogenic crust.
It has been demonstrated that changes of coulomb stress ranging between 0.1
and 1 bar (sometimes even smaller as 0.01 bar) can influence the occurrence of future
earthquakes. This seems surprising because variations of ∆CFS of an earthquake are
commonly much larger, e.g. several bars to several hundred bars. Hence such a small
change in stress might be too small to affect any crustal fault. The stress change
beneath 2001 Bhuj mainshock hypocenter from the postseismic relaxation is 4-7 times
greater than the immediate co-seismic loading (Figures 6.3-6.4 & 6.5-6.6), which
points to the importance of considering the contribution from viscoelastic relaxation
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of the lower crust and upper mantle. Thus, viscosities of the lower crust and upper
mantle are of importance for the stress calculations (Xiong et al., 2009).
Coulomb stress assessments indicate that mainshock of the Bhuj earthquake is
strongly stressed up to the surface near the mainshock (Figures 6.3 -6.4 & 6.5-6.6). At
the depth of 20-25 km, the suddenly stress variations are more in co- and postseismic. The negative changes at this depth are responsible for the aftershocks
activity. Our analysis (Chapter-3, 4 and 5) on mapping of b-value, a-values, Ds-values,
slip ratio, Vp, Vs, σ, ε, ξ, and ψ confirmed that the 2001 Bhuj earthquake driven by
fluid-fractures rock matrices at the depth of 20-25 km (Kayal et al., 2002; Mishra and
Zhao, 2003; Mandal and Chadha, 2008; Singh et al., 2011; Singh et al., 2012a). Thus
our estimates of Coulomb stress validate our tomographic imaging beneath the 2001
Bhuj source region, which vindicates that rupture nucleation, is associated with fluid
related anomalies. However, the fluid filled materials matrix exhibit time-dependent
stress fields, we examine if this behavior can explain the triggering of aftershocks. We
compare solutions for a fluid filled material matrix with those for previous
observations beneath the mainshock, and evaluate the effect of cohesion between
deformation and fluid diffusion.
Our results as described in previous Chapter-3, 4, and 5, also show that
aftershocks migrated towards the NE and SW of the mainshock, where positive ∆CFS
are observed. The results of ∆CFS modeling including viscoelastic processes show the
variations of patterns are almost similar in the co-seismic and post-seismic. However,
we observed some significant difference within 2001 Bhuj mainshock zone in both
modeling i.e co-seismic and post-seismic features It is also found that the more than 3
strong events M≥4.5 occurred in NE direction during the period of 2006-2010. A
recent analysis of the aftershock sequences in the KRB region indicates a cluster of
aftershocks to the NW of hypocenter at the depth range of 10-25 km. The correlation
between the aftershocks and increased ∆CFS region is provides better explanation of
the assumed slip model taken for this study. Regions exhibiting repeating earthquakes
with small magnitude are usually characterized by asesimic slip. Because of large
variations of coulomb stress surrounding the creeping zone, a large earthquake is
likely to nucleate there and eventually grow into areas, which are more tightly bonded
in the rock masses. In particular there are zones around some of faults (e.g., KMF,
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GF, and IBF) where the co-seismic Coulomb stress changes are observed negative.
However, changes in Coulomb stress because positive in the NW and SE directions of
the 2001 Bhuj mainshock source zone because of redistribution of pore fluids, which
could be zone for future failure in the region.
In fact it is found that the co-seismic negative coulomb stress (Figures 6.3-6.4)
changes became positive at some locations after the 2001 Bhuj mainshock, and then
becomes positive in post seismic coulomb stress (Figures 6.5-6.6), and vice versa. The
post seismic coulomb stress pattern becomes stable during 10-years from the
occurrence of the 2001 Bhuj mainshock as the processes of stability id dictated by
permeability through the diffusivity (Piombo et al., 2005), if the crust were to behave
as an isotropic, fluid-filled, and poro-elastic medium. Coulomb stress changes may be
related to pore fluid diffusion as one of the important mechanism for aftershock
generation. The presence of fluid-filled fractured rock matrix at 23–25 km depth,
which is characterized by high crack density, high saturation rate and high porosity as
revealed by the tomographic study, could enhance the fluid pressure resulting in
lubrication of the fault zone (Mishra and Zhao 2003). The presence of fluids at the
2001 Bhuj mainshock earthquake hypocenter may lead to negative Coulomb stress
changes at varying depths. Further, the upward advection of fluids along fractures
could locally raise the temperature along the fault zone (Bickle and McKenzie, 1987;
Hoisch, 1991), which could also provide an alternative explanation for the variation in
seismogenic base along faults.
6.6 Conclusions
Estimated 3-D co-seismic and post-seismic stress in the ∆CFS changes at
different depths in the 2001 Bhuj earthquake source region suggests that viscoelastic
stress relaxation in the lower crust in the Kachchh region is main driving force for
continued seismogenesis in the region. The positive changes in ΔCFS at the depth of
0-30 km indicate the increase in likelihood failure of small faults in terms of
aftershocks towards NW and east of the Bhuj 2001 mainshock, which made the region
seismically very active. It is observed that the variations in co-seismic ΔCFS towards
east increases, it confirms that viscoelastic coulomb stress plays an important role
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inside the lower crust of KRB. The postseismic coulomb stress changes after 10 years
are more prominent in these regions due to viscoelastic stress relaxation processes,
which causing occurrence of future aftershocks in these regions. At the depth of 20-25
km, ∆CFS represents variations of coulomb stress in the range of -0.10.0 to 300.0
bars, is closer to the hypocentral depth of mainshock. It is also found that the
mainshock of the Bhuj earthquake is strongly stressed up to the surface near the
mainshock. The migration of aftershocks has been noticed towards the NW and SE of
the mainshock. It is deciphered that there are zones around the fault where the static
coseismic Coulomb stress changes in negative. But it becomes positive as pore fluids
are redistributed; in this zone future failure may happen provided other consideration
of rupture nucleation apt for seismogenesis. Regions exhibiting repeating earthquakes
with small magnitude are usually characterized by asesimic slip. Because of high
stress surrounding the creeping zone, a large earthquake is likely to nucleate there and
eventually grow into areas those are more tightly bounded in the plate interior of the
2001 Bhuj earthquake source zone.
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