Chapter VI
STRAIN ENERGY RELEASE PATTERN IN INDOBURMA OROGENIC BELT
6.1 Introduction
Crustal strain energy is accumulated in the earth’s crust because of
the stress developed due to tectonic movements of different fractured blocks
present in a region and the same has been released in the form of an
earthquake
Thus, the source of energy of an earthquake is the potential
energy stored in rocks due to the accumulation of strain over a period of
time. When the accompanying elastic stress accumulates beyond the
competence of the rocks, an earthquake occurs. The elastic rebound of the
volume of rock under stress generates the earthquake (Benioff, 1949) and
most of the strain energy stored in the rocks has been released in the form
of seismic waves, which radiate in all directions from its source region.
However, a part of the original potential energy of strain stored in the rock
mass must goes into mechanical work as in raising crustal blocks against
gravity or in crushing material in the fault zone and other part is dissipated
as heat. The strain rebounded during an earthquake is proportional to the
square root of the energy released (Benioff, 1951). Thus, earthquakes are
the manifestation of strain energy released from a region and its spatiotemporal behaviour represents the seismotectonic characteristics of the
region. The strain energy release pattern of a region helps in identifying the
probable source region for the occurrence of an earthquake in near future
Moreover, temporal variation of strain energy released from a seismotectonic
block reveals the earthquake generation potentiality of the block at any
specified time
Assuming that an earthquake is generated by the elastic rebound of
strain energy stored in rocks, Gutenberg and Richter (1956) derived the
energy, E of an earthquake having magnitude, M by the relation,
log E = 12 +1,8 M
...
(6.1)
Later on, they modified the relationship between strain energy released and
the magnitude of the associated earthquake by using unified magnitude Mfa
derived from body waves recorded at teleseismic distance and represented
as follows log E = 5.8 + 2.4 Mb
...
(6.2)
Chouhan (1979, 1966) applied this relationship to study the regional
strain release characteristics of Indian earthquakes. He observed that every
region is characterized by a certain minimum level of strain energy and strain
may persist in a tectonic block even after the release of the accumulated
strain by a large earthquake. Therefore, taking it as a reference, it is possible
to estimate the amount of strain energy available in a tectonic block at a
given time in near future. Thus, in turn, it may give an estimate about the
possible maximum magnitude earthquake that may occur in the block, if the
accumulated strain energy releases simultaneously (Goswami, 1984).
100
The importance of strain release studies is that the characteristic
features and trends of stress field in a tectonic block can be investigated by
examining the pattern of strain energy release over it. Assuming a linear
probability of earthquake occurrences over a period of few years, a direct
extrapolation of the pattern of strain release into immediate future can
through some light on the probable future earthquake activity expected in the
region. The areal distribution of strain energy released helps in identifying
the setsmically active zones. Here, the strain energy accumulation and its
release pattern in Indo-Burma orogenic belt demarcated by latitude 20°N 28°N and longitude 93°E - 980E have been studied for the period 1964 to
2005.
6.2 Database and M ethodology
The comprehensive data file prepared by using different earthquake
catalogues available for the study region as discussed in Chapter HI have
been used for this analysis. Since the data file as a whole is not complete,
only epicentral data from 1964 to 2005 having magnitude Mb£4 have been
used to analyse the strain release pattern in the study region. Small
magnitude earthquakes (Mb< 4) have been rejected on the assumption that
they might be the representatives of an incomplete data set as the seismic
network density is increasing year after year in recent years. Moreover,
energy associated with small magnitude earthquakes is negligible compared
to moderate and large magnitude earthquakes. Besides, some of the small
magnitude earthquakes may be caused by gravity force as suggested by
101
Verma and others (Verma et al, 1976) on the basis of the relationship
observed between elevation and seismicity.
(a) Theory and Application of the Method
Benioff (1949) has shown that the potential energy Ep of a volume of
rock (w) is given by Ep = 0 5 p w S2
...
(6.3)
where p is the coefficient of shear and S is average strain just before the
earthquake. Thus the energy released by seismic waves is given by,
ET = 0 5 p f w S2
...
(6.4)
where f is the function of energy released as seismic waves.
If the strain is reduced to zero during the earthquake by some
movement along a fault, then the average strain S is proportional to the fault
displacement (Xf). Thus,
S = C Xf
..
(6.5)
where C is the constant of proportionality. Putting the value of S in equation
(6.4) we get,
Et = 0.5 p f w C 2 Xf2
= G 12 Xf2
...
(6.6)
Where G ,2 = 0.5 p f w C2.
Equation (6.6) implies that fault displacement (Xf) is proportional to the
square root of the energy released. Hence, using the magnitude energy
relation of Guttenberg and Richter in equation (6.6), we get -
102
log Et 1/2 = log Gi X,
= 2.9 +1.2 Mb
or
X f = (1/G i) 1029 + 1 2Mb
...
(6.7)
The equation (6.7) gives the relation between fault displacement (Xf)
and
earthquake
magnitude
Mb. Thus,
if the
magnitudes
of all
the
earthquakes occurring in any fault system over a period are known, one can
plot the fault displacement (strain) that may be occurred during that time
period. Such plots, which give strain release characteristics, represent spurts
of seismic activity separated by relatively quiescent periods. The resulting
curve
will
be
a
saw-toothed
curve
representing
the
exhaustion
of
accumulated strain caused by earthquakes.
(b) Application of the Method in the Study Region
(i) Temporal variation
To apply the method put forth by Benioff (1949), the whole region is
divided into three tectonic blocks - (i) Block-1 (20° - 22.6° N Lat.), (ii) Block-2
(22.5°- 25° N Lat.) and (iii) Block-3 (25°-28° N Lat.) as discussed in Chapter IV
(Fig. 4.2). The strain energy released in a tectonic block has been computed
for each year by considering strain energy as the sum of the square root of
energy for all earthquakes occurred in the tectonic block in the specified year
(Benioff, 1951 and Ritsma, 1954). Then, strain energy has been added up
year-by-year to get the strain release characteristics. Hence, the vertical line
represents the release of earthquake energy in E1/2, which again represents
the strain factor in earg1/2. Since S is proportional to E1/2, the continuous
103
lines
represent
the
secular
strain
generation
showing
that
strain
accumulation is linear and may be represented asI E1/z = A + Bt
...
(6.8)
where A and B are constants and t is time in years.
(ii) Spatial variation
The energy of each earthquake is determined by using equation (6.2)
and iso-strain release map has been prepared. For this, the whole study
region is divided into small grids having dimension 1/2° by 1/2°. The sum
total of the energy released by all the earthquakes those occurred in a
particular grid are computed out and plotted at the center of the grid. Since
the variation of energy (E) is very high, Log E (strain energy) has been used
in preparing the iso-strain release map. Then the isolines of energy have
been drawn to prepare the iso-strain release map. Relatively high strain
release points are demarcated as A, B, C and D in the strain release map.
6.3 Results and Discussion
(a) Temporal Variation of Strain Energy Released
Generally, it is assumed that the strain energy accumulation in a
tectonic block is almost linear. But the strain energy release pattern is not
linear because of the frictional force developed in the rock-fractures and the
temporal variation pattern of strain energy release forms a saw-toothed
curve, commonly known as Benioff Graph. It gives an idea about the strain
energy accumulation in a tectonic block at a particular time and is
considered as complementary to the study of the magnitude-frequency
104
relationship which defines the mean recurrence rate of any particular
magnitude earthquake in a tectonic block or region. Moreover, it indicates
the trend of activity with time and allows extrapolation for estimating strain
energy accumulation in the block in near future
As mentioned earlier, the study region is divided into three tectonic
blocks on the basis of the concentration of earthquakes, convergence,
alignment and direction of thrusting as well as structural pattern (Fig 4.2 of
Chapter IV), Benioff graphs have been prepared for all the three blocks and
for the region as a whole. Thus, altogether four Benioff graphs are obtained
as shown in Fig. 6.1 to Fig. 6.4. The lower and the upper bound in the
variation of strain energy represented by the Benioff graphs are also shown
in the figures The difference between this upper bound and the lower bound
gives an idea about the strain energy bearing capacity of the respective
block. The rate of strain energy accumulation estimated from the graphs,
approximate strain energy bearing capacity and the strain energy stored as
off 2005 in each block and in the region as a whole are presented in Table
6.1.
On the basis of the strain accumulation as off 2005, the magnitudes of
the maximum possible earthquakes that may occur in near future in different
blocks and the region as a whole are computed and present in the table
It has been observed that the estimated rate of strain energy
accumulation is low in Block-I and Block-Ill compared to Block-ll, which
reflects comparatively low seismic activity in these two blocks. Similar to the
seismic activity as observed in the epicentral plot (Fig. 4.2), the strain energy
105
accumulation rate is found to be the highest in Block-ll, in which subduction
is different from those of Block-1 and Block-Ill containing the arcuate bend
Table 6.1: Strain Energy Accumulation in the study region as off 2005
Rate of Strain
Region
Energy (E)
Accumulation
(in ergs)
Estimated
Strain Energy (E) due for
Maximum strain
release as of 2005
energy bearing
capacity
(in ergs)
in
ergs
in terms of
earthquake
(mb)
Block - 1
0.203 x 1020
39.69 x 1020
21.809 x 1020
6.47
Block - li
1 716 x 1020
184.96 x 1020
74.823 x 1020
6.70
Block - ill
0.185 x 1020
11.56 x 1020
0.624 x 1Q20
5.83
Whole Study
Region
4.840 x 1020
272.25 x 10120 129.277 x 1020
6.80
The strain release characteristic of Block-I as shown in Fig.6.1
indicates gradual accumulation of strain energy from 1964 to 1975,
which was released by an earthquake of magnitude 6.1 mb in 1975
normalizing the seismic stress developed in the block. Thereafter,
seismic activity in the block is very low indicating a quiescent period
and accumulation of strain energy up to 1994 and then again remains
silent till 2005 As of 2005, estimated accumulated strain in the block is
about 21.809 x 1020, which is equivalent to an earthquake of magnitude
6.47 mb.
106
i
year
. 6.1: Strain release characteristics for the earthquakes of the Block-1
Square root of energy
31
Square root of energy
25
107
Fig. 6.3: Strain release characteristics of earthquake of Block - III.
Benioff graph of Block-ll shows that most of the accumulated strain
energy in the block was probably released in 1988 by an earthquake of
magnitude 6.6 mb The rest of the accumulated energy is released almost
regularly and got neutralize in 1995 by an earthquake of magnitude 6 3 mb.
After 1995, strain accumulation in the block is going on and as off 2005, the
accumulated strain energy in the block is estimated to be about 74.823 x
1020 ergs.
The strain release pattern of Block-Ill indicates gradual accumulation
of strain energy in the block during the study period that has been released
mainly in three steps - in 1970 by an earthquake of magnitude of 5.4 mb, in
1976 by an earthquake of magnitude 6.2 mb and in 2000 by an earthquake
108
of magnitude of 6.2 mb As off 2005, the estimated stock of strain energy in
the block is about 0.624 x lQ20er9S.
The strain release pattern of the region as a whole is shown m Fig.6 4.
It shows three relatively quiescent periods - 1964-1974, 1976-1987and
1996-1999 having low s e is m ic activity followed by moderate earthquakes of
magnitudes 6 2 mb (1975), 6.6 mb (1988) and 6.2 mb (2000) respectively.
The strain energy accumulated during the period 1976 to 1987 could not
released completely by the earthquake occurred in 1988 (6.6 mb) and there
was regular release of energy in the region during the period 1989 to 1995
neutralizing the stress developed in the whole region by 1995. As a whole, the
Square root of energy
region shows strain energy accumulation of about 129.277 x 1020ergs by 2005.
U 1 ------------------- 1------------------- 1------------------- 1-------------------1------------------- 1------------------- 1------------------- 1------------------- 1------------------- 1------------------- 8
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
year
Fig. 6.4: Strain release characteristics for the earthquakes of the study
region as a whole.
109
2010
The strain accumulation and relaxation curves obtained in the above
analysis, in each of the three tectonic blocks and the region as a whole,
provide a very useful meaning of estimating the size of the future
earthquakes that may strike the region. These curves provide a reference
and minimum strain level, which gives an estimate about the amount of
stored strain in the region and hence, the possible size of the earthquake
that may occur, if the entire stored strain would release at a time. The
maximum possible magnitude of an earthquake that might be occurred in
near future in Indo-Burma orogenic belt as a whole and in the three tectonic
blocks as already mentioned are also presented in Table 6.1. It shows that
the magnitude of the maximum possible earthquake that may occur in near
future in Biock-I, Block-ll and Bloc-Ill are found to be about 6,47 mb, 6.7
mb and 5.8 mb respectively. The region as a whole, as of 2005 accumulates
strain energy equivalent to an earthquake of magnitude 6 8 mb.
The above results show that the earthquake risk in the area is not so
high. The size of the maximum magnitude earthquake that may occur in
each of the three tectonic blocks as well as in the region as a whole indicate
that only moderate earthquakes are likely in near future.
(b) Spatial Variation o f Strain En erg y R eleased
Iso-strain release map of the study region have been prepared as
discussed in section 6.2 and is presented in Figure 6.5. It reveals that high
strain energy release took place along the Eastern Boundary Thrust (EBT),
which is shown as dark shade in the map. Strain energy release is
110
com paratively low along Shan Boundary Fault and in the schupoen
comprising
Naga-Disang thrust system
Thus
from the seismic
seism otectonic co-relation it is observed that earthguakes occur mo>e
Eastern
Boundary Fault (EBT) compared to Schuppen belt and Sha
boundary fault. Thus, the strain energy release map reflects the areas
high and low seismic activity, which is very much applicable in the sei
(/)
03
C
03
(/)
L T
M T
T .S .
N T
D T
E .0 T .
F (B & C T i
F fB )
. . .
»
---------
L o h it T h r u s t
M ish w n i T h r u s t
T id d in u S u t u i e
N a g a Th r ust
D is a n u T h r u s t
E a s t e r i B o o u n c ia
B a s e rre rrt & Cov*
Basem ent F
Th ru s t
N e o t e c to* » c F
A n t if o r m
L in e a m e n t
F a u lt
Iso sliain Lines
t —
1
r ~
i
ETZ~]
Fig. 6.5 : Iso-strain Release map of the Study region
111
> 20 0
so
n
<18 5
1 8
5
6.4 Conclusion
The main purpose of the strain release characteristic study is to see
the pattern of strain release and to foresee its continuation Blockwise strain
release characteristics study shows a definite pattern with active and
quiescence periods during the last four decades. It is also observed that the
accumulated strain energy in the study region is released by some moderate
shocks in installment basis. The highest amount of strain energy due for
release is found in Block—II (74.82 x 1020 ergs) which has also experienced
highest seismic activity during the last four decades. The accumulated strain
energy in the region as a whole is estimated to be about 129.277 x 1020ergs
by 2005 which is equivalent to an earthquake of magnitude 6.8 mb
Iso-strain release map of the region reveals that high strain energy
release took place along the Eastern Boundary Thrust (EBT) Strain energy
release is comparatively low along Shan Boundary Fault and in the
Schuppen Belt comprising Naga Thrust and Disang Thrust.
-oOo-
112