CHAPTER 7
STRAIN RELEASE AND ACCUMULATION
The s t r a i n energy released i n earthquake i s
through and around the earth as e l a s t i c wave and i s
dissipated.
ra d i a te d
eventually
The s t r a i n rebound d u r i n g an earthquake is taken
t o be p r o p o r t i o n a l to the square r o o t of the energy r e l e a s e .
P a r t of the o r i g i n a l p o t e n t i a l energy of s t r a i n s to r e d i n the
rock roust go i n t o mechanical work,
as i n r a i s i n g c r u s t a l blocks
a g a i n s t g r a v i t y , o r i n cr u s h i n g m a t e r i a l i n the f a u l t zone;
p a r t roust be d i s s i p a t e d as he at.
Ac co rdin g to Gutenberg and
R i c h t e r , the r e l a t i o n s h i p between energy and magnitude of an
earthquake i s of th e form o f ,
log E -
5 .8 + 2 . 4 mb
(7)
where E i s the energy in ergs and mb i s the u n i f i e d magnitude
d e r i v e d from body waves recorded at t e l e s e i s m i c d i s t a n c e s .
B e n io ff
(1951) has s t u d i e d the s t r a i n r e l e a s e by the
w o r l d ' s sh all ow earthquakes and concluded t h a t the r e l e a s e is
appr o xi m a t e l y c o ns ta n t s i n c e 1907 (Chouhan,
1979).
Chouhan
( 1 9 6 6 ) ha 3 st u d ie d the r e g i o n a l s t r a i n r e l e a s e c h a r a c t e r i s t i c s
o f I n d i a n earthquakes. He observed two n o n - l i n e a r segments
fo l l o w e d by a l i n e a r one.
The s t r a i n c h a r a c t e r i s t i c s of no r t h e a s t
I n d i a was s tu di ed i n d e t a i l by Chouhan at a l .
(1984).
( 19 66 ) and Goswami
Assuming a l i n e a r p r o b a b i l i t y of earthquake occu rre nc e
o v e r a period of feu y e a r s , a d i r e c t e x t r a p o l a t i o n of the p a t t e r n
65
of s t r a i n relaaae i n t o immediate f u t u r e can throu some l i g h t
on f u t u r e earthquake a c t i v i t y expected in a given r e g i o n .
Here, s t r a i n release and accumulation
of the earthquakes
of nor th ea st Ind ian region as a uhole f o r the period
1897 to
1987, and bl oc kuise energy rele ase studi es have been done.
Theory and Wethod of A p p l i c a t i o n
D r . Hugo Benioff
:
(1949) has shown that the p o t e n t i a l
energy £p of a volume ( U ) of a rock i s given by,
Ep « 0 . 5 yU US2
where
(8)
i s the c o e f f i c i e n t of shear and S i s the average
j u
s t r a i n j u s t before the earthquake.
Then energy released by
s eis m ic waves i s given by
Et - 0.5
tJd
US2
(9 )
where f i s the f r a c t i o n of energy released as seismic waves.
I f the s t r a i n i s reduced to zero d u r i n g the earthquake
by some movement along a f a u l t , then the average s t r a i n S is
p r o p o r t i o n a l to the f a u l t displacement (Xj.) i . e .
S
oC
or,
Xjj
S * C Xf
(10)
uhere C i s the co ns ta n t.
From equation ( 9 ) we get ,
et
-
0.5 f
LX
UC2X2
66
- °i
et
4
( id
where G2 » 0 .5 f J U
UC2
The equation ( 1 1 ) i m p l i e s t h a t f a u l t displacement ( X ^ ) is
p r o p o r t i o n a l to the square r o ot of the energy r e l e a s e .
Hence using the magnitude energy r e l a t i o n of Gutenberg
and R i c h t e r i n equation ( 1 1 ) ue g e t ,
i
l o g E-j-
*» l o g G^Xp
1
G
or, Xf
10
= 2 .9 + 1.2 mb
2.9 + 1,2mb
( 12 )
1
This equation ( 1 2 ) gives the r e l a t i o n between the f a u l t
displ aceme nt ( X p ) and earthquake magnitude mb.
Thus i f magnitude
o f a l l earthquakes o o c u r r i n g i n any one f a u l t system over a
per io d of y e a r s ,
(stra in )
are known, one can p l o t f a u l t displacement
that occur d u r i n g t h a t time p e r i o d .
g i v e s t r a i n rebound c h a r a c t e r i s t i c s
Such p l o t s , which
represents s p u r t s of s eismi c
a c t i v i t y separated by r e l a t i v e l y quiescent p e r i o d .
The r e s u l t i n g
c u r v e w i l l be a saw toothed c u r v e , the upper peaks of which
r e p r e s e n t the exhaustion of accumulated s t r a i n caused by e a r t h ­
quakes. A l i n e drawn through these peaks, t h e r e f o r e ,
t h e r a t e of s t r a i n ge ne ra ti on i n gi ve n r e g i o n .
r a t e of s t r a i n g e n e ra t io n i n a r e g i o n , i t
represents
Assuming t h i s
i s then p o s s i b l e to
i l l u s t r a t e the probable s t r a i n l e v e l s obtained at d i f f e r e n t times
by means of a s t r a i n accumulation and r e l a x a t i o n c u r v e .
In the
b e g i n n i n g of the period df s t u d y , the s t r a i n accumulation and
r e l a x a t i o n cur ve s t a r t e d from an a r b i t r a r y l e v e l which r epr ese n t
the s t o r e of s t r a i n at th at time.
I t i s then made to f o l l o w a
67
s lo p e
equal to
s t r a in
p la c e
t im e
to
rebound
th e
ra te
c h a r a c t e r is t ic s
w h ich r e p r e s e n t
a x is ,
o ccu rre n ce o f
A p p lic a t io n
e a r th q u a k e .
o f mean r a t e
to
th e n o r t h e a s t
to n o rth e a s t
Burma t e c t o n i c
and
u n it
a d ja c e n t
t e c t o n ic
1897 t o
ta k e n
th e
u n it ,
g e n e r a t io n
a g a in
u n t il
S h illo n g
F ig u r e
I n d ia n
t e c t o n ic
p la t e a u ,
equal
h ills
in
s e p a r a t e ly d u r in g
of
th e t h r e e
t h e p e r io d
r e g io n
c h a r a c t e r is t ic
u s in g
d ia g ra m
a ll
cu rv e
e a rth q u a k e s
4 .0 from
each b la c k
1897 t o
th a t
S i n ^ /erg,
s in c e
e n e rg y r e le a s e have been c a l c u l a t e d
s t r a in
re p re s e n ts
a c c u m u la t io n i s
in
fo r
h a v in g
1987
lin e
a g a in
S®Cs/T ( e q u a t io n
t'wo y e a rs
th e s e c u la r s t r a in
lin e a r
been
v e r t ic a l
th e s t r a in
lin e
th e
m a s s if
re p re se n t
fa c to r
In d o -
and
t h e r e le a s e o f e a r t h q u a k e e n e rg y i n s/T w h ich
c o n t in u o u s
th e
e a rth q u a k e m a g n itu d e has
r e le a s e
In t h i s
P lik ir
i.e .
u n it
a re a b lo c k s
4 .0 o r g r e a t e r th a n
12.
u n it s
re p re se n t
T h is
on th e
j
E a s t e r n H im a la y a
The lo w e r l i m i t
n o rth e a s t
shown i n
p o in t ,
th e c u r v e
th re e t e c t o n ic
The n in e t e e n
body wave m a g n itu d e
is
ta k e s
and a c c u m u la t io n method h a v e been
4 .0 mb. The s t r a i n
e n t ir e
At t h is
from
by an amount e q u iv a le n t
o f s t r a in
u n it s h a v e been s t u d ie d
as
an e a rth q u a k e
T h e re fo re
I n d ia
I n d ia i n
c o v e r in g
a re a .
1987.
o b t a in e d
th e n e x t e a r th q u a k e .
The e n e rg y r e le a s e
t e c t o n ic
u n t il
r e le a s e o f s t r a i n .
o f th a t
th e s lo p s
a p p lie d
g e n e r a t io n
th e c u r v e d ro p s v e r t i c a l l y
th e s t r a i n
f o llo w s
o f s t r a in
u n it.
9 ).
The
g e n e r a t io n s h o w in g
and may be r e p r e s e n t e d
by t h e
68
The Northeast Indian Region
1200
1000
800
600
400
200
0
400
200
0
10
1920
1940
1960
1980
YEARS
(a) Strain release characteristics and (b)
Strain accumulation and relaxation curve ».
for the entire northeast India using all
earthquakes having magnitude 4.0 or
greater from 1897 to 1987
69
equation,
Z S - A + Bt
(13)
where A 4 B are c o n s t a n t s and t i s the time in y e a r s .
From F ig u r e 1 2 (a ) i t i s seen t h a t a f t e r the o c c u r r e n c e
o f th e g r e a t Indi an earthquake
of
12th 3une,
quake energy r e l e a s e was s p o r a d i c .
1897, the e a r t h ­
In between 1918 t o
r e l e a s e uas found to be a p p r o x im a t e ly unif or m.
In 15th August,
1950 another g r e a t earthquake o c c u r r e d i n t h i s r e g i o n .
1954 t h e energy r e l e a s e seems t o be s m a ll i . e .
1950, the
After
the t e c t o n i c
a c t i v i t y o f the region i s r e l a t i v e l y l e s s .
The F ig u r e 12 (b ) shows s t r a i n ac c u m u la ti on and r e l a x a t i o n
c u r v e f o r the n o r t h e a s t I n d i a .
This F ig u re has been drawn,
t h e r a t e o f s t r a i n g e n e r a t i o n from F i g u r e 1 2 ( a ) .
lin e
The i n c l i n e d
r e p r e s e n t s t r a i n a c c um ul a ti on and the v e r t i c a l l i n e s ,
stra in release.
usi ng
the
The h o r i z o n t a l l i n e drawn through t h e l o w e s t
p o i n t s o f t h e s t r a i n accum ula ti on and r e l a x a t i o n c u r v e i s th e
r e f e r e n c e s t r a i n l e v e l , which may be used t o d e t e r m in e the
accumulated s t r a i n at any t i m e .
uas c o n t in u e d upto
After
1948 and 1949.
1897, s t r a i n ac cum ula ti on
In 1950 and 1951 a l l the
accumulated energy uas c o m p l e t e l y r e l e a s e d by a b i g earthquake
and i t s
is
aftershocks.
continuing.
There a f t e r , s t r a i n energy accum ulat io n
Feu accumulated energy uas r e l e a s e d
quake o c c u r r e d on 6th August,
It is
1988.
This i s
by an e a r t h ­
a v e r y minor r e l e a s e .
i n d i c a t e d t h a t f o r r e l e a s i n g a l l the accumulated s t r a i n
t h e r e must be a b i g earthquake in t h i s r e g i o n o f magnitude o f
ab out 8 . 0 mb
70
Block 1 and 2 (a part of eastern Himalaya)
:
The st r a i n re l e a s e c h a r a c t e r is t ic curve f o r blocks 1
and 2 i s shoun in Figure 13. I t is observed that the seismic
a c t i v i t y of this region uas r e l a t i v e l y low before 1954. There
may be omission of fewer earthquakes in the e a r l i e r period
because of the incomplete reporting.
The accumulated st ra i n
energy was completely released in 1954*1955 and 1959-1960.
But a f t e r I960, the seismic a c t i v i t y in this region decreasing,
in d ic a tin g that a gradual increase in stored s t r a i n energy.
The highest magnitude earthquake may be occur in this region
i s to be in the order of 6.4 mb.
Block 3. 4 and 5 (a Part of eastern Himalaya)
s
For blocks 3, 4 and 5, the s t r a i n re lea se and accumulation
curve shoun in Figure 14. From f i g u r e i t i s found that from 1928
to 1956 the energy re lea se i s not continuous year by year.
But
a f t e r that there uas gradual accumulation of s t r a i n u p t i l l nou.
This indicates that stored st ra i n energy in those three blocks
is
f a i r l y high.
The highest magnitude earthquake may be occur
in near future about 6.9 mb.
Block 6 and 19 i
Th8 3 e tuo blocks 6 and 19 f a l l s
in Assam Syntix region.
The s t r a i n rebound c h a r a c t e r is t ic curve for these tuo tectonic
blocks is shoun in Figure 15. I t is observed that the seismic
a c t i v i t y of north eastern part of northeast India uas lou before
1950 except feu energy release for the earthquake in 1906. After
"
3
4
2
Figure 13.
lu
ce
LLl
o 4
UJ
UJ
QZ
3
^rsj
'
rel^a^ > Cearar
Pigure 14- (a) strain
ib ?train
(b) Strain
accumulation and relaxatio
curve for
n curve for the earthquablocks 3,
kes of the blocks 1 and 2
YEARS
release characteristics and
accumulation and relaxation
the earthquakes of the
4 and 5
YEARS
71
72
1906 the r a t e of energy r e l e a s e has been seem to in c r e a s e
g r a d u a l l y and almost a l l the sto re d
energy was relea sed in
1950 due to the major earthquake of
15th August,
Figure
1950. The
15 does not i n d i c a t e any l i n e a r s t r a i n ge n e ra t i o n as
i n the o th e r blocks of n o r t h e a s t I n d i a . Hence no l i n e a r s t r a i n
g e ne ra t io n l i n e of the form o f equa ti on ( 1 3 ) can be draun f o r
the block 6 and 19. T h e r e f o r e , u i t h o u t knowledge of s t r a i n
g e n e r a t i o n r a t e , no s t r a i n accumulation and r e l a x a t i o n can be
draun.
Block 7. 8 and 9 :
The s t r a i n r e l e a s e c h a r a c t e r i s t i c s f o r the blocks 7, 8
and 9 which comprise of N i k i r massif, K o p i l i lineament etc are
shown i n F i g u r e
16. T hi s f i g u r e i n d i c a t e s
a lin e a r strain
g e n e r a t i o n r a t e n e a r l y uniform earthquake a c t i v i t y
upto 1950.
The s t r a i n accumulation and r e l a x a t i o n curve are shown i n
Figure
16(b).
After
1950 the energy r e l e a s e i s
also c o nt in u e
by some s m a l l magnitude earthquakes but n u m e r i c a l l y i t
sm all.
is very
So we may i n f e r t h a t v e r y hig h l e v e l of accumulation has
been found f o r an earthquake h a v i n g magnitude 7.5 mb may occur
i n this region.
Slock
10.
Figure
11.
12 and 13 j
17 shows the s t r a i n r e l e a s e c h a r a c t e r i s t i c s
t h e earthquakes of S h i l l o n g p l a t e a u ,
fa u lt.
of
Bengal bas in and Dauki
The f i g u r e also i n d i c a t e s a l i n e a r r a t e of s t r a i n
generation.
A f t e r t h e great earthquake r f 12th Jun e,
1897, a
m
m
XJ
o
Figure 15. Strain release characteristics
curve for the earthquakes of
the blocks 6 and 19
YEARS
Figure 16.
(a) Strain release character­
istics and (b) Strain accu- w
mulation and relaxation
curve for the earthquakes of
the blocks 7, 8 and 9
YEARS
Block 7,8 and 9
B lo c k 10,11,12 a n d 13
1900
1940
1920
1960
1980
YEARS
Figure 17.
(a) Strain release characteristics and (b)
Strain accumulation and relaxation curve
for the earthquakes of the blocks 10, 11,
12 and 13
75
m ajor
and
part of
1923 i n
curve
le ve l
re su ltin g
the
stra in
uas
two m a j o r e a r t h q u a k e s .
re la xa tio n
the high
accum ulated
in
release
are shoun in
of stored
released
The s t r a i n
Figu re
energy
in
17(b)
very
low i n d i c a t i n g
till
1 9 87 .
a high
Accum ulated
instantan eously
stra in
le vel
stra in
may r e s u l t
in
in
of
the
1918
four
and
if
and
that
1923
Thereafter
blocks
accumulated
by t h a t y e a r
1918
de picts
above 7 .0 .
th is
year
accum ulation
cle arly
the year
an e a r t h q u a k e o f m a g n i t u d e
o f accum ulated
in
w er e
stra in
energy
released
an e a r t h q u a k e h a v i n g m a g n i t u d e
7 . 7 mb.
Block
14 and
The s t r a i n
from
in
blocks
Figu re
a ctivity
in
release
14 and
18.
15,
T his
th is
15 ( a P a r t
Figu re
18(b).
in
1938,
1 9 5 4 and
energy
release
in d icate s that
accum ulated
of
th is
region
Block
a part
of
is
shows
alm ost
The c u r v e s h o u s
19 87.
In
the
that
stra in .
u ill
and
16,
1 9 38 .
relaxatio n
three d is t in c t
the p r e v io u s
are shoun
earthquake
uniform s in c e
The
curve is
high
two y e a r
shoun
accum ulation
accumulated
the t h i r d
e a rth q u a k e must o c c u r t o
The maximum m a g n i t u d e f o r
the
peak
release
the
earthquake
be 7 . 3 mb.
16 ( a p a r t
For block
earthquakes
the
by some m o d e r a t e e a r t h q u a k e s . B u t
big
i
Indo-Burm a b ord er
accum ulation
a fa irly
region
ch a ra cte ristics of
curve c le a r ly
correspondin g s t r a in
in
Indo-Burm a b o rd e r)
of
I n d o r Burma b o r d e r )
the s t r a i n
earthquakes are shoun i n
Figu re
release
19.
This
j
c h a r a c t e r is t ic s of
figu re
the
ind ic a te s l i n e a r
Block 1^and15
YEARS
Figure 18.
(a) Strain release characteristics and (b)
Strain accumulation and relaxation curve
for the earthquakes of the blocks 14 and
15
Figure 19. (a) Strain release characteristics
and (b) Strain accumulation and
relaxation curve for the earth­
quakes of the block 16
70
s t r a i n g e n e r a t i o n r a t e and n e a r l y uniform earthquake a c t i v i t y .
The s t r a i n accu m ul a ti on and r e l a x a t i o n cu r ve are shown in
Figure
19(b).
The s t o r e d
energy a f t e r
by an earthquake i n t h e year
stored
1954.
1926 was c o m p l e t e l y r e l e a s e
T h e r e a f t e r s m a ll amount o f
energy uas released by a number o f i n t e r m e d i a t e and sma ll
magnitude e ar t h qu ak es .
In the year
1987, the accumulated s t r a i n
energy uas enough t o produce an earthquake o f magnitude
On August 6,
1908 an earthquake o f
7.4 mb.
magnitude 6.8mb ac t u a l l y
o c c u r r e d in t h i s b l o c k uhich p r o b a b l y r e l e a s e d
most of
the accumulated s t r a i n .
Block
17 (a Part o f Indo-Burma b o r d e r )
:
For t h i s b l o c k t h e s t r a i n r e l e a s e c u r v e are shoun in
F i g u r e 20.
The Figu re 2 0 (b ) shows t h e s t r a i n accu m ul a ti on and
r e la x a tio n o f this
r e l e a s e d in
7.0 .
tecton ic block.
The accumulated s t r a i n uas
1932 and 1950 by t h e earthquake having magnitude
T h e r e a f t e r t h e r e was grad ua l ac cum ula ti on o f s t r a i n , some
o f uhich uas r e l e a s e d in 1970 by an earthquake o f magnitude
6 .5 mb. The accumulated s t r a i n u n t i l
u ill
1987, i f
released
at o n c e ,
be an earthquake e q u i v a l e n t t o magnitude 7 . 2 mb.
Block
18 (a pa rt o f Indo-Burma b o r d e r )
:
The s t r a i n r e l e a s e c h a r a c t e r i s t i c s f o r the earthquakes
o f t h i s b l o c k are shown in F ig u r e 21.
This f i g u r e i n d i c a t e s t h e
l i n e a r s t r a i n g e n e r a t i o n . The c o r r e s p o n d i n g s t r a i n ac cum ula ti on
and r e l a x a t i o n cur ve i s shoun i n F i g u r e 2 1 ( b ) .
uas r e l e a s e d by an earthquake o f magnitude 6 . 4 .
The s t o r e d energy
A f t e r th at the
79
(ERG) '2
E / 2 (ERG)' 2
Block 17
YEARS
Figure 20.
(a) Strain release characteristics and (b)
Strain accumulation and relaxation curve
for the earthquakes of the block 17
Block 18
Figure 21.
(a) Strain release characteris
-tics and (b) Strain accumula­
tion and relaxation curve for
the earthquakes of the block
18
strain
energy seems to be accumulating t i l l
I s o s t r a l n r e l e a s e cont o u r
1987.
j
The s t r a i n energy r e l e a s e of a re g i o n i s a good represen­
t a t i o n of s eis m ic a c t i v i t y of the region*
To determine the energy
r e l e a s e from n o r t h e a s t I n d i a ' s earthquakes and to compare the
r e l a t i v e a c t i v i t y of d i f f e r e n t pa r t s o f the r e g i o n , ue have
a l r e a d y d i v i d e d the whole n o r t h e a s t I n d i a i n t o
F o r drawing such type o f con tou r map i t
19 equal b l o c k s .
i s necessary to c a l c u l a t e
the energy r e l e a s e from each bl ock . The energy released c a l c u l a t e d
from 1897 to 1987 i n each block are shown i n Table 11.
The values
of s t r a i n energy r e l e a s e have been p l o t t e d i n the c e n t r e of
each b l o c k . Then the i s o - s t r a i n r e l e a s e contour at the i n t e r v a l
of energy va l u e of
10 are drawn.
The contour i n F i g u r e 22 gives
th e s e i s m i c i t y i n terms of energy r e l e a s e .
Again the energy r e l e a s e from each block has been c a l c u l a t e d
from 1954 to
1987 as shown i n T a b l e
12.
The r e s u l t has been shown
i n F i g u r e 23.
F i g u r e 22 shows t h a t n o r t h e a s t e r n and southwestern parts
o f n o r t h e a s t I n d i a are s e i s m i c a l l y v e r y a c t i v e .
T hi s may be due
t o the l a r g e amount of energy r e l e a s e by the earthquakes of
1897 (FI * 8 . 7 ) and 1950 (M * 8 . 6 ) ,
energy r e l e a s e a f t e r
( F ig u r e 23).
because c o n s i d e r a t i o n of
1954 does not show these two maxima
But i n both cases, h i g h e r a c t i v i t y i n Indo-Burma
82
TABLE 11 j Energy release calculated from each block in
northeast India (from 1897 to 1987)
Block
Energy release
s / l
X 1 0 10 ergs
Block 1
2.5
Block 2
2.5
Block 3
20.9
Block 4
7.6
Block 5
66.7
Block 6
335.5
Block 7
2.4
Block 8
24.4
Block 9
1.7
Block 10
338.8
Block 11
21.6
Block 12
50.0
13
0.5
Block 14
13.5
Block 15
57.4
Block 16
93. 1
Block 17
51.4
Block 18
8.7
Block 19
24.8
Block
border is evident. The eastern Himalayan region along the
northern main boundary fault also shows relatively higher
strain release. Shillong plateau which was the seat of 1897
earthquake seems to show little energy release during the last
80 years or so.
83
Figure 22. Seismic energy release due to earthquakes
in northeast India during the period 1897
to 1987
(contour interval is 10 X lO^ergs)
TABLE
12 s Energy release calculated
from each block in
northeast India (from 1954 to 1987)
Energy release
Block
s/E
X 1010 ergs
1
2.5
Block 2
2.5
Block 3
3. 1
Block 4
1.3
Block 5
3.6
Block 6
Block 7
1.1
0.2
Block 8
0.7
Block 9
0. 4
Block
10
1.4
Block
11
0.5
Block
12
3. 1
Block
13
0.5
Block
14
13.5
Block
15
29. 6
Block
16
31.0
Block
17
10.2
Block 18
8.7
Block
4.9
Block
D iscussion
19
:
The pattern of strain release seems to show some
correlation uith the geological structure.
The strain
accumulation and relaxation curves, obtained in the above
85
Figure 23. Seismic energy release due to earthquakes
in northeast India during the period 1954
to 1987 (contour interval is 10 X 109ergs)
86
a n a l y s i s , i n each of the nineteen blocks and n o r t h e a s t I n d i a
as a whole, provided a v e r y u s e f u l means of e s t i m a t i n g the
s i z e of the f u t u r e earthquake* The s t r a i n energy r e l e a s e
cu rve s show
a r e f e r e n c e and minimum s t r a i n l e v e l which i s
used to estimate the amount of s to re d s t r a i n and hence the
p o s s i b l e s i z e of earthquakes i f
relea sed at a time.
the e n t i r e sto re d s t r a i n is
The maximum p o s s i b l e magnitude of e a r t h ­
quakes t h a t may occur i n the near f u t u r e i n the no r t h e a s t
I n d i a as a whole and the blocks s e p a r a t e l y are shown i n Table
13.
These r e s u l t from energy accum ula tion and r e l a x a t i o n show
t h a t the p o s s i b i l i t y of b i g earthquake i n n o r t h e a s t I n d i a is
ve ry high.
TABLE 13 j The magnitude of the f u t u r e earthquakes that, may
o ccu r i n no r t h e a s t I n d i a on the basis of energy r e le ase
Regions
Blo ck
Maximum p o s s i b l e body wave magnitude
6.4
1, 2
Block 3,
4, 5
6.9
Block 6,
19
cannot be assigned
Block 7, 8,
7.5
9
12,
13
7.7
Block
10,
11,
Block
14,
15
Block
16
6.6
Block
17
7.2
Block
18 '
6 .4
N o r th e a s t I n d i a
8.0
7.3