Seismicity and tectonics in the Hindukush mountains, Sulaiman and Kirthar ranges in the light of plate tectonics theory.

Abstract

A seismicity map of the Pakistan-Afghanistan region that lies between the latitudes 280 & 380 N and longitudes 660 & 750 E is given using all available data for the period 1890-1972. The earthquakes of magnitude 4.5 and above were considered in preparation of this map. A strain-energy release map for the same period was prepared following the method used by Allen et al. (1965). On the basis of these two maps, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent mutually orthogonal seismicity lineaments, namely, the northwestern and the northeastern trends characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well defined zones of intermontane seismicity exhibiting north-south alignment. Thirty-two new focal mechanism solutions for the above mentioned region have been determined. These, together with the results obtained by earlier workers suggest the prevalence of thrust as well as strike-slip faulting in the area. The Salt range; the Sulaiman wrench zone, the Kirthar wrench zone as well as the supposed extension of the Murray ridge upto the Karachi coast appear to be mostly undergoing strike-slip movements. In the Hindukush region, thrust as well as strike-slip faulting are found to be equally prevalent. Almost all the thrust type mechanisms belonging to the Hindikush area have both the nodal planes in NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of F-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast to Uzbekistan on the northwest through the Hindukush. Almost perpendicular to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting. The above results are discussed in the light of convergence between the Indian and predicted by plate tectonics theory.

Full text

Mavsom, (1979), 3
0.2,
s:
3,
nl
551.2+ (2 35· 2 1)
Seismicity and tectonics in the
Hindukush
mountains,
Sulaiman and Kirthar ranges in the li
ght
of
plate
tectonics theory
R. K.
rERl\
l A , 1\IAI\ OJ M U KH O PA
DH
YAY and A. K . lIH A
I\JA
Indian S
cho
ol of.\Jillts,
Dhanbad
A
BSTRACT
Aseismicity
In':lP
of th Pakistan-Afghanistan region that lies between th e latitudes 28° &38° X
and
longitude; 66' &75
'E
is given using all avail able
data
for the period 1890-1972.
The
earthquakes
of
mag
-
nitude
4-.
5
and
above were conside red in p
reparat
ion of th is
map
. A strain-energy release
map
for the same
period
was
pr
epar
ed
following the m
ethod
used by Allen et
01.
(1965).
On
the basis of these two
maps
,
it
is
observed
that
the seismicity
patt
ern
over the well-known Hind ukush region is
quite
complex.
Two
promi
-
nent
mutua
lly ort hog onal seismicity lin
eam
ents,
namely.
the northwestern
and
the northeastern
tren
ds
ch arac terize the Hin d ukush area.
Th
e
nort
hwestern trend appears to ex
tend
from
the
M ain Bounda ry
Fault
of
the
Ka
shmir Himalaya on the southeast to the plains of th c Amu
Darya
in Uzbekistan on
the
northw
est beyond the Hindukush.
Th
e
Sulaiman
and
Kirtha
r
rang
es
of
Paki
stan
are
well defined zones
of
intermontan
e seismicity exhi biting north-s
outh
alignment.
Thirt
y-two new focal me
chani
sm solutions for the above ment ioned region
hav
ebeen
determined.
T hese, togeth er wit h the results
obtained
by ea rlier wor kers suggest the prevalence
of
th rust a s well as
strike-slip faulting in the ar ea.
Th
e Salt ra nge ; the
Sulaiman
wrench zone, the Kirthar wrench zone as \ \
'(.
11
as the
sup
posed extension
of
the
Mu
rray ridge
upto
the
Kara
chi coast
appear
to be mostly und
er
going stri-
ke-slip movement s. In th e Hi
ndu
kush region, thrust as well as st rike-slip faulting are f
ound
to be equally
prevalent . Alm ost all the th rust type mechanisms belonging to the H indu kush
area
have both the
noda
l
planes in
~
\V
·SE
dir
ection for shallow as well as
intermediat
e
dept
h
earthquakes
. '111e
dip
of
P·axes
for
the
event
s indi
cating
thru
st type mechanisms
rarely
exc eeds 35°. T he d irect ion of seismic sli p ve
ctor
obtain
ed
thr
ough
thru
st type solutions is always dir ected t
oward
s the
north
east.
Th
e epieentral
pattern
together
with
these results suggest a deep-seat ed fault zone paral1eling the n
orthw
esterl y seismic
ZOne
underneath
the
Hinduku
sh.
Thi
s N\V·lin eam
ent
has a preference for thrust faulting,
and
it
appears
to
ext
end
from the vicinity of the
Main
Boundary
Fault of the K ashmir
Hima
laya
on the southeast
to
Uzbekis
tan
on the
nort
hwest
thro
ugh the
Hindu
kush.
Alm
ost perpendic
ular
to this 1\'\V-scislllic zo ne,
there
is a
NE·
scismic lineam
ent
in which there is a preference for strike-slip faulting.
The
above results ar e discussed in the li
ght
of convergence between th e
Indian
and
Eurasian plates
predicted by
plat
e t
ecton
ics th
eor
y.
(221)