International Journal of Geosciences, 2011, 2, 318-325
doi:10.4236/ijg.2011.23034 Published Online August 2011 (http://www.SciRP.org/journal/ijg)
Clusters of Moderate Size Earthquakes along Main
Central Thrust (MCT) in Himalaya
Basab Mukhopadhyay
Geodata and Database Division, Geological Survey of India, Kolkata, India
Received April 25, 2011; revised June 6, 2011; accepted July 21, 2011
Abstract
The Main Central Thrust (MCT) in Himalaya is seismically active in segments. In recent times, strain release
within these active segments produce five spatial clusters (A to E; Figure 1). The seismicity within the clus-
ter zones occurs in two depth bands; corresponding to the base of upper and lower crust. Depth sections
across the clusters illustrate gently dipping subducted Indian Plate, overriding Tibetan Plate and compressed
Sedimentary Wedge in between, with mid crustal ramping of MCT. Several presumptions / hypotheses have
been put forward to decipher the causes of clustering along MCT. These are segmental activation of MCT,
cross fault interactions, zones of arc parallel and arc perpendicular compressions, pore pressure perturbations,
low heat flow zones etc. But these hypotheses need to be evaluated in the future after more ground level data
are available. The maximum size of seismic threat that MCT can produce is inferred to be around Mw 7.0 in
those clusters.
Keywords: MCT, Himalaya, Earthquake, Seismic Clusters, Seismic Potential
1. Introduction
The E-W bow like shape with trend reversal and higher
elevations at terminal ends, Nanga Parbat (Western Syn-
taxis) in the West and Namcha Barwa (Eastern Syntaxis)
in the East (Figure 1) characterise the physiography of
the Himalaya. Himalaya came into existence due to
collision of Indian shield with Eurasian/Tibetan Plate,
compression and sequential thrusting along major faults
such as Main Central Thrust (MCT), Main Boundary
Thrust (MBT) and Main Frontal Thrust (MFT) [1,2].
MCT, a major shear zone separating the Higher Hima-
layan Crystallines from the Lesser Himalayan Series, is a
major thrust fault that has contributed to the formation of
the Himalaya [3]. MCT is thought to be an early Pale-
ozoic Suture Zone [4-6]. It extends for nearly 2500 km
along strike and has been the site of at least 120 - 140 km
and perhaps more than 600 km of displacement [7-9]
from its formational site. In recent times, due to locking
of Himalayan thrust in the frontal part, the interactions
between MFT, MBT and allied thrust planes have
generated many great earthquakes (1905 Kangra, 1934
Bihar, 2005 Kashmir etc.) in the frontal belt of Himalaya.
Though Lesser Himalayan earthquakes and related seis-
mic hazards have been studied in details [10], the seis-
mogenesis due to MCT and clustering of earthquakes
related to this are somewhat neglected or oversimplified.
At the same time, the seismic potentiality of the MCT to
generate moderate size events (1980 Gangtok, 1991
Uttarkashi, 1999 Chamoli, 2009 Bomdila etc.) in the
MCT proper and/or along its mid crustal ramp zone is
also debated. Over and all, the assessment of earthquake
hazard due to MCT is always been under-played. In this
particular note, the author has tried to unearth causes of
moderate size seismic clustering along different parts of
MCT, its seismotectonic behaviour, interaction between
sub-surface structures, and also to judge its seismic
hazard potentiality.
2. Himalaya and Some Salient Features on
MCT
The pioneering research of Seeber, Armbruster and
Quittmeyer [11], was the first to suggest a tectonic model
of the Himalaya from seismic data analysis and suggested
a gently dipping Indian slab, overriding Tethyan slab
(Tibetan Plate) and a tapering Sedimentary Wedge (SW)
(as shown in Figure 2(a)). The Sedimentary Wedge is
decoupled from the Indian and Tethyan slabs at its
boundary. Simultaneous activation of MCT and MBT is

B. MUKHOPADHYAY 319
Figure 1. Seismo-tectonic map of Himalaya with earthquake (mb ≥ 4), major thrusts, faults and lineaments. Earthquakes (mb
≥ 5) generated by MBT (black square) and MCT (black circle with white outer rim) and its allied thrusts are shown. A- A’ to
E- E’ is section lines. Note the CMT beachballs in the map with its number plotted. BF – Bomdila Fault, DKF – Dhansiri –
Kopili Fault, EL – Everest Lineament, EPF – East – Patna Fault, ES – Eastern Syntaxis, GBF – Great Boundary Fault, GL –
Gaurishankar Lineament, ITS - Indus – Tsangpo Suture, JF – Jhelum Fault, KL- Kanchenjungha Lineament, KS – Kalyani
Shear, LF – Lucknow Fault, MBT – Main Boundary Thrust, MCT – Main Central Thrust, MDF – Mahendragarh –
Dehradun Fault, MF – Moradabad Fault, MKF – Malda – Kishanganj Fault, MSRF- Munger-Saharsa Ridge Fault, MSMF –
Munger-Saharsa Ridge Marginal Fault, PEL – Purnia – Everest Lineament, RF- Ropar Fault, SF – Sundernagar Fault, TL –
Tista Lineament, WPF – West Patna Fault, WS – Western Syntaxis, Al – Allahabad, Bo – Bomdila, Dd – Dehradun, Ga –
Gangtok, Ja – Jaipur, Jam – Jammu, K- Kathmandu, Nd – New Delhi, Sl – Simla.
also proposed in this model. However, Ni and Barazangi
[12] argued that MCT is dormant presently and MBT is
active. In their model, the interface between the subduct-
ing slabs and sedimentary wedge is a ‘plane of detach-
ment’. The zone between ‘plane of detachment’ and
junction between Sedimentary Wedge and Tibetan Plate
roughly coincides with the high topographic gradient
between Lesser and Higher Himalaya and characterised
by steep dip, ramping of MCT and Himalayan crust at
the northern edge of Indian Plate [7,13-15]. Further, the
geometry of the Himalayan collision boundary is wedge-
shaped; the base of this wedge is defined by a decolle-
ment, Main Himalayan Thrust (MHT), a major reflector
that has been identified by seismic investigation [16].
The wedge defines crustal scale fault bend folds, forma-
tion of Lesser Himalayan Duplex that forms taper and
controlled the foreland-ward propagation of the thrust
sheets [17]. The metamorphic grade within the Lesser
Himalaya increases towards the MCT with highest-grade
rocks (kyanite and sillimanite gneisses) is found within
the MCT shear zone. The MCT shear zone is chara-
cterized by a well-documented inverted metamorphism.
Arita [18] placed two thrusts (MCT-I and MCT-II) on
each side of the MCT shear zone.
MCT, since its inception, has been activated, de-
activated and again reactivated several times in the
geological history and produces recurrent seismicity as it
is accomplishing today. The movement along MCT and
corresponding metamorphism, anatexis, followed by
granite emplacement is found around 25 - 15 Ma with a
peak at 21  0.5 Ma [19]. The youngest deformation
episode is marked around 3.1 Ma [20]. Recent SHRIMP
monazite and zircon geochronology provides evidences
of anatexis affecting the upper portion of the MCT shear
zone occurred during Early Oligocene (~31 Ma) [21].
The samples from the base of Main Central Thrust yield
4.5 ± 1.1 Ma (T = 540 ± 25˚C and P = 700 ± 180 MPa
from coexisting assemblage) and 4.3 ± 0.1 Ma (five grains)
matrix monazite ages, suggesting Pliocene reactivation
of the structure [22]. Cooling ages and geomorphic
considerations have been taken to suggest that the MCT
zone was activated in the early Miocene [23] and may
have been reactivated as an out of sequence thrust in the
Pliocene [24] and may even still be active [25]. Further,

B. MUKHOPADHYAY 320
Figure 2. Schematic seismo-tectonic sections across Himalaya. Position of section lines (A-A’ to E-E’) is marked in Figure 1
and corresponding sections are numbered (a) to (e) respectively. Note the subducting Indian plate, overriding Tibetan plate
and an ever-compressed Sedimentary Wedge (SW) in between. Also make a note of the mid-crustal ramp of MCT in all the
sections. Earthquakes of table-1 are plotted with star symbol and annotated with year and magnitude (mb). In these depth
sections, relocated seismic events using EHB technique (courtesy: Prof. E. R. Engdahl) are also used.

B. MUKHOPADHYAY 321
knickpoints in the gradient of major rivers is southern
part of Himalaya corresponds to the traces of MCT
which in turn suggest that some segments of MCT may
still be active [26]. Supporting evidences of neotectonic
activities along MCT is obtained from topographic and
geodetic data [27]. From the above discussion, it is
apparent that parts of MCT are still active today as an out
of sequence thrust and produce seismicity.
3. Data
Well-located earthquake data for the time period between
1905 and 2009 (source ISS/ISC/NEIC) with magnitude
(mb ≥ 4) has been used for this study. The earthquake
epicentre plot on map (Figure 1) over a tectonic base
map [after [28]] has brought out the earthquake distribu-
tion pattern in this terrain. The earthquakes (magnitude
5) that have been generated by possible movements
along MBT & allied thrust planes (solid black boxes) are
marked. Further, the events with magnitude 5 that may
have been spawned due to the movement along MCT
and/or by sympathetic thrust planes are plotted with
symbols (black circles with white outer rim). The CMT
solutions of these events are extracted from HRVD web
site (Table 1) and plotted with corresponding numbers
and beach ball circles. The earthquake events of Table 1
are found to cluster in certain geographical locales from
west to east (A to E). Five depth sections (A-A’ to E-E’),
position marked in Figure 1, were drawn through these
clusters (Figures 2(a) - (e)) to unravel the crustal con-
figurations, relation of earthquakes with seismogenic
surfaces and to access the overall tectonic scenario. In
these depth sections, relocated seismic events using EHB
technique (Dr. E. R. Engdahl, personal communication)
are also used.
4. Seismotectonics and Depth Sections across
MCT
The beach-ball diagrams of 12 earthquakes of Table 1
are plotted in Figure 1. Except for numbers 6, 9 and 10,
by and large all the solutions show predominant thrust
movement along WNW-ESE / NW-SE trending thrust
planes dipping low angle towards north. The solutions 6,
9 and 10 belong to clusters B, D and E respectively
(Table 1) show strike slip movements along N-S/NW-SE
trending fault planes dipping moderately towards east.
Beach balls (Figure 1) of larger earthquakes like 1991
Uttarkashi (plot no. 1), 1999 Chamoli earthquake (plot no.
2) and 2009 Bomdila earthquake (plot no. 12) show con-
sistent thrust fault plane solutions along NW-SE trending
thrust planes dipping low angle 6˚ - 14˚ towards nor-
theast. It indicates the strike and dip of the seismically
active thrusts in Western and Eastern Himalaya.
In depth sections (Figure 2) the general seismicity
(mb  4) as filled circles; projections of tectonic planes,
Indian & Tibetan Plates and intervening Sedimentary
Wedge, and Table 1 earthquakes with star symbols are
plotted. Gently dipping Indian plate and its existence
below Tibet is inferred from the seismic data distribution.
Further, mid crustal ramping of MCT as shown in the
INDEPTH profile [15] is also inferred in all these
sections from the distributions of seismicity and topo-
graphic relief data. From the sections, it is inferred that
recent seismicity in Himalaya is restricted in two distinct
places: one along the zone between MFT/MBT and MCT,
and another beyond Indus Tsangpo Suture (ITS) in Tibet.
The section along cluster ‘A’ (section A-A/ of Figure
2(a)) shows cluster of seismicity in the interface between
MCT and upper part of down going Indian Plate. This
section contains two important and well-studied earth-
quakes of Himalaya, Uttarkashi earthquake of 1991 and
Chamoli earthquake of 1999. The Uttarkashi earthquake
of 1991 was associated with the Main Central Thrust
zone of Himalaya (just south of the Vaikrita thrust, also
interpreted as MCT 1) occurred at a depth of 15  1.5 km
(constrained by depth phase pP-P, ISC) [29]. Whereas,
the 1999 Chamoli mainshock (30.512  0.04°N, 79.403
 0.024˚E) occurred on a 9° north dipping thrust at a
depth of 15 km beneath a region about 25 km NNE of
Chamoli in Himachal Pradesh. A total of 204 aftershocks
of magnitude varying from 1.4 to 4.8 were recorded by
NGRI during 4 April 1999 through 20 May 1999. The
estimated hypocentral parameters for these well-located
aftershocks delineate three distinct seismic trends corre-
sponding to: 1) a detachment surface dipping at 15° to
NNE at depths of 10 - 16 km; 2) Munsiari (MCT 2)
thrust dipping at 45˚ to NE at depths 2 - 16 km; and 3)
NE trending transverse fault dipping at 45˚ to SE
extending from a depth of 4 to 10 km [30,31]. They also
revealed that the mainshock occurred near a junction
between the detachment surface and MCT 2 at a depth of
15 km. In the contrary, the occurrence of Chamoli earth-
quake from a seismogenic fault, south of the Main Central
Thrust (MCT) by thrust faulting as well as by strike-slip
faulting was also advocated [32]. From the above dis-
cussion, it is apparent that the 1991 Uttarkashi and 1999
Chamoli earthquakes (Mw 6.8 and 6.6 respectively,
Table 1) have been generated due to thrust movement
along MCT and/or crustal ramp zone of MCT. Thus the
MCT proper, its sympathetic faults and crustal ramps
below the higher Himalaya are seismogenic in this
sector.
Similar scenario exists in sections B-B′ and C-C′
(Figures 2(b) and (c)) drawn through clusters B and C,
where the thrust movement along MCT or in the crustal

B. MUKHOPADHYAY 322
Table 1. Earthquakes (mb ≥ 5) occurred due to movement of MCT and related thrusts are tabulated. The CMT solutions
(Source www.seismology.harvard.edu) for 12 earthquakes due to MCT are tabulated. The solution parameters are discussed
in text. The value of column marked as (No) is plotted on the map (Figure 1). YR – Year, MO – Month, DT – Day. HR – Hour,
MN – Minute, SEC – Second, T – Tension, N – Neutral, P – Compression, FP – Fault Plane.
Cluster No. YR MO DT HR MN SECLAT LONG mb MwT axis
Plunge
T axis
Azimuth
N axis
Plunge
N axis
Azimuth
P axis
Plunge
P axis
Azimuth
FP – I
Strike
FP –
I
Dip
FP –
I
Slip
FP –
II
Strik
FP
– II
Dip
FP – II
Slip
Name of the
earthquake
A
1963 11 27 21 10 4030.8 79.10 5.1
A
1969 6 22 1 33 2330.5 79.40 5.3
A
1979 12 28 1 59 1830.8 78.57 5
A 1 1991 10 19 21 23 1530.8 78.79 6.4 6.857 14 6 113 32 207 31714 115 112 78 84 Uttarkashi
Earthquake
A 2
1999 3 28 19 5 1230.5 79.40 6.3 6.652 27 2 295 38 203 2807 75 115 83 92 Chamoli
Earthquake
A 3
2005 12 14 7 9 5430.5 79.25 5.3 5.168 30 1 296 22 206 29323 86 117 67 92
A
2009 9 21 9 43 5130.9 79.06 5
B
1967 12 18 10 51 3629.5 81.71 5
B
1981 5 15 17 22 4329.5 81.92 5.1
B
1984 5 18 4 28 5229.5 81.79 5.6
B
1991 12 9 1 2 4229.5 81.61 5.6
B 4
2001 11 27 7 31 5229.7 81.72 5.5 5.548 16 2 284 42 192 2574 63 104 87 92
B 5
2001 11 27 8 53 5429.6 81.7 5.3 5.470 46 8 295 18 202 28028 73 119 64 99
B
2001 11 27 17 56 5729.6 81.74 5
B 6 2005 10 31 5 51 1629.7 81.86 5 4.828 331 51 103 24 227 8 52 176 100 87 39
B
2005 11 6 1 36 5729.7 81.87 5
C
1936 2 11 4 48 027.5 87 5.5
C
1970 2 26 19 30 1427.6 85.7 5
C
1974 3 24 14 16 127.7 86 5.4
C
1978 10 4 13 53 5127.8 85.93 5.2
C
1988 4 20 6 40 2627 86.72 5.4
C 7
1988 10 29 9 10 5327.9 85.64 5.5 5.271 352 10 112 16 205 30930 109 106 62 79
C
1997 11 27 16 11 5727.6 87.31 5
C 8
2006 2 3 1 57 4727.3 86.38 5 4.875 6 0 98 15 188 27930 91 98 60 90
D
1964 8 30 2 35 727.4 88.21 5.1
D 1964 3 27 23 3 4127.1 89.36 5
D
1972 8 21 18 55 727.2 88.02 5.1
D 9
1980 11 19 19 0 4527.4 88.80 6 6.325 68 51 302 28 172 20951 –2 301 89 –141Gangtok
Earthquake
D
1989 5 22 19 24 3127.4 87.86 5
D
1998 11 26 10 24 2327.7 87.86 5.1
D
2008 12 2 5 11 4227.4 88.05 5.2
E
1967 9 15 10 32 4427.4 91.86 5.8
E
1983 10 2 21 3 2428.1 92.52 5
E 10 1995 2 17 2 44 2527.6 92.40 5.1 5.525 282 46 40 34 174 32246 –172 226 84 –44
E 11 1996 6 9 23 25 1628.4 92.26 5.1 5.224 303 10 37 64 149 1223 –117 221 69 –79
E
1998 7 8 3 44 5927.3 91.07 5.1
E
1998 8 18 4 10 2327.7 91.10 5
E
2000 1 25 16 43 1927.9 92.65 5.3
E
2001 11 6 14 9 2427.4 91.97 5.1
E 12
2009 9 21 8 53 627.3 91.44 6.1 6.151 10 1 279 39 189 2746 85 99 84 91 Bomdila
Earthquake

B. MUKHOPADHYAY 323
ramp zone of MCT again defines seismicity. The 1980
Gangtok earthquake (Mw 6.3, Table 1) generated due to
interaction of MCT and the upper part of Indian Plate
boundary (section D – D′, Figure 2(d)) by a strike slip
motion. Further, the 2009 Bomdila earthquake (Mw 6.1.
Table 1) has also been generated due to thrust movement
along MCT (section E – E′, Figure 2(e)).
5. Some Assumption for Clustering of
Moderate Size Earthquakes along MCT
It is found that moderate size earthquake events (magni-
tude ≥5) clusters in only five places (A to E, Figure 1)
due to probable movement of MCT. Inter-cluster zones,
which are comparatively large in area, are almost devoid
of MCT induced seismicity. Some propositions/explana-
tions are summarised below to explain such phenome-
non.
1) It is known that active movements along fault plane
occur in segments. The cluster zones are those segments
of MCT that are presently active.
2) The cluster zones are the zones of strain accumu-
lation. It is also found elsewhere that domains of inter-
secting major discontinuity surfaces are favourable locales
for stress build up and can be considered as sites of
seismic potentiality [33]. In cluster zone, strain is accu-
mulated due to cross fault interactions between trans-
gressing northerly trending deep seated faults from
Peninsular Shield with E-W trending shallower Hima-
layan thrusts (see Figure 1). Such interaction is inferred
in cluster A [where NNE-SSW trending Mahendragarh –
Dehradun Fault (MDF) is in interaction with WNW-ESE
trending MBT & MCT]; in cluster ‘B’ [where NE-SW
trending Great Boundary Fault (GBF), N-S trending
Lucknow Fault (LF) is in interaction with WNW-ESE
trending MBT & MCT]; in cluster ‘C’ [NE-SW trending
West Patna Fault (WPF), East – Patna Fault (EPF),
Munger-Saharsa Ridge Fault (MSRF) with E-W trending
MBT & MCT]; in cluster ‘D’ [NNW-SSE trending
Purnia – Everest Lineament (PEL), Malda – Kishanganj
Fault (MKF), Tista Lineament (TL) with E-W trending
MBT & MCT] and finally in cluster ‘E’ [NW-SE
trending Dhansiri – Kopili Fault (DKF), Bomdila Fault
(BF) with ENE-WSW trending MBT & MCT]. Such
scenario is either absent or not active in inter-cluster
zones.
3) In these cluster zones, the seismic slips release the
accumulated strain whereas in the inter-cluster zones,
probable aseismic creep (?) release the strain gradually.
4) In cluster zones, both Indian Plate and overriding
Sedimentary Wedge have participated equally in re-
leasing the seismogenic strain by earthquakes (Thick
Skinned Tectonics). This is probably absent in the inter-
cluster zones.
5) Following the model of Seeber and Pecher [34] on
Himalaya, the Himalayan arc to maintain its arcuate
geometry need arc parallel extensions in several places,
in addition to the layer perpendicular compressional
stress due to plate convergence. The zones between two
extension zones, thus, experience both layer parallel and
layer perpendicular compressions. The cluster zones de-
fine those areas where crust experiences both arc parallel
and arc perpendicular compressions. This is primarily
manifested by thrust type of earthquakes with subordi-
nate strike slip motion. The CMT solution also conforms
this hypothesis by showing: a) two perpen- dicular sets
of causative fault planes, one along WNW- ESE/NW-SE
trending thrust planes dipping low angle towards north
and other N-S/NW-SE trending fault planes dipping
moderately towards east, and b) change in the orientation
of compression (P) axis from NNE-SSW/ NE-SW to
NW-SE plunging low to moderately (Table 1).
6) Due to cross-faulting and repeated rupturing, the
cluster zones are places of newly developed micro and
major cracks within a highly fractured crust. These micro-
cracks accumulate strains at its tips [as indicated by
‘Griffith’s crack theory’] and encourage repeated failure
over a threshold stress limit. Further, the percolations of
rainwater within these cracks induce pore-pressure per-
turbation and thus enhance the chance of failure in lower
stress followed by generation of moderate size earth-
quakes at crustal level.
7) The cluster zones are the places of probable low
heat flow due to depression in Moho. Depression in
Moho suggests more seismogenic volume. The experi-
ments to determine Moho in several parts of Himalaya
reveal variable depths. In western Himalaya, the Moho
deepens northward across the foredeep and is at 50 km
depths beneath the foothills of the Himalaya and 60 - 65
km depth beneath the highest part of the Himalaya [35].
In Nepal Himalaya, the subsurface images from HIMNT
teleseismic receiver functions and local earthquake to-
mography show an increase in Moho depth from ~45 km
beneath Nepal to ~75 km beneath Tibet [36]. Bhatta-
charya et al. [37] inferred a Moho depth of ~(45 - 47) km
below lesser Himalayan part of Western Arunachal
Himalaya. It is also evident from seismic depth sections
(Figure 2) that earthquakes generated due to MCT and
sympathetic thrust occurs in two depth bands, 15 - 25 km
corresponding to the base of upper Crust and 35 - 45 km
corresponding to the base of lower Crust. Thus, it can be
inferred that in the zone of clusters, the Moho depth may
be higher (~50 km) with lower heat flow, more brittle
deformation, in comparison to non-cluster zones.
6. Conclusions
The results in this study suggest that MCT is not dormant

B. MUKHOPADHYAY 324
but active in segments. The active segments produce
seismic clusters (A to E) of moderate size earthquake
events in recent time. The magnitude of the events
generated so far is restricted below Mw 7.0 (Table 1).
Thus the seismic potentiality of MCT within these
clusters can be inferred as Mw 7.0. This value can pro-
bably be used for engineering design purposes. But this
inference needs to be exercise with caution as this an-
alysis has only considered seismic record for the last 105
years. The earthquake data prior to 1905 are mainly
historical and have large locational errors and thus are
not used in this analysis.
The depth sections show gently dipping subducted
Indian plate, overriding Tibetan Plate and compressed
Sedimentary Wedge, steep dip and mid crustal ramping
of MCT. The seismicity along the clusters are also found
to be restricted to two depth bands, 15 - 25 km corre-
sponding to the base of upper Crust and 35 - 45 km
related to the base of lower Crust, with an inferred Moho
depth of nearly 50 km. Several inferences has been drawn
for the cause of clustering of earthquakes along MCT;
like segmental activation of MCT, cross fault intera-
ctions, zones of arc parallel and arc perpendicular com-
pressions, pore pressure perturbations, low heat flow etc.
But these hypotheses need to be backed by experi-
mental data. These inferences hold good for clustering of
earthquakes in the Himalaya, especially in the lesser Hi-
malaya where a characteristic earthquake model is appa-
rent [10].
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