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Tectonophysics 524–525 (2012) 135–146
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Earthquake occurrence processes in the Indo-Burmese wedge and
Sagaingfault region
Bhaskar Kundu, V.K. Gahalaut ⁎National Geophysical Research
Institute (CSIR), Hyderabad, India
⁎ Corresponding author. Tel.: +91 40 23434700.E-mail address:
[email protected] (V.K. Gahala
0040-1951/$ – see front matter © 2011 Elsevier B.V.
Alldoi:10.1016/j.tecto.2011.12.031
a b s t r a c t
a r t i c l e i n f o
Article history:Received 25 January 2011Received in revised form
11 November 2011Accepted 21 December 2011Available online 29
December 2011
Keywords:Indo-Burmese wedgeSagaing faultSeismogenesisEarthquake
focal mechanismsIntra-slab earthquakes
Earthquakes in the Indo-Burmese wedge and Sagaing fault regions
occur in response to the partitioning of theIndia–Sunda motion
along these two distinct boundaries. Under the accretionary wedge
of the Indo-Burmesearc, majority of the earthquakes occur in the
depth range of 30–60 km and define an eastward gently
dippingseismicity trend surface that coincides with the Indian
slab. The dip of the slab steepens in the east directionand
earthquakes occur down to a depth of 150 km, though the slab can be
traced up to the 660 kmdiscontinuity. Although these features are
similar to a subduction zone, the nature of the earthquakes andour
analysis of their focal mechanisms suggest that these earthquakes
are of intra-slab type which occuron steep plane within the Indian
plate and the sense of motion implies a northward relative motion
withrespect to the Sunda plate. Thus these earthquakes and the
stress state do not support active subductionacross the
Indo-Burmese arc which is also consistent with the relative motion
of India–Sunda plates. Theabsence of inter-plate earthquakes, lack
of evidence of the occurrence of great earthquakes in the
historicalrecords and non-seismogenic nature of the plate interface
under the accretionary wedge suggest that seismichazard due to
earthquakes along the plate boundary may be relatively low.
However, major intra-slabearthquakes at shallow and intermediate
depths may still cause damage in the sediment filled valley
regionsof Manipur and Cachar in India and Chittagong and Sylhet
regions of Bangladesh. In the Sagaing fault region,earthquakes
occur through dextral strike slip motion along the north–south
oriented plane and the stressstate is consistent with the plate
motion across the Sagaing fault.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The Indo-Burmese wedge along with the Sagaing fault forms
thenorthern part of the northwestern Sunda arc (Chandra, 1984;
Chenand Molnar, 1990; Curray, 2005; Fitch, 1972; Le Dain et al.,
1984;Nandy, 2001; Ni et al., 1989; Verma et al., 1976). The
approximatelynorth–south trending convex westward Indo-Burmese
wedge joinsthe approximately east–west trending eastern Himalaya in
thenorth (Fig. 1). The region of this junction is referred as the
EasternHimalayan Syntaxis, which is marked with complex tectonics,
highexhumation and erosion rates, etc. (Zeitler et al., 2001). In
thesouth, the Indo-Burmese wedge joins with the north–south
trendingAndaman–Sumatra subduction zone. In the region of
Indo-Burmesewedge, the northward motion of about 35 mm/year of the
Indiaplate with respect to the Sunda plate (Maurin et al., 2010;
Nielsenet al., 2004; Vigny et al., 2003) is assumed to be
accommodatedthrough slip partitioning in the Indo-Burmese arc and
on the Sagaingfault (Fig. 1). The plate reconstruction models
suggest that subduc-tion probably occurred in the Indo-Burmese
wedge in the geological
ut).
rights reserved.
past when it was predominantly southeast–northwest
trending.Though the precise timing of this transition is not known,
it appearsto have occurred prior to about 50 Ma (Hall, 1997).
However, afterthe collision of the India plate with the Eurasian
plate, the Burmaplate, consisting of the Indo-Burmese wedge,
Myanmar CentralBasin along with the Andaman–Sumatra arc, rotated
clockwise tobecome predominantly north–south trending (Hall, 1997).
The extru-sion and clockwise rotation of the Burma plate in the
late tertiaryperiod created compressional structure in the Myanmar
Centralbasin (MCB) (Everett et al., 1990; Le Dain et al., 1984;
Tapponnieret al., 1982). Recent geochronology of the Mogok
metamorphic beltin Burma (Searle et al., 2007) supports that
right-lateral motion onthe Sagaing fault which might have initiated
after 16–22 Ma. TheBurma plate appeared to have originated through
three major phases.In the first phase, by the end of the Eocene
(~35 Ma), Burma Plate col-lided with the northeast edge of the
Indian plate and was draggednorthward as a fore arc sliver. In the
Miocene (~15 Ma), this acceler-ated motion led to the formation of
NE–SW trending extensionalbasins bounded by NE–SW striking normal
faults, and to the creationof Andaman sea rift (Curray, 2005).
Finally in the Pliocene (~5 Ma),when the northern end of the Burma
Plate collided with Asia,extensional deformation ceased and
transpressional deformationcaused reverse faults, positive flower
structures, inversion of normal
http://dx.doi.org/10.1016/j.tecto.2011.12.031mailto:[email protected]://dx.doi.org/10.1016/j.tecto.2011.12.031http://www.sciencedirect.com/science/journal/00401951hp高亮
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Fig. 1. Major tectonic features of the Sunda and Himalayan
arc.
136 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
faults and extensional basin (Bertrand and Rangin, 2003; Maurin
andRangin, 2009).
Presently, the motion between the India and Sunda plates
ispartitioned between Indo-Burmese arc and Sagaing fault. Our
recentGPS measurements in the Indo-Burmese arc region (Gahalaut,
V.K.et al., manuscript in preparation) and the available
measurementsin the Sagaing fault region (Maurin et al., 2010; Vigny
et al., 2003)suggest that about 60% of the relative motion between
India andSunda plates is taken up by the Sagaing fault. Both
regions arecharacterized by earthquake occurrences. One of the
majordifferences between the earthquakes in the Indo Burmese
wedgeand in the Sagaing fault is their focal depth. Earthquakes are
generallyvery shallow in the latter region, whereas in the former,
they occur upto a depth of 150 km (Guzman-Speziale and Ni, 1996).
Another majordifference is that the earthquakes predominantly occur
through strikeslip motion on the Sagaing fault while in the Indo
Burmese wedge,they occur through strike slip and thrust and oblique
motion. In theIndo Burmese wedge, it is not known whether these
earthquakes areof inter-plate or intra-plate (or intra-slab) type
(Guzman-Spezialeand Ni, 1996). Several geological studies (arc
magmatism and meta-morphism, occurrence of ophiolitic rock
sequences, surface as wellsubsurface expression of fold and thrust
belt structures), geophysicalobservations (tomographic images and
gravity anomaly), and platereconstruction studies confirm that the
subduction occurred acrossIndo Burmese wedge between India and
Burma plates (Bannert andHelmcke, 1981; Barley et al., 2003;
Bertrand et al., 1998; Guzman-Speziale and Ni, 1996; Hall, 1997; Li
et al., 2008; Mukhopadhyay andDasgupta, 1988; Ni et al., 1989;
Pivnik et al., 1998; Sengupta et al.,1990) and there are evidence
of subducted India slab, but whetherthe subduction is still active,
is a debatable topic. Several investigatorshave analyzed earthquake
occurrence processes in the Indo Burmesewedge (Angelier and Baruah,
2009; Guzman-Speziale and Ni, 1996;Rao and Kalpna, 2005; Rao and
Kumar, 1999; Satyabala, 1998;Satyabala, 2003). However, none of
them could address all the aboveissues. In some cases it is due to
lack of sufficient and accurate data(e.g., Guzman-Speziale and Ni,
1996).
Maurin and Rangin (2009) analyzed structures and kinematics
ofthe Indo-Burmese wedge which defines the western margin of
theBurma plate, a sliver between the India and Sunda plates
(Gahalautand Gahalaut, 2007; McCaffrey, 1992). On the basis of the
age, gradeof metamorphism and rock type, they classified it into
outer, innerand core wedges. The outer wedge mainly consists of
Tripura Foldbelt and the eastern part of the Bengal basin and is
made of Neogeneclastic sequences. Age of the sediments in the outer
wedge rangesfrom lower Miocene submarine deposits, upper Miocene
shelfaldeposits to Plio-Pleistocene fluvial deposits (Maurin and
Rangin,2009). The inner Indo-Burmese wedge is composed of Eocene
flyschsaffected by N–S trending strike-slip right faults, such as
theChurachandpur–Mao fault, CMF, discussed later in the text. The
coreof the wedge is made of high-grade metamorphic rocks,
tectonicallyimbricated with Mesozoic ophiolites and sedimentary
sequencesranging from Late Triassic to Late Cretaceous (Bender,
1983). Adetailed discussion on these units may be found in Maurin
andRangin (2009). In this article, we analyze the seismicity of the
regionand address all these issues to suggest that no subduction
occursacross the Indo Burmese wedge and the earthquakes are of
intra-slab type that occur within the Indian plate through
reactivation ofthe old geologic fabric.
2. Seismicity of the region
2.1. Historical major earthquakes
There are two most notable great earthquakes, namely, the
1897Shillong Plateau and the 1950 Assam earthquakes (Molnar,
1990;Seeber and Armbruster, 1981) that have occurred close to the
Indo-Burmese arc and Sagaing fault (Fig. 2). The 1897 Shillong
Plateauearthquake occurred primarily under the Shillong Plateau and
islinked with the tectonics of the Shillong Plateau (Bilham
andEngland, 2001). Thus this earthquake is considered as an
intra-plateearthquake which probably occurred through reverse
motion on thesouth dipping steep fault. The 1950 Assam earthquake
occurred inthe Arunachal Himalaya and the Eastern Himalayan
Syntaxis regionand is considered as the interplate earthquake that
occurred due tothe ongoing India–Eurasia convergence, part of which
is accommo-dated in the Himalaya (Molnar, 1990; Seeber and
Armbruster,1981). Thus the two great earthquakes are probably not
linked withthe tectonics of the Indo-Burmese arc and the Sagaing
fault regions.
We compiled a catalogue of major earthquakes in the Indo-Burmese
arc and Sagaing Fault regions (Fig. 2 and Table 1). Weexcluded
earthquakes occurring in the Shan Plateau and Red Riverfault region
as they are linked with the tectonics of the Tibet Plateauextrusion
due to the India–Eurasia convergence. Although there are afew
unverifiable reports of earthquake occurrence as early as 1548
inTripura, Assam or Bangladesh (Iyengar et al., 1999; Steckler et
al.,2008), there are large uncertainties in the earthquake
location. Wefind that the catalogue is probably reliable only after
1762. As manyof these historical earthquakes are located on the
basis of maximumdamage, their epicentral locations are not very
reliable. Thus a fewearthquakes which may probably be linked with
the tectonics of theShillong plateau and Himalaya, may erroneously
be included here asthey caused damage in the Indo-Burmese Arc
region. Similarly, theearthquakes which caused damage in the
Shillong Plateau region,and hence they were excluded, might have
actually occurred in theIndo-Burmese arc region.
The May 23, 1912 earthquake (M~8) is probably the only
greatearthquake that occurred in the Sagaing fault region
(Guzman-Spezialeand Ni, 1996; Maurin et al., 2010; Richter, 1958;
Tsutsumi and Sato,2009). This earthquake occurred in the eastern
Burma and causedbending of the railroad tracks. Other than this,
severalmajor earthquakeshave occurred along the Sagaing Fault. In
the Indo-Burmese arc region afewnotablemajor earthquakes are the
April 2, 1762 andAugust 24, 1858
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Fig. 2. General tectonics, seismicity and earthquake focal
mechanisms in the Indo-Burmese wedge and the Sagaing fault. (A)
First panel shows the tectonics of the region. Faults rked with
yellow color are mapped in the present study. Thebold arrow shows
the relative motion of the India plate with respect to the Sunda
plate. (B) Middle panel shows seismicity of the region. Filled
circles with different colors, de ing focal depths, are the
earthquakes from EHB catalogue forthe period from 1964 to 2008.
Small squares with four numerals indicate epicenters of major
earthquakes with their year of occurrence that have occurred in the
past 250 yea Approximate locations of the 1762 Arakan and 1839
SagaingFault earthquakes are also indicated. Locations of 1897
Shillong Plateau and 1950 Assam earthquakes which occurred in the
Shillong Plateau and Eastern Himalayan regions, a also shown.
Imphal is located in the Manipur valley. (C) Lastpanel shows
earthquake focal mechanisms. Black and gray color focal mechanisms
denote shallow (b75 km) and intermediate (75–150 km) depth
earthquakes. Dashed lines in Bay of Bengal region represent the
oceanic fracture zones orthe geologic fabric trend (Desa et al.,
2006). CMF — Churachandpur Mao Fault, EHS — Eastern Himalayan
Syntaxis, KDF — Kaladan Fault, KF — Kabaw Fault, MCB — Myanma ntral
Basin.
137B.K
undu,V.K.G
ahalaut/Tectonophysics
524–525
(2012)135
–146
manotrs.rether Ce
image of Fig.�2
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Table 1Major earthquakes in the Indo-Burmese arc and Sagaing
fault regions. Earthquakes in the Red River fault region and South
China have been excluded.
S. no. Date Latitude °N Longitude °E Depth Magnitude Region
1. 1762/04/02 Chittagong, Ramree and Cheduba Major Arakan
Coast2. 1839/03/23 Near Amarapura, Mandalay Major Sagaing Fault3.
1843/02/06 19.5 95.5 Major Sagaing Fault/Ramree Island4. 1843/10/30
19 95 Major Sagaing Fault/Cheduba Island5. 1848/01/03 19.5 95.5
Major South Myanmar6. 1858/08/24 19 95.25 Major Arakan Coast, South
Mynmar, Sagaing Fault
1869/01/10 25 93 Major (7.4) Manipur and Cachar1885/07/14
Manikganj, about 50 km west o
DhakaMajor Bangladesh
1889/01/10 Major Jaintia Hills7. 1906/08/31 27 97 Major Sagaing
Fault8. 1908/12/12 26.5 97 7.5 Sagaing Fault9. 1912/05/23 21 97 8
Sagaing Fault10. 1918/07/08 24.5 91 7.6 Srimangal, Bangladesh12.
1923/09/09 25.25 91 7.1 Shillong Plateau, Durgapur14. 1929/08/08 19
96.5 7 Sagaing Fault15. 1930/05/05 17 96.5 7.3 Sagaing Fault16.
1930/07/02 25.25 90 7.1 Dhubri, Assam17. 1930/12/03 18 96.5 7.3
Sagaing Fault18 1931/01/27 25.6 96.8 7.6 Sagaing Fault19.
1932/08/14 26 95.5 7 India–Myanmar20. 1938/08/16 23.5 94.25 7.2
Sagaing Fault21. 1941/12/26 21 99 7 Shan Plateau22. 1943/10/23 26
93 7.2 Shillong Plateau Assam23. 1946/09/12 23.5 96 7.5 Sagaing
Fault24. 1946/09/12 23.5 96 7.7 Sagaing Fault26. 1954/03/21 24.2
95.1 7.1 India–Myanmar
1957/07/01 24.4 93.8 7.2 Manipur1970/07/29 26.02 95.37 68 7.0
Myanmar–Bangladesh
27. 1975/07/08 21.44 94.59 116 7.0 Indo Burmese wedge29.
1988/08/06 25.09 95.11 99 7.3 India–Myanmar31. 1991/01/05 23.58
95.88 13 7.0 Sagaing Fault
138 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
Arakan earthquakes, and the January 10, 1869 Cachar
earthquake(Guzman-Speziale and Ni, 1996; Richter, 1958). Other than
these earth-quakes, several instances of damage due to earthquakes
are reportedfrom Chittagong, Sylhet, Manipur valley and Cachar
regions (Bilham,
Fig. 3. (A) Historical Govindaji temple and the accompanying
structure, the Beithoub, inhistorical temples in the region did not
appear to have suffered any damage from the e(B and C) which may
not necessarily be ascribed to the earthquake.
2004; Guzman-Speziale and Ni, 1996; Le Dain et al., 1984; Martin
andSzeliga, 2010). However, it is possible that the high damage in
theseregions was due to relatively large population, local site
effects, asthese places are located in the valley with thick
sediment cover and
the Kangla palace, Imphal, was damaged during the 1869 Cachar
earthquake. Otherarthquakes. However a small tilt (~4°) was
observed at the Madan Mohonji temple
image of Fig.�3
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139B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
prevalence of non-traditional construction practices in these
regions incontrast to those in the surrounding hilly regions where
houses weremainly made with bamboos to escape damage due to the
earthquakes.The April 2, 1762 Arakan earthquake has been considered
as the greattsunamigenic earthquake (Cummins, 2007). It caused
extensive damagein the Chittagong region through shaking,
liquefaction, damming ofchannels, seiches, etc. (Gulston, 1763).
The reported uplift of ChedubaIsland due to this earthquake was not
found to be singularly due to thisearthquake (Martin and Szeliga,
2010; Oldham, 1883). Halsted (1843)reported the effects of the
earthquakes which were mostly found to behighly exaggerated (Gupta
and Gahalaut, 2009). It is now consideredthat this earthquake
probably did not cause any major tsunami (Guptaand Gahalaut, 2009)
and was probably only a major earthquake(Martin and Szeliga, 2010).
The August 24, 1858 Arakan earthquakewas felt in many parts of
Burma and was severely felt at Kyauk Pyu,Ramree Island, Myanmar. It
caused liquefaction, damage to buildingsand Pagodas (Martin and
Szeliga, 2010). In this region, the distancebetween the Sagaing
fault and the structurally mapped Arakan trenchis less than 200 km
and hence based on the scanty reports of damage itis difficult
whether this and the 1843 and 1848 earthquakes are linkedwith the
Arakan frontal arc or with the Sagaing fault.
The January 10, 1869 Cachar–Manipur earthquake was the
mostsevere earthquake in the available 2000 years of written
historical
Fig. 4. Depth section across the Indo Burmese wedge and Sagaing
fault (SF) showing EHB sprojected position of the CMF and SF. West
of the CMF, the thickness of the sediments ofwith the sediment
thickness estimated at a site KMG from the receiver function
technique (wedge is conjectural and is partly based on the
available geological sections (Alam et al., 20ages (Li et al.,
2008; Pesicek et al., 2010). A progressive steepening in the slab
may be notevelocity (VT) from north to south. VS and VR are the
subduction and rollback velocity (see F
records of Manipur (Parratt, 1999; Singh, 1965). Manipur, now
astate of India, was an independent kingdom which was ruled
byMeitei kings since 35 CE, at least (Parratt, 1999). Kangla
(nowImphal) in the Manipur valley was the capital of the princely
stateand the historical records were maintained in the “The
CheitharolKumpapa, The Court Chronicle of the Kings of Manipur”
(Parratt,1999; Singh, 1965). The sediment filled valley region has
remainedthe center of inhabitation since historical times. The
valley is locatedat the center of the Indo-Burmese arc. Thus this
region could nothave escaped from the damage due to any great
earthquake in theIndo-Burmese arc. Hence, it may be appropriate to
state that no largerearthquake than the January 10, 1869
earthquake, occurred in theIndo-Burmese arc during the period of
written historical records.The 1869 earthquake caused severe damage
in the Cachar valley,near Silchar and Manipur valley, near Imphal.
Five deaths werereported from Silchar while three from Imphal
(Oldham, 1882;Singh, 1965). Extensive liquefaction, wide cracks
occurred in theSilchar region, near the Barak river and in Imphal,
near the Imphalriver (Oldham, 1882). In Imphal a few bridges and
the palace of theking, the Kangla Palace, and a temple within it
were damaged. Theearthquake was followed by more than 15
aftershocks in the follow-ing two day period which were strongly
felt in Imphal (Singh, 1965).Ambraseys and Douglas (2004) estimated
the magnitude of this
eismicity (Engdahl et al., 1998). Filled and hollow triangles in
each panel indicate thethe accretionary wedge is estimated to be
about 25–30 km, which is also consistent
Mitra et al., 2005). The dashed portion of the slab under the
Indo-Burmese accretionary03). The deeper part (>150 km) of the
slab geometry is based on the tomographic im-d from north to south
which may be explained as due to the increase in trench retreatig.
8).
image of Fig.�4
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140 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
earthquake as 7.4. Oldham (1882) described the damage caused
bythis earthquake in the Cachar valley near Silchar and at far off
placesbut did not visit the Manipur valley due to security reasons,
wherethe damage was equally severe. In April 2010, we visited
various his-torical monuments in Imphal and adjoining regions of
Manipur valleyand tried to assess the damage to these monuments due
to this orother historical earthquakes. In Imphal, the Govindaji
temple andthe inner walls of the Kangla Fort palace are reported to
have suffereddamage due to this earthquake; however, massive
restoration worktaken up by the successive rulers did not allow us
to assess the extentand pattern of damage due to the earthquake to
these buildings.Nevertheless, there are clear signs of damage and
restoration work.In fact some of the 12 pillars (with heights of
about 5 m and diameterof about 1 m) in the Beithoub, in front of
the Govindaji temple(Fig. 3A), which were extensively damaged
during the earthquakeand were restored, are still tilted. In
addition, we could locateadditional seven temples within Imphal and
adjoining regionswhich were built before the 1869 earthquake and
some of themwere more than 300 years old. The architecture of all
these templesis simple, unlike the temples of north India. These
temples generallyhave only one dome with the maximum height of
about 10 m witha ground base of about 8 m x 8 m. These are very
massive structureswith thickness of walls being about 1 m. In some
structures (e.g.,the temple of Brindabanchandra at Imphal) there is
a gallery aroundthe sanctum sanctorum. The arch shaped ceiling and
openings at
Fig. 5. A depth section across the Indo-Burmese wedge and
Sagaing fault showing the seismtopography. Note the steep nodal
planes of the earthquake focal mechanisms which are naccretionary
wedge. The lower panel shows a composite field photograph of the
Churacharoad is shown by the yellow dash line.
the doors and windows provide added strength to these
structures.None of these structures appears to have suffered damage
due tothe earthquake, though a small tilt (~4°) at two temples,
namelythe Madan Mohonji temple at Imphal, and the Vishnu temple
atBishanpur, was noticed (Fig. 3B and C). However, it is difficult
toascertain whether this tilt occurred due to the shaking caused
bythe 1869 earthquake or it is due to slow and continuous
settlementof the foundation since the construction of the
temple.
In summary we did not find much evidence of extensive damagein
the Manipur valley due to the 1869 earthquake. The damage wasmainly
confined to the Kangla palace in Imphal. At other places theold
historical structures did not suffer damage either due to the
factthat they were small and massive or the site conditions were
better.
2.2. Current seismicity
We used the updated catalogue of relocated earthquakes
(EHBcatalogue of Engdahl et al., 1998 from International
SeismologicalCentre, 2009) since 1964. In the past 50 years no
great earthquakehas occurred in the region. The August 6, 1988
earthquake of M 7.3,that occurred in the Indo-Burmese arc region at
the India–Myanmarborder and about 120 km east–northeast of Imphal
with an interme-diate focal depth of 99 km, was the largest
magnitude earthquake inthe region in the past 50 years. Three
people died and there wassome damage to the buildings in the
northern Myanmar due to this
icity, earthquake focal mechanisms (east–west vertical cross
sectional projections) andot consistent with the gentle dip of the
seismicity trend under the Indo-Burmese arcndpur Mao Fault (CMF)
zone from a region 60 km southwest of Imphal. The winding
image of Fig.�5
-
Fig. 6. Rose diagram showing the azimuth and histogram showing
the plunges of P, T and N axes of the earthquakes focal mechanisms
from the Indo Burmese wedge and Sagaingfault.
141B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
earthquake. Majority of the earthquakes occur in the Indo
Burmesewedge and Sagaing fault region while the intervening region
of theMyanmar Central Basin rarely experiences earthquakes (Fig.
2). Inthe Indo Burmese wedge earthquakes occur down to a depth
ofabout 150 km, while they are very shallow in the Sagaing fault
region.In the Indo Burmese wedge, they appear to define the
underlyingIndian slab which has been traced to extend at least or
beyond410 km discontinuity in the regional tomographic studies (Li
et al.,2008). Recent tomographic studies (Li et al., 2008; Pesicek
et al.,2010) and EHB seismicity sections (Fig. 4) suggest that
there is aprogressive increase in the dip of the subducted slab
from north tosouth profiles.
Under the Indo-Burmese accretionary wedge earthquakes occur
atdepths of 30–60 km and define a very gently eastward
dippingseismicity trend surface which lies below the base of the
accretionarywedge. Rarely, the earthquakes occur within the
accretionary wedge.Though, several faults, namely, the Kaladan
(KDF) and Kabaw fault(KF) have been mapped on the surface, none of
them appears to beassociated with the earthquakes. Further south in
the Irrawaddyregion, where seismicity is very low, the slab is not
traceable becauseof the tear in the Indian slab (Kundu and
Gahalaut, 2010; Richardset al., 2007). Seismicity in the Sagaing
fault region is quite scanty,particularly in the central portion.
The Sagaing fault is seismicallymost active in the northern
portion. All the earthquakes occur atshallow depth (b25 km). A few
earthquakes occur in the centralMyanmar basin. Here, we have
excluded the earthquakes associatedwith the Red River fault in the
South China from our discussion.
2.3. Earthquake focal mechanisms
We use previously published earthquake focal mechanisms andthose
listed in the Harvard Centroid Moment Tensor (CMT) catalogue(Fig.
2). However, for the location of the earthquakes, we use
thecorresponding hypocenter from the EHB catalogue. In the
Sagaingfault region all the earthquakes occur through predominant
strikeslip motion on steep plane with one nodal plane parallel to
thenorth–south trending Sagaing fault which exhibit dextral strike
slipmotion. In the Indo-Burmese wedge, earthquake focal
mechanismsare quite mixed. Though, they are predominantly of strike
slip orthrust types, a few earthquakes with normal motion are also
noticed.Contrary to the earlier observations (Rao and Kalpna, 2005;
Rao andKumar, 1999), we did not find any clear segregation in the
earth-quake focal mechanisms with depth in the Indo Burmese
wedge(Fig. 5). In fact, Stork et al. (2008) also suggested that
probablythere is no depth-wise segregation in earthquake focal
mechanismsin the Indo-Burmese arc. The directions and plunge of the
P, T andN axes in the Indo-Burmese wedge and Sagaing fault regions
areshown in Fig. 6. At shallow depth (b75 km) in the
Indo-Burmesewedge the general orientation of the sub-horizontal P
axes is in theNNE–SSW direction. The orientation of the T axes is
in the east–west to ESE–WNW direction with generally gentle plunge
and theorientation of the N axes is approximately in the NW–SE
directionwith no consistent plunge. At intermediate depth (>75
km) in theIndo-Burmese wedge, the orientation of the sub-horizontal
P axes isin the NNE–SSW direction, similar to that at shallow
depth. The
image of Fig.�6
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Fig. 7. Results of inversion of earthquake focal mechanisms of
the Indo-Burmese wedge and Sagaing fault. Scatter in the estimation
of principal stresses corresponds to 95%confidence level. The
scatter is due to the variation in the earthquake focal
mechanisms.
142 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
predominant orientation of the T axes is in the ESE–WNW
directionwith steep plunge while the N axes have orientations
varying be-tween SW–NE to NW–SE with moderate plunge. These
orientationsof P, T and N axes in the Indo-Burmese wedge region
imply strikeslip faulting on the NNE–SSW or ENE–WSW oriented planes
or thrustfaulting on the WNW–ESE oriented planes. Further, the
orientation ofP and T axes in the Indo-Burmese wedge suggest that
at present thereis no active subduction along this margin (Rao and
Kumar, 1999). Infact our recent results of GPS measurements (our
unpublishedresults) across the Indo-Burmese wedge of Indian region
show novariation in the arc normal motion, implying no active
subduction.In the Sagaing fault region, the orientation of the
sub-horizontal Paxes is in the SW–NE to east–west direction while
the orientation ofthe T axes is in the NW–SE direction with very
gentle plunge. The az-imuth of N axes is quite diffused in all
directions but it has a steepplunge. These orientations of P, T and
N axes in the Sagaing faultregion imply dextral strike slip motion
on the steep north–south ori-ented plane or sinistral strike slip
motion on the steep east–westplane. The former is consistent with
the motion on the Sagaing fault.
There is one most interesting aspect of the earthquake focal
mech-anisms in the Indo-Burmese wedge. The earthquake hypocenters
ap-pear to define a sub-horizontal seismicity trend surface which
couldeither be the basement of the accretionary wedge or the top
surfaceof the Indian plate. However, none of the two nodal planes
of theseearthquakes focal mechanisms is consistent with the dip of
the seis-micity trend surface (Fig. 5). The dip of the two nodal
planes issteep and hence is inconsistent with the gentle dip of the
seismicitytrend and the plate interface. Thus the earthquake
hypocenters andthe seismicity trend surface under the Indo Burmese
accretionarywedge disguise to be the inter-plate earthquakes, while
they actuallyare of intra-plate type, which possibly occur within
the Indian plate.The results of GPS measurements suggest that the
outer and innerwedge and the underlying Indian plate are strongly
coupled andbehave as a single unit with no relative displacement.
It is only thecore part of the Indo Burmese wedge which
accommodates therelative partitioned motion between India and
Sunda. In the Indiangeographical region, this motion occurs on the
CMF which could beequivalent or same as the Lelon fault, identified
by Maurin and
Table 2Results of inversion of focal mechanisms for estimating
the principal stress directions.
Region σ1 azimuth,plunge
σ2 azimuth,plunge
σ3 azimuth,plunge
Indo-Burmese arc (focal depth 0–75 km) 202°, 10° 301°, 51° 104°,
37°Indo-Burmese arc(focal depth 75–150 km)
358°, 1° 268°, 32° 91°, 58°
Sagaing Fault 226°, 3° 117°, 86° 236°, 2°
Rangin (2009). Other faults in the outer and inner wedge, e.g.,
theChittagong Coastal fault, Kaladan fault, and Kabaw fault, do
notappear to accommodate any motion between the India and
Sundaplates. Since majority of the earthquakes occur at depth
greaterthan 25 km, it is inappropriate to relate the sense of
motion duringthese earthquakes with these and other geologically
mapped faultson the surface.
2.4. Inversion of the earthquake focal mechanism
We used earthquake focal mechanism solutions from CMTcatalogue
to estimate the directions of principal stress in the Indo-Burmese
arc and Sagaing fault regions (Fig. 2). We divided theearthquake
focal mechanisms into three categories (i) the shallowearthquakes
(focal depth b75 km) of the Indo-Burmese arc, (ii) theintermediate
depth earthquakes (focal depth >75 km) of the Indo-Burmese arc
and (iii) the earthquakes of the Sagaing fault region.We used the
linear least square inversion approach (Michael, 1984,1987) to
estimate the best fitting maximum (σ1), intermediate (σ2)and
minimum (σ3) stress directions. The results of the inversion
areshown in Fig. 7 and Table 2. At shallow depth in the
Indo-Burmesearc region, σ1 is very gentle and is oriented in the
NNE–SSW direction.σ2 and σ3 are moderately steep and have azimuth
in NW and ESEdirection. At intermediate depth in the Indo-Burmese
arc region, σ1is again very gentle and is oriented in the
north–south direction.The azimuth of σ2 is not well constrained but
is approximately inthe west direction with moderate plunge. σ3 is
moderately steepand has azimuth in the east direction.
We suggest that within the assumed uncertainties in the trendand
plunge of the slip vector of an individual earthquake's
focalmechanism, there is no significant difference in the stress
state atshallow and intermediate depth levels in the Indo-Burmese
arc re-gion and the stress state is generally consistent with the
strike slipand thrust faulting in the region. In the Indo-Burmese
arc the relativeplate motion between the India–Sunda plates is
predominantlytoward north (N10°). Thus in this region the derived
stress state isgenerally consistent with the relative plate motion.
Although manyearthquakes show thrust dominated focal mechanism, it
does notimply that subduction is currently active in the region, as
thedirection of maximum principal stress (σ1) is NEN–SWS, rather
thanbeing east–west to support subduction in that direction.
Further, itmay be noted that the nodal planes of these thrust
earthquakes areoriented in the WNW–ESE direction, whereas, if the
eastwardsubduction of the Indian plate was still active, they
should havebeen oriented in the north–south direction. The unusual
WNW–ESEorientation of the planes is probably linked with the
orientation ofthe old oceanic fabrics present in the Bay of Bengal
and has beendiscussed in a subsequent section.
image of Fig.�7
-
Fig. 9. The upper panel shows a hypothetical subducting oceanic
slab, dipping at anangle of θ, in which a fault system is shown at
a position P1 prior to its subduction.After the subduction its
position is represented by P2. In this case we assume that thedip
of the fault system changes according to the dip of the subducting
slab. If thefault system created at P1 is reactivated at position
P2, then the fault system shouldbe comparable, when the subducted
slab is rotated in the counterclockwise directionby θ, back to its
horizontal position (represented by dashed lines) at P3 (Jiao et
al.,2000). Middle panel shows the poles of the nodal planes of the
focal mechanisms ofearthquakes before and after the rotation. Lower
panels show contours of the densityof poles. Blue, brown, pink and
red contours represent 2%, 4%, 8% and more than 16%of the data
(i.e., poles) respectively. Note the segregation in poles after the
rotationalong the trend, f, marked with the arrows. This trend
corresponds to the generaltrend of the fabric (f) in the Bay of
Bengal (Desa et al., 2006).
Fig. 8. A conceptual model demonstrating the steepening of the
Indian slab from northto south. On the top panel, O represents the
hinge or the pivot position along which theBurmese plate and the
arc rotated from its original position from OA1 to OA2. Thiscaused
a difference in trench retreat velocity VT along the arc, as VT
along the arc isproportional to the distance (r) from O. The lower
panel shows that the resultantvelocity is the vector sum of the
constant subduction velocity along the arc and VTwhich is varying
along the arc. Thus increase in VT will result in increase in
rollbackvelocity VR, which may lead to the steepening of the
slab.
143B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
In the Sagaing fault region, σ1 and σ3 are almost horizontal and
areoriented in the northeast–southwest and
northwest–southeastdirections, respectively, and σ2 is vertical.
The stress state in the Saga-ing fault region is perfectly suited
for the strike slip faulting on thenorth–south or east–west
oriented vertical planes, of which north–south plane with dextral
strike slip motion is the fault plane. Thestress state in the
Sagaing fault region is also consistent with thecurrent crustal
deformation across it (Vigny et al., 2003).
Several investigators have attempted inferring stress
directionsfrom the inversion of earthquake focal mechanisms from
the Indo-Burmese wedge region (Angelier and Baruah, 2009; Rao and
Kalpna,2005). However, one of the problems with this method is that
it isassumed that the direction of the maximum principal stress
(σ1) isoptimally oriented with respect to the fault (i.e., the σ1
makes anangle of π/4−Φ/2, where Φ is the angle of friction), which
may notbe true in case of fault reactivation. It is possible that
during the for-mation of the fault in the geological past, it was
optimally orientedwith respect to the stress directions. However,
the stress directionmight change with time and the region and the
faults might rotateor undergo change in dip, which may make these
faults not so opti-mally oriented with respect to the stress
direction. Nevertheless,these faults continue to be the weak zones
where earthquakes canoccur through reactivation as they still are
favorably but not optimal-ly oriented with respect to the stress
directions. Thus it is not neces-sary that the stress state derived
from the inversion of the earthquakefocal mechanisms should be
consistent with the present day motionas the present day motion on
these faults is the motion along thepre-existing faults which are
only favorably oriented rather than op-timally oriented (McKenzie,
1969). It is more appropriate to estimatethe orientation of
principal stress, which is consistent with the esti-mated or
measured direction of slip on the fault. Further, uncer-tainties of
±15° in trend and ±5° in plunge of the slip vector froman
individual earthquake are typical in the CMT focal
mechanisms(McCaffrey, 1992). Thus we may expect similar error in
our analysisof directions of principal stress.
3. Discussion
3.1. Progressive increase in the dip of the subducted Indian
slab fromnorth to south
The seismicity depth sections across the Indo-Burmese arc in
Fig. 4suggest that there is a progressive increase in the dip of
the subductedslab from north to south. Tomographic studies in the
Indo-Burmesearc have provided evidence of presence of Indian slab
and its progres-sive steepening toward south. However about its
depth extent,rollback, and tear, there are several conflicting
views. Bijwaard et al.(1998) suggested that the Indian slab
experiences a west directedrollback at 410 km discontinuity and
probably does not penetratefurther. Li et al. (2008) suggested that
the slab extends up to adepth of at least 300 km. Recently, Pesicek
et al. (2010) proposedthat it extends up to a depth of 660 km
discontinuity and thepresence of tear in the slab can neither be
confirmed nor refuted.However, all these studies indicate a
southward steepening in thesubducted Indian slab. On the contrary,
due to increase in the age ofthe lithosphere under the Indo-Burmese
wedge toward north, anorthward steepening is expected. We attempt
to explain it inFig. 8. From plate reconstruction models of the
Indian subcontinent,
image of Fig.�9image of Fig.�8hp高亮
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144 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
it has been suggested that the general trend of the Indo
Burmesewedge has changed dramatically since 60 Ma, from NW–SE to
almostN–S, at present (Bannert and Helmcke, 1981; Hall, 1997). We
assumethat the region of extreme eastern Himalaya, where India
plate firstcollided with the Eurasia plate and the Eastern
Himalayan Syntaxiswhich acted as a pivot point for the rotation of
the Indo Burmesewedge and the Burma plate, did not change its
position significantlyduring the course of rotation. It thus
implies that there must havebeen differential trench retreat
velocity (VT) in geological past alongthe trench which increased
from north to south, as VT is directly pro-portional to the length
of the arc (r) measured from the pivot point.Now if we assume
constant subduction velocity (VS) across thetrench, then it must
have affected the rollback velocity (VR) signifi-cantly, as VR is
the vector sum of VT and VS. Thus increase in VT towardsouth might
have led to increase in the subducted slab dip (as VR in-creased
from north to south) which might have progressively steep-ened the
slab in the south direction. However, the presence ofsubhorizontal
tear in the Irrawaddy region (Kundu and Gahalaut,2010; Richards et
al., 2007) may also enhance the rollback-inducedflow which occurs
at the lateral slab edges (e.g., Kincaid andGriffiths, 2003;
Schellart, 2004). Alternatively, the subduction of the90°E ridge
under the Andaman (Gahalaut et al., 2010) and furthernorth may also
control the dip of the subducted slab.
Another noticeable feature in the seismicity is the increase
inseismicity at two depth levels, one at depths greater than 50
kmand another at depths greater than 70 km (Fig. 4). These
twoincreases in the seismicity mark the change in dip in the
underlyingIndian slab. Thus the simplest explanation for the
increase inseismicity could be due to the increase in flexure in
Indian platewith depth. The increase in flexure may lead to
reduction in normalstress from the steep faults, thereby
facilitating the occurrence ofearthquakes and hence causing
increase in seismicity level withdepth.
3.2. Reactivation of faults in the Indo-Burmese arc region
As discussed above, majority of the earthquakes in the
Indo-Burmese arc occur through reactivation on the pre-existing
faults ofthe Indian plate. However, such faults might have
experiencedsignificant dip change during subduction in the
geological past. Itmay be noted that only the faults at depth
greater that 75 km mighthave been affected by this, as the dip of
the Indian slab changesquite significantly at that depth. The
faults at the shallow depthmight not have experienced any
significant change in the dip andorientation. To test the
reactivation hypothesis we adopt the methodof Jiao et al. (2000).
We demonstrate it in Fig. 9. We consider ahypothetical fault system
on the subducting plate at a position P1prior to its subduction.
After subduction its position is representedby P2 on the inclined
subducted slab. If we rotate the inclined sub-ducted slab segment
by the local dip angle of θ, back to its horizontalposition, then
it is expected that the orientation of the fault systemshould be
the same as that at P1. Although the trend of Indo-Burmese arc has
changed significantly in geological past, we assumethat it has not
affected the fault orientations of the subducted Indianplate.
Further, we neglected the effect of the rotation of the Indianplate
on the considered fault.
We rotated the poles of the two nodal planes of the
earthquakefocal mechanisms by the local dip angle θ, of the Indian
slab at thatdepth. The rotation angle or the local dip angle of the
subductedslab is determined from the seismicity depth sections
(Fig. 4) acrossthe Indo Burmese wedge. We measured average value of
θ as ~60°.We perform the rotation of the fault with the help of
GEOrient, ver-sion 9.4.4
(www.holcombe.net.au/software/rodh_software_georient.htm). Before
rotation, there appears to be no preferential segregationin the
poles of the nodal planes of earthquake focal mechanisms.However,
after counter-clockwise rotation by the local dip angle
(~60°), we find a distinct segregation (Fig. 9) in the poles
along thetrend NNW–SSE, which probably corresponds to the older
trend ofthe fault planes that were reactivated during the
earthquakes.
We suggest that the preservation of old oceanic fabric or
weakzones by the presence of hydrous mineral phase and their
geometri-cal orientation plays a vital role. There is no direct
evidence whetherthere are any old oceanic crust fabrics or weak
zones present beneaththe thick cover of Bengal fan sediments, as
high resolution swathbathymetric data of the Bay of Bengal are not
available. However, ithas been reported (Desa et al., 2006), that
the Bay of Bengal ispredominantly characterized by the fabric
(possibly transform faults)with a strike of about N140° to N150°,
which is considered to be ofCretaceous age that developed during
early sea floor spreading epi-sode. It is also associated with the
N50°E trending marine magneticanomalies. These fabrics are traced
right up to 21.5°E latitude onoceanic crust and their general
trends are also consistent with thegeneral trend obtained above in
the pole segregation (Fig. 9). Hencewe suggest that it is possible
that the old oceanic crust fabrics atintermediate depth on the
subducted Indian slab beneath Burmeseplate are reactivated during
the earthquakes. In the region there isan increase in earthquake
frequency at depths beyond 70 km whichcould be due to dehydration
embrittlement and volume change dueto gabbro/basalt to eclogite
transition (Ranero et al., 2005).
There could be another hypothesis in favor of fault
reactivation.From the plate reconstruction studies of the Indian
subcontinent ithas been suggested that at about 60–40 Ma time
period, the generaltrend of the subducted margin was NW–SE (Bannert
and Helmcke,1981; Hall, 1997). So it is possible that during 60–40
Ma time periodwhen subduction was active, some faults/weak zones
were formed onthe subducting oceanic plate due to bending in the
outer-rise whichwas generally parallel to the trench. These old
faults were reactivatedduring metamorphic dehydration process at
intermediate depth(Fig. 8). Hence, unusual orientations of some of
the thrust planesseen in the earthquake focal mechanisms are
possibly the indicatorof the fossil trend of the subducted
margins.
3.3. Lack of inter-plate great earthquakes in the Indo-Burmese
arc region
The earthquake focal mechanisms of the earthquakes in
theaccretionary wedge of the Indo-Burmese arc suggest that all
theseearthquakes are of intra-slab type that occurred through
reactivationof preexisting faults on the underlying Indian slab.
These earthquakesare quite similar to those that occur in a
subduction zone at interme-diate depth (e.g., Lamarche and Lebrun,
2000; Lister et al., 2008;Ranero et al., 2005). The contact surface
between the underlyingIndia plate and the overlying accretionary
wedge does not appear tobe seismogenic. The historical records of
earthquakes do not suggestoccurrence of great earthquakes in past
several centuries. Even thelargest earthquake in past 50 years,
namely, the M 7.3 August 6,1988 earthquake, was of intra-slab type
that occurred at intermediatedepth. All the above observations
imply that seismic hazard due togreat earthquakes in the region is
relatively low. For the occurrenceof great earthquake, a large
fault area is required which may not begenerally available in the
Indo-Burmese arc region. However, majorintraplate earthquakes may
cause damage in the sediment filledvalleys, e.g., the Manipur
valley around Imphal, Cachar valley aroundSilchar in India and
Chittogong and Sylhet region in Bangladesh, asthey have done in
past. GPS observations in the region may furtherhelp in assessing
the status of strain accumulation, if any, across theplate
boundary.
3.4. Formation of the Imphal valley: a pull apart basin
The Imphal valley is an oval shaped valley. This valley
stretches toabout 1843 km2 which accounts for less than one tenth
of the totalland area of Manipur state. The southern portion
contains a number
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145B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012)
135–146
of lakes and marshes, of which Loktak lake is the most famous.
It isthe largest fresh water lake in the northeast India and is the
onlyone in the world with floating biomass which is also the
naturalhabitat of one of the most endangered deers, the
brow-antlereddeer, locally known as the ‘Sangai’ deer. The
remaining part of theIndo-Burmese arc is hilly. There is no
information about how thisvalley was formed. We conjectured that it
was formed only after thetransition from active subduction
tectonics to transform tectonicstook place. We suggest that in a
manner similar to the formation ofthe Myanmar Central Basins
(Maurin and Rangin, 2009), this valleytoo was formed as a pull
apart basin when the activity on the NNE–SSW oriented dextral
strike slip fault, which was located somewhereeast of the valley,
(e.g., Kabaw fault) shifted to a similar fault locatedto the west
of the valley. In fact we could map an active fault, referredas the
Churachandpur–Mao Fault (CMF) on the western edge of thefault. It
is marked with intense deformation, pulverized material,and fault
gauge (Fig. 5). It is also marked with high number ofoccurrence of
landslides near the fault (Kumar and Sanoujam, 2007).
4. Conclusions
Following conclusions may be drawn from our analysis.
(1) The earthquakes in the Indo-Burmese arc and Sagaing
faultoccur due to slip partitioning of the predominantly
northwardrelative motion between the India and Sunda plates.
(2) The earthquake focal mechanisms in the Indo-Burmese
arcregion suggest predominantly northward relative motionthrough
dextral strike slip on the NNE–SSW oriented planesand thrust motion
on the WNW–ESE oriented planes (Fig. 4).The derived stress state
suggests that there is no significantvariation in the stress state
with depth.
(3) The earthquakes under the accretionary wedge of the
Indo-Burmese arc occur at a depth of about 30–60 km and define
agently eastward dipping seismicity trend surface. However,their
focal mechanisms suggest that all these earthquakes areof
intra-slab type earthquakes that occur on steep planes inthe Indian
plate (Fig. 5).
(4) Our analysis support fault reactivation hypothesis for
theintermediate-depth earthquakes of the Indo Burmese wedgeregion
that occur within the subducted Indian slab. We suggestthat
preservation of the old oceanic crust fabrics in the pres-ence of
hydrous mineral phases lead to shear failure by meta-morphic
dehydration at higher pressure temperature regime,which give rise
to the intermediate-depth earthquakes.
(5) The earthquake focal mechanisms and their inversion for
thestress state suggest that no active subduction occurs at
presentalong the Indo-Burmese arc. The earthquake focal
mechanismsin the Sagaing fault region are consistent with the
dextralmotion on the north–south oriented steep fault plane (Fig.
5).
(6) Lack of evidence of occurrence of great earthquakes in
thehistorical records, and absence of occurrence of
inter-plateearthquakes in the Indo-Burmese arc, probably suggest
thatthe seismic hazard due to the great earthquakes in the
Indo-Burmese arc is relatively low. However, major intra-slab
earth-quakes may cause damage in some regions due to local
siteeffects.
(7) We suggest that the contact surface between the outer
andinner Indo-Burmese wedge and the underlying Indian plate(Fig. 5)
is non-seismogenic.
Acknowledgements
Nixon Singh, Dolendra Singh, Arun Kumar, Sunil
Laishram,Tamonava, helped in the field work. Historians R.K.
Jhaljit Singh,Khelchandra Singh, N Devendra Singh, and N Tombi
Singh provided
help in accessing historical records of Manipur. Kalpna helped
instress inversion. We are thankful to the Editor and two
anonymousreviewers whose comments helped us in improving the
manuscript.This work is financially supported by MoES. BK
acknowledgesfinancial support from CSIR.
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Earthquake occurrence processes in the Indo-Burmese wedge and
Sagaing fault region1. Introduction2. Seismicity of the region2.1.
Historical major earthquakes2.2. Current seismicity2.3. Earthquake
focal mechanisms2.4. Inversion of the earthquake focal
mechanism
3. Discussion3.1. Progressive increase in the dip of the
subducted Indian slab from north to south3.2. Reactivation of
faults in the Indo-Burmese arc region3.3. Lack of inter-plate great
earthquakes in the Indo-Burmese arc region3.4. Formation of the
Imphal valley: a pull apart basin
4. ConclusionsAcknowledgementsReferences