earthquake occurrence processes in the indo-burmese wedge...

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

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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.

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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


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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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


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

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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.


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

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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.


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.


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


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

http://www.holcombe.net.au/software/rodh_software_georient.htmhttp://www.holcombe.net.au/software/rodh_software_georient.htm


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.

References

Alam, Mahmood, Alam, M. Mustafa, Curray, Joseph R., Chowdhury, M. Lutfar Rahman,Gani, M. Royhan, 2003. An overview of the sedimentary geology of the BengalBasin in relation to the regional tectonic framework and basin-fill history.Sedimentary Geology 155, 179–208.

Ambraseys, N., Douglas, J.J., 2004.Magnitude calibration of north Indian earthquakes. Geo-physical Journal International 159, 165–206. doi:10.1111/j.1365-246X.2004.02323.x.

Angelier, J., Baruah, Saurabh, 2009. Seismotectonics in Northeast India: a stress analysis offocalmechanism solutions of earthquakes and its kinematic implications. GeophysicalJournal International 178, 303–326. doi:10.1111/j.1365-246X.2009.04107.x.

Bannert, D., Helmcke, D., 1981. The evolution of the Asian Plate in Burma. Band 70 (2),446–458.

Barley, M.E., et al., 2003. Jurassic to Miocene magmatism and metamorphism in theMogok metamorphic belt and the India–Eurasia collision in Myanmar. Tectonics.doi:10.1029/2002TC001398.

Bender, F., 1983. Geology of Burma, Beitr. Reg. Geol. Erder 16, Gebr. Borntraeger, Berlin.Bertrand, G., Rangin, C., 2003. Tectonics of the western margin of the Shan plateau

(central Myanmar): implication for the India–Indochina oblique convergencesince the Oligocene. Journal of Asian Earth Sciences 21, 1139–1157.

Bertrand, G., et al., 1998. The Singu basalt (Myanmar): new constraints for the amountof recent offset on the Sagaing Fault. Earth & Planetary Sciences 327, 479–484.

Bijwaard, H., Spakman, W., Engdahl, E.R., 1998. Closing the gap between regional andglobal travel time tomography. Journal of Geophysical Research 103,30,055–30,078.

Bilham, R., 2004. Earthquakes in India and the Himalaya: tectonics, geodesy andhistory. Annals of Geophysics 47, 839–858.

Bilham, R., England, P., 2001. Plateau pop-up in the 1897 Assam earthquake. Nature410, 806–809.

Chandra, U., 1984. Tectonic segmentation of the Burmese–Indonesian arc.Tectonophysics 105, 279–289.

Chen, W.-P., Molnar, P., 1990. Source parameters of earthquakes and intraplatedeformation beneath the Shillong Plateau and northern Indoburman Ranges.Journal of Geophysical Research 95, 12,527–12,552.

Cummins, P.R., 2007. The potential for giant tsunamigenic earthquakes in the northernBay of Bengal. Nature 449, 75–78.

Curray, J.R., 2005. Tectonics and history of the Andaman Sea region. Journal of AsianEarth Sciences 187–232.

Desa, M., Ramana, M.V., Ramprasad, T., 2006. Seafloor spreading magnetic anomaliessouth off Sri Lanka.MarineGeology 229, 227–240. doi:10.1016/j.margeo.2006.03.006.

Engdahl, E.R., Hilst, R.V.D., Buland, R., 1998. Global teleseismic earthquake relocationwith improved travel time and procedures for depth determination. Bulletin ofthe Seismological Society of America 88, 722–743.

Everett, J.R., Russell, O.R., Staskowski, R.J., Loyd, S.P., Malhotra, V., 1990. Regionaltectonics of Myanmar (Burma) and adjacent areas (abstract). AAPG Bulletin 74,651.

Fitch, T.J., 1972. Plate convergence, transcurrent faults and internal deformationadjacent to Southeast Asia and the western Pacific. Journal of Geophysical Research77, 4432–4460.

Gahalaut, V.K., Gahalaut, K., 2007. Burma Plate motion. Journal of Geophysical Research112, B10402 doi:10.1029/2007JB004928.

Gahalaut, V.K., Subrahmanyam, C., Kundu, B., Catherine, J.K., Ambikapathy, A., 2010.Slow rupture in Andaman during 2004 Sumatra-Andaman earthquake: conse-quence of subduction of 90ºE ridge. Geophysical Journal International 180,1181–1186.

Gulston, E., 1763. An account of an earthquake at Chittagaon: translated from thePersian by Mr. Edward Gulston, in the service of the Honourable East IndiaCompany, and communicated by him to the Reverend Mr. Hirst. PhilosophicalTransactions 53, 251–256.

Gupta, H., Gahalaut, V., 2009. Is the northern Bay of Bengal tsunamigenic? Bulletin ofthe Seismological Society of America 99, 3496–3501.

Guzman-Speziale, M., Ni, J.F., 1996. Seismicity and active tectonics of the westernSunda Arc. In: Yin, A., Harrison, T.M. (Eds.), The Tectonic Evolution of Asia.Cambridge Univ. Press, New York, pp. 63–84.

Hall, R., 1997. Cenozoic plate tectonic reconstructions of SE Asia. In: Fraser, A.J.,Methews, S.J., Murphy, R.W. (Eds.), Petroleum Geology of Southeast Asia:Geological Society of London Special Publication, 126, pp. 11–23.

Halsted, E.P., 1843. Report on the Island of Chedooba. Journal Asiatic Society of Bengal114, 319–446.

International Seismological Centre, 2009. EHB Bulletin. http://www.isc.ac.ukInternational Seismological Centre, Thatcham, United Kingdom. (2010).

Iyengar, R.N., Sharma, S.D., Siddiqui, J.M., 1999. Earthquake history of India in medievaltimes. Indian Journal of History of Science 34, 181–237.

Jiao, W., Silver, P.G., Fei, Y., Prewitt, C.T., 2000. Do intermediate- and deep-focusearthquakes occur on preexisting weak zones? An examination of the Tongasubduction zone. Journal of Geophysical Research 105, 28,125–28,138.

Kincaid, C., Griffiths, R.W., 2003. Laboratory models of the thermal evolution of themantle during rollback subduction. Nature 425, 58–62. doi:10.1038/nature01923.

http://dx.doi.org/10.1111/j.1365-.2004.02323.xhttp://dx.doi.org/10.1111/j.1365-.2009.04107.xhttp://dx.doi.org/10.1029/2002TC001398http://dx.doi.org/10.1016/j.margeo.2006.03.006http://dx.doi.org/10.1038/nature01923hp高亮

hp高亮


Kumar, Arun, Sanoujam, Manichandra, 2007. Landslide studies along the nationalhighway (NH 39) in Manipur. Natural Hazards 40, 603–614.

Kundu, B., Gahalaut, V.K., 2010. An Investigation into the seismic potential of theIrrawaddy region, Northern Sunda arc. Bulletin of the Seismological Society ofAmerica 100, 891–895.

Lamarche, G., Lebrun, J.-F., 2000. Transition from strike-slip faulting to obliquesubduction: active tectonics at the Puysegur Margin, South New Zealand.Tectonophysics 316, 67–89.

Le Dain, A.Y., Tapponnier, Paul, Molnar, Peter, 1984. Active faulting and tectonics ofBurma and surrounding regions. Journal of Geophysical Research 89, 453–472.

Li, C., van der Hilst, R.D., Meltzer, A.S., Engdahl, E.R., 2008. Subduction of the Indianlithosphere beneath the Tibetan Plateau and Burma. Earth and Planetary ScienceLetters 274, 157–168.

Lister, G., Kennett, B., Richards, S., Forster, M., 2008. Boudinage of a stretching slabletimplicated in earthquakes beneath the Hindu Kush. Nature Geoscience 1, 196–201.

Martin, S., Szeliga, Walter, 2010. A catalog of felt intensity data for 570 earthquakes inIndia from 1636 to 2009. Bulletin of the Seismological Society of America 100,562–569. doi:10.1785/0120080328.

Maurin, T., Rangin, C., 2009. Structural and kinematics of the Indo-Burmese wedge: recentand fast growth of the outerwedge. Tectonics 28, TC2010. doi:10.1029/2008TC002276.

Maurin, T., Masson, Frederic, Rangin, Claude, Min, U. Than, Collard, Philippe, 2010. Firstglobal positioning system results in northern Myanmar: constant and localized sliprate along the Sagaing fault. Geology 38, 591–594.

McCaffrey, R., 1992. Oblique plate convergence, slip vectors, and forearc deformation.Journal of Geophysical Research 97, 8905–8915.

McKenzie, D.P., 1969. The relation between fault plane solutions for earthquakes andthe direction of the principal stresses. Bulletin of the Seismological Society ofAmerica 59, 591–601.

Michael, A.J., 1984. Determination of stress from slip data: faults and folds. Journal ofGeophysical Research 89, 11,517–11,526.

Michael, A.J., 1987. Use of focal mechanisms to determine stress: a control study.Journal of Geophysical Research 92, 357–368.

Mitra, S., Priestley, K., Bhattacharyya, A., Gaur, V.K., 2005. Crustal structure andearthquake focal depths beneath northeastern India and South Tibet. GeophysicalJournal International 160, 227–248.

Molnar, P., 1990. A review of the seismicity and the rates of active underthrusting anddeformation at the Himalaya. Journal of Himalayan Geology 1, 131–154.

Mukhopadhyay, M., Dasgupta, S., 1988. Deep structure and tectonics of the Burmesearc: constraints from earthquake and gravity data. Tectonophysics 149, 299–322.

Nandy, D.R., 2001. Geodynamics of Northeastern India and Adjoining Region. ACBPublications.

Ni, J.F., Speziale, M.G., Bevis, M., Holt, W.E., Wallace, T.C., Seager, W.R., 1989.Accretionary tectonics of Burma and the three dimensional geometry of theBurma subduction. Geology 17, 68–71.

Nielsen, C., Chamot-Rooke, N., Rangin, C., the ANDAMAN Cruise Team, 2004. Frompartial to full strain partitioning along the Indo-Burmese hyper-obliquesubduction. Marine Geology 209, 303–327.

Oldham, R.D., 1882. The Cachar earthquake of 10th January, 1869. Memoirs of theGeological Society of India 19, 1–98.

Oldham, T., 1883. A catalogue of Indian earthquakes. Memoirs of the Geological Surveyof India 19, 163–215.

Parratt, Saroj Nalini, 1999. The Court Chronicle of the Kings of Manipur: 33–1763 CE. TheCheitharol Kumpapa, 1. Routledge, Taylor and Francis Group, United Kingdom, p. 305.

Pesicek, J.D., Thurber, C.H., Widiyantooro, S., Zhang, H., DeShon, H.R., Engdahl, E.R.,2010. Sharpening the tomographic image of the subducting slab below Sumatra,the Andaman Islands and Burma. Geophysical Journal International 182, 433–453.

Pivnik, D.A., Nahm, J., Tucker, R.S., Smith, G.O., Nyein, K., Nyunt, M., Maung, P.H., 1998.Polyphase deformation in a fore-arc/back-arc basin, Salin subbasin, Myanmar(Burma). AAPG Bulletin 82, 1837–1856.

Ranero, C.R., Villaseñor, A., Phipps Morgan, J., Weinrebe, W., 2005. Relationshipbetween bend-faulting at trenches and intermediate-depth seismicity.Geochemistry, Geophysics, Geosystems 6, Q12002. doi:10.1029/2005GC000997.

Rao, N.P., Kalpna, 2005. Deformation of the subducted Indian lithospheric slab in theBurmese arc. Geophysical Research Letters 32, L05301. doi:10.1029/2004GL022034.

Rao, N.P., Kumar, M.R., 1999. Evidences for cessation of Indian plate subduction in theBurmese arc region. Geophysical Research Letters 26, 3149–3152.

Richards, S., Lister, G., Kennett, B., 2007. A slab in depth: three dimensional geometryand evolution of the Indo-Australian plate. Geochemistry, Geophysics, Geosystems12, Q12003. doi:10.1029/2007GC001657.

Richter, C.F., 1958. Elementary Seismology. Freeman, San Francisco. (768 pp.).Satyabala, S.P., 1998. Subduction in the Indo-Burma Region: is it still active? Geophysical

Research Letters 25, 3189–3192.Satyabala, S., 2003. Oblique plate convergence in the Indo-Burma (Myanmar)

subduction region. Pure and Applied Geophysics 160, 1611–1650.Schellart, W.P., 2004. Kinematics of subduction and subduction-induced flow in the upper

mantle. Journal of Geophysical Research B07401. doi:10.1029/2004JB002970.Searle, M.P., Noble, S.R., Cottle, J.M., Waters, D.J., Mitchell, A.H.G., Hliaing, T., 2007.

Tectonic evolution of the Mogok metamorphic belt, Burma (Myanmar)constrained by U–Th–Pb dating of metamorphic and magmatic rocks. Tectonics26, TC3014. doi:10.1029/2006TC002083.

Seeber, L., Armbruster, J., 1981. Great detachment earthquakes along the HimalayanArc and long-term forecasting. In: Simpson, D.W., Richards, P.G. (Eds.), EarthquakePrediction: An International Review. Maurice Ewing Series: American GeophysicalUnion, 4, pp. 259–277.

Sengupta, S., Ray, K.K., Acharyya, S.K., 1990. Nature of ophiolite occurrences along theeastern margin of the Indian plate and their tectonic significance. Geology 18,439–442.

Singh, R.K. Jhaljit, 1965. A Short History of Manipur. Manipur Sahitya Parishad, Imphal,India, p. 365.

Steckler, M.S., Akhter, S.H., Seeber, L., 2008. Collision of Ganges–Brahmaputra Deltawith the Burma Arc: implication for earthquake hazard. Earth and PlanetaryScience Letters 273, 367–378.

Stork, L., Selby, N.D., Heyburn, R., Searle, M.P., 2008. Accurate relative earthquakehypocenters reveal structure of the Burma subduction zone. Bulletin of theSeismological Society of America 98, 2815–2827.

Tapponnier, R., Peltzer, G., Le Dain, A.Y., Armijo, R., Cobbold, P., 1982. Propagatingextrusion tectonics in Asia; new insights from simple experiments with plasticine.Geology 10, 611–616.

Tsutsumi, H., Sato, Takayuki, 2009. Tectonic geomorphology of the SouthernmostSagaing Fault and surface rupture associated with the May 1930 Pegu (Bago)Earthquake, Myanmar. Bulletin of the Seismological Society of America 99,2155–2168. doi:10.1785/0120080113.

Verma, R.K., Mukhopadhyay, Manoj, Ahluwalia, M.S., 1976. Earthquake mechanismsand tectonic features of Northern Burma. Tectonophysics 32, 387–399.

Vigny, C., Socquet, A., Rangin, C., Chamot-Rooke, N., Pubellier, M., Bouin, M., Bertrand, G.,Becker, M., 2003. Present-day crustal deformation around Sagaing fault, Myanmar.Journal of Geophysical Research 108 (B11), 2533. doi:10.1029/2002JB001999.

Zeitler, P.K., Meltzer, Anne S., Koons, Peter O., Craw, David, Bernard Hallet, C.,Chamberlain, Page, Kidd, William S.F., Park, Stephen K., Seeber, Leonardo, Bishop,Michael, Shroder, John, 2001. Erosion, Himalayan geodynamics, and thegeomorphology of metamorphism. GSA Today 11, 4–9.

http://dx.doi.org/10.1785/0120080328http://dx.doi.org/10.1029/2008TC002276http://dx.doi.org/10.1029/2005GC000997http://dx.doi.org/10.1029/2004GL022034http://dx.doi.org/10.1029/2007GC001657http://dx.doi.org/10.1029/2004JB002970http://dx.doi.org/10.1029/2006TC002083http://dx.doi.org/10.1785/0120080113http://dx.doi.org/10.1029/2002JB001999

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

earthquake occurrence processes in the indo-burmese wedge...

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