Earthquake Occurrence Processes in the Indo-Burmese Wedge and Sagaing Fault Region

Tectonophysics 524–525 (2012) 135–146

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Tectonophysics

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Earthquake occurrence processes in the Indo-Burmese wedge and Sagaing fault region

Bhaskar Kundu, V.K. Gahalaut ⁎

National Geophysical Research Institute (CSIR), Hyderabad, India article info abstract

Article history: Earthquakes in the Indo-Burmese wedge and Sagaing fault regions occur in response to the partitioning of the Received 25 January 2011 India–Sunda motion along these two distinct boundaries. Under the accretionary wedge of the Indo-Burmese Received in revised form 11 November 2011 arc, majority of the earthquakes occur in the depth range of 30–60 km and deﬁne an eastward gently dipping Accepted 21 December 2011 seismicity trend surface that coincides with the Indian slab. The dip of the slab steepens in the east direction Available online 29 December 2011 and earthquakes occur down to a depth of 150 km, though the slab can be traced up to the 660 km discontinuity. Although these features are similar to a subduction zone, the nature of the earthquakes and Keywords: Indo-Burmese wedge our analysis of their focal mechanisms suggest that these earthquakes are of intra-slab type which occur Sagaing fault on steep plane within the Indian plate and the sense of motion implies a northward relative motion with Seismogenesis respect to the Sunda plate. Thus these earthquakes and the stress state do not support active subduction Earthquake focal mechanisms across the Indo-Burmese arc which is also consistent with the relative motion of India–Sunda plates. The Intra-slab earthquakes absence of inter-plate earthquakes, lack of evidence of the occurrence of great earthquakes in the historical records and non-seismogenic nature of the plate interface under the accretionary wedge suggest that seismic hazard due to earthquakes along the plate boundary may be relatively low. However, major intra-slab earthquakes at shallow and intermediate depths may still cause damage in the sediment ﬁlled valley regions of 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 stress state is consistent with the plate motion across the Sagaing fault. © 2011 Elsevier B.V. All rights reserved.

1. Introduction past when it was predominantly southeast–northwest trending. Though the precise timing of this transition is not known, it appears The Indo-Burmese wedge along with the Sagaing fault forms the to have occurred prior to about 50 Ma (Hall, 1997). However, after northern part of the northwestern Sunda arc (Chandra, 1984; Chen the collision of the India plate with the Eurasian plate, the Burma and Molnar, 1990; Curray, 2005; Fitch, 1972; Le Dain et al., 1984; plate, consisting of the Indo-Burmese wedge, Myanmar Central Nandy, 2001; Ni et al., 1989; Verma et al., 1976). The approximately Basin along with the Andaman–Sumatra arc, rotated clockwise to north–south trending convex westward Indo-Burmese wedge joins become predominantly north–south trending (Hall, 1997). The extru- the approximately east–west trending eastern Himalaya in the sion and clockwise rotation of the Burma plate in the late tertiary north (Fig. 1). The region of this junction is referred as the Eastern period created compressional structure in the Myanmar Central Himalayan Syntaxis, which is marked with complex tectonics, high basin (MCB) (Everett et al., 1990; Le Dain et al., 1984; Tapponnier exhumation and erosion rates, etc. (Zeitler et al., 2001). In the et al., 1982). Recent geochronology of the Mogok metamorphic belt south, the Indo-Burmese wedge joins with the north–south trending in Burma (Searle et al., 2007) supports that right-lateral motion on Andaman–Sumatra subduction zone. In the region of Indo-Burmese the Sagaing fault which might have initiated after 16–22 Ma. The wedge, the northward motion of about 35 mm/year of the India Burma plate appeared to have originated through three major phases. plate with respect to the Sunda plate (Maurin et al., 2010; Nielsen In the ﬁrst phase, by the end of the Eocene (~35 Ma), Burma Plate col- et al., 2004; Vigny et al., 2003) is assumed to be accommodated lided with the northeast edge of the Indian plate and was dragged through slip partitioning in the Indo-Burmese arc and on the Sagaing northward as a fore arc sliver. In the Miocene (~15 Ma), this acceler- fault (Fig. 1). The plate reconstruction models suggest that subduc- ated motion led to the formation of NE–SW trending extensional tion probably occurred in the Indo-Burmese wedge in the geological basins bounded by NE–SW striking normal faults, and to the creation of Andaman sea rift (Curray, 2005). Finally in the Pliocene (~5 Ma), when the northern end of the Burma Plate collided with Asia, ⁎ Corresponding author. Tel.: +91 40 23434700. extensional deformation ceased and transpressional deformation E-mail address: [email protected] (V.K. Gahalaut). caused reverse faults, positive ﬂower structures, inversion of normal

Maurin and Rangin (2009) analyzed structures and kinematics of the Indo-Burmese wedge which defines the western margin of the Burma plate, a sliver between the India and Sunda plates (Gahalaut and Gahalaut, 2007; McCaffrey, 1992). On the basis of the age, grade of metamorphism and rock type, they classified it into outer, inner and core wedges. The outer wedge mainly consists of Tripura Fold belt and the eastern part of the Bengal basin and is made of Neogene clastic sequences. Age of the sediments in the outer wedge ranges from lower Miocene submarine deposits, upper Miocene shelfal deposits to Plio-Pleistocene fluvial deposits (Maurin and Rangin, 2009). The inner Indo-Burmese wedge is composed of Eocene flyschs affected by N–S trending strike-slip right faults, such as the Churachandpur–Mao fault, CMF, discussed later in the text. The core of the wedge is made of high-grade metamorphic rocks, tectonically imbricated with Mesozoic ophiolites and sedimentary sequences ranging from Late Triassic to Late Cretaceous (Bender, 1983). A detailed discussion on these units may be found in Maurin and Rangin (2009). In this article, we analyze the seismicity of the region and address all these issues to suggest that no subduction occurs across the Indo Burmese wedge and the earthquakes are of intra- slab type that occur within the Indian plate through reactivation of the old geologic fabric.

2. Seismicity of the region

2.1. Historical major earthquakes

Fig. 1. Major tectonic features of the Sunda and Himalayan arc. There are two most notable great earthquakes, namely, the 1897 Shillong Plateau and the 1950 Assam earthquakes (Molnar, 1990; Seeber and Armbruster, 1981) that have occurred close to the Indo- faults and extensional basin (Bertrand and Rangin, 2003; Maurin and Burmese arc and Sagaing fault (Fig. 2). The 1897 Shillong Plateau Rangin, 2009). earthquake occurred primarily under the Shillong Plateau and is Presently, the motion between the India and Sunda plates is linked with the tectonics of the Shillong Plateau (Bilham and partitioned between Indo-Burmese arc and Sagaing fault. Our recent England, 2001). Thus this earthquake is considered as an intra-plate GPS measurements in the Indo-Burmese arc region (Gahalaut, V.K. earthquake which probably occurred through reverse motion on the et al., manuscript in preparation) and the available measurements south dipping steep fault. The 1950 Assam earthquake occurred in in the Sagaing fault region (Maurin et al., 2010; Vigny et al., 2003) the Arunachal Himalaya and the Eastern Himalayan Syntaxis region suggest that about 60% of the relative motion between India and and is considered as the interplate earthquake that occurred due to Sunda plates is taken up by the Sagaing fault. Both regions are the ongoing India–Eurasia convergence, part of which is accommo- characterized by earthquake occurrences. One of the major dated in the Himalaya (Molnar, 1990; Seeber and Armbruster, differences between the earthquakes in the Indo Burmese wedge 1981). Thus the two great earthquakes are probably not linked with and in the Sagaing fault is their focal depth. Earthquakes are generally the tectonics of the Indo-Burmese arc and the Sagaing fault regions. very shallow in the latter region, whereas in the former, they occur up We compiled a catalogue of major earthquakes in the Indo- to a depth of 150 km (Guzman-Speziale and Ni, 1996). Another major Burmese arc and Sagaing Fault regions (Fig. 2 and Table 1). We difference is that the earthquakes predominantly occur through strike excluded earthquakes occurring in the Shan Plateau and Red River slip motion on the Sagaing fault while in the Indo Burmese wedge, fault region as they are linked with the tectonics of the Tibet Plateau they occur through strike slip and thrust and oblique motion. In the extrusion due to the India–Eurasia convergence. Although there are a Indo Burmese wedge, it is not known whether these earthquakes are few unverifiable reports of earthquake occurrence as early as 1548 in of inter-plate or intra-plate (or intra-slab) type (Guzman-Speziale Tripura, Assam or Bangladesh (Iyengar et al., 1999; Steckler et al., and Ni, 1996). Several geological studies (arc magmatism and meta- 2008), there are large uncertainties in the earthquake location. We morphism, occurrence of ophiolitic rock sequences, surface as well find that the catalogue is probably reliable only after 1762. As many subsurface expression of fold and thrust belt structures), geophysical of these historical earthquakes are located on the basis of maximum observations (tomographic images and gravity anomaly), and plate damage, their epicentral locations are not very reliable. Thus a few reconstruction studies confirm that the subduction occurred across earthquakes which may probably be linked with the tectonics of the Indo Burmese wedge between India and Burma plates (Bannert and Shillong plateau and Himalaya, may erroneously be included here as Helmcke, 1981; Barley et al., 2003; Bertrand et al., 1998; Guzman- they caused damage in the Indo-Burmese Arc region. Similarly, the Speziale and Ni, 1996; Hall, 1997; Li et al., 2008; Mukhopadhyay and earthquakes which caused damage in the Shillong Plateau region, Dasgupta, 1988; Ni et al., 1989; Pivnik et al., 1998; Sengupta et al., and hence they were excluded, might have actually occurred in the 1990) and there are evidence of subducted India slab, but whether Indo-Burmese arc region. the subduction is still active, is a debatable topic. Several investigators The May 23, 1912 earthquake (M~8) is probably the only great have analyzed earthquake occurrence processes in the Indo Burmese earthquake that occurred in the Sagaing fault region (Guzman-Speziale wedge (Angelier and Baruah, 2009; Guzman-Speziale and Ni, 1996; and Ni, 1996; Maurin et al., 2010; Richter, 1958; Tsutsumi and Sato, Rao and Kalpna, 2005; Rao and Kumar, 1999; Satyabala, 1998; 2009). This earthquake occurred in the eastern Burma and caused Satyabala, 2003). However, none of them could address all the above bending of the railroad tracks. Other than this, several major earthquakes issues. In some cases it is due to lack of sufficient and accurate data have occurred along the Sagaing Fault. In the Indo-Burmese arc region a (e.g., Guzman-Speziale and Ni, 1996). few notable major earthquakes are the April 2, 1762 and August 24, 1858 .Knu ..Ghlu etnpyis524 Tectonophysics / Gahalaut V.K. Kundu, B. – 2 21)135 (2012) 525 – 146

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 marked with yellow color are mapped in the present study. The bold 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, denoting focal depths, are the earthquakes from EHB catalogue for the 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 years. Approximate locations of the 1762 Arakan and 1839 Sagaing Fault earthquakes are also indicated. Locations of 1897 Shillong Plateau and 1950 Assam earthquakes which occurred in the Shillong Plateau and Eastern Himalayan regions, are also shown. Imphal is located in the Manipur valley. (C) Last panel shows earthquake focal mechanisms. Black and gray color focal mechanisms denote shallow (b75 km) and intermediate (75–150 km) depth earthquakes. Dashed lines in the Bay of Bengal region represent the oceanic fracture zones or the geologic fabric trend (Desa et al., 2006). CMF — Churachandpur Mao Fault, EHS — Eastern Himalayan Syntaxis, KDF — Kaladan Fault, KF — Kabaw Fault, MCB — Myanmar Central Basin. 137 138 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146

Table 1 Major 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 Coast 2. 1839/03/23 Near Amarapura, Mandalay Major Sagaing Fault 3. 1843/02/06 19.5 95.5 Major Sagaing Fault/Ramree Island 4. 1843/10/30 19 95 Major Sagaing Fault/Cheduba Island 5. 1848/01/03 19.5 95.5 Major South Myanmar 6. 1858/08/24 19 95.25 Major Arakan Coast, South Mynmar, Sagaing Fault 1869/01/10 25 93 Major (7.4) Manipur and Cachar 1885/07/14 Manikganj, about 50 km west o Major Bangladesh Dhaka 1889/01/10 Major Jaintia Hills 7. 1906/08/31 27 97 Major Sagaing Fault 8. 1908/12/12 26.5 97 7.5 Sagaing Fault 9. 1912/05/23 21 97 8 Sagaing Fault 10. 1918/07/08 24.5 91 7.6 Srimangal, Bangladesh 12. 1923/09/09 25.25 91 7.1 Shillong Plateau, Durgapur 14. 1929/08/08 19 96.5 7 Sagaing Fault 15. 1930/05/05 17 96.5 7.3 Sagaing Fault 16. 1930/07/02 25.25 90 7.1 Dhubri, Assam 17. 1930/12/03 18 96.5 7.3 Sagaing Fault 18 1931/01/27 25.6 96.8 7.6 Sagaing Fault 19. 1932/08/14 26 95.5 7 India–Myanmar 20. 1938/08/16 23.5 94.25 7.2 Sagaing Fault 21. 1941/12/26 21 99 7 Shan Plateau 22. 1943/10/23 26 93 7.2 Shillong Plateau Assam 23. 1946/09/12 23.5 96 7.5 Sagaing Fault 24. 1946/09/12 23.5 96 7.7 Sagaing Fault 26. 1954/03/21 24.2 95.1 7.1 India–Myanmar 1957/07/01 24.4 93.8 7.2 Manipur 1970/07/29 26.02 95.37 68 7.0 Myanmar–Bangladesh 27. 1975/07/08 21.44 94.59 116 7.0 Indo Burmese wedge 29. 1988/08/06 25.09 95.11 99 7.3 India–Myanmar 31. 1991/01/05 23.58 95.88 13 7.0 Sagaing Fault

Arakan earthquakes, and the January 10, 1869 Cachar earthquake 2004; Guzman-Speziale and Ni, 1996; Le Dain et al., 1984; Martin and (Guzman-Speziale and Ni, 1996; Richter, 1958). Other than these earth- Szeliga, 2010). However, it is possible that the high damage in these quakes, several instances of damage due to earthquakes are reported regions was due to relatively large population, local site effects, as from Chittagong, Sylhet, Manipur valley and Cachar regions (Bilham, these places are located in the valley with thick sediment cover and

Fig. 3. (A) Historical Govindaji temple and the accompanying structure, the Beithoub, in the Kangla palace, Imphal, was damaged during the 1869 Cachar earthquake. Other historical temples in the region did not appear to have suffered any damage from the earthquakes. However a small tilt (~4°) was observed at the Madan Mohonji temple (B and C) which may not necessarily be ascribed to the earthquake. B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 139 prevalence of non-traditional construction practices in these regions in records of Manipur (Parratt, 1999; Singh, 1965). Manipur, now a contrast to those in the surrounding hilly regions where houses were state of India, was an independent kingdom which was ruled by mainly made with bamboos to escape damage due to the earthquakes. Meitei kings since 35 CE, at least (Parratt, 1999). Kangla (now The April 2, 1762 Arakan earthquake has been considered as the great Imphal) in the Manipur valley was the capital of the princely state tsunamigenic earthquake (Cummins, 2007). It caused extensive damage and the historical records were maintained in the “The Cheitharol in the Chittagong region through shaking, liquefaction, damming of Kumpapa, The Court Chronicle of the Kings of Manipur” (Parratt, channels, seiches, etc. (Gulston, 1763). The reported uplift of Cheduba 1999; Singh, 1965). The sediment ﬁlled valley region has remained Island due to this earthquake was not found to be singularly due to this the center of inhabitation since historical times. The valley is located earthquake (Martin and Szeliga, 2010; Oldham, 1883). Halsted (1843) at the center of the Indo-Burmese arc. Thus this region could not reported the effects of the earthquakes which were mostly found to be have escaped from the damage due to any great earthquake in the highly exaggerated (Gupta and Gahalaut, 2009). It is now considered Indo-Burmese arc. Hence, it may be appropriate to state that no larger that this earthquake probably did not cause any major tsunami (Gupta earthquake than the January 10, 1869 earthquake, occurred in the and Gahalaut, 2009) and was probably only a major earthquake Indo-Burmese arc during the period of written historical records. (Martin and Szeliga, 2010). The August 24, 1858 Arakan earthquake The 1869 earthquake caused severe damage in the Cachar valley, was felt in many parts of Burma and was severely felt at Kyauk Pyu, near Silchar and Manipur valley, near Imphal. Five deaths were Ramree Island, Myanmar. It caused liquefaction, damage to buildings reported from Silchar while three from Imphal (Oldham, 1882; and Pagodas (Martin and Szeliga, 2010). In this region, the distance Singh, 1965). Extensive liquefaction, wide cracks occurred in the between the Sagaing fault and the structurally mapped Arakan trench Silchar region, near the Barak river and in Imphal, near the Imphal is less than 200 km and hence based on the scanty reports of damage it river (Oldham, 1882). In Imphal a few bridges and the palace of the is difﬁcult whether this and the 1843 and 1848 earthquakes are linked king, the Kangla Palace, and a temple within it were damaged. The with the Arakan frontal arc or with the Sagaing fault. earthquake was followed by more than 15 aftershocks in the follow- The January 10, 1869 Cachar–Manipur earthquake was the most ing two day period which were strongly felt in Imphal (Singh, 1965). severe earthquake in the available 2000 years of written historical Ambraseys and Douglas (2004) estimated the magnitude of this

Fig. 4. Depth section across the Indo Burmese wedge and Sagaing fault (SF) showing EHB seismicity (Engdahl et al., 1998). Filled and hollow triangles in each panel indicate the projected position of the CMF and SF. West of the CMF, the thickness of the sediments of the accretionary wedge is estimated to be about 25–30 km, which is also consistent with the sediment thickness estimated at a site KMG from the receiver function technique (Mitra et al., 2005). The dashed portion of the slab under the Indo-Burmese accretionary wedge is conjectural and is partly based on the available geological sections (Alam et al., 2003). The deeper part (>150 km) of the slab geometry is based on the tomographic images (Li et al., 2008; Pesicek et al., 2010). A progressive steepening in the slab may be noted from north to south which may be explained as due to the increase in trench retreat velocity (VT) from north to south. VS and VR are the subduction and rollback velocity (see Fig. 8). 140 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 earthquake as 7.4. Oldham (1882) described the damage caused by the doors and windows provide added strength to these structures. this earthquake in the Cachar valley near Silchar and at far off places None of these structures appears to have suffered damage due to but did not visit the Manipur valley due to security reasons, where the earthquake, though a small tilt (~4°) at two temples, namely the damage was equally severe. In April 2010, we visited various his- the Madan Mohonji temple at Imphal, and the Vishnu temple at torical monuments in Imphal and adjoining regions of Manipur valley Bishanpur, was noticed (Fig. 3B and C). However, it is difficult to and tried to assess the damage to these monuments due to this or ascertain whether this tilt occurred due to the shaking caused by other historical earthquakes. In Imphal, the Govindaji temple and the 1869 earthquake or it is due to slow and continuous settlement the inner walls of the Kangla Fort palace are reported to have suffered of the foundation since the construction of the temple. damage due to this earthquake; however, massive restoration work In summary we did not find much evidence of extensive damage taken up by the successive rulers did not allow us to assess the extent in the Manipur valley due to the 1869 earthquake. The damage was and pattern of damage due to the earthquake to these buildings. mainly confined to the Kangla palace in Imphal. At other places the Nevertheless, there are clear signs of damage and restoration work. old historical structures did not suffer damage either due to the fact In fact some of the 12 pillars (with heights of about 5 m and diameter that they were small and massive or the site conditions were better. of about 1 m) in the Beithoub, in front of the Govindaji temple (Fig. 3A), which were extensively damaged during the earthquake 2.2. Current seismicity and were restored, are still tilted. In addition, we could locate additional seven temples within Imphal and adjoining regions We used the updated catalogue of relocated earthquakes (EHB which were built before the 1869 earthquake and some of them catalogue of Engdahl et al., 1998 from International Seismological were more than 300 years old. The architecture of all these temples Centre, 2009) since 1964. In the past 50 years no great earthquake is simple, unlike the temples of north India. These temples generally has occurred in the region. The August 6, 1988 earthquake of M 7.3, have only one dome with the maximum height of about 10 m with that occurred in the Indo-Burmese arc region at the India–Myanmar a ground base of about 8 m x 8 m. These are very massive structures border and about 120 km east–northeast of Imphal with an interme- with thickness of walls being about 1 m. In some structures (e.g., diate focal depth of 99 km, was the largest magnitude earthquake in the temple of Brindabanchandra at Imphal) there is a gallery around the region in the past 50 years. Three people died and there was the sanctum sanctorum. The arch shaped ceiling and openings at some damage to the buildings in the northern Myanmar due to this

Fig. 5. A depth section across the Indo-Burmese wedge and Sagaing fault showing the seismicity, earthquake focal mechanisms (east–west vertical cross sectional projections) and topography. Note the steep nodal planes of the earthquake focal mechanisms which are not consistent with the gentle dip of the seismicity trend under the Indo-Burmese arc accretionary wedge. The lower panel shows a composite ﬁeld photograph of the Churachandpur Mao Fault (CMF) zone from a region 60 km southwest of Imphal. The winding road is shown by the yellow dash line. B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 141

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 Sagaing fault. earthquake. Majority of the earthquakes occur in the Indo Burmese 2.3. Earthquake focal mechanisms wedge and Sagaing fault region while the intervening region of the Myanmar Central Basin rarely experiences earthquakes (Fig. 2). In We use previously published earthquake focal mechanisms and the Indo Burmese wedge earthquakes occur down to a depth of those listed in the Harvard Centroid Moment Tensor (CMT) catalogue about 150 km, while they are very shallow in the Sagaing fault region. (Fig. 2). However, for the location of the earthquakes, we use the In the Indo Burmese wedge, they appear to define the underlying corresponding hypocenter from the EHB catalogue. In the Sagaing Indian slab which has been traced to extend at least or beyond fault region all the earthquakes occur through predominant strike 410 km discontinuity in the regional tomographic studies (Li et al., slip motion on steep plane with one nodal plane parallel to the 2008). Recent tomographic studies (Li et al., 2008; Pesicek et al., north–south trending Sagaing fault which exhibit dextral strike slip 2010) and EHB seismicity sections (Fig. 4) suggest that there is a motion. In the Indo-Burmese wedge, earthquake focal mechanisms progressive increase in the dip of the subducted slab from north to are quite mixed. Though, they are predominantly of strike slip or south profiles. thrust types, a few earthquakes with normal motion are also noticed. Under the Indo-Burmese accretionary wedge earthquakes occur at Contrary to the earlier observations (Rao and Kalpna, 2005; Rao and depths of 30–60 km and define a very gently eastward dipping Kumar, 1999), we did not find any clear segregation in the earth- seismicity trend surface which lies below the base of the accretionary quake focal mechanisms with depth in the Indo Burmese wedge wedge. Rarely, the earthquakes occur within the accretionary wedge. (Fig. 5). In fact, Stork et al. (2008) also suggested that probably Though, several faults, namely, the Kaladan (KDF) and Kabaw fault there is no depth-wise segregation in earthquake focal mechanisms (KF) have been mapped on the surface, none of them appears to be in the Indo-Burmese arc. The directions and plunge of the P, T and associated with the earthquakes. Further south in the Irrawaddy N axes in the Indo-Burmese wedge and Sagaing fault regions are region, where seismicity is very low, the slab is not traceable because shown in Fig. 6. At shallow depth (b75 km) in the Indo-Burmese of the tear in the Indian slab (Kundu and Gahalaut, 2010; Richards wedge the general orientation of the sub-horizontal P axes is in the et al., 2007). Seismicity in the Sagaing fault region is quite scanty, NNE–SSW direction. The orientation of the T axes is in the east– particularly in the central portion. The Sagaing fault is seismically west to ESE–WNW direction with generally gentle plunge and the most active in the northern portion. All the earthquakes occur at orientation of the N axes is approximately in the NW–SE direction shallow depth (b25 km). A few earthquakes occur in the central with no consistent plunge. At intermediate depth (>75 km) in the Myanmar basin. Here, we have excluded the earthquakes associated Indo-Burmese wedge, the orientation of the sub-horizontal P axes is with the Red River fault in the South China from our discussion. in the NNE–SSW direction, similar to that at shallow depth. The 142 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146

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 direction Rangin (2009). Other faults in the outer and inner wedge, e.g., the with steep plunge while the N axes have orientations varying be- Chittagong Coastal fault, Kaladan fault, and Kabaw fault, do not tween SW–NE to NW–SE with moderate plunge. These orientations appear to accommodate any motion between the India and Sunda of P, T and N axes in the Indo-Burmese wedge region imply strike plates. Since majority of the earthquakes occur at depth greater slip faulting on the NNE–SSW or ENE–WSW oriented planes or thrust than 25 km, it is inappropriate to relate the sense of motion during faulting on the WNW–ESE oriented planes. Further, the orientation of these earthquakes with these and other geologically mapped faults P and T axes in the Indo-Burmese wedge suggest that at present there on the surface. is no active subduction along this margin (Rao and Kumar, 1999). In fact our recent results of GPS measurements (our unpublished results) across the Indo-Burmese wedge of Indian region show no 2.4. Inversion of the earthquake focal mechanism variation in the arc normal motion, implying no active subduction. In the Sagaing fault region, the orientation of the sub-horizontal P We used earthquake focal mechanism solutions from CMT axes is in the SW–NE to east–west direction while the orientation of catalogue to estimate the directions of principal stress in the Indo- the T axes is in the NW–SE direction with very gentle plunge. The az- Burmese arc and Sagaing fault regions (Fig. 2). We divided the imuth of N axes is quite diffused in all directions but it has a steep earthquake focal mechanisms into three categories (i) the shallow plunge. These orientations of P, T and N axes in the Sagaing fault earthquakes (focal depth b75 km) of the Indo-Burmese arc, (ii) the region imply dextral strike slip motion on the steep north–south ori- intermediate depth earthquakes (focal depth >75 km) of the Indo- ented plane or sinistral strike slip motion on the steep east–west Burmese arc and (iii) the earthquakes of the Sagaing fault region. plane. The former is consistent with the motion on the Sagaing fault. We used the linear least square inversion approach (Michael, 1984,

There is one most interesting aspect of the earthquake focal mech- 1987) to estimate the best fitting maximum (σ1), intermediate (σ2) anisms in the Indo-Burmese wedge. The earthquake hypocenters ap- and minimum (σ3) stress directions. The results of the inversion are pear to define a sub-horizontal seismicity trend surface which could shown in Fig. 7 and Table 2. At shallow depth in the Indo-Burmese either be the basement of the accretionary wedge or the top surface arc region, σ1 is very gentle and is oriented in the NNE–SSW direction. of the Indian plate. However, none of the two nodal planes of these σ2 and σ3 are moderately steep and have azimuth in NW and ESE earthquakes focal mechanisms is consistent with the dip of the seis- direction. At intermediate depth in the Indo-Burmese arc region, σ1 micity trend surface (Fig. 5). The dip of the two nodal planes is is again very gentle and is oriented in the north–south direction. steep and hence is inconsistent with the gentle dip of the seismicity The azimuth of σ2 is not well constrained but is approximately in trend and the plate interface. Thus the earthquake hypocenters and the west direction with moderate plunge. σ3 is moderately steep the seismicity trend surface under the Indo Burmese accretionary and has azimuth in the east direction. wedge disguise to be the inter-plate earthquakes, while they actually We suggest that within the assumed uncertainties in the trend are of intra-plate type, which possibly occur within the Indian plate. and plunge of the slip vector of an individual earthquake's focal The results of GPS measurements suggest that the outer and inner mechanism, there is no significant difference in the stress state at wedge and the underlying Indian plate are strongly coupled and shallow and intermediate depth levels in the Indo-Burmese arc re- behave as a single unit with no relative displacement. It is only the gion and the stress state is generally consistent with the strike slip core part of the Indo Burmese wedge which accommodates the and thrust faulting in the region. In the Indo-Burmese arc the relative relative partitioned motion between India and Sunda. In the Indian plate motion between the India–Sunda plates is predominantly geographical region, this motion occurs on the CMF which could be toward north (N10°). Thus in this region the derived stress state is equivalent or same as the Lelon fault, identified by Maurin and generally consistent with the relative plate motion. Although many earthquakes show thrust dominated focal mechanism, it does not imply that subduction is currently active in the region, as the

direction of maximum principal stress (σ1) is NEN–SWS, rather than Table 2 being east–west to support subduction in that direction. Further, it Results of inversion of focal mechanisms for estimating the principal stress directions. may be noted that the nodal planes of these thrust earthquakes are – Region σ1 azimuth, σ2 azimuth, σ3 azimuth, oriented in the WNW ESE direction, whereas, if the eastward plunge plunge plunge subduction of the Indian plate was still active, they should have Indo-Burmese arc (focal depth 0–75 km) 202°, 10° 301°, 51° 104°, 37° been oriented in the north–south direction. The unusual WNW–ESE Indo-Burmese arc 358°, 1° 268°, 32° 91°, 58° orientation of the planes is probably linked with the orientation of (focal depth 75–150 km) the old oceanic fabrics present in the Bay of Bengal and has been Sagaing Fault 226°, 3° 117°, 86° 236°, 2° discussed in a subsequent section. B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 143

3. Discussion

3.1. Progressive increase in the dip of the subducted Indian slab from north to south

The seismicity depth sections across the Indo-Burmese arc in Fig. 4 suggest that there is a progressive increase in the dip of the subducted slab from north to south. Tomographic studies in the Indo-Burmese arc have provided evidence of presence of Indian slab and its progressive 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 directed rollback at 410 km discontinuity and probably does not penetrate further. Li et al. (2008) suggested that the slab extends up to a depth of at least 300 km. Recently, Pesicek et al. (2010) proposed that it extends up to a depth of 660 km discontinuity and the presence of tear in the slab can neither be conﬁrmed nor refuted. However, all these studies indicate a southward steepening in the subducted Indian slab. On the contrary, due to increase in the age of the lithosphere under the Indo-Burmese wedge toward north, a northward steepening is expected. We attempt to explain it in Fig. 8. From plate reconstruction models of the Indian subcontinent, Fig. 8. A conceptual model demonstrating the steepening of the Indian slab from north to south. On the top panel, O represents the hinge or the pivot position along which the

Burmese plate and the arc rotated from its original position from OA1 to OA2. This caused a difference in trench retreat velocity VT along the arc, as VT along the arc is proportional to the distance (r) from O. The lower panel shows that the resultant velocity is the vector sum of the constant subduction velocity along the arc and VT which is varying along the arc. Thus increase in VT will result in increase in rollback velocity VR, which may lead to the steepening of the slab.

In the Sagaing fault region, σ1 and σ3 are almost horizontal and are oriented in the northeast–southwest and northwest–southeast directions, respectively, and σ2 is vertical. The stress state in the Saga- ing fault region is perfectly suited for the strike slip faulting on the north–south or east–west oriented vertical planes, of which north– south plane with dextral strike slip motion is the fault plane. The stress state in the Sagaing fault region is also consistent with the current crustal deformation across it (Vigny et al., 2003). Several investigators have attempted inferring stress directions from 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 is assumed that the direction of the maximum principal stress (σ1)is optimally oriented with respect to the fault (i.e., the σ1 makes an angle of π/4−Φ/2, where Φ is the angle of friction), which may not be true in case of fault reactivation. It is possible that during the formation of the fault in the geological past, it was optimally oriented with respect to the stress directions. However, the stress direction might change with time and the region and the faults might rotate or undergo change in dip, which may make these faults not so optimally oriented with respect to the stress direction. Nevertheless, these faults continue to be the weak zones where earthquakes can occur through reactivation as they still are favorably but not optimally oriented with respect to the stress directions. Thus it is not neces- sary that the stress state derived from the inversion of the earthquake Fig. 9. The upper panel shows a hypothetical subducting oceanic slab, dipping at an focal mechanisms should be consistent with the present day motion angle of θ, in which a fault system is shown at a position P1 prior to its subduction. as the present day motion on these faults is the motion along the After the subduction its position is represented by P2. In this case we assume that the pre-existing faults which are only favorably oriented rather than op- dip of the fault system changes according to the dip of the subducting slab. If the fault system created at P1 is reactivated at position P2, then the fault system should timally oriented (McKenzie, 1969). It is more appropriate to estimate be comparable, when the subducted slab is rotated in the counterclockwise direction the orientation of principal stress, which is consistent with the esti- by θ, back to its horizontal position (represented by dashed lines) at P3 (Jiao et al., mated or measured direction of slip on the fault. Further, uncer- 2000). Middle panel shows the poles of the nodal planes of the focal mechanisms of tainties of ±15° in trend and ±5° in plunge of the slip vector from earthquakes before and after the rotation. Lower panels show contours of the density of poles. Blue, brown, pink and red contours represent 2%, 4%, 8% and more than 16% an individual earthquake are typical in the CMT focal mechanisms of the data (i.e., poles) respectively. Note the segregation in poles after the rotation (McCaffrey, 1992). Thus we may expect similar error in our analysis along the trend, f, marked with the arrows. This trend corresponds to the general of directions of principal stress. trend of the fabric (f) in the Bay of Bengal (Desa et al., 2006). 144 B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 it has been suggested that the general trend of the Indo Burmese (~60°), we find a distinct segregation (Fig. 9) in the poles along the wedge has changed dramatically since 60 Ma, from NW–SE to almost trend NNW–SSE, which probably corresponds to the older trend of N–S, at present (Bannert and Helmcke, 1981; Hall, 1997). We assume the fault planes that were reactivated during the earthquakes. that the region of extreme eastern Himalaya, where India plate first We suggest that the preservation of old oceanic fabric or weak collided with the Eurasia plate and the Eastern Himalayan Syntaxis zones by the presence of hydrous mineral phase and their geometri- which acted as a pivot point for the rotation of the Indo Burmese cal orientation plays a vital role. There is no direct evidence whether wedge and the Burma plate, did not change its position significantly there are any old oceanic crust fabrics or weak zones present beneath during the course of rotation. It thus implies that there must have the thick cover of Bengal fan sediments, as high resolution swath been differential trench retreat velocity (VT) in geological past along bathymetric data of the Bay of Bengal are not available. However, it the trench which increased from north to south, as VT is directly pro- has been reported (Desa et al., 2006), that the Bay of Bengal is portional to the length of the arc (r) measured from the pivot point. predominantly characterized by the fabric (possibly transform faults)

Now if we assume constant subduction velocity (VS) across the with a strike of about N140° to N150°, which is considered to be of trench, then it must have affected the rollback velocity (VR) signifi- Cretaceous age that developed during early sea floor spreading epi- cantly, as VR is the vector sum of VT and VS. Thus increase in VT toward sode. It is also associated with the N50°E trending marine magnetic south might have led to increase in the subducted slab dip (as VR in- anomalies. These fabrics are traced right up to 21.5°E latitude on creased from north to south) which might have progressively steep- oceanic crust and their general trends are also consistent with the ened the slab in the south direction. However, the presence of general trend obtained above in the pole segregation (Fig. 9). Hence subhorizontal tear in the Irrawaddy region (Kundu and Gahalaut, we suggest that it is possible that the old oceanic crust fabrics at 2010; Richards et al., 2007) may also enhance the rollback-induced intermediate depth on the subducted Indian slab beneath Burmese flow which occurs at the lateral slab edges (e.g., Kincaid and plate are reactivated during the earthquakes. In the region there is Griffiths, 2003; Schellart, 2004). Alternatively, the subduction of the an increase in earthquake frequency at depths beyond 70 km which 90°E ridge under the Andaman (Gahalaut et al., 2010) and further could be due to dehydration embrittlement and volume change due north may also control the dip of the subducted slab. to gabbro/basalt to eclogite transition (Ranero et al., 2005). Another noticeable feature in the seismicity is the increase in There could be another hypothesis in favor of fault reactivation. seismicity at two depth levels, one at depths greater than 50 km From the plate reconstruction studies of the Indian subcontinent it and another at depths greater than 70 km (Fig. 4). These two has been suggested that at about 60–40 Ma time period, the general increases in the seismicity mark the change in dip in the underlying trend of the subducted margin was NW–SE (Bannert and Helmcke, Indian slab. Thus the simplest explanation for the increase in 1981; Hall, 1997). So it is possible that during 60–40 Ma time period seismicity could be due to the increase in flexure in Indian plate when subduction was active, some faults/weak zones were formed on with depth. The increase in flexure may lead to reduction in normal the subducting oceanic plate due to bending in the outer-rise which stress from the steep faults, thereby facilitating the occurrence of was generally parallel to the trench. These old faults were reactivated earthquakes and hence causing increase in seismicity level with during metamorphic dehydration process at intermediate depth depth. (Fig. 8). Hence, unusual orientations of some of the thrust planes seen in the earthquake focal mechanisms are possibly the indicator 3.2. Reactivation of faults in the Indo-Burmese arc region of the fossil trend of the subducted margins.

As discussed above, majority of the earthquakes in the Indo- 3.3. Lack of inter-plate great earthquakes in the Indo-Burmese arc region Burmese arc occur through reactivation on the pre-existing faults of the Indian plate. However, such faults might have experienced The earthquake focal mechanisms of the earthquakes in the significant dip change during subduction in the geological past. It accretionary wedge of the Indo-Burmese arc suggest that all these may be noted that only the faults at depth greater that 75 km might earthquakes are of intra-slab type that occurred through reactivation have been affected by this, as the dip of the Indian slab changes of preexisting faults on the underlying Indian slab. These earthquakes quite significantly at that depth. The faults at the shallow depth are quite similar to those that occur in a subduction zone at interme- might not have experienced any significant change in the dip and diate depth (e.g., Lamarche and Lebrun, 2000; Lister et al., 2008; orientation. To test the reactivation hypothesis we adopt the method Ranero et al., 2005). The contact surface between the underlying of Jiao et al. (2000). We demonstrate it in Fig. 9. We consider a India plate and the overlying accretionary wedge does not appear to hypothetical fault system on the subducting plate at a position P1 be seismogenic. The historical records of earthquakes do not suggest prior to its subduction. After subduction its position is represented occurrence of great earthquakes in past several centuries. Even the by P2 on the inclined subducted slab. If we rotate the inclined sub- largest earthquake in past 50 years, namely, the M 7.3 August 6, ducted slab segment by the local dip angle of θ, back to its horizontal 1988 earthquake, was of intra-slab type that occurred at intermediate position, then it is expected that the orientation of the fault system depth. All the above observations imply that seismic hazard due to should be the same as that at P1. Although the trend of Indo- great earthquakes in the region is relatively low. For the occurrence Burmese arc has changed significantly in geological past, we assume of great earthquake, a large fault area is required which may not be that it has not affected the fault orientations of the subducted Indian generally available in the Indo-Burmese arc region. However, major plate. Further, we neglected the effect of the rotation of the Indian intraplate earthquakes may cause damage in the sediment filled plate on the considered fault. valleys, e.g., the Manipur valley around Imphal, Cachar valley around We rotated the poles of the two nodal planes of the earthquake Silchar in India and Chittogong and Sylhet region in Bangladesh, as focal mechanisms by the local dip angle θ, of the Indian slab at that they have done in past. GPS observations in the region may further depth. The rotation angle or the local dip angle of the subducted help in assessing the status of strain accumulation, if any, across the slab is determined from the seismicity depth sections (Fig. 4) across plate boundary. the Indo Burmese wedge. We measured average value of θ as ~60°. We perform the rotation of the fault with the help of GEOrient, ver- 3.4. Formation of the Imphal valley: a pull apart basin sion 9.4.4 (www.holcombe.net.au/software/rodh_software_georient. htm). Before rotation, there appears to be no preferential segregation The Imphal valley is an oval shaped valley. This valley stretches to in the poles of the nodal planes of earthquake focal mechanisms. about 1843 km2 which accounts for less than one tenth of the total However, after counter-clockwise rotation by the local dip angle land area of Manipur state. The southern portion contains a number B. Kundu, V.K. Gahalaut / Tectonophysics 524–525 (2012) 135–146 145 of lakes and marshes, of which Loktak lake is the most famous. It is help in accessing historical records of Manipur. Kalpna helped in the largest fresh water lake in the northeast India and is the only stress inversion. We are thankful to the Editor and two anonymous one in the world with floating biomass which is also the natural reviewers whose comments helped us in improving the manuscript. habitat of one of the most endangered deers, the brow-antlered This work is financially supported by MoES. 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