Andaman Trench Research Articles

Teleseismic traveltime tomography of the Andaman subduction zone

The Andaman subduction zone is characterized by great earthquakes and strong tectonic activity, which are closely related to the subduction of the Indian plate. To gain insight into the subduction processes, the geometry of the downgoing Indian plate must be investigated. At present, the geometry of the subducting Indian plate remains unclear owing to the historical lack of a seismic network deployed in the Andaman Sea and the surrounding area. In this study, we employed a teleseismic traveltime tomography method to image the deep structures beneath the Andaman Sea. The principle of the tomography method is that regional earthquakes are regarded as seismic stations, and seismic stations at teleseismic distances are treated as teleseismic events based on the reciprocity theorem. With this type of teleseismic traveltime tomography, we selected a total of 179 regional earthquakes that were recorded by 4515 stations, and we manually selected 40,369 high-quality P-wave first arrivals. From the inversion of these seismic traveltime data, we obtain P-wave images at depths from 300 km to 700 km. Our tomographic images reveal three distinct features. First, we observe high-velocity anomalies beneath the Andaman Sea at depths of 300–700 km. The orientation of the high-velocity anomalies is nearly consistent with that of the Andaman trench and volcanic arc. The high-velocity anomalies in the upper mantle are laterally discontinuous; a gap at approximately 9°N divides the high-velocity anomalies into two parts. Second, high-velocity anomalies are found in the mantle transition zone (MTZ) below the Andaman trench. Third, strong low-velocity anomalies are detected below the volcanic group in the Mergui Basin. Based on our tomographic results, we infer the currently subducting Indian plate beneath the Andaman Sea, an ancient slab segment in the MTZ, and upwelling hot materials below the Mergui Basin.

Seismotectonics and Spatio-temporal Variations in Seismicity Rates along Andaman Trench

Abstract The earthquake Frequency Magnitude Distribution (FMD) or b values have been meticulously used in the last two decades as a deterministic earthquake precursor or indicator, in earthquake forecasting or probabilistic seismic hazard assessment studies, in a variety of tectonic and stress regimes. In the present study, temporal fluctuations of b values were analysed by means of 185 well located (Mw ≥ 5) earthquakes, in the region of Andaman and Nicobar Islands (N10° to 14°) for a time period of ~100 years (1918 to 2018). Variation in b value rates before and after the 2004 great Sumatra-Andaman earthquake (Mw 9.3) was analysed, preceded by a comprehensive geodynamic and seismotectonic-crustal deformation overview along the Andaman trench section. The analysis suggest a precursory drop in b values from 1.05 ± 0.05 (1918 to 2004) to 0.90 ± 0.05 post 2004 earthquake (2004 to 2018) reflecting a greater slip deficit and strain localization, that exceeds considerably the pattern of energy release in this region. Further, the region of Andaman and Nicobar Islands have witnessed a poor slip distribution (on-fault reactivation) from the great earthquakes that nucleated near the Sumatra segment, and have ruptured few large earthquakes, with no accounts of great earthquake (>Mw 8) during the last century. This emphasizes that, strain accumulation has been concentrated over a long period of time, and can contribute a destructive deformation event in near future.

Andaman–Nicobar–Sumatra Margin Revisited: Analysis of the Lithospheric Structure and Deformation Based on Gravity Modeling and Distribution of Seismicity

The Andaman–Nicobar–Sumatra subduction margin, with a well-developed Benioff zone down to ~ 200 km depth, extends over 1300 km along strike and has a lateral extent of ~ 200 km. Two-dimensional (2D) profiles based on generalized inversion of free-air gravity anomaly data across different segments of the Andaman–Nicobar sector of the margin were analyzed by reconstructing the geometry of the converging India–Eurasia lithospheric plates. Detailed 2D structures of the Ninetyeast Ridge (NER), fore-arc basin, volcanic arc, back-arc basin, spreading ridge, Sewell Rise, Mergui Ridge and Mergui Basin, and depths of Moho and the lithosphere–asthenosphere boundary were delineated. The volcanic arc is located at a distance of ~ 150–200 km from the trench and is marked by a zone of positive gravity anomaly separated from the trench by the fore-arc basin with a zone of gravity low. The topographic and gravity anomaly patterns show complex geometrical patterns over the volcanic arc due to the presence of a number of regional faults striking parallel to the trench axis developing a series of pull-apart (transtensional) basins as well as the interaction of the spreading ridges on the overriding plate. A narrow zone of negative gravity anomaly (~ − 185 to − 110 mGal) observed in the gravity model of the margin is interpreted to be associated with fore-arc shear fault above a zone of fractured oceanic crust with a thick sedimentary layer, located above a zone of sharp bending of the eastward converging Indian lithosphere in the fore-arc basin. A high concentration of seismicity and a trench-parallel band of moderate moment energy release along this fault might have been caused by concentrated deformation within the zone of flexing of the descending plate. A wide fore-arc (> 200 km) and the enhanced deformation of the subducting Indian oceanic plate in central sectors possibly resulted from increased interaction between the NER and the Andaman trench and extension in the Andaman back-arc. Appearance of a second moderate energy band and shifting of seismicity toward the trench axis following the 2004 MW 9.2 mega-event are apparently caused by the migration of the stress field from deeper to shallower part of the Indian lithosphere. A similar type of seismicity migration toward shallower part of the descending plate has been recorded along other subduction margins around the globe.

Neotectonics and related crustal deformation along Carbyn thrust fault, South Andaman, India: implications of the frontal surface faulting and propagation of tectonic activity towards Andaman trench

Paleoseismic investigations have been carried out across an ~ 10-m-high scarp along the Carbyn thrust fault, that marks the structural contact between the overriding Ophiolite suite of the Cretaceous age and under riding Andaman Flysch group (Oligocene) in the outer-arc setting of the Northern segment of Sunda-Andaman subduction zone. The Carbyn thrust fault is one of the important concomitant faults in the accretionary prism of the Andaman subduction zone and the surface trace marks a N-S trending thrust fault with Easterly dip which extends for a length of about 30 km in the Eastern half of South Andaman, starting from Carbyn area up to Chidiyatapu (Southernmost tip of the South Andaman) through Kodiyaghat and Burmanallah areas. The thrust has emplaced dismembered Cretaceous Ophiolite slices over Oligocene siliciclastic turbidites (Andaman Flysch) almost in a N-S trending linear zone of deformation. The 10-m-high scarp expressed at the interface between Ophiolite suite and Andaman Flysch group borders the topographic front of Ophiolite suite and breaks the Quaternary and late Holocene succession almost up to a length of 3.7 km from Carbyn to Burmanallah area, registers the youngest surface deformation along the thrust. The scarp is abrupt and sharp trending in N-S direction shows variations in displacement along the strike and has preserved its initial geometry without erosion. Trench exposure and Georadar survey across the scarp at Carbyn area reveals a very low angle thrust fault that cuts the bedrock surface (Ultramafics of Ophiolite Suite) rocks against the young alluvial surface deposited by Prothrapur Nallah and Carbyn creek at Carbyn area. The thrust fault shows flattening of the fault geometry around 16° at the base of the trench and dips 8–10° at near surface. The expression of the scarp at the frontal position of Ophiolite Suite and verging towards the West displacing late Holocene sediments reflects advancing of thrusting and propagation of tectonic activity towards Andaman trench.

Lava flows and cinder cones at Barren Island volcano, India (2005–2017): a spatio-temporal analysis using satellite images

Barren Island volcano erupted during January–February 2017. Located near the Andaman trench and over a subduction zone, it is the only active volcano in India. It comprises a prominent caldera within which there is a polygenetic intra-caldera cinder cone system, with a record of eruptive events which date back to eighteenth century (1787–1832). Major eruptions occurred in 1991, 1994–1995, 2005 and, since 2008, the volcano has been showing near continuous activity with periodic eruptions. We used coarse spatial resolution “fire” products (Band I4) from Suomi National Polar-Orbiting Partnership Visible Infrared Imaging Radiometer Suite to detect days of eruption during the January–February 2017 period. Moderate spatial resolution (23.5 m) short-wavelength infrared (SWIR) data of Resourcesat-2 Linear Imaging Self Scanning Sensor–III available for specific days during this period were used to verify signatures of volcanic eruption. Thermal infrared band data from the Landsat series over the 2005–2017 periods were used to estimate the brightness temperature and location of the active vent within the polygenetic cinder cone field. High-spatial resolution images (1–5.8 m) in the visible bands (Resourcesat-2 LISS-IV, Cartosat-1 and 2) were used to delineate the changes in overall morphology of the volcano and to identify an inner crater ring fault, new paths of lava flow and the formation of a new cinder cone on the old crater. These multi-temporal data sets show significant changes in the paths of lava flows from 2005 to 2017. The observations also document periodic shifts in the location of effusive vents. Morphogenetic changes in recent eruptive phases of the Barren Island volcano were successfully delineated using a combination of multi-temporal and multi-resolution satellite images in visible, SWIR and thermal infrared regions of the electromagnetic spectrum.

Effects of Ninetyeast Ridge Magmatism and Pre India-Eurasia Collision Dynamics on Basement and Crust-lithospheric Structures of the Northeastern Indian Ocean

Abstract: The coexistence of multifaceted geodynamical situation in the Northeastern Indian Ocean (Bay of Bengal) lithosphere inspires to investigate the origin, present day orientation and gravitational stability of its basement, crust and lithospheric structures. The origin, evolution and the present day configuration of these structures such as the northward down dipping of the Bay of Bengal basement and its lithosphere, the Moho of the 85°E ridge, the formation of the Ninetyeast ridge median graben and ridge normal faults and their interaction with slow convergence and stress diffusion processes in the north Andaman trench, remain conspicuous. In this paper, we explain the above mentioned processes and the existence of the Bay of Bengal basement features through plausible tectonic scenarios that related with past time dynamics of the Indian Plate prior to the India-Eurasia collision and lithospheric thermal perturbation due to the Ninetyeast Ridge magmatism. Our analysis on the basis of satellite altimeter-derived geoid, gravity, sediment thickness, basement undulation, isostatic and prospecting geoid anomaly; suggests apart from the enhanced Oligocene Himalayan erosion and subsequent increased sediment load, the gravitational stability of the basin load (like formation of ridge graben) was also affected by altering the basin rheology to inelastic, owing to the enhanced thermal perturbation by the ninetyeast ridge magma. This made the Bay of Bengal lithosphere rheologically weak. Pre India-Eurasia collision, fast movement and fast convergence of the Indian plate at erstwhile north Andaman subduction zone made the lithosphere relatively stretchable due to the prevalence of intra-plate extensional stresses. The NE-SW orientation of the Central basin depression and the northward (NE-SW) dipping of the Bay of Bengal (BOB) lithosphere are pointing to such a stretchable lithosphere and its remnants are present at the northeast of the northern Ninetyeast ridge, as observed from the low prospecting geoid anomaly. The presence of such a thermally altered, weak lithosphere as a subducting slab at the north Andaman trench caused the present day observed low convergence rate, lack of seismicity, as well as the cessation of fault rupture followed by the Great Sumatra Andaman earthquake of Mw (9.1) on 26 December 2004. The Ninetyeast ridge oblique collision with the north Andaman trench was presumed around Pleistocene based on the timing of abandonment of the Nicobar fan from the Bengal head fan. The discontinuity of the ridge along the oblique convergence zone of the north Andaman and its isostatic stability are mainly controlled by the subduction zone dynamics, unlike the 85°E ridge.

Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (Te) of the Andaman subduction zone. The recovered Te map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0<Te<3km) in that region, where the NER is close to the Andaman trench (north of 10°N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, Te<Ts (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low Te and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region.

Seismogenic sources in the Bay of Bengal vis-à-vis potential for tsunami generation and its impact in the northern Bay of Bengal coast

Geodynamic status, seismo-tectonic environment, and geophysical signatures of the Bay of Bengal do not support the occurrence of seismogenic tsunami. Since thrust fault and its intensity and magnitude of rupture are the key tectonic elements of tsunamigenic seismic sources, the study reveals that such characteristics of fault-rupture and seismic sources do not occur in most of the Bay of Bengal except a small segment in the Andaman–Nicobar subduction zone. The inferred segment of the Andaman–Nicobar subduction zone is considered for generating a model of the deformation field arising from fluid-driven source. The model suggests local tsunami with insignificant inundation potential along the coast of northern Bay of Bengal. The bathymetric profile and the sea floor configuration of the northern Bay of Bengal play an important role in flattening the waveform through defocusing process. The direction of motion of the Indian plate makes an angle of about 30° with the direction of the opening of Andaman Sea. The opening of Andaman Sea and the direction of plate motion of the Indian plate results in the formation of Andaman trench where the subducting plate dives more obliquely than that in the Sunda trench in the south. The oblique subduction reduces significantly the possibilities of dominant thrust faulting in the Andaman subduction zone. Further, north of Andaman subduction in the Bengal–Arakan coast, there is no active subduction. On the otherhand, much greater volume of sediments (in excess of 20 km) in the Bengal–Arakan segment reduces the possibilities of mega rupture of the ocean floor. The water depth (≈1,000 m) along most of the northern Bay of Bengal plate margin is not optimum for any significant tsunami generation. Hence, very weak possibility of any significant tsunami is suggested that based on the interpretation of geodynamic status, seismo-tectonic environment, and geophysical signatures of the Andaman subduction zone and the Bengal–Arakan coast.

Updip rupture of the 2004 Sumatra earthquake extended by thick indurated sediments

Sediments accumulated in a subduction trench are usually unconsolidated and impede the updip propagation of fault rupture during an earthquake. Seismic images of the southern Sumatra–Andaman trench reveal blocks of consolidated sediment that may have enabled fault rupture in 2004 to propagate up fault dip, thus further seaward, increasing the tsunami magnitude.

Structure and tectonics of 85°E Ridge from analysis of Geopotential data

The Bay of Bengal (BOB) sedimentary basin is one of the largest offshore sedimentary basins of the world having the thickest accumulation of sediments which mask the underlying crust and it is the Geopotential data that offers the unique opportunity of seeing below the thick sediments. Satellite derived Free Air Gravity (FAG) data, marine magnetic data, bathymetry, available seismic profiles and isopach maps over the Bay of Bengal are utilized in the present paper to throw light on the nature and tectonics of the BOB with particular emphasis on the 85°E Ridge. Combined analysis of gravity and magnetic data along six EW profiles of length varying from 325 to 440 km is undertaken, incorporating seafloor spreading anomalies in the model. Of the several theories put forth for the evolution of the 85°E Ridge, the present study supports the sagging of the crust and rules out the hotspot trace and magmatic under-plating theories. The crustal model derived from the magnetic profile analysis suggests that the 85°E Ridge does not extend to the North of 15°N latitude and this is supported by the Euler solutions of the FAG and the long wavelength filtered FAG. However, the intermediate wavelength filtered FAG suggests that the 85°E Ridge extends up to the Ocean Continent Boundary thereby suggesting that the 85°E Ridge thins out dramatically to the North of 15°N latitude. Analysis of gravity data along 17°N latitude profile predicts that in the northern segment the 85°E Ridge is a geo-morphological feature within the sediments above the basement whereas to the South of 15°N latitude it is a feature within the oceanic crust (below the basement). Further the reverse magnetization associated with the seafloor spreading anomalies lying on the 85°E Ridge belongs either to the Albian period or the anomaly A34/A33. The 85°E Ridge would be younger than these seafloor spreading anomalies and would have formed due to horizontal compressional forces of the lithosphere preceding development of the subduction zone at the Andaman trench.

Rupture Properties of the Giant Sumatra Earthquake Imaged by Empirical Green’s Function Analysis

Abstract Empirical green’s function (egf) analysis has remained little used to image the rupture properties of the giant 26 December 2004 Sumatra earthquake. The 2 November 2002 foreshock (Mw 7.2), close to the mainshock epicenter, gives us the opportunity to use its waveforms to empirically simulate the Rayleigh-wave propagation of the Sumatra earthquake. We first show that the exceptional size of the Sumatra earthquake does not prevent use of the egf technique. Four aftershocks (Mw 5.9–6.1), distributed along the Sumatra–Andaman trench, are shown to have consistent Rayleigh waves for periods between 100 and 200 sec. At a lower frequency, we present two large earthquakes of the Mexican subduction zone (Mw 7.2– 7.3, close to the selected egf magnitude) for which long-period Green’s functions (100–2000 sec) remain very similar, even if event epicenters are separated by about 650 km. This justifies the possibility of using the 2002 foreshock as an egf for the whole rupture process of the Sumatra earthquake and shows more generally the very broad range of application of egf technique. Then, a specific analysis reveals that seismic moment magnitude is close to 9.1 (seismic moment equal to 5.6 × 1022 N m). Moment release analysis along the Sumatra–Andaman trench shows two main slip episodes: one next to the northern extremity of Sumatra (∼20-m slip) and the other one along the Nicobar Islands (∼10-m slip), with a global extent of 1150–1200 km. Rupture velocity varies between values around 2.5 km/sec in the first half of the rupture and values closer to 2 km/sec in the second half. Total duration imaged by Rayleigh waves is 580 sec (±20 sec) and no activity of the fault is found in the time scale between 600 and 2000 sec. In the hypothesis of even longer timescale slip, this phenomenon would be of the order of 10%–20% of the global moment and likely restricted to the Andaman Islands.

Present‐day crustal deformation around Sagaing fault, Myanmar

Global Positioning System (GPS) measurement campaigns in Myanmar, conducted in 1998 and 2000, allow quantifying the present‐day crustal deformation around the Sagaing fault system in central Myanmar. Both a regional network installed at four points within the country and a local 18‐station network centered on the city of Mandalay across the Sagaing fault demonstrate that active deformation related to the northward motion of India is distributed across Myanmar in a platelet that extends from the western edge of the Shan Plateau in the east to the Andaman Trench in the west. In this platelet, deformation is rather diffuse and distributed over distinct fault systems. In the east, the Sagaing/Shan Scarp fault system absorbs &lt;20 mm/yr of the 35 mm/yr India/Sundaland strike‐slip motion. Along this major plate boundary, strain is partitioned along the N‐S trending Sagaing fault and the transtensile N160°E trending Shan Scarp fault. Shortening and wrenching within the inverted central Myanmar basins, strike‐slip faults affecting the Arakan Yoma fold‐and‐thrust belt, and oblique subduction along the Andaman trench should absorb the remaining India/Sundaland motion (&gt;10 mm/yr). This GPS study combined with an on land geotectonic survey demonstrate that oblique slip of India along the rigid Sundaland block is accommodated by a partitioned system characterized by distribution of deformation over a wide zone.

Shallow seismicity, stress distribution and crustal deformation pattern in the Andaman-West Sunda arc and Andaman Sea, northeastern Indian Ocean

Shallow seismicity and available source mechanisms in the Andaman–westSunda arc and Andaman sea region suggest distinct variation in stressdistribution pattern both along and across the arc in the overriding plate.Seismotectonic regionalisation indicates that the region could be dividedinto eight broad seismogenic sources of relatively homogeneousdeformation. Crustal deformation rates have been determined for each oneof these sources based on the summation of moment tensors. The analysisshowed that the entire fore arc region is dominated by compressive stresseswith compression in a mean direction of N23°, and the rates ofseismic deformation velocities in this belt decrease northward from 5.2± 0.65 mm/yr near Nias island off Sumatra and 1.12 ±0.13 mm/yr near Great Nicobar islands to as much as 0.4 ±0.04 mm/yr north of 8°N along Andaman–Nicobar islandsregion. The deformation velocities indicate, extension of 0.83 ±0.05 mm/yr along N343° and compression of 0.19 ±0.01 mm/yr along N73° in the Andaman back arc spreadingregion, extension of 0.18 ± 0.01 mm/yr along N125° andcompression of 0.16 ± 0.01 mm/yr along N35° in NicobarDeep and west Andaman fault zone, compression of 0.84 ±0.12 mm/yr N341° and extension of 0.77 ± 0.11 mm/yralong N72° within the transverse tectonic zone in the Andamantrench, N-S compression of 3.19 ± 0.29 mm/yr and an E-Wextension of 1.24 ± 0.11 mm/yr in the Semangko fault zone ofnorth Sumatra. The vertical deformation suggests crustal thinning in theAndaman sea and crustal thickening in the fore arc and Semangko faultzones. The apparent stresses calculated for all major events range between0.1–10 bars and the values increase with increasing seismic moment.However, the apparent stress estimates neither indicate any significantvariation with faulting type nor display any variation across the arc, incontrast to the general observation that the fore arc thrust events showhigher stress levels in the shallow subduction zones. It is inferred that theoblique plate convergence, partial subduction of 90°E Ridge innorth below the Andaman trench and the active back arc spreading are themain contributing factors for the observed stress field within the overridingplate in this region.

Detection of major geological structures in the Eastern offshore of India using Geosat altimeter data

Satellite altimetry can be used to infer subsurface geological structures analogous to gravity anomaly maps generated through ship-borne survey. The Eastern offshore was taken up for analysis using Geosat Exact Repeat Mission (ERM) altimeter data. A methodology is developed to use altimeter data as an aid to offshore hydrocarbon exploration. Processing of altimeter data involves corrections for various atmospheric and oceanographic effects, stacking and averaging of repeat passes, cross-over correction, removal of deeper earth and bathymetric effects, spectral analysis and conversion into free-air gravity anomaly. The final processed results were derived for Eastern offshore in the form of prospecting geoid and gravity anomaly maps and their spectral components. The highs and lows observed in those maps were derived in terms of a number of prominent megastructures e.g., gravity linears, 85°E and 90°E ridges, the Andaman trench complex etc. Satellite-derived gravity profiles along 12°N latitude match well with the existing structures.

Gravity anomalies and deep structure of the Andaman arc

The Andaman arc is associated with a major Free-air anomaly pair of mean amplitude 180 mgal. Two-dimensional gravity interpretation suggests significant mass anomalies below the arc that presumably have resulted due to subduction of the Indian plate below the Burma plate. It is inferred that the Andaman trench is of asymmetric V-shape containing about 7 km sediments. An outer bathymetric rise seaward of the trench possibly corresponds to a lithospheric flexure by 500 m. The Cretaceous-Tertiary sediments constituting the Andaman sedimentary arc attain their maximum thickness of about 13 km under the Nicobar. Deep at the subduction zone. At this location a mafic mass is emplaced within the sedimentary section. The underlying oceanic crust apparently experiences phase transition at about 27 km depth in a Benioff zone environment. The Andaman volcanic arc underlies a low density zone that is at least 60 km wide. Along the east margin of the Andaman Sea, cuustal transition presumably occurs below the Mergui Terrace at the Malayan coast.

Seismotectonics of subduction and back-arc rifting under the Andaman Sea

Active underthrusting of the Indian plate below the Burma plate produces seismic activity under the Andaman and Burmese arcs. It is suggested that significant differences exist in the configuration of the Benioff zone underlying the two arcs. Several parallel and linearly extensive fault zones are outlined by epicentral distribution in the Andaman arc. The Andaman back-arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients NNE to SSE in north and south parts of the Andaman Sea respectively. The spreading ridge is offset by active transform faults. On the far north the spreading ridge appears to connect to a major transform fault in Burma across the Burmese coast. Focal mechanism study provides evidence for tensional stress near the inner wall of the Andaman trench, dominant compressive deviatoric stress within the overriding Burma plate, and tensional stress under the back-arc spreading ridge.

Geodynamics of Burma and Andaman-Nicobar Region, on the basis of tectonic stresses and regional seismicity

The nature and distribution (in depth and latitude) of earthquakes and deformation in different regions of Burma and Andaman-Nicobar indicate rather unequal horizontal compression and relative movement of different segments. The (east-west) longitudinal distribution of earthquakes delineates well defined zones of seismicity dipping 30–50 degrees from the Andaman trench to a depth of about 180 km under the Andaman Islands. Fault-plane solutions show that the strike slip, normal and thrust faults are associated with seismic and aseismic regions. The change in nature of volcanism from andesite (south) to basalt (north) in Burma suggested that the stress field changed from compressional to tensional. The mechanical coupling, by the deformation of two different segments, caused an extensive contortion of the sediments in the southern Assam massif.