Seismicity, Gravity and Tectonics in the Andaman Sea

J. Phys.Earth, 26, Suppl., S 233-S 248, 1978

SEISMICITY, GRAVITY AND TECTONICS IN THE ANDAMAN SEA

R.K. VERMA, Manoj MUKHOPADHYAY, and N.C. BHUIN Department Appliedof Geophysics, Indian School of Mines,Dhanbad, India (ReceivedJune 19,1978; Revised October 15, 1978)

An analysis of the available bathymetric, gravity and seismicity data of the Andaman

Sea that lies between the latitudes 4° to 16°N and longitudes 91° to 97°E has been carried

out. A seismicity map of the Andaman Sea has been prepared using all available data for the period 1916-1975. The epicentral distribution parallels the structural lineation of the

Andaman Arc showing larger activity under the Andaman Basin. An inclined seismic zone

extending at least up to a depth of 150km is found to be present underneath the Andaman

Basin. The seismic zone dips towards the continental side, attaining its deepest part not

right below the axis of the negative gravity anomalies, but is rather deflected further east

towards the volcanic islands. For a more complete understanding of plate motions in Andaman Sea area, all available focal mechanism solutions incorporating 12 new ones have

been analysed. It is observed that normal, thrust as well as strike-slip faulting take place

in the area. The direction of seismic slip vectors for thrust-type solutions is generally

eastward over the areas of outer sedimentary arc. In the southern part of the Andaman

Sea, the direction of seismic slip vectors is towards south. Over the southeastern part of

the Andaman Sea, in continuation of the Semangko Fault (rift) of Sumatra, strike-slip

mechanism predominates. This gives a clear evidence of the continuation of the Semangko Fault beyond Sumatra underneath the Andaman Sea up to 9°N. The results obtained

through the focal mechanism studies further suggest that thrust faulting is prevalent be-

tween 30 to 90km depth under the Andaman Sea. Normal faulting prevails at shallow

as well as at greater depths (more than 90km). The transcurrent type of movement ap-

pears to affect a considerable thickness of the lithosphere in the area, at least up to a depth of 150km. On the basis of these results it is suggested that an active subduction zone is

present underneath the Andaman Basin. The present structural form as well as seismicity of the Andaman and the west-Indonesian Arc seems to have resulted as a result of subduc-

tion of the Indian Plate at the continental margin of the SE-Asian Plate.

1. Introduction

The Andaman-Nicobar Group of islands and the associated Andaman Sea form a part of the north-south trending Tertiary Folded Belt, in continuation of the Arakan Folded Belt of Burma towards the north and islands approximately between 4° to 16°N and 91° to 97°E. Geology and bathymetry of the area has been studied by RODOLFO (1966, 1969), who has made an attempt to reconstruct the geologic history of the area, and WEEKS et al. (1967), who have delineated major segments of the island arc system including fore deep, outer sedimentary arc, inner volcanic arc and back deep through a distance of about 1,100km. Gravity and magnetic fields and their relationship to surface features has been studied by PETER et al. (1966). They have suggested the continuation of the structural trend of the Barisan Ranges of Sumatra into the Andaman Sea up to 10°N.

S 233 S 234 R.K. VERMA et al.

Detailed seismicity of the area has not so far been studied. SANTO (1969) briefly discussed the seismicity of the area and presented a seismic section across the structural trend of the area. Focal mechanism solutions for few earthquakes have been given by RITSEMA and VELDKAMP (1960) as well as by FITCH (1970, 1972). In this paper an analysis of the results of the gravity field, seismicity and 32 focal mechanism solutions (12 solutions added from the present studies) from the area mentioned above is presented in terms of plate tectonics model taking into account the geological and tectonic history of evolution of the area.

2. Geological and Tectonic History of the Area

According to RODOLFO (1969), the sedimentation in the Andaman Sea started some- time during the Late Cretaceous. The oldest rocks in the Andaman Sea are of this age. Following a period of sedimentation in troughs and basins, an episode of orogeny com- menced in the Oligocene and resulted in the formation of the Arakan-Yoma Mountains and the Andaman-Nicobar Ridge. Late Miocene dilational movements formed the An- daman Basin. The orogenic forces which started during the Late Pliocene times have continued till Recent. In Burma, the orogeny has been marked by volcanism in Mt. Popa-Pegu Yoma trend as well as the Shan Scarp. Similar orogenic forces working in the Andaman Basin have resulted in the formation of Narcondam and Barren Group of volcanoes. The dominant structural features in the area are related to the Indonesian Arc. This arc consists primarily of a double arc with a recognised inner arc characterised by volcanic activity and a younger outer sedimentary arc. The volcanic trend is well defined in central Burma and can be traced across the Irrawaddy Delta through Narcondam and Barren Islands, the Invisible Banks and then into Sumatra where it manifests itself in the form of Barisan Ranges. These ranges have been elevated during Plio-Pleistocene times (VAN BEMMELEN, 1949). The mountains are split up longitudinally by a well-known fault known as the Semangko Fault, a graben or a rift valley extending along the full length of Sumatra. This rift valley probably developed as a post-elevation collapse feature (WEEKS et al., 1967). In northern Sumatra the trend of the fault zone is identified by Atjeh

graben which extends offshore into the Andaman Sea (PETER et al., 1966). Prominent features of the arc are shown in Fig. 1.

The sedimentary oil producing basins of Sumatra and Burma are interconnected through the Andaman Sea which forms a trough like structure. According to KRISHNAN

(1960), the Andaman Sea probably acquired its present status during Cretaceous times. West of the sedimentary trough lies the Tertiary Folded Belt or outer Island Arc which can be traced from the Arakan Yoma in the north through Andaman, Nicobar Islands, Simeu- lue Islands, and Mentawai Islands west of Sumatra.

West of the outer Island Arc (Indonesian Islands) is the fore deep or the trench which on the south is called the "Java Trench." This trench does not seem to extend north of 3°N and is not present west of Andaman and Nicobar Islands. Major topographic features of the area are shown in Fig. 2, after RODOLFO (1966).

The Tertiary Folded Belt comprising the Andaman, Nicobar and other islands clearly shows up as a topographic high. The Andaman Basin reaches an average depth of about

3,000m below sea level. The Invisible Banks, Barren and Narcondam Islands, also show up as topographic highs. Seismicity, Gravity and Tectonics in the Andaman Sea S 235

Fig. 1. Prominent structuralfeatures of the Andaman Arc depicting the sitesfor various orogenic belts traced from Burma on the north to Sumatra on the south through the Andaman Sea (afterWEEKS et al.,1967).

Volcanic activityin the inner arc during the period Tertiaryto historictimes isshown in Fig. 3.

3. Free-AirAnomaly Map The distributionof free-airanomalies in the area (afterPETER etal., 1966) isshown in Fig. 4. The free-airanomalies are seen to be stronglypositive along the volcanic arc extending from Barisan Ranges of Sumatra through InvisibleBanks, Narcondam and Barren Islands. These are alsopositive over Mergui Archipelago and to the west of An- daman-Nicobar IslandArc. The largestpositive anomalies (ofthe order of +50 to +100 mgals) are found over the InvisibleBanks, Narcondam and Barren Islands. In between thesevolcanic islands and the sedimentary arc (Andaman and Nicobar Islands)the anom- aliesare seen to be stronglynegative being of the order of -100 to -150 mgals, con- S 236 R.K. VERMA et al.

Fig. 2. Prominent topographic features in the Andaman Sea showing the Andaman Group of islands asa major bathymetric rise extended in north-south directionforming the Andaman sedimentary arc.Note that the major part of the Andaman Basin has depths of the order of 3km (afterRODOLFO, 1966).

fined within a relatively narrow zone which extends in N-S direction for about 1,000km, following the arcuate pattern of the Andaman Arc. The distribution of these anomalies suggests large abnormalities so far as mass distribution with depth is concerned. The outer sedimentary arc also has negative anomalies as compared to the inner volcanic arc.

4. Seismicity Map of the Area

Seismicity map of the area lying between 4° to 16°N and 91° to 97°E for the period 1916-1975 is shown in Fig. 5. The source of data has been the reports of I.S.S., Bull. Seismicity, Gravity and Tectonics in the Andaman Sea S 237

Fig. 3. Volcanicityin the Andaman Sea. Sitesfor volcanism arc traced from Burma on the north to Sumatra on the south (afterRODOLFO, 1969). Refer to discussionin text for associationof strong positive free-airanomalies with the chain of volcanicislands.

I.S.C., and reports from India Meteorological Department, as well as P.D.E. Bulletins issued by NOAA. The seismicity shows a well-defined pattern almost parallel to the structural trend of the area. Most of the earthquakes in the area originate from shallow depths. However, the epicenters become deeper (exceeding 100km) towards the eastern side, close to the above-mentioned volcanic arc. It is evident that the whole of Andaman Basin is seismically very active, the seismic zone dipping towards the continental side to the east. The seismicity, however, is confined within a zone of about 4°, longitudinally. S 238 R.K. VERMA et al.

Fig. 4. Principalzones of free-airanomalies in the Anda- man Sea (contour interval 50 mgal), after PETER et al. (1966). Note the pronounced zone of narrow- cum-elongated negative free-air anomalies immediately to the east of Andaman islandsextending over the fulllength of the Andaman Sea in north- south direction.The anomalies have littlecorrespond- ence with bathymetry.

5. Focal Mechanism Solutions The mechanism of releaseof seismicenergy by the earthquakesthrows a considerable lighton the nature of tectonicforces active in the area. Focal mechanism solutionsfor few earthquakes have been studiedearlier by RITSEMA and VELDKAMP (1960) and FITCH (1970, 1972). Twelve new focal mechanism solutionswere determined for earthquakes which occurred during the period 1961-1972. The solutionswere determined using P- wave firstmotion directionsreported in the I.S.S.and the I.S.C.Bulletins. First motion Seismicity, Gravity and Tectonics in the Andaman Sea S 239

Fig. 5. The seismic activity in the Andaman Sea for the period 1916-1975. The free-air anomalies are also shown in the figure. A larger concentration of seismic activity under the Andaman Basin can be clearlyseen. Also note the deepening of earthquake foci eastward towards the continental side. S 240 R.K. VERMA et al.

Table 1. Parameters of earthquake mechanism solutions

directions were plotted on an equal area projection of the lower hemisphere using the

(i, Δ) tables of NUTTLI (1969). A double couple source mechanism was assumed. EGUCHI et al. (1978) have reported the results of 5 new mechanism solutions. The results of all the solution parameters (in all 32 solutions) are listed in Table 1, and are schematically illustrated in Fig. 6. The main points brought out by these results are discussed below. It may be seen from Table 1 and Fig. 6 that normal, thrust as well as strike-slip faulting take place in the area. Over the western sedimentary arc, characterized by relatively shallow seismicity, mostly thrust as well as normal faulting prevails. The transcurrent faulting prevails over the eastern and southeastern parts of the Andaman Sea.

Thrust mechanisms. Out of a total 32 mechanisms, 10 relatively shallow-focus mechanisms (up to a depth of about 90km) discussed in this paper indicate thrust faulting (mechanisms 2, 7, 8, 9, 13, 15, 18, 21, 25 and 26). Mechanisms 2, 9, 13, 15, 18,

25 and 26 are for the earthquake events located over the western sedimentary arc, and mechanism 7, 8 and 21 are for earthquake events located to the east of the axis of the Seismicity, Gravity and Tectonics in the Andaman Sea S 241

of Andaman region (azimuth and plunge in degrees).

centralnegative gravity anomaly zone discussedearlier. One of the nodal planes suggestiveof the faultplane isgenerally oriented in north-southdirection in the centralpart of the outer sedimentary arc but thisorientation changes to northeastand northwest in the northern and southern extremitiesconforming with the arcuate trend of the arc. This change in orientationof the nodal plane from north to south can be clearlyseen in Fig. 6. In solution13 locatedimmediately to the north of the North Andaman Island,both the nodalplanes are in northeasterndirection. In solution18 near theTen Degree Channel, both the nodalplanes are in north-southdirection. With gradualpassage of latitudinal distancetowards south, this orientation of nodal planeschanges to northwestor WNW near Sumatra on thefar south (see solutions 2, 26,7, 8,9 etc.in Fig.6). The near paral- lelismof the nodal plane with thearcuate trend of thesedimentary arc over the full length of theAndaman Sea over a distanceof 1,000km suggestsit to be the faultplane. The onlyexception being the solution 15 foran earthquakeevent located near LittleAndaman Island,in which both the nodal planesare in east-westdirection, which isorthogonal to S 242 R. K. VERMA et al.

Fig. 6. Schematic orientation of the nodal planes for the 32 focal mechanism solutions considered in the present work. The numbers refer to the earthquake events for which solution parameters are listed in Table 1. Direction of seismic slip vector for thrust type mechanism solutions is marked by thick arrows. the generalstructural trend of the outerarc. Obviously,few more solutionswould be necessaryto establishthat faults orthogonal to theouter arc are alsoexisting in thearea, ifat all. Normal mechanisms.The mechanism solutions1, 3, 5, 12,14, 16, 17,19, 20 and 22 indicatenormal faultingin the area. Normal mechanisms are found forshallow as well as sub-crustal(as deep as 149km) shocks. In general,one nodal planein the solutions suggestiveof normal mechanisms nearlyparallels the localtrend of the outerarc. So- Seismicity, Gravity and Tectonics in the Andaman Sea S 243

lutions 3, 5, 14 and 19 for earthquake events located under the outer arc suggest pure normal mechanisms, whereas other solutions have got some amount of strike-slip components.

The near parallelism of the nodal planes of these solutions with the outer arc suggests them

to be fault planes in their respective areas.

The normal mechanism solutions (Nos. 1 and 16) are observed at the northeastern

corner of the Andaman Sea. These two solutions are for earthquake events located at the

continental slope (about 1,000m), near Malayan Peninsula. Another mechanism solu-

tion suggestive of normal faulting (solution 17) is located near the Ten Degree Channel.

The orientation of extensional stress axis in southern 17 is analogous to that found from the

solutions belonging to the outer Island Arc which is in east-west direction. However, near

the locations of solutions 1 and 16, the extentional stress axis is found to be in NW-SE

direction (near 14° N, 96° E). It may be observed from Fig. 5 that there is a curious

gap in seismic activity between this place to the Coco Island further west. Another interesting feature being the near orthogonality of the T axis found from these two solutions

(1 and 16) to that of the strike direction of the volcanic islands (e.g., Barren and Narcon- dam Islands etc.) and their associated gravity anomaly pattern. It is felt that these normal

mechanism solutions and derived direction of extentional stress is a normal consequence of

plate consumption on the inner side of the Island Arc. Transcurrent mechanisms. Over the southeastern part of the Andaman Sea, in continuation of the Semangko Fault of Sumatra, strike-slip mechanism predominates (solutions 4, 6, 10, 11, 23, 24 and 27). This is a clear evidence of the continuation of the Semangko Fault beyond Sumatra underneath the Andaman Sea up to 9° N. It also suggests that the tectonic forces which were responsible for the creation of Semangko Fault during Ter- tiary times are still very active.

EGUCHI et al. (1978) have determined five focal mechanism solutions for earthquake events located near the Ten Degree Channel and in the north eastern part of the Andaman Sea. All the five solutions (Nos. 28, 29, 30, 31 and 32), whose solution parameters are listed in Table 1, suggest pure strike-slip mechanisms. Event 32 is located near the Ten Degree Channel. This solutionin conjunctionwith otherstrike-slip mechanism solutions belongingto the southeastern Andaman Sea offersa few interestinginferences to be drawn. These being: (a)the Semangko Faultpossibly extends northward beyond Sumatra up to asfar as Ten Degree Channelcontinuing underneath the Andaman Sea,(b) the fault zone almostparallels the outer sedimentary arc formed by theAndaman Group of islandsup to thisdistance. This calls for a closeinterrelationship between the formation of the sedimentaryarc and theactive fault zone, (c) the negative free-air gravity anomalies immediately to the eastof the sedimentaryarc discussedearlier area ssociatedwith the faultzone. The anomaliesmay be partlyrelated to the sedimentsaccumulated in the faultzone. But, sincethe negativeanomalies also encompass the sedimentaryarc, and the free-air anomaliesover the postulatedfault zone and over the Andaman Basinshow somewhat inverserelationship with bathymetry,one isled to concludethat the free-airanomalies are due to deepermass distribution.The earthquakeevents for which mechanism solutionsare discussed above have focaldepths ranging up to 120km (seeTable 1). In other words,the faultzone, characterized by strike-slipfaulting is a deep-seatedfracture zone extendingthrough the major partof thelithosphere. (d) Another interestingfeature of theouter Island Arc and thefault zone is the presence of ophiolitesin the Andaman Islands. Through a detailedstudy of stratigraphyand platemotions associated with upper Meso- zoicto Plioceneophiolite nappes of the Pacific,Indian and Mediterranean,BROOKFIELD S 244 R. K. VERMA et al.

Fig. 7. A plot of dip of P axis with focal depth for the focal mechanism solutions listed in Table 1.

(1977) has shown that the transcurrentfaulting during changes in relativeplate motions is the major cause of initialophiolite nappe emplacement (see discussionon ophiolite nappes of the Andaman Islandsin Brookfield'spaper). All thesesuggest that the negative gravity anomalies,the transcurrentmotion in the faultzone, the presence of ophiolites, the outer Island Arc and inner volcanicarc are all relatedto plate motions under the Andaman Sea. A plot of the dip of P axis obtained through focal mechanism studies with focal depth is shown in Fig. 7. It may be noted that the transcurrent type of faulting takes place at shallow as well as deeper levels up to a depth of about 120km. The P axes for these solutions dip at 10° to 20°. Thrust faulting takes place up to a depth of about 90km and the dip of P axes for these vary from 15° to 40°. Normal faulting takes place up to a depth of about 150km and the dip of P axes for these varies from 60° to 80°. The shallow nature of P axes over the outer zone of Island Arc is typical of conditions prevailingat the outerpart of the subductionzones. These resultsfurther suggest that practicallyall the threemechanisms of faultingare active in the area and are at present prevailingover a considerablepart of thelithosphere. An analysisof the gravityand seismicitydata alongthe ProfileAA' (locationshown in Fig.5) ispresented in Fig.8. Keeping in view the geologicalhistory of the area,the Seismicity, Gravity and Tectonics in the Andaman Sea S 245

Fig.8. Free-airgravity anomalies, bathymetry, Bouguer anomaliesand epicentraldistribution in a verticalsection along the Profile AA'. The locationof the profile is shown in Fig.5. A schematic representationof the subducting lithosphere under the Andaman Sea isalso shown in thefigure.

evidence from gravity, seismicity and the nature of focal mechanism solutions, a model for the present day conditions prevailing in the lithosphere is suggested. For purposes of

plotting the seismicity, the locations of earthquake foci (between 6° and 12° N) were pro- jected along the profile. It is clear from the figure that the depth of foci increases towards the east and reaches a maximum close to the inner volcanic arc. The distribution of earthquake foci does not seem to be related to the narrow belt of strong negative free-air anomalies prevailing close to the Andaman Group of islands. The prevailing anomalies also do not seem to have any correlation with the observed bathymetry. The strong negative anomalies close

to the sedimentary arc suggest, abnormal mass distribution resulting in departure from isostatic conditions at least locally. Over the deepest part of the Andaman Sea, the free- air anomalies show inverse relationship with bathymetry. Strong positive gravity anoma- S 246 R. K. VERMA et al. liesover the innervolcanic arc, and a biastoward positive gravity field over major partof the Andaman Basinmay be attributedto largescale submarine volcanism under theAn- daman Basin. The deepestpart of theinclined seismic zone is attained not right below the axisof the negativegravity anomalies, but israther deflected east towards the Andaman Basin(over which a positivegravity bias is noted). The model suggeststhe existence of a subductionzone underneaththe Andaman Sea. As a resultof thissubduction, thrusting, normal faultingas wellas strike-slipfaulting is taking place at depth. It isalso suggested that as a resultof relativemovement of the lithosphere,magma has been createdin the past (perhapsis also being createdat present)in the upper mantle. The movement of this magma along faultshas been manifested in the form of severalvolcanic features such as the InvisibleBanks, the Narcondam and Barren Islandsetc. The present day seismicevidence suggeststhat theseprocesses are continuingat present.

6. InferredTectonic Movements For a more complete understanding of platemotion in Andaman Sea region,one has to take into considerationthe historyof evolutionof the Indian Ocean apart from the historyof evolutionof the Andaman Sea. A great deal of evidence obtained from magnetic anomalies (LE PICHON and HEIRTZLER, 1968; MCKENZIE and SCLATER, 1968) has shown that the Indian Ocean has been evolved as a resultof driftof the Indian Landmass towards the north. As a resultof thismovement, the Indian Plate has collidedwith the Burmese part of the Asian Plate towards the east and the Tibetan part towards the north. From the analysisof seismicityand gravity data for the northern Burma, VERMA et al. (1976) have suggested that an activesubduction zone existsat present underneath the Arakan Yoma and the Irrawady Basin in Burma. However, apart from subduction,large scalestrike-slip movements in the lithosphereseem to be takingplace in the entireeastern belt extending from Himalaya to Indonesia. Thus it appears that along the eastern margin of the Indian Plate,probably a substantialpart of subductionis transforminginto transcurrent motion. To illustratethis point further, the senseof transcurrentmotion derivedthrough the focalmechanism studiesconsidered in the presentwork was plottedfor the individual solutionswhich areshown in Fig.9. In additionto theresults discussed above, the results obtainedfor earthquakes located over inlandof Burma (VERMA etal., 1977) to thenorth of the presentarea have alsobeen includedand are shown in Fig.9. VERMA etal. (1977) have reportedstrike-slip as well as thrustmechanism solutionsfor earthquakes located in Burma. The directionof thrusting deduced for all the thrust mechanism solutionsreported forthe Andaman Sea and northernBurma isalso shown in thefigure. It may be observed from thisfigure that transcurrent motions are takingplace right from the EasternHima- layanSyntaxis on the extremenorth to Sumatra on thefar south, across the full length of the IrrawaddyBasin of Burma and theAndaman Sea. Itis interesting toobserve that the zone affectedby transcurrentmotions is relativelynarrow in width but widespreadin north-southdirection over a distanceof about 2,500km. Thiszone of transcurrentmotion iscurrently undergoing right-lateral motion as suggestedby thefocal mechanism studies. On the farsouth in Sumatra,earthquakes of theright lateral shear motion typeare known to occuralong the Semangko Faulttrending NW-SE (KATILIand HEHUWAT, 1967). Seismicity, Gravity and Tectonics in the Andaman Sea S 247

Fig. 9. Sense of transcurrent motion deduced from focalmechanism solutionsfor earthquake events studied from Andaman Sea and inland Burma. The directionof seismicslip vector for thrust mechanism solutionsare also shown in the figure.The resultsfor Burmese earthquakes are afterVERMA et al. (1977).

It may be further observed from this figure that the direction of seismic slip vectors for various thrust mechanism solutions (shown in Figs. 6 and 7) are in the direction of the northeast in the southern part of the area, it swings towards east in the central part following the arcuate trend of the islands, and then turns towards east-southeast in the northern part (north of 12°). This suggests that the plate margin itself may be taking an arcuate shape in this area.

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