Geometry of Burmese- Andaman Plate 2003

8/8/2019 Geometry of Burmese- Andaman Plate 2003

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Journal of Seismology 7: 155174, 2003.

2003 Kluwer Academic Publishers. Printed in the Netherlands.155

The geometry of the Burmese-Andaman subducting lithosphere

Sujit Dasgupta1, Manoj Mukhopadhyay2,, Auditeya Bhattacharya1 & Tapan K. Jana11Geological Survey of India, 27 J.L. Nehru Road, Calcutta 700016, India; 2Indian School of Mines, Dhanbad

826004, India; Author for correspondence

Received 6 February 2001; accepted in revised form 9 January 2003

Key words: Benioff zone contortions, Burmese-Andaman arc, Indian plate, lithosphere, seismicity, subduction

Abstract

The gross seismotectonic features for the Burmese-Andaman arc system which defines the northeast margin of

the Indian plate are rather well known but variations in the subduction zone geometry along and across the arc

and fault pattern within the subducting Indian plate have not been studied. Present work aims to study these by

using seismicity data whose results are presented in the form of: (a) Lithospheric across-the-arc sections at aboutevery 100120 km (approximately one degree latitude apart) covering the 3500 km long Burmese-Andaman arc

system, (b) a structure contour map showing the depth to the top surface of the seismically active lithosphere and

(c) interpretation of focal mechanism solutions for 148 Benioff zone earthquakes. Both penetration depth and the

dip of the Benioff zone vary considerably along the arc in correspondence to the curvature of the fold-thrust belt

which varies from concave to convex in different sectors of the arc. Several extensive Hinge faults that abut at

high angles to the arc orientation, are inferred from an interpretation of the structure contour map. Active nature

of the hinge faults is established in several areas by their association with earthquakes and corroborated through

fault plane solutions. At shallow level of the Benioff zone along these faults, focal mechanism solutions display

left lateral strike slip movement while at deeper levels reverse fault solutions are common.

Introduction

The Burmese-Andaman Arc System (BAAS) presents

nearly 3500 km long subducting margin in northeast-

ern part of the Indian plate where varying degrees of

seismic activity, volcanism and active tectonism are

evidenced. The region is of particular interest due to

the following features: (a) It serves as an important

tectonic link between the Eastern Himalayas (a typ-

ical collisional margin) with the Sunda Arc (which is

a part of the Western Pacific arc system), (b) An initial

collisional phase has already set in the northernmost

segment of BAAS (in the Naga Hills) within an overallsubducting regime (Brunnschweiler, 1974; Mitchell

and Mckerrow, 1975) (Figure 1), (c) Burma is one

of the few regions in the world where a subduction

zone upto about 180 km depth is clearly discernible

in a land environment (Mukhopadhyay and Dasgupta,

1988); (d) Coastal Burma and north part of the An-

daman Sea are largely aseismic, suggesting that sub-

duction of the Indian plate in this region has stoppedrecently or occurs aseismically, and the hanging litho-

spheric slab is being dragged northward through the

surrounding lithosphere (Le Dain et al., 1984), (e) the

Andaman back-arc spreading ridge (ASR) underlying

the Andaman Sea relates to the oblique convergence of

the Indian plate at the Asian continental margin (Cur-

ray et al., 1979; Mukhopadhyay, 1984; Mukhopad-

hyay and Krishna, 1995); actual spreading occurred

through several short leaky-transforms, producing the

pull-apart Andaman basin in southern half of the

BAAS (cf. Curray et al., 1982), and (f) further south is

the intense seismic zone of the West Sunda Arc with

its attendant volcanism (Hamilton, 1974).Although the gross features underlying the BAAS

subduction zone are quite well known (Brunnsch-

weiler, 1974; Mitchell and McKerrow, 1975; Curray et

al., 1979, 1982; Mukhopadhyay and Dasgupta; 1988.

Rajendran and Gupta; 1989; Dasgupta et al., 1990;

Dasgupta and Mukhopadhyay, 1993), details of the


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Figure 1. Tectonic features of the Burmese-Andaman Arc System

in northeastern part of the Indian plate. (redrawn after Curray et al.,

1979). More tectonic details of the arc are shown on Figure 2. Active

subduction occurs below the arc upto intermediate focal depth of

earthquakes.

Table 1. Summary Statistics on numbers and magnitudes of

2135 earthquakes used in the study

Magnitude No. of events Data coverage period+

8 1 A

7.07.9 11 A

6.06.9 30 | A

| 4818 | B

5.05.9 13 | A| 473

460 | B

4.04.9 1576 B

3.53.9 26 C

Ms for A and Mb for B & C.+ A:18971962; B:19631993; C: 19911993.

subduction zone geometry and deformation created at

the convergent margin are yet to be studied. Here we

aim to study these using a large number of select-

ive earthquakes from an Earthquake Data Base Filefor the Indian Sub-continent created recently at the

Geodata Division of the Geological Survey of India,

Calcutta (Anon, 1999). This permits to investigate

the 2-D geometry of the BAAS subduction zone, to

construct a structure contour map defining top surface

of the seismically active lithosphere for the 3500 km

strike length of the BAAS in north-south direction,

and to infer the presence of several hitherto un-

known transverse faults which are developed in the

downgoing lithosphere. Many moderate to large mag-

nitude earthquakes relate to activity along such faults.

To substantiate the deformation pattern in the sub-

ducting lithosphere we have also examined the resultsavailable from a large number of fault plane solutions.

Analysis of seismicity data

a) BAAS Seismicity

We scanned through the Earthquake Data Base File

(based mainly on ISS/ISC catalogue) to list a total of

3476 events that occurred in the study area covered by

latitudes 028N and longitudes 9098E during the

period 1897 to 1993. Out of these we select only 2202earthquake events to study the seismicity pattern in

plan view (Figure 2) while 2135 events with known fo-

cal depths were utilised to constrain the Benioff zone

in sections (Figure 3). These 67 earthquakes (10 events

of mag. 7.07.9; 28 events of mag. 6.06.9, and 29

events of mag. 5.05.9) whose focal depths are uncer-

tain are otherwise well located events from the period


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18971962 and are useful for correlating large earth-

quakes with major tectonic features. To select these

2135 earthquakes (Table 1) from the entire database

we imposed certain selectivity criteria to reject the

followings: (a) earthquakes whose epicentral and hy-

pocentral parameters are poorly determined (reported

by only a few stations); (b) earthquakes whose focaldepths are not available; (c) earthquakes of unknown

magnitudes, and (d) earthquakes of magnitude less

than 4.0 [except for a few deeper events (70 km)

in the magnitude range 3.53.9 that occurred during

19911993, to better constrain the Benioff zone at

lithospheric levels]. The map area is suitably segmen-

ted into a number of blocks (A1 through L2) across

which some 29 depth sections are taken in east-west

direction for illustrating the Benioff zone geometry

underlying the BAAS.

Figure 2 illustrates that the entire BAAS is seis-

mically active whose most intense seismic zones are

located in north Burma, mid and south parts of the

Andaman Sea and northern Sumatra. Large magnitude

earthquakes (M 6.0) mainly occur in association

with the Benioff zone and forearc part of the BAAS

as well as with the Shan-Sagaing transform in Burma

and its southern continuation with the ASR (see Muk-

hopadhyay, 1984). Table 2 summarizes the spatial

relationship of the large magnitude earthquakes as-

sociated with the tectonic features of the BAAS in

blocks A1 through L2. A total of 98 large earthquakes

(M 6.0) have occurred in BAAS out of which 72

alone were interplate events at the Indian plate margin.

Notice that subduction-related large interplate eventsare prevalent in blocks A1 to D2 and also in blocks

H3 to L2 but they are conspicuously absent in coastal

Burma. Intense seismic zones characterize the BAAS

where the arc convexity is westward. That this rela-

tionship is more than fortuitous is evidenced by a clear

absence of well defined Benioff zone in coastal Burma

and the Gulf of Martaban where the arc convexity

changes eastward.

Another noticeable feature of the BAAS seismi-

city is that most of the large magnitude earthquakes

have their focal depths in upper part of the lithosphere.

Table 3 gives a summary status on the focal depth

distribution for the large interplate earthquakes. How-

ever, for nearly one-third of the reported events, focal

depth is not known. Following Abe (1981), we can

only speculate that most of the large magnitude events

of unknown focal depths are also of shallow foci.

Shallow foci large magnitude interplate earthquakes

are known for their capabilities for producing sub-

Table 2. Spatial distribution of large earthquakes in different

tectonic domains of the BAAS

Block Number of Tectonic domain

events with

magnitude

Interplate Shan- Andaman7.0 6.06.9 subduction Sagaing spreading

& forearc fault ridge

and

Sumatra

fault

A1 2 3 +

1 +

A2 1 1 +

A3 1 5 +

B1 2 1 +

4 +

C1 2 8 +

D1 1 +

3 +

D2 1 +

1 +

E1

E2

E3

F1 1 +

G1 1 +

F2

H1

H2 1 +

H3 1 +

1 +

H4 2 4 +

2 +

H5 1 +

H6 1 +

I1 2 +

I2 1 3 +

J1 3 +

J2 2 +

J3 1 3 +

J4 3 +

K1 1 1 +

1 +

K2 1 3 + 6 +

L1 1 2 +

L2 2 6 +

1 1 +


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Figure 2. Seismotectonic map of the Burmese-Andaman Arc System (seismicity data for the period 18971993). The entire area is divided

into 29 blocks (A1 through L2) in north-south direction to study depth sections illustrated on Figure 3. BS, Belt of schuppen in the Naga Hills;

EBT, Eastern Boundary Thrust; DF, Dauki fault; VA, Volcanic Arc; OC, Oceanic crust; CC, Continental Crust; SF, Sumatra fault. K, Kohima;

I, Imphal; A, Agartala; S, Shillong; B, Bhamo; C, Chittagong; M, Mandalay; R, Rangoon; star symbol, volcanic province.


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160

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Continue

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161

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Figure3.

Continue

d.


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163

Table 3. Focal depth distribution for large

interplate earthquakes of the BAAS

Focal depth (km) No. of earthquakes

Unknown 24

060 21

61100 12

101150 13

>150 2

stantial damages, particularly, if they are thrust-type

earthquakes.

b) Benioff Zone configuration 2D sections

The BAAS Benioff Zone configuration is represented

by 29 depth sections taken across several blocks (Fig-

ure 2). Each block is approximately of 1 width in

north-south direction; 2135 earthquake data out of a

total of 2202 plotted on Figure 2 are used for this pur-

pose. Orientation of each block is set perpendicular

to the local tectonic trend of the BAAS, such as the

fold axis in Burma or the trench axis in the Andaman

Sea. These blocks are grouped into 12 classes (A to

L) depending on the major changes in the orientation

of the tectonic trend between north Burma and south

Andaman Sea. For example, under class A, there are

actually 3 blocks A1, A2 and A3 each of1 width and

the area occupied by them has demonstrably the sim-

ilar tectonic trend which is the Burmese fold mountainbelt. With the change in the local trend, separate block

class is therefore designated. As the regional trend

of the arc changes, there is certain overlapping in

some of the blocks, consequently the earthquakes in

the overlapped area are also plotted in both of the

depth sections. For instance; 143 hypocentres are plot-

ted in A3 and 118 hypocentres are plotted in B1 but

there are 17 earthquakes common to both A3 and B1.

Similarly, between B1 and C1, 44 earthquakes are

common. Maximum number of such common earth-

quakes is found to be 52 occurring between blocks

K2 and L1. However, between each block of the sameblock class (e.g., between A2 and A3 or between D1

and D2 etc.) there may or may not be any common

earthquake which needs to be plotted on the boundary

of the adjoining blocks. The Benioff zone depth sec-

tions are illustrated on Figure 3. In plotting the depth

sections, a computer program is utilized for projecting

all earthquakes in each block on the center plane of

Table 4. Different parameters defining the geometry of the

Benioff zone below the BAAS

Block Average Penetration Arc-Trench

dip of the depth (km) into gap (km)

Benioff zone the mantle

A1 50 150

A2 42 180

A3 45 200

B1 48 160

C1 50 190

D1 48 190

D2 35 140

E1 42 140

E2 32 110 300

E3 25 110 270

G1 30 110 290

F1 22 70 280

F2 30 70 250

H1 30 80 220

H2 30 90 200

H3 43 130 220

H4 53 160 220

H5 35 170 220

H6 45 220 280

I1 50 220 280

I2 45 180 300

J1 38 180 290

J2 43 200 280

J3 40 240 260

J4 53 280 290

K1 50 270 280

K2 36 230 320

L1 37 240 350L2 42 330

the block where the hypocenters are plotted according

to their depths. The enveloping surface defining the

subducting and overriding plates are manually adjus-

ted using the surface disposition of the various tectonic

elements (e.g., the location of the volcanic arc) and the

pattern of hypocentral distribution across the BAAS in

general.The depth sections thus prepared are utilized to

investigate the followings: (a) the average dip of the

Benioff zone in different parts of the BAAS, (b) pen-

etration depth of the subducting lithosphere, (c) the

arc-trench gap, (d) the subduction zone geometry

underlying the BAAS and (e) the probable contor-

tions created therein due to plate deformations. The


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main results on the first three parameters are given in

Table 4. For nearly 500 km stretch in northern and

central Burma covered by blocks A1 through D1, in-

clination of the Benioff zone varies from 4250 as

subduction reaches down to 200 km depth. This region

also houses the most intense seismicity of the en-

tire BAAS. The Benioff zone dip gets gradually moremoderate southward. In blocks D2 and E1, the dip of

the Benioff zone varies from 3542 with shallow pen-

etration depth up to 140 km. Southwards, in the area

defined by blocks E2 to H2, the dip of the Benioff zone

is still shallower (varying between 2232) wherepen-

etration depth barely reaches to 110 km. In coastal

Burma and below the Gulf of Martaban, the level of

seismicity is largely subdued (covered by blocks E2 to

H1). As a result, no meaningful data can be presented

about the penetration depth or the arc-trench gap for

coastal Burma and the Gulf of Martaban. Notice that in

this region, the BAAS is convex eastward as compared

to its westward convexity in Burma and Andaman

respectively. Again from block H2 southward, the

Benioff zone gradually develops below the Andaman

arc where it increases from 30 below H2 through 43

below H3 to about 53 below H4. The volcanic islands

of Narcondam and Barren are located in blocks H3

and H4 respectively. The Indian lithosphere penetrates

up to 160 km below the Barren Island which is the

only active volcano in the BAAS at present (Dasgupta

and Mukhopadhyay, 1997) (see below). Smaller dip of

the Benioff zone is usually accompanied with shallow

penetration depth of the lithosphere in the mantle but

there are some exceptions; e.g., under block H5 wherethe dip is around 35 and penetration depth is 170 km

in contrast to block H4 where, though the penetration

depth is somewhat less (160 km) but the Benioff zone

is having a higher inclination (53). Active spreading

of the Andaman back arc has commenced since the

Neogene in areas covered by blocks H3 to H5, where,

except the constant arc-trench gap (of about 220 km)

other parameters are variable (Table 4). The dip of

the Benioff zone in south Andaman-north Sumatra

(covered by blocks H6K1) varies again almost in

this range (3850) with an important distinction that

seismically defined portion of the Indian lithosphere

is much thinner below the Nicobar Islands. An in-

spection of Table 4 data also suggests that intrablock

changes in dip angle are more than inter-block vari-

ation in the dip of the Bemoff zone. For example,

the variation in dip angle between A3 and B1 (inter-

block), F2 and H1 or J4 and K1 are negligible, as

compared to intra-block variation between D1 and D2,

F1 and F2 or K1 and K2, etc., which are somewhat on

the higher side and indicates abrupt variation in the

Benioff zone dip. Such variations have resulted due

to the presence of transverse faults within the Benioff

zone. Though inter-block variations in dip angle are

negligible, in a few cases there are some variations

which are seemingly influenced by the orientation of ablock in relation to the true dip direction of the Benioff

zone. For instance, in the case of the three overlapping

blocks E3, G1 and F1 with respective dip angles 25,

30 and 22 respectively, where, it is evident that G1

best represents the true depth sections as compared to

the other two which are apparent sections only. An ex-

amination of the depth sections illustrated on Figure 3

suggests the followings:

(a) Average dip of the Benioff zone varies signific-

antly along the length of the BAAS. This has

consequently produced a wide ranging configur-

ation for the dipping lithosphere changing from

relatively flat to steep dips.

(b) Though uncertainties in calculation of focal depths

constrain the vertical thickness of seismic lay-

ers, nevertheless, seismically active lithosphere is

relatively thick below Burma than in Andaman.

Considering the thickness variation real, this is

probably an outcome of the directional approach

of the descending Indian plate in respect of the

overriding plate.

(c) A tectonic relationship is apparently manifested

between the dip of the Benioff zone and the BAAS

curvature. Seismicity is highly intense under the

Fold Thrust Belt in Burma or its continuationinto the Outer Sedimentary Arc in Andaman

where the arc convexity is towards the descending

Indian plate. This is in contrast to coastal Burma

and the Gulf of Martaban where the arc convexity

is in the opposite direction. Seismicity in the latter

area is highly subdued or practically absent (refer

above).

(d) Considerable deformations seemingly affect the

dipping lithosphere under the BAAS as postu-

lated by several hinge-faults whose throw decrease

on the up-dip side (Figure 4). They are the dis-

continuities created on the upper surface of theBenioff zone; the deep faults orient at high angles

to the strike direction of the BAAS. Sixteen such

deep faults: f1 through f16, are identified on Fig-

ure 4. Their existence is further supported from the

results of focal mechanism solutions for a large

number of earthquakes occurring at lithospheric

depths below the BAAS (see below). For a great


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majority of them, the nodal planes do not corrob-

orate to surficial features or general trend of the

arc, rather they help substantiating the presence

of the inferred transverse faults slicing the des-

cending lithosphere. The contortions created by

the transverse faults in the lithosphere under the

BAAS therefore merit particular attention.

Constraints on the geometry of Benioff zone

The foregoing analysis of the 2-D sections taken

across the BAAS suggests that the dipping Indian

lithosphere is by no means a smoothly dipping slab

rather short wavelength flexures aided by transverse

faults provide ample evidences for contortions in the

subduction zone. It was therefore felt necessary to

inspect the subduction zone geometry through three-

dimensional perspective imaging so that the Benioff

zone upper surface could be presented on a plan view

with its contortions and faults. Bevis and Isacks (1984)

adopted hypocentral trend surface analysis through

least-square fitting using data from local network and

other teleseismic events to infer the Benioff zone

geometry, particularly, for the mid-surface of the

lithosphere of presumed thickness. Trend-surface con-

tour maps showing configuration for such mid-surface

of the subducting lithosphere below the Andes were

prepared by these authors.

Here we use a simpler technique for imaging the

upper surface of the Benioff zone (rather than itsmid-surface), by utilizing the shallowest earthquake

epicenters in a number of pre-designed unit cells both

along and across the arc in order to trace a surface to

represent the top of the Benioff zone. This approach is

adopted since no local seismic network data are avail-

able in the present case to justify the application of the

trend-surface technique. Therefore the best that can be

done is to image the upper surface of the Benioff zone

by depicting it as a Structure Contour Map. Res-

ults from the 2-D sections discussed in the preceding

section are utilized to constrain the contouring of the

structure contour map. The 2D sections demonstratethat a large variation exists in the thickness of the seis-

mically active lithosphere below the BAAS, but the

structure contour map representing the top surface of

the dipping lithosphere clearly remains unaffected by

this thickness variation of the active lithosphere. De-

tails of the map preparation and its main results are

discussed below.

Figure 4. Structure Contour Map representing top surface of thesubducting Indian lithosphere as imaged through the shallow foci

earthquake distribution; details are in text. A total of 460 such shal-

lowest Benioff zone earthquakes for unit mesh of 0.25 0.25 are

plotted. Sixteen faults (f1 through f16) are inferred on the map based

on contour trends. Transverse orientation of the faults to the strike

of the arc suggests for contortions affecting the dipping lithosphere.

Abbreviations and symbols as in Figures 2 and 3. Contour interval:

20 km.


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a) Structure contour map

Judging by the disposition of hypocentral data distri-

bution in the 2-D sections and their respective blocks

(Figures 2 and 3), it was found that some 1260 earth-

quake events actually belong to the Benioff zone

below the BAAS. The entire area was then gridded

into 0.25 0.25 mesh (where these 1260 events

originated) and the shallowest event for each unit

that supposedly comes from the top surface of the

Benioff zone was programmatically separated out to

infer the depth to the top surface of the dipping litho-

sphere. A total of 460 shallowest hypocenters were

thus sorted out and utilized to generate the structure

contour map (Figure 4). The limitations in the tech-

nique adopted here are: (a) All meshes do not have

earthquake incidence within the sample period, and

(b) the shallowest hypocenter registered within certain

meshes may not actually represent the top surface of

the Benioff zone when they show anomalously greaterhypocentral depths compared with those from adjoin-

ing meshes, if they did not have a shallower event

to mimic the top surface of the Benioff zone. In the

present case, 20 such anomalous events were detected

which have been excluded from contouring. Instead,

the nearest hypocentral value from the general trend of

the Benioff zone was adopted for contouring purposes.

Initial contouring was done by using standard software

which was subsequently upgraded by manual contour-

ing through inverse square technique by introducing

the faults at sharp contour kinks (localized deflection

of contours along narrow zones) and hanging contours

(abrupt termination of a particular contour).

b) Hinge-type tear faults

Figure 4 shows that the BAAS is fragmented by at

least 16 hinge-type tear faults (f1 through f16) that

orient at high angles to the structural trend of the

arc. Seven each of them are inferred for Burma and

Andaman and two for north Sumatra. The discon-

tinuities present on the structure contour map for the

Benioff zone is best explained by invoking these tear

faults. Faults transverse to the arc orientation in Burma

and Andaman have also been inferred by other work-ers merely on the basis of earthquake hypocentral

distribution that orient at high angles to the overall

tectonic trend (e.g., Hamilton, 1974; Page et al., 1979;

Mukhopadhyay, 1984).

Between the faults f1 and f2 in northern Burma,

the dipping lithosphere is traceable upto 180 km depth

where the fault bounded block has clearly subsided.

The next two faults (f3 and f4) below Wuntho province

trends east-west in Burma but disposition and off-

set of shallow-level contours imply that both faults

swerve to the southwest continuing below Burma and

coastal Bengal basin. Lithosphere has penetrated to

about 140 km depth between faults f3 and f4; both

faults have southerly throw. The lithospheric segmentbetween faults f4 and f5 represents another subsided

block, on which, locates the Chindwin-Mt. Popa Vol-

canic Arc with Mio-Pleistocene explosive volcanoes

of Letpadaung and Pleistocene Recent volcanics at

Mt. Popa (Dasgupta et al., 1990). This is clearly a case

of fault bounded lithospheric flexuring, atop which,

giant volcanic structures like that of Mt. Popa are loc-

ated. This part of the subducting lithosphere exhibits

flattening of dip of the Benioff zone, thereby restrict-

ing the penetration depth of lithosphere to 120 km.

The two southernmost faults (f6 and f7) underlying

Arakan-Yoma and coastal Burma orient ENE, both

indicate northerly throw. In general, the dip of the

Benioff zone gets shallower by about 100 km in

Burma alone as the penetration depth reduces from

180 km in north Burma to around 80 km below Pegu

Yoma in south Burma. The subduction has practically

ceased in coastal Burma.

Inferred fault f8 delineates southeast corner of the

Narcondam volcanic Island. Tectonically this situation

is comparable to that for Mt. Popa in Burma where

fault f5 defines its eastern limit. The Benioff zone sur-

face stops short of the volcanoes in either case, though,

both these have remained active in the Holocene. The

Benioff zone is however steeper (50

) to its imme-diate south; it penetrates to 140 km depth where the

faults f9 and f10 are inferred. At this location, the

Barren Island volcano that erupted during 199194

is developed (Dasgupta and Mukhopadhyay, 1997).

Variable dip and penetration depth of the descending

Indian plate below blocks H3H5 produce a contorted

picture of the lithosphere at depth that corresponds not

only to the locus of active volcanism but also to active

backarc spreading (Figures 1 and 2) through splitting

of the overriding Andaman plate almost longitudinally

in NNE direction. However, with the advancement

of the subducting slab, the gap between the volcanic

arc and the spreading ridge gradually reduces from

150 km in the Narcondam areato 135 km in the Barren

Island area and thence to 100 km further south where

the faults f10 and f11 are conjectured. The spreading

axis ultimately merges with the volcanic arc near the

Little Andaman Island in block H6.


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167

A set of three ENE oriented faults (f11f13)

between the Little Andaman and Great Nicobar Is-

lands is inferred to rupture the width of the lithosphere

into a northern 225 km and a southern 125 km long

segments (Figure 4). The northern segment registers

a shallow dip for the subducting lithosphere down

to 200 km depth while the southern slab is narrowerbut steeper, plunging to about 180 km depth. The

fault f13 possibly extends further southwest beyond

the trench axis to delineate the shallow foci seismicity

distribution. Similar argument appears to hold good

for another transverse fault f16 in offshore Sumatra

(refer Hamilton, 1974 for a description on transverse

seismicity across the Sunda and Indonesian trenches).

The foregoing discussion on the transverse faults

f1 through f16 commonly suggests for a hinge-type

geometry of the faults with throw decreasing on the

updip side of the Benioff zone thereby producing the

maximum vertical displacement at the leading edge

of the subducting slab. On a plan view, the faults

display a fan-shaped distribution that appears to con-

verge towards the continental side of the BAAS. The

convergent pattern implies that the transverse faults

genetically relate to the curvature of the arc-trench,

and possibly for the Benioff zone as well. Implicit

in this observation being that the oceanic and con-

tinental side of the BAAS are under the influence of

extensional and compressional stress regimes respect-

ively. We investigate this problem and other fault types

in the Benioff zone by using results from 148 fault

plane solutions of earthquakes originating within the

Burmese-Andaman subducting lithosphere.

c) Results from fault plane solutions

A large number of fault plane solutions for Benioff

zone earthquake occurring in the area covered by

blocks A1 through L2 up to 1993 were compiled from

published literature, including our previous work. For

selecting the focal mechanism solutions, weightage

was given to HRVD best double couple solutions as

they are considered more representative, complete and

less influenced by subjective interpretations (see Froh-

lich and Apperson, 1992). Out of a total of 148 solu-tions 87 are from the HRVD catalogue. Of the remain-

ing 61 P-wave solutions, 54 solutions are from Muk-

hopadhyay and Dasgupta (1988), Dasgupta (1992)

and Dasgupta and Mukhopadhyay (1993) (these are

carefully selected well constrained solutions with ho-

mogeneous distribution of stations from all the quad-

rants, polarity considered from long-period stations

and use of impulsive phase data etc.); 7 from other

published papers (Fitch, 1970, 1972; Ritsema and

Veldkamp, 1960; Ritsema, 1956; Bergman and So-

lomon, 1985). These Benioff zone focal mechanism

solutions are reviewed to study the faulting mechan-

ism and stress pattern that characterise the Burmese-

Andaman subducting plate particularly in relation tothe geometry of the Benioff zone and to correlate with

the lithospheric structural features that have been de-

tected through the present study. Locations for such

88 Benioff zone earthquakes with their solutions are

schematically depicted in Figure 5a; another 60 solu-

tions whose nodal planes are obliquely oriented to the

trend of the Benioff zone are shown in Figure 5b. Fo-

cal mechanism parameters for all the 148 earthquakes

are given in Table 5. Here we first review the focal

mechanism solutions whose nodal plane orientation

agrees with the inferred geometry of the Benioff zone,

followed by further discussion on those solutions that

are seemingly associated with the lithospheric hinge

faults.

Out of 148 focal mechanism solutions, there are

88 events whose orientation of nodal planes match

with the overall trend and geometry of the Benioff

zone (Figure 5a). They are both compressional (51

solutions) and tensional (37 solutions) events which

give an idea on the stress distribution acting along and

across the Benioff zone. 15 typical interplate shallow

and 16 deeper foci pure thrust earthquakes (blocks

A1-D1:16; H4-I2:9; J3:3; and L1-L2:3) character-

ise different segments of the Bz. In addition, there

are 12 shallow and 8 deeper foci events (B1-D1:5;F2:2; H4-H5:4; J3-J4:5 and K1-L1:4) that display high

angle reverse fault mechanism, of which 10 (shal-

low) and 4 (deep) are downdip compressional (DDC)

earthquakes. It may be noted that such compress-

ive earthquakes are not known from southern Burma

(blocks E1H3, except F2) nor from blocks J1-J2 in

the Car-Little Nicobar sector. Of the 37 earthquakes

that display normal fault solutions, 36 are downdip

tensional (DDT) events. 10 shallow and 6 deeper foci

DDT events locate in blocks A2-D2. In blocks C1 and

D1 the DDT events locate below the shallow interplate

thrust earthquakes and clustered just below the bend-

ing inflexion point in D2. 6 shallow and 3 deeper DDT

earthquakes are located in blocks E1, E3, H2 and J1-

J2, which are devoid of any compressive events. In

block K2, one deeper and 4 shallow DDT events loc-

ate below the shallow DDC earthquakes. Further, one

shallow foci DDT event locate in each of the blocks

I2, K1 and L1 while one deeper event each occurs


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168

Figure 5a. Tectonic map of the Burmese-Andaman Arc where the transverse lithospheric faults inferred during the present study are shown.

Focal mechanism solutions of 88 Benioff zone earthquakes whose nodal planes are conformable with the subduction zone geometry are

schematically depicted. Solution parameters are listed in Table 5. For other features refer to Figures 1 and 3.


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169

Figure 5b. Tectonic map of the Burmese-Andaman Arc where the transverse lithospheric faults inferred during the present study are shown.

Focal mechanism solutions of 60 earthquakes that are related to transverse lithospheric and other faults are schematically depicted. For solution

parameters refer to Table 5 and for other features refer to Figures 1 and 3.


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170

in blocks H4, I2, K2 and L2. Predominance of DDT

earthquakes in the Benioff zone also suggest that slab

pull extensional tectonics significantly contributes to

the subduction process of the Indian lithosphere below

the Burmese plate.

In addition to the above said 88 compressive and

tensional earthquakes, there are 7 strike slip fault solu-tions found in Burma (37, 45 and 71 in Figure 4b),

Nicobar (104) and Sumatra (119, 121 and 123). While

the Burmese shallow foci events cannot be correlated

with any known faults, the other earthquakes may

be related to the West Andaman fault and the Great

Sumatra fault respectively. For the entire BAAS arc we

have detected another 52 fault plane solutions whose

nodal planes mismatch (see Figure 5b) with the trend

and geometry of the Benioff zone. A closer examin-

ation reveals that they can be better explained when

correlated to the activity of the transverse hinge faults

discussed in the foregoing. Their details are given

below.

Solution 5 (a deep foci event) shows reverse fault

mechanism with left lateral shear which is related to

activity along the inferred fault f1, while at least five

deeper event solutions (10, 15, 19, 20 & 21) indicate

thrust or reverse slip mechanism along NW trending

nodal plane parallel to fault f2. Further, solution 22

though located slightly off the fault f2 shows similar

mechanism along plane parallel to f2. It is likely that

with a slight change in trend, f2 penetrates the shal-

lower section of the Benioff zone where another five

shallow foci earthquakes (1114 & 16) indicate left

lateral shear along roughly E-W nodal plane (see alsoMukhopadhyay and Dasgupta, 1988). Earthquake 32

is associated with f3 and gives a reverse fault solution

along WNW nodal plane parallel to f3. Similar solu-

tion is shown by events 24 and 25 though not spatially

disposed to f3. Three more shallow foci earthquakes

(34, 38 & 39) with left lateral strike slip mechanism

along NE to ENE nodal planes also relate to activity

along the fault f3. Though not depicted in Figure 4 (as

it cannot be predicted from the present technique to

decipher fault along shallower section of the Benioff

zone), a fault conjugate to f3, passes through solutions

2831 which indicate right lateral strike slip mech-

anism along NW trending nodal plane. At least four

solutions (46, 5052; all deeper events) can be cor-

related with activity along f4; with this fault is also

associated solution 63 that gives left lateral strike slip

mechanism along NE fault plane. Nodal planes of re-

verse slip solutions 68 and 70, and strike slip (left

lateral) solution 64 along NW planes clearly relate to

fault f5. Though for the faults f6, f7 and f8 there is no

supporting focal mechanism available, nodal planes of

at least two events (80 and 83) matches with the fault

f9. Similarly though no solution directly corroborates

activity of fault f10, the E-W nodal plane of event

92 that indicates reverse with strike slip mechanism,

could be associated with f10. There are four solutionsthat support activity along f11; two deeper events (95

96) at the leading edge of Benioff zone indicate high

angle reverse fault along roughly E-W nodal plane,

while two shallower events (9798) display left lateral

shear along NE trending plane. Earthquake solutions

105, 106 and 107 suggest reverse fault mechanism

along nodal plane parallel to f12. Events 109 (deeper

foci normal fault solution) and 113 (shallow foci left

lateral shear) are clearly associated with f13 and if

the fault is extended beyond the trench axis (see also

Dasgupta and Mukhopadhyay, 1993), NE trending

nodal planes of solutions 110, 111 and 117 indicate

left lateral shear along f13. Only a small segment of

the fault f14 could be mapped and possibly solution

118 showing a high angle reverse slip mechanism, is

related to f14. Solutions 128 and 129 match well with

f15 and possibly 131 is also related to this fault. Both

solutions 140 and 142 display normal fault mechan-

ism along nodal plane parallel to f16. This fault could

be traced through events 144 and 145 with left lateral

strike slip along NNE plane.

Conclusions

Gross features of the BAAS Benioff zone were knownfrom earlier studies but the present work brings out

the details of the Benioff zone and the contortions

created in it. The dip of the Benioff zone, depth of

penetration of the subducting Indian lithosphere, and

the arc-trench gap vary along the BAAS. Significant

changes are noticed in the dip of the Benioff zone

within relatively short distances along the arc sug-

gesting the presence of several transverse faults which

dissect the subducting lithosphere into segments that

undergo deformation. The top surface of the down-

going slab is imaged through foci distribution of the

shallowest earthquake in each unit area of the Benioffzone below the BAAS. Such hypocentral values are

next utilized to construct the structure contour map

representing the top surface of the seismically act-

ive portion of the Indian plate that helps identifying

the transverse faults within the subducting lithosphere.

A large number of fault plane solutions are analysed

which indicate that apart from shallow foci interplate


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171

Table 5. Source parameters and focal mechanism solutions for Burmese-Andaman subducting plate earthquakes

No Yr Mo Dt Hr Mn Sec Lat Long Ms Mb Depth Ppl Paz Tpl Taz Bpl Baz NP1st NP1dp NP2st NP2dp Source

(km)

1 1970 04 06 05 07 59.8 26.45 96.34 5.0 98 18 309 72 142 6 40 30 28 223 64 MD

2 1972 11 01 21 53 45.8 26.44 96.37 5.2 93 19 307 71 135 4 38 32 26 220 64 MD

3 1969 08 29 10 02 49.6 26.35 96.06 5.2 72 13 308 77 143 4 39 32 32 222 58 MD

4 1985 04 24 06 47 45.2 26.18 96.08 5.2 5.3 42 10 256 68 139 19 350 324 38 182 58 HRV

5 1983 11 16 00 54 11.4 26.16 96.12 5.0 139 3 51 44 144 45 318 177 58 286 62 HRV

6 1970 07 29 10 16 20.4 26.02 95.37 6.4 68 40 239 18 1 26 45 26 11 76 265 50 MD

7 1969 04 28 12 50 17.2 25.93 95.20 5.0 68 57 257 21 1 33 23 33 24 70 257 31 MD

8 1964 06 03 02 49 17.2 25.88 95.69 5.4 121 50 288 40 102 3 194 14 84 171 6 MD

9 1964 03 27 04 30 36.1 25.82 95.71 5.3 115 16 276 72 62 10 184 23 30 178 62 MD

10 1988 08 13 19 59 51.0 25.29 95.13 5.0 87 12 232 73 98 12 325 307 35 152 58 HRV

11 1987 05 18 01 53 51.3 25.23 94.21 5.9 5.7 55 25 27 6 294 64 192 67 68 163 77 HRV

12 1971 12 29 22 27 03.5 25.17 94.73 5.6 46 7 32 21 124 68 286 260 80 166 70 MD

13 1989 04 03 19 39 31.5 25.15 94.66 4.8 5.3 69 12 27 1 297 78 202 71 81 163 82 HRV

14 1983 08 30 10 39 27.2 25.04 94.67 5.7 64 9 23 33 118 55 279 155 60 255 74 HRV

15 1 988 08 06 00 36 25.5 25.13 95.15 7.2 6.6 108 5 217 65 117 24 310 284 45 148 54 HRV

16 1965 02 18 04 26 34.7 24.97 94.21 5.4 45 17 58 17 153 66 288 286 90 196 66 MD

17 1979 07 13 23 20 08.8 24.88 95.22 4.3 4.9 108 17 308 72 114 4 217 44 28 215 62 HRV

18 1964 07 12 20 15 58.8 24.88 95.31 5.5 152 28 240 49 8 27 145 16 30 129 78 MD

19 1981 04 25 11 32 23.0 24.89 95.34 5.0 5.7 146 2 52 83 308 7 143 135 44 329 47 HRV

20 1992 03 25 22 32 34.2 24.82 95.25 5.2 106 22 2 16 59 85 21 315 272 30 144 70 HRV

21 1990 01 09 18 51 29.2 24.74 95.26 6.1 118 20 1 6 58 142 24 276 140 32 267 69 HRV

22 1983 08 23 12 12 17.5 24.55 95.12 5.2 126 5 229 67 123 22 321 297 44 158 54 HRV

23 1971 06 26 02 16 36.9 24.60 94.78 5.0 74 14 280 72 100 12 6 350 26 190 56 MD

24 1984 03 05 21 26 42.6 24.52 94.62 5.2 70 30 3 43 54 127 17 242 114 21 238 78 HRV

25 1 979 05 29 00 39 52.1 24.50 94.74 4.6 5.2 82 21 348 60 120 20 250 109 30 241 69 HRV

26 1989 04 13 07 25 33.0 24.40 92.43 5.1 5.0 29 43 265 47 97 6 1 291 6 181 88 HRV

27 1984 12 30 23 33 35.0 24.66 92.85 5.5 02 4 238 67 338 22 146 350 45 128 53 HRV

28 1991 12 20 02 06 05.2 24.69 93.12 4.9 5.3 41 7 206 45 109 44 304 258 54 150 66 HRV

29 1973 05 31 23 39 52.4 24.31 93.52 5.7 5.8 1 8 48 14 139 73 287 274 86 183 74 MD

30 1984 05 06 15 19 11.3 24.22 93.53 5.8 5.7 54 2 25 27 116 62 290 157 69 254 73 HRV

31 1991 05 11 02 15 22.2 24.26 93.68 4.5 5.0 64 3 25 16 116 74 285 159 77 251 81 HRV

32 1992 04 15 01 32 11.3 24.27 94.93 5.5 116 4 214 66 114 24 306 281 45 145 54 HRV

33 1979 08 11 20 32 07.9 24.20 94.93 3.9 5.0 113 22 288 67 116 3 19 11 22 200 68 MD

34 1970 05 29 10 33 58.6 23.96 94.06 5.1 49 1 22 15 112 75 294 247 80 155 80 MD

35 1975 05 21 03 16 18.3 23.86 94.09 5.3 51 43 196 13 95 44 350 331 70 225 48 MD

36 1973 07 04 21 04 46.2 23.60 94.86 5.0 126 16 105 72 310 7 196 20 60 185 30 MD

37 1986 02 08 00 28 54.0 23.87 93.00 5.0 5.2 38 29 186 10 90 59 344 224 62 321 77 HRV

38 1977 10 13 11 32 09.3 23.47 93.33 5.2 61 37 3 54 27 107 41 223 145 41 228 84 HRV

39 1980 05 20 13 19 52.2 23.72 94.20 5.4 4.8 83 4 204 14 114 75 300 251 78 160 82 MD

40 1986 07 26 20 24 49.6 23.71 94.19 4.9 5.2 35 2 83 23 352 67 179 130 70 36 76 HRV

41 1975 12 13 22 35 44.2 23.62 94.27 5.2 62 40 314 50 145 6 48 5 9 230 84 MD

42 1973 07 27 20 23 48.6 23.27 94.49 5.4 60 33 250 56 5 6 8 156 10 12 156 78 MD

43 1993 04 0 1 16 30 0 9.8 23.21 94.46 5.3 105 37 309 25 59 43 174 99 44 1 83 HRV

44 1964 06 13 17 35 58.3 23.00 93.95 5.2 60 31 278 24 24 49 144 63 50 329 86 MD

45 1966 10 22 03 03 24.4 23.04 94.28 5.1 72 22 259 15 356 63 118 39 63 308 85 MD

46 1987 08 24 09 24 40.0 23.05 94.41 5.1 94 14 9 54 119 33 270 135 42 254 67 HRV

47 1969 10 17 01 25 11.5 23.09 94.70 6.1 124 63 276 25 120 9 26 22 70 228 22 MD

48 1 978 02 23 23 18 34.0 23.08 94.70 4.8 5.0 113 21 274 59 144 22 12 331 31 201 69 HRV


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172

Table 5. Continued


(km)

49 1978 02 03 23 46 42.4 23.02 94.70 5.1 92 6 81 37 170 52 347 312 66 208 62 MD

50 1981 05 01 04 08 10.0 22.94 94.56 4.1 4.8 98 34 210 49 70 21 314 247 22 137 82 HRV

51 1986 04 26 00 25 58.4 22.85 94.51 4.9 116 21 324 59 95 21 226 87 30 217 69 HRV

52 1989 07 15 00 09 14.9 22.79 94.54 5.4 98 19 337 43 86 40 230 111 44 217 75 HRV

53 1969 01 25 23 34 28.4 22.98 92.40 5.2 49 29 280 60 118 8 14 348 18 196 74 MD

54 1980 11 20 18 14 11.4 22.74 93.92 5.1 5.2 30 30 85 57 239 12 348 208 19 345 76 HRV

55 1964 01 22 15 58 43.7 22.33 93.58 6.3 60 54 254 25 123 24 22 14 74 254 30 MD

56 1988 07 03 08 19 18.6 22.07 94.26 5.2 88 63 244 27 54 4 146 133 18 327 72 HRV

57 1983 04 17 23 16 33.8 22.03 94.36 5.1 100 49 2 29 31 95 24 349 236 26 345 81 HRV

58 1983 10 2 1 08 44 4 7.3 22.00 94.38 5.3 93 47 259 43 76 2 167 122 2 347 88 HRV

59 1965 12 15 04 43 47.4 22.00 94.47 5.2 109 66 254 23 60 3 152 334 68 142 22 MD

60 1964 02 27 15 10 47.8 21.65 94.40 6.0 91 35 258 55 73 2 166 0 10 166 80 MD

61 1966 12 15 02 08 03.1 21.51 94.43 5.4 84 25 274 65 1 08 6 6 352 21 189 70 MD

62 1975 07 08 12 04 38.0 21.42 94.62 5.9 112 25 251 63 5 0 8 159 0 22 156 70 MD

63 1977 05 12 12 20 00.6 21.68 92.96 5.4 39 10 172 15 79 72 295 216 72 125 87 HRV

64 1974 04 05 03 46 29.7 21.33 93.70 5.0 47 19 85 28 185 55 326 316 84 223 56 MD

65 1989 12 02 19 44 26.8 21.21 93.82 4.6 5.2 51 63 187 9 79 25 344 196 42 328 59 HRV

66 1989 12 08 00 04 26.7 21.19 93.80 4.5 5.6 47 51 199 19 83 32 340 213 38 328 71 HRV

67 1992 07 08 10 09 47.8 21.06 93.68 4.7 5.4 42 75 237 13 90 8 358 191 33 353 58 HRV

68 1979 01 01 18 51 10.9 20.89 93.69 4.7 5.3 60 16 23 69 248 14 117 93 32 305 62 HRV

69 1992 03 27 00 05 18.4 20.87 94.59 5.3 97 57 242 33 65 1 334 159 12 334 78 HRV

70 1989 09 24 10 55 20.2 20.69 94.95 5.2 135 1 317 45 48 45 226 83 59 192 61 HRV

71 1967 02 15 05 57 30.5 20.33 93.99 5.4 51 20 176 30 74 53 295 218 54 124 84 MD

72 1992 11 22 11 42 45.4 20.33 94.32 4.4 5.3 69 58 197 23 63 21 324 187 29 317 71 HRV

73 1988 10 23 11 43 09.4 20.30 94.41 5.1 71 64 198 21 54 14 319 168 27 313 68 HRV

74 1965 06 01 04 32 48.5 20.13 94.83 5.2 81 47 240 39 86 13 344 343 85 232 14 MD

75 1988 02 19 23 17 14.1 18.41 95.07 4.4 5.3 66 56 250 33 52 8 148 114 14 329 78 HRV

76 1979 10 03 11 35 14.1 18.11 94.80 4.9 5.6 54 58 235 31 70 7 336 181 16 334 76 HRV

77 1972 04 28 11 30 18.1 16.99 94.85 5.3 28 2 320 41 230 49 52 12 62 264 63 D

78 1980 08 27 04 30 16.7 15.83 94.67 4.9 5.4 29 5 27 84 234 3 117 114 40 299 50 HRV

79 1967 09 06 07 30 10.8 14.65 93.55 5.5 36 59 286 31 106 0 16 16 76 196 15 F

80 1968 01 12 04 17 43.1 13.27 93.12 5.5 33 35 190 17 88 50 338 322 78 222 52 DM

81 1986 09 20 10 05 01.3 13.02 93.39 4.8 56 25 266 62 114 12 1 332 23 185 71 HRV

82 1984 03 22 05 36 37.4 12.93 93.56 5.0 92 41 74 49 2 54 0 344 344 86 164 4 DM

83 1983 01 24 23 09 21.7 12.91 93.59 6.1 85 7 235 52 136 37 330 291 50 174 62 HRV

84 1978 02 07 20 31 54.6 12.89 93.04 5.6 5.6 17 9 68 81 230 3 338 162 36 336 54 HRV

85 1978 02 07 12 30 40.4 12.81 93.00 5.3 5.5 03 10 69 60 321 28 164 129 43 2 61 HRV

86 1941 06 26 11 52 03.0 12.50 92.50 7.7 8.0 60 40 3 26 50 146 0 56 56 5 236 85 RV

87 1969 12 04 00 34 58.6 12.45 93.62 5.2 93 65 254 25 74 0 344 344 70 166 20 DM

88 1991 07 10 09 49 31.0 12.59 93.94 5.0 138 27 3 33 58 119 16 235 97 23 231 74 HRV

89 1981 11 02 21 10 25.5 12.18 92.87 5.5 5.7 24 28 50 59 259 13 147 110 21 331 74 HRV

90 1979 07 05 15 39 41.7 11.98 92.88 4.5 5.0 45 14 332 60 216 27 70 30 38 263 64 DM

91 1993 09 30 17 04 48.0 11.84 92.58 4.8 5.3 23 20 260 70 90 3 351 344 25 173 65 HRV

92 1982 12 16 08 56 35.3 11.70 92.99 5.4 60 9 216 46 117 43 315 268 52 158 67 HRV

93 1992 09 16 04 23 17.3 11.64 93.65 5.2 149 14 271 74 62 7 179 11 32 175 60 HRV

94 1974 02 16 01 51 10.8 11.47 92.32 5.2 19 38 238 50 42 9 142 10 10 140 84 DM

95 1988 02 28 03 19 36.2 11.07 93.53 5.0 119 19 354 50 109 34 251 125 39 238 72 HRV

96 1980 O6 01 23 11 24.0 10.70 93.83 4.1 4.9 138 2 28 45 120 45 296 154 58 264 62 HRV


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173

Table 5. Continued


(km)

97 1973 07 09 16 19 46.8 10.66 92.59 5.6 44 12 182 22 86 63 305 135 86 223 65 DM

98 1972 02 22 18 43 42.0 10.42 92.48 5.4 4 27 336 10 70 61 182 22 80 116 64 DM

99 1976 04 21 19 09 59.6 10.29 92.86 5.3 52 20 287 64 157 18 29 355 30 215 66 DM

100 1971 11 05 22 11 15.5 10.18 92.98 5.7 55 25 284 64 120 7 19 0 20 200 70 DM

101 1968 10 06 07 42 25.2 09.98 93.61 5.0 124 39 206 23 98 43 343 335 81 234 42 DM

102 1970 05 06 15 21 55.1 09.81 92.91 5.3 32 55 240 35 60 0 330 330 80 150 10 DM

103 1971 06 05 01 38 10.9 09.38 92.46 5.3 25 55 240 35 60 0 333 333 80 153 10 DM

104 1992 03 17 02 14 48.8 09.13 92.89 4.9 71 13 1 83 14 90 71 314 227 71 317 89 HRV

105 1992 12 08 07 08 42.1 09.27 93.53 5.9 94 12 3 33 101 55 256 137 58 235 76 HRV

106 1986 06 02 17 51 56.1 09.12 93.51 5.6 101 16 346 46 94 39 243 118 45 227 72 HRV

107 1964 09 15 15 29 32.2 08.90 93.03 6.3 89 17 204 52 88 33 308 141 70 258 38 F

108 1983 09 17 04 40 36.8 07.94 93.21 4.5 5.2 54 58 186 11 77 29 341 199 43 324 62 HRV

109 1986 06 19 18 12 30.4 07.81 94.56 5.8 191 60 66 23 204 18 302 263 27 128 70 HRV

110 1973 04 07 03 00 5 8.8 07.00 91.32 5.8 39 10 350 1 78 82 168 34 84 124 86 BS

111 1979 10 16 22 51 23.0 06.37 91.20 5.2 5.2 38 15 331 5 239 74 131 14 76 106 83 HRV

112 1980 02 19 17 27 36.5 06.73 92.59 5.5 5.1 32 19 254 56 134 27 354 308 35 186 69 HRV

113 1991 03 08 01 42 00.5 07.27 93.45 4.5 5.2 54 31 180 31 73 43 309 219 43 309 90 HRV

114 1979 06 08 20 36 40.6 07.30 94.43 5.0 5.1 120 32 336 58 163 3 68 55 13 249 77 HRV

115 1976 08 05 13 37 16.7 07.00 94.31 5.7 112 15 296 56 173 30 27 340 40 220 70 DM

116 1971 07 17 05 32 42.9 06.98 94.65 5.6 138 8 292 54 192 35 29 350 48 230 62 DM

117 1989 02 10 16 59 15.0 06.25 92.23 5.4 5.3 39 16 154 11 247 70 10 291 71 200 87 HRV

118 1993 08 28 20 14 43.0 06.50 94.65 5.7 122 5 159 50 255 40 65 283 53 38 61 HRV

119 1986 12 07 05 40 39.5 06.81 95.13 5.3 204 8 230 8 139 79 5 274 79 4 90 HRV

120 1973 11 09 23 26 39.0 05.98 93.90 5.1 44 33 38 51 248 19 139 321 82 80 20 DM

121 1937 08 04 23 35 18.0 06.00 94.50 6.0 120 19 113 9 205 69 322 158 83 249 70 RV

122 1984 08 11 11 56 51.7 06.05 95.29 5.3 136 21 100 52 222 30 356 348 74 232 34 DM

123 1982 02 13 19 56 13.2 05.75 94.77 5.1 77 4 320 21 52 68 219 94 72 188 78 HRV

124 1983 07 02 09 34 05.1 05.71 94.68 5.6 78 8 325 61 69 28 230 83 44 211 59 HRV

125 1983 04 04 02 51 34.5 05.71 94.72 6.5 95 1 143 61 51 29 234 207 51 78 53 HRV

126 1983 01 30 01 26 06.2 05.47 94.96 5.2 67 59 1 73 19 47 23 308 169 32 298 68 HRV

127 1981 09 10 14 17 44.2 05.50 95.37 5.1 97 8 254 50 155 39 352 309 50 194 64 DM

128 1936 08 23 21 12 13.0 05.00 95.00 7.1 7.3 40 5 170 42 78 47 264 116 65 224 59 RV

129 1988 04 03 14 27 10.0 04.71 94.46 5.8 5.8 32 14 23 63 143 22 287 141 36 275 63 HRV

130 1983 09 17 05 56 56.7 04.76 95.05 5.7 57 45 1 57 17 49 40 304 182 45 289 73 HRV

131 1989 07 20 06 27 26.4 05.07 95.66 5.8 93 8 118 43 20 46 216 169 54 62 68 HRV

132 1986 04 29 13 59 22.1 04.48 95.03 5.0 5.2 39 17 11 49 261 36 114 61 43 308 71 HRV

133 1974 01 01 14 07 40.1 04.64 95.87 5.1 74 45 1 89 39 46 19 300 299 86 200 20 DM

134 1987 06 10 16 03 55.7 04.18 94.84 5.3 5.5 61 45 150 16 44 41 300 176 46 284 72 HRV

135 1976 11 03 09 54 38.2 04.22 95.19 5.5 54 18 9 0 42 198 43 344 330 74 222 44 DM

136 1978 12 18 08 26 20.1 04.20 95.44 5.1 5.3 66 61 191 22 53 17 316 173 27 309 70 HRV

137 1977 05 25 14 55 45.0 04.21 95.74 5.7 67 54 2 37 32 28 14 127 78 19 310 78 HRV

138 1991 08 06 02 17 33.0 03.86 95.41 5.5 5.9 21 2 33 87 277 3 123 120 44 306 47 HRV

139 1967 08 21 07 33 00.6 03.72 95.74 6.1 40 33 203 57 23 0 293 293 12 113 78 F

140 1991 07 23 13 25 48.9 03.81 95.96 5.8 52 52 158 36 359 10 261 132 13 260 82 HRV

141 1990 01 22 17 26 12.3 03.92 96.13 5.9 6.0 59 18 223 72 44 0 313 313 27 133 63 HRV

142 1983 03 16 09 13 11.9 03.51 95.80 4.6 5.3 22 48 187 12 292 40 36 342 45 232 70 DM

143 1977 12 03 13 41 20.9 03.52 95.91 5.8 21 30 3 02 24 47 50 170 87 50 354 87 HRV

144 1979 09 29 18 37 12.5 01.16 94.20 6.8 6.2 30 2 149 0 239 88 332 284 88 194 89 HRV

145 1969 11 21 02 05 3 5.3 01.94 94.61 6.4 20 2 147 20 236 69 56 282 74 15 76 DM


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174

Table 5. Continued


(km)

146 1982 10 31 02 48 11.8 02.93 96.06 5.1 5.5 48 51 145 7 244 38 340 299 51 184 63 HRV

147 1993 09 01 14 03 19.0 02.99 96.14 6.2 5.8 35 36 221 54 43 1 311 306 9 132 81 HRV

148 1984 05 29 04 36 09.7 03.64 97.14 5.7 72 65 1 99 24 30 4 299 130 21 297 70 HRV

MD Mukhopadhyay and Dasgupta, 1988; HRV Harvard (Dziewonski et al.); D Dasgupta, 1992; F Fitch, 1970, 1972; DM

Dasgupta and Mukhopadhyay, 1993; RV Ritsema and Veldkamp, 1960; R Ritsema, 1956; BS Bergman and Solomon, 1985.

thrust earthquakes there are many downdip tensional

events within the Benioff zone suggesting slab-pull

extensional tectonics as a contributing force for sub-

duction of the Indian plate. Further, results from a

large number of focal mechanism solutions suggest

for contemporary activity along the inferred transverse

hinge faults which thereby segment the Benioff zone

into smaller blocks. However, it should be pointed

out that the structure contour map given in this paperand faults inferred at discontinuities can be improved

upon as and when local seismic networks are run in the

region and their data become available for analysis.

Acknowledgements

We are thankful to the reviewers for their constructive

suggestions in improving the manuscript. The work

was carried out under the Geological Survey of In-

dia Programme: 001/SEI/CHQ/GDB/199497 and we

thank the Directors, Geodata Division, GSI for their

support provided during the work.

References

Abe, K., 1981, Magnitudes of large shallow earthquakes from 1904

1980, Phys. Earth Planet. Int. 27, 7292.

Anon., 1999, Earthquake database for the Indian subcontinent

(037N/6898E) from 18971995. (database in db3plus main-

tained in GSI Geodata centre, Calcutta), Geol Surv. India, 133,

2, 24.

Bergman, E.A. and Solomon, S.C., 1985, Earthquake source mech-

anisms from bodywaveform inversion and intraplate tectonics in

the northern Indian Ocean, Phys. Earth Planet. Int. 40, 123.

Bevis, M. and Isacks, B.L., 1984, Hypocentral trend surface ana-

lysis: Probing the geometry of Benioff zones, J. Geophys. Res.

89, 61536170.

Burnnschweiler, R.O., 1974, Indoburman Ranges. In: Spencer,

A.M. (ed.), Mesozoic-Cenozoic Orogenic Belts, Spec. Publ.

Geol. Soc. London 4, 279299.

Curray, J.R., Moore, D.G., Lawver, L.A., Emmel, F.J., Raitt, R.W.,

Henry, M. and Kieckhefer, R., 1979, Tectonics of the Andaman

Sea and Burma, Am. Assoc. Pet. Geol. Mem. 29, 189198.

Curray, J.R., Emmel, F.J., Moore, D.G. and Raitt, R.W., 1982,

Structure, tectonics and geological history of the northeastern In-

dian Ocean, In: Nairn, A.E.M. and Stehli, F.-G. (eds), The Ocean

Basins and Margins, vol. 6, Plenum, New York, pp. 399450.

Dasgupta, S., 1992, Seismotectonics and stress distribution in the

Andaman plate, Mem. Geol. Soc. India 23, 319334.

Dasgupta, S. and Mukhopadhayay, M., 1993, Seismicity and plate

deformation below the Andaman arc, Tectonophysics 225, 529

542.

Dasgupta, S. and Mukhopadhyay, M., 1997, Aseismicity of the An-

daman subduction zone and recent volcanism, J. Geol. Soc. India

49, 513521.

Dasgupta, S., Mukhopadhyay, M. and Nandy, D.R., 1990, Magmat-

ism and tectonic setting in the Burmese-Andaman arc, Indian J.

Geol. 62, 117141.Eguchi, T., Uyeda, S. and Maki, T., 1979, Seismotectonics and

tectonic history of the Andaman Sea, Tectonophysics 57, 3551.

Fitch, T.J., 1970, Earthquake mechanisms in the Himalayan,

Burmese and Andaman regions and continental tectonics in

central Asia, J. Geophy. Res. 75, 26992709.

Fitch, T.J., 1972, Plate convergence, transcurrent faults, and internal

deformation adjacent to southeast Asia and western Pacific, J.

Geophys. Res. 77, 44324460.

Frohlich, C. and Apperson, K.D., 1992, Earthquake focal mechan-

ism, moment tensors and the consistency of seismic activity near

plate boundaries, Tectonics 11, 279296.

Hamilton, W., 1974. Tectonics of the Indonesian Region, U.S. Geol.

Survey Prof. Paper 1078, pp. 345.

Le Dain, A.Y., Tapponnier, P. and Molnar, P., 1984, Active faulting

and tectonics of Burma and surrounding regions, J. Geophys.

Res. 89, 453472.Mitchell, A.H.G. and McKerrow, W.S., 1975, Analogous evolution

of the Burma orogen and the Scottish Caledonides, Geol. Soc.

Am. Bull. 86, 305315.

Mukhopadhyay, M., 1988, Gravity anomalies and deep structure of

the Andaman arc, Marine Geophys. Res. 9, 197210.

Mukhopadhyay, M. and Dasgupta, S., 1988, Deep structure and

tectonics of the Burmese arc: Constraints from earthquake and

gravity data, Tectonophysics 149, 299322.

Mukhopadhyay, M. and Krishna, M.B.R., 1995, Gravity anomalies

and deep structure of Ninetyeast Ridge north of the equator, East-

ern Indian Ocean A hot spot trace model, Marine Geophys. Res.

17, 201216.

Page, B.G.N., Bennett, J.D., Cameron, N.R., Bridge, D.M., Jeffery,

D.H., Keats, W. and Thaib, J., 1979, A review of the main struc-

tural and magmatic features of northern Sumatra, J. Geol. Soc.

London 136, 569579.Rajendran, K. and Gupta, H.K., 1989, Seismicity and tectonic stress

field of a part of the Burma-Andaman-Nicobar arc, Bull. Seis.

Soc. Amer. 79, 9891005.

Ritsema, A.R., 1956, The mechanism in the focus of 28 southeast

Asian earthquakes, Lem. Metero. Dan Geofisik. 50, 172.

Ritsema, A.R. and Veldkamp, J., 1960, Fault plane mechanism of

southeast Asian earthquakes, Med. En Verhandelingen 76, 63

85.

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Geometry of Burmese- Andaman Plate 2003

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