Seismic Cluster Analysis From Burma-Andaman- West Sunda Arc

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Seismic cluster analysis for the Burmese-Andaman and West Sunda Arc:

insight into subduction kinematics and seismic potentialityBasab Mukhopadhyaya; M. Fnaisb; Manoj Mukhopadhyayb; Sujit Dasguptaa

a Geological Survey of India, Central Headquarters, Kolkata, India b Department of Geology &Geophysics, King Saud University, Riyadh, Kingdom of Saudi Arabia

First published on: 13 September 2010

To cite this Article Mukhopadhyay, Basab , Fnais, M. , Mukhopadhyay, Manoj and Dasgupta, Sujit(2010) 'Seismic clusteranalysis for the Burmese-Andaman and West Sunda Arc: insight into subduction kinematics and seismic potentiality',Geomatics, Natural Hazards and Risk,, First published on: 13 September 2010 (iFirst)

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Seismic cluster analysis for the Burmese–Andaman and West Sunda Arc:

insight into subduction kinematics and seismic potentiality

BASAB MUKHOPADHYAY*{, M. FNAIS{, MANOJ MUKHOPADHYAY{

and SUJIT DASGUPTA{

{Geological Survey of India, Central Headquarters, Kolkata 700016, India

{Department of Geology & Geophysics, King Saud University, PO Box 2455,

Riyadh 11451, Kingdom of Saudi Arabia

(Received 13 April 2010; in final form 3 May 2010)

The Burmese–Andaman Arc System (BAAS) and the West Sunda Arc (WSA) in

NE Indian Ocean are well known for their high seismic hazard and tsunami

potentiality. Seismicity is caused by eastward subduction of the Indian plate to

intermediate focal depths below the BAAS, but the penetration depth goes even

deeper to about 500 km below the WSA. The seismicity map and its correlation to

crustal and mantle faults for this extensive plate margin are presented. This is

achieved by using frequency–magnitude relationship to select larger (mb 5.0)

and comparatively well-recorded events from the available earthquake catalogue

that span for a period of little more than a century (1906–2008). Barely 14% of

the events qualify the treatment, and the events so selected are subjected to cluster

analysis using a statistical function ‘point density’. The clusters found for the arc

demonstrate significant relationship to subduction geometry in their respectiveareas; 11 out of a total of 13 clusters commonly originate below the fore arc.

Earthquakes within the individual clusters have linear fractal geometry consistent

with the traces of seismogenic surfaces that actually produce them. Correlation of

clusters to seismologic depth sections and the composite results derived from 518

CMT solutions of earthquakes establish a close spatial relationship between the

shape and orientation of the clusters with stress axes and regional tectonics. This

provides a three-dimensional perspective on the stress distribution within the

respective clustered seismic zones. Seismic potentiality for five most conspicuous

clusters is also inferred.

1. Introduction

The Burmese–Andaman Arc System (BAAS) and West Sunda Arc (WSA) together

constitute a subducting plate margin in the NE Indian Ocean, nearly 2800 km in

length, that serves as the tectonic link between the Western Pacific Arc System with

the Himalayas. BAAS and the WSA have the following tectonic domains: (1) a

northernmost segment of BAAS in Burma where a subduction zone is clearly

discernible in a land environment delimited by Eastern Boundary Thrust (EBT); (2)

further west, the trench zones of Andaman and WSA where the Indian plate

*Corresponding author. Email: [email protected]

Geomatics, Natural Hazards and Risk

2010, 1–32, iFirst Article

Geomatics, Natural Hazards and RiskISSN 1947-5705 Print/ISSN 1947-5713 online ª 2010 Taylor & Francis

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subducts; (3) outer sedimentary ridge of Andaman–Nicobar–Nias Islands in between

trench and the arc, followed by a continuous volcanic arc from Sumatra to Burma

with active Barren Island volcano and dormant Narcondam in the central part and

many dormant volcanoes in Burma; (4) Andaman back-arc spreading ridge (ASR)

underlying the Andaman Sea between Alcock Rise (AR) and Sewell Rise (SR)

relating to the oblique convergence of the Indian plate at the Asian continentalmargin in the east; actual spreading occurred through several short leaky-

transforms, producing the ‘pull-apart’ Andaman basin in southern half of the

BAAS; (5) further south is the intense seismic zone of the WSA with volcanism in

Sumatra (figure 1). Sixteen hinge faults (figure 1) across the trend of the arc with

Figure 1. Tectonic domains in the Burmese–Andaman Arc System (BAAS) and the WestSunda Arc (WSA) in NE Indian Ocean. AR: Alcock Rise; ASR: Andaman Spreading Ridge;

B: Barren Island volcano; BS: Belt of Shuppen; DF: Dauki Fault; EBT: Eastern BoundaryThrust; MR: Margui Ridge; N: Narcondam volcano; SR: Sewell Rise. The locations of twogreat earthquakes from 2004 and 2005 are shown.

2 B. Mukhopadhyay et al.



fixed western end marked by our earlier study (Dasgupta et al. 2003) have delimited

the entire study area into several blocks of individual seismic characters.

The arc is unique in several ways. First is the intense seismic and tsunami hazard

potentiality of the arc; the last example was the 2004 Sumatra earthquake.

Seismically, the arc is not equally active throughout; a stretch of nearly 800 km

between coastal Burma and the Gulf of Martaban is seismically almost passive.Second, 40% of the arc lies within the Burmese mainland where the Indian plate

currently subducts to almost 200 km depth. The Neogene tectonics in central Burma

is controlled by the India-Indochina oblique convergence, which is in response to a

major Miocene regional plate kinematic reorganization (Bertrand and Rangin 2003).

Third, the ASR underlying the Andaman Sea relates to the oblique convergence of

the Indian plate at the Asian continental margin (Curray et al. 1979, Mukhopadhyay

1984). Fourth, the subduction depth in central Andaman extends to about 220 km

depth (Dasgupta et al. 2003), but the subduction proceeds to about 500 km depth

below the WAS further south. Fifth, the arc houses several active to dormant

volcanoes, including the active Barren Island volcano in central Andaman. In thepresent study, our aim is to search for spatial seismic clusters in the intense seismic

zones of the BAAS and WSA, in order to gain an understanding on the relationship

of potential seismic clusters to structural elements of the subduction zone. In spite of

more than 100 years of instrumental recording in the region (the first seismological

observatory in SE Asia was established in Calcutta as early as 1897), a major

limitation in the study is the non-uniform status of seismic monitoring for the arc as

a whole, in particular, for the lower-magnitude shocks. This is an outcome of the

typical geometric orientation of the arc, most of which lies offshore, whose remote

islands locate far off from the mainland of SE Asia. These factors introduce

inconsistency and incompleteness in any earthquake catalogue, resulting in inherent

uncertainty in the search for ‘long-term earthquake clustering’ (Kagan and Jackson

1991), no matter what methodology is adopted. Notwithstanding these limitations,

we present here an analysis of seismic clusters for the BAAS and WSA, and also

investigate their respective stress characters and seismic potentiality.

2. Seismic data treatment and analysis

It is known that similar events occurring close together in space produce spatial

clusters; particularly, moderate to large magnitude earthquakes often occur in

spatial seismic clusters. A seismic cluster is suspected in a region if it consists of

multiple events with a magnitude greater than a threshold value originating withinan acceptable time period. Any statistical treatment for cluster analysis essentially

therefore depends on the completeness of the earthquake catalogue (Ansari et al.

2009). The earthquake catalogue used in the present study (source: ISS, ISC and

NEIC-USGS) for the BAAS and WSA consists of a total of 13,057 earthquake

records for the period of 1906–2008 covering a rather wide range of magnitude (2.7

to 8.6) with focal depths extending to as deep as 500 km. Both single and cumulative

earthquake frequency curves are constructed using the catalogue data. The b-value

of earthquakes with mb 4.0 calculated by the regression method is 1.0382

(figure 2(a)). Only 1752 events of magnitude 5.0 and above in the catalogue actually

qualify for cluster analysis. Of these, barely 47 seismic events occurred prior to 1964,with a magnitude-frequency break-up as follows: 11 events in the magnitude range of

5.0–5.5, 20 events in the magnitude range 5.6–6.0 and 16 events of magnitude

Seismic cluster analysis for the Burmese–Andaman and West Sunda Arc 3



exceeding 6.0. Earthquake frequency in the lower-magnitude range (mb¼ 5.0–5.5)

shows a clear increase postdating 1964 (table 1) as a consequence of an increase in

the number of monitoring stations. The selected events exhibit a smoother

cumulative frequency curve (figure 2(b)) with a b-value of 0.9731 calculated by the

regression method; this is similar to the global average of b-value of 1.0. Table 1

earthquakes, classified according to their magnitude, are superposed on a generalizedtectonic map for the BAAS and WSA (figure 3).

Though several visual clusters are apparent on the map (figure 3), the following

spatial statistical functionalities are applied on the dataset in table 1 to constrain

their configuration and extents:

Figure 2. Frequency magnitude relationships for: (a) 13,057 events of magnitude mb¼ 2.7 andabove as listed in catalogue, and (b) 1752 events of mb¼5.0 and above. The respective a are b

values are given.

Table 1. Earthquakes for the Andaman–Burmese arc classified according to their magnitudeand frequency (used for cluster analysis).

Earthquake magnitude (mb) range Number

5.0–5.5 143045.5–6.0 24146.0–6.5 6046.5–7.0 10

47.0 11

Total 1752




Figure 3. Epicentral map for the Burmese–Andaman and West Sunda Arc, period: 1906–2008.AR: Alcock Rise; ASR: Andaman Spreading Ridge; BS: Belt of Shuppen; DF: Dauki Fault;

EBT: Eastern Boundary Thrust; RF: Renong Fault; SF: Semangko Fault; SR: Sewell Rise; SSF:Shan-Sagaing Fault; VA: Volcanic Arc; WAF: WEST Andaman Fault; WST: West SundraTrench. Tectonic features are adopted after Curray et al. (1982) and Dasgupta et al. (2003).




. A point density function is applied to constrain the extent of the clusters. Point

density is a classical spatial statistical tool to identify areas where data points

are concentrated more or vice versa. To calculate the point density, the

distance between the adjacent earthquakes is measured, and a mean distance

(*8 km) is calculated. Half of the mean distance (i.e. 4 km) is taken as the

radius of the circular neighbourhood. The point density is then calculated asthe total number of earthquake epicentral points that fall within a circular

neighbourhood with a specific radius (in this case 4 km) divided by the area of

the neighbourhood. A factor resulting from the size of the earthquake is also

introduced for deriving the point density value, e.g. 6 points are counted

instead 1 for an earthquake of magnitude 6 in the selected neighbourhood.

This is done to offer more weight to larger earthquakes in the calculation. The

measurement is then carried out in an overlapping grid pattern where the

centre of the circle has been moved across the map (both along latitude and

longitude) by a sliding distance of 4 km. The calculated point density value is

stored in a grid point at the centre of the circle. The resulting values obtainedby this sliding grid process have a mean (M) of 0.0027 and standard deviation

(SD) of 0.008772. The areas with anomalous point density (value 4 (Mþ 2

SD), i.e. 0.020244) have been marked as zones of spatial clusters and shown as

closed grey polygons (figure 4). This process identifies 13 numbers of spatial

clusters of variable sizes with numbers C1 to C13 across the entire study area.

. Comparative statistics between clustered and non-clustered events presented in

figure 5 show that almost half the population (table 1) actually originates

within the cluster domains. The figure also illustrates their respective single and

cumulative frequency–magnitude relationships. The ‘b’ value calculated by the

regression method for the cluster zone is somewhat lower (0.8195) than that for

the non-cluster zone (0.9967). However, this apparent increase in b-value is

probably attributable to data inhomogeneity, rather than any true significance

in data coverage for smaller-magnitude shocks.

. The geometric configuration of the clustered and non-clustered earthquakes is

further constrained by fractal analysis to understand the cause–effect

relationship between the occurrence of seismicity and the underlying processes

that produce them. Further, once the fractal or scale-independent nature of

occurrence of seismicity is established, the ‘fractal dimension’ gives the exact

geometric relationship between the earthquakes points and the underlying

causative surfaces. The Box-counting method (Feeder 1988) is used to calculate

the fractal dimensions. The seismicity points are overlain with a grid of squareboxes (pixels), which progressed from the smallest to successively larger boxes

by combining the pixels in an up-scale manner (Cheng 1999). The numbers of

boxes (N n) of size ‘rn’ required to cover data are plotted on a log–log scale as a

function of ‘rn’. To denote the distribution as fractal, ‘N n’ with a characteristic

linear dimension greater than ‘rn’ must satisfy the relation N n¼C /N nD, where

‘C ’ is the proportionality constant, and ‘D’ is the fractal dimension. ‘D’ is

calculated as factor log(N nþ1/N n)/log(rn/rnþ1) (Turcotte 1997). The seismicity

data for both clustered and non-clustered domains (the plots of log (N n) versus

log(1/rn)) give straight lines (right panel in figure 5). These plots indicate that

the seismicity of both cases obeys scale-invariant fractal geometry, but thecausative processes that generate such geometric configuration are different.

The seismicity in a cluster indicates line fractal geometry with fractal




dimension 1.059, whereas the non-clustered seismicity designates a pointfractal geometry with fractal dimension 0.8796. This also implies that the

seismicity points in clusters are arranged to form a linear pattern on the map,

Figure 4. Seismic cluster analysis results for the Burmese–Andaman and West Sunda Arcusing data plotted in figure 3. In all, 13 clusters (C1–C13) are detected; see text for discussion.




analogous to the traces of major seismogenic surfaces represented as a

curvilinear line. Strain release by well-connected seismogenic surfaces at depth

control the orientation of the clusters. On the contrary, such linear geometry is

absent in case of non-clustered earthquakes with fractal dimension (0.8796),

which indicates agglomeration of points. The disposition of such point fractals

is not directly controlled by any seismogenic surface at depth, instead

indicating sporadic strain release.

The visual relationship, thus illustrated in the foregoing section between seismic

clusters and tectonic features, shows their significant distribution largely in the fore-

arc for this extensive plate margin but to a much lesser extent with the ASR.

3. Discussion on cluster characteristics

3.1 Burmese Arc Clusters

Clusters C1, C2 and C3 are recognized for the Burmese Arc (figures 4 and 6(a)); these

are located in north to north-central Burma in the zones of moderate seismicity.

Earthquake parameters of the clusters (earthquake statistics embodied in a cluster,

magnitude and depth ranges, length of the major axis) are given in table 2. Of these,

cluster C2 is the largest with a maximum number of earthquakes; it is elliptical inshape with a strike length of 130 km, trends north-east and is thus sub-parallel to the

local structural orientation of the arc. C1 is a small elliptical cluster in the north of

Figure 5. Frequency–magnitude distribution and fractal plots for the clustered and non-clustered events; see table 1 for their magnitude distribution.




Burma. C3 is a north-trending elliptical cluster located south of C2 in the same

structural trend. While C2 and C3 clusters originate from the subducting Indian

plate (refer to the corresponding depth sections in figures 6(c) and 6(d )), cluster C1 is

from the overriding Burmese plate (figure 6(b)). None of these clusters, however,

shows any association with the Burmese Volcanic Arc, which implies their tectonic

origin, rather than their volcanic affinity.The clusters C1–C3 are separated by lithospheric hinge faults, where subduction

penetrates to depths of 150 km at the location of cluster C1 to 200 km at cluster C3,

Figure 6. (a) Tectonic map with results of CMT solutions for the three clusters, C1–C3, for theBurmese Arc. Digits refer to CMT solutions indexed in table 4. Grey circles are epicentres of earthquakes with mb5. Notice that thrust mechanisms dominate in all three clusters, wherethe P – T axes are orientated NE–SW and NW–SE directions. Seismological depth sections inBAAS: across the clusters (b) C1, (c) C2 and (d ) C3. DF: Dauki Fault; EBT: Eastern BoundaryThrust; SF: Sagaing Fault; VA: Volcanic Arc.

Table 2. Earthquake parameters defining the clusters.

Clusters

Number of

earthquakes (mb 5)

Range of

mb

Depth

range (km)

Length of major

axis (km)

C1 11 5–6.2 9–43 58C2 26 5–6.3 69–152 130C3 10 5.1–5.3 60–126 64C4 12 5–6.2 12–44 60C5 10 5–8 20–60 55C6 30 5–6.6 5–44 121C7 13 5–5.3 15–39 43C8 37 5–5.4 10–36 122C9 225 5–6.1 3–128 324C10 130 5–7.3 21–95 202C11 16 5–6.1 30–68 55

C12 24 5–5.6 22–33 135C13 343 5–8.9 1–105 460




but then diminish to 160 km to the immediate south (Dasgupta et al. 2003). The

average dip of the Benioff zone below the three clusters ranges from 428 to 508 E

(figure 6). Most of the Burmese Arc seismic potentiality is therefore expected at these

locales where the clusters are seen. Note that the remainder of the Burmese Arc for a

distance of 1000 km up to north Andaman (where the next neighbouring clusters C4

and C5 are seen) is a cluster-free region (figure 4), as signified by its rather low levelof seismicity. This is probably an outcome of passive tectonic process where the

hanging lithospheric slab is being dragged northward through the surrounding

lithosphere, as postulated by Le Dain et al. (1984). A somewhat analogous ‘low

seismic’ region and an example of time-dependent earthquake occurrence are known

for the Lower Rhine Embayment (Faenza et al. 2007).

3.2 Andaman Arc Clusters

Seismic clusters C4–C9 are located between central and south parts of the Andaman

Arc (figures 4, 7(a) and 8(a)). As before, the earthquake parameters of the clustersare also indexed in table 2. All of them are shallow depth clusters but contain higher-

magnitude shocks (often exceeding mb¼ 6.0, sometimes as high as mb¼ 8.0).

Clusters C4 and C5 are located over the Andaman fore-arc (figures 7(b) and (c)),

whereas cluster C6 is found at the south Andaman trench (figure 7(d )) (this is similar

to cluster C12 seen for the West Sunda Trench; see below). Clusters C7 and C8 are

correlated with the ASR (figures 7(a), (e) and ( f )). Cluster C9 is the longest of all; it

has a strike length of 324 km, contains maximum number of earthquakes and is

clearly associated with the West Andaman Fault and its southern continuation to the

Figure 7. (a) Tectonic map with results of CMT solutions for clusters C4–C6 below theAndaman fore arc and clusters C7 and C8 below the ASR; see table 4 for results of the CMTsolutions. Composite CMT is in the inset with grey colour circle for P axes and black circle forT axes. Other symbols are as in figure 6. Thrust mechanisms dominate in the Andaman forearc, while normal mechanisms dominate the ASR. Notice that P–T axes are orientated E–W

for the fore arc but N–S for the ASR. Seismological depth sections in BAAS: across theclusters (b) C4, (c) C5, (d ) C6, (e) C7 and ( f ) C8. ASR: Andaman Spreading Ridge; B: BarrenVolcano; N: Narcondam; SR: Sewell Rise; VA: Volcanic Arc; WAF: West Andaman Fault.




Semangko Fault in Sumatra (figures 7(a) and (b)). The penetration depth of the

subducting Indian plate in central and south Andaman extends from 130 to 220 km,

where the dip of the Benioff zone varies from 308 to 458 E (Dasgupta et al. 2003).

Such large variations in both the penetration depth and dip angle of the Benioff zone

are typical of plate kinematics in the central and southern parts of the Andaman arc.

3.3 West Sunda Arc Clusters

Seismic clusters C10–C13 are seen in offshore Sumatra (figures 4 and 9(a)). As

before, the earthquake parameters of the clusters are indexed in table 2. These areshallow to moderate depth clusters with a high seismicity. The configuration for all

four clusters follows the structural outline of the outer sedimentary arc of the Nias

and other islands. Seismically, this presents one of the most destructive regions in the

world, where the 26 December 2004 Sumatra earthquake (M w 9.3) and 2005 Banyak

Island earthquake (M w 8.7) hit. Cluster C13 is the widest of all in this zone

(figure 9(a)) with a strike length of 460 km. Clusters C10 and C13 have earthquakes

belonging to both plates (figures 9(b) and 9(e)), whereas cluster C11 belongs only to

the overriding SE Asian plate (figure 9(c)). Cluster C12 is located below the West

Sunda Trench (figure 9(d )).

The present understanding on subduction kinematics for this complex arc can beimproved only when a comparison of results of clustering analysis with more

homogeneous data sets becomes available with a combination of greater

Figure 8. (a) Cluster 9 skirting the Sewell Seamount, where a nascent rift was describedelsewhere (Mukhopadhyay et al. 2010). Other symbols are as in figure 6. Cluster 9 typicallydemonstrates normal solutions for the rift, whereas strike-slip mechanisms correspond to theregional faults transverse to the rift. T axes are orientated WNW–ESE, suggesting the riftingdirection perpendicular to it. (b) Seismological depth section across C9 and (c) summary plotfor focal mechanism stress axes. SFS: Semangko Fault System; VA: Volcanic Arc; WAF: WestAndaman Fault. The black dotted line in the depth sections traces the top of the subductingplate; the trajectories are from our earlier work (Dasgupta et al. 2003).




instrumental data coverage, in particular, for smaller magnitude earthquakes. A

temporary digital network established on the Andaman Islands by the Geological

Survey of India recorded about 18,000 small aftershocks in about 10 weeks following

the 26 December 2004 Sumatra earthquake; the recorded magnitude is 3.0 and above

(Mishra et al. 2007).

4. Stress distributions and strain partitioning within seismic clusters

The Indian plate moves in a N108 E to N178 E direction between latitude 28 N (958 E

longitude) and 48 N (938 E) at rates of 52 mm and 61 mm per year, respectively,beneath the Sunda Plate (Sieh and Natawidjaja 2000). The motion changes its

orientation to N238 E at 98 N latitude (928 E longitude) at a rate of about 54 mm

per year (DeMets et al. 1990). GPS data also indicate a non-negligible east–west

convergence along the Andaman arc (Paul et al. 2001). Such differential movement

vectors making a small angle to the trench axis in this oblique deformation front are

partitioned between two stress components, one parallel to the trench and the other

perpendicular to it. The trench parallel motion has been consumed along a large-

scale crustal structure with a dominant strike-slip motion (WAF, SFS and Shan-

Sagaing fault) parallel to the trench along fore-arc and also by back-arc spreading in

ASR by leaky transform tectonics. The trench perpendicular component generateslarge-scale thrust-related motions in segments along the fore-arc (Eastern Boundary

Thrust; gap between the Andaman trench and WAF, etc.).

Figure 9. (a) Clusters C10–C13 in offshore Sumatra. This is by far the largest cluster found forthe study area and is also seismically the deadliest. Other symbols are as in figure 6. Mostlythrust mechanisms prevail in the region of all four clusters, where P–T axes are orientatedNE–SW. A summary plot for focal mechanism stress axes is shown in the inset. Seismologicaldepth sections: (b) across C10 and summary plot for focal mechanism stress axes; ( c) acrossC11; (d ) across C12; and (e) seismologic section across cluster C13 of WSA, and summary plotfor focal mechanism stress axes. Note the coupling between the lower and upper plates in theC10 and C13 clusters. OAR: Outer Arc Ridge; SFS: Semangko Fault system; T: Andaman

Trench; VA: Volcanic Arc; WAF: West Andaman Fault.




The stress distribution and strain partitioning along different sectors have been

studied in details on map and in depth sections across the clusters. Tectonic analyses

of clusters are carried out with the help of composite CMT plots generated from 518

well-constrained CMT solutions collected from the HRVD website. The zone from

north Burma to Sumatra has been associated with both positive and negative slips;

the positive slip is accommodated primarily by thrust motion, whereas the negativeslip is by normal fault or oblique slip movements. The pure strike–slip motion is in

between positive and negative slips. The composite CMT plots are constructed for

the clusters (figures 6–9) distributed along the entire length of the Burmese–

Andaman arc to illustrate the variation in stress axes and orientation of causative

fault planes. By composite CMT plot, here we mean that all the P, T axes and pole of

nodal planes of the earthquakes belonging to one cluster/multiple clusters (as the

case may be) are plotted on a stereographic projection for tectonic analysis,

irrespective of their type of fault plane solutions.

4.1 Burmese Arc Clusters: C1–C3

CMT solutions corresponding to all three clusters found for the Burmese Arc are

listed in table 3. This is a thrust-dominated domain, with subordinate strike-slip

movements. Cluster C1 is a shallow focus thrust–strike-slip domain in the overriding

Burma plate; C2 is an intermediate focus (4100 km) thrust-dominated domain with

occasional strike–slip earthquakes along the plate interface/subducting Indian Plate.

Similarly, C3 indicates a predominantly thrust domain in the subducting Indian

Plate. It can also be seen that the strike-slip earthquakes within this predominantly

thrust domain occur along the terminal ends of the elliptical cluster boundary. These

strike slip earthquakes occur due to the movement of adjacent cross-cutting hinge

faults, inferred by Dasgupta et al. (2003). Thus, the three clusters in between EBT

and the volcanic arc indicate earthquake concentration predominantly along the

plate interface or within plate. The CMT solution shows an overall NE–SW

compression at a shallow angle (108) and a NW–SE extension at a moderately high

angle (568) along this sector (table 4). The CMT data clearly show a dominance of

thrust movement in the C2–C3 clusters with subordinate strike-slip motion both

occurring even beyond a depth of 100 km; this was indicated previously by Stork

et al. (2008) in the Burma subduction zone.

The right-lateral strike-slip motion in both the plates is consistent with the

geometry in Burmese arc and surrounding terrains in China where right-lateral

strike-slip movement predominates along N–S to NW–SE planes. The motion of the Indian plate is primarily accommodated by the positive slips associated with

the thrust movement along EBT zones resulting in formation of C2 and C3

clusters. The counter motion on the overriding plate generates the cluster C1. The

residual slip adjustment takes place along the Shan-Sagaing fault that principally

accommodates a sizeable amount of the motion by aseismic slip/creep along its

length up to the Gulf of Martaban and may have contributed to the extension

along ASR.

4.2 Andaman Arc Clusters: C4–C8

Seismic clusters C4, C5 and C6 underlie the Andaman fore arc; they are thrust-

dominated clusters with normal fault events (figure 7(a)). These shallow focus thrust




























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/ 2 0 0 5

8 : 3 8 : 2 2 . 5

0 . 2

0

9 7 . 2

7

1

4 . 5

4 . 8

6 8

3 0 9

1 7

1 6 9

1 4

7 5

1 4 3

3 5

5 9

3 5 9

6 1

1

1 0

C 1 3

T h r u s t

5 0 4

1 3 / 1

/ 2 0 0 6

1 0 : 5 0 : 5 . 5

1 . 2

2

9 7 . 5

8

3

1 . 2

5 . 1

7 1

3 5 8

1 3

1 3 1

1 4

2 2 4

3 3 2

3 4

1 1 5

1 2 3

6 0

7 4

C 1 3

T h r u s t

5 0 5

1 3 / 1

/ 2 0 0 6

2 2 : 4 7 : 3 5 . 8

1 . 3

5

9 7 . 2

8

3

4 . 6

4 . 9

3 3

1 1 8

5 4

3 2 6

1 4

2 1 7

2 6 3

5 7

1 5

1 6 4

7 7

1

4 6

C 1 3

S t r i k e - s l i p

5 0 6

3 1 / 1

/ 2 0 0 6

1 9 : 1 5 : 5 3 . 9

2 . 3

4

9 5 . 9

0

2

3 . 0

5 . 9

6 2

5 5

7

3 1 2

2 7

2 1 9

2 9 3

1 9

7 0

1 3 4

7 2

9 7

C 1 3

T h r u s t

5 0 7

6 / 2 / 2 0 0 6

1 2 : 4 0 : 4 0 . 9

2 . 5

9

9 5 . 8

8

2

9 . 2

4 . 8

9

7 1

2 1

3 3 8

6 7

1 8 2

1 8 5

4 1

7 5 6

3 2 3

5 7

7 1

1 6

C 1 3

N o r m a l

5 0 8

6 / 2 / 2 0 0 6

2 3 : 5 5 : 1 3 . 9

1 . 4

2

9 6 . 8

7

2

5 . 0

5 . 2

6 0

2 3

9

1 2 8

2 8

2 2 3

3 3 6

1 9

1 1 9

1 2 6

7 4

8 1

C 1 3

T h r u s t

5 0 9

1 / 3 / 2 0 0 6

1 4 : 3 6 : 4 . 8

2 . 6

2

9 5 . 4

4

2

6 . 2

5 . 0

6 4

7 6

1

3 4 4

2 6

2 5 4

3 4 1

1 9

8 7

1 6 4

7 1

9 1

C 1 3

T h r u s t

5 1 0

4 / 4 / 2 0 0 6

1 0 : 3 6 : 5 9 . 7

0 . 3

6

9 7 . 2

2

1

2 . 0

5 . 2

2 3

5 3

1 5

3 1 7

6 2

1 9 6

1 7 1

2 6

7 5 2

3 1 1

7 0

7 1

0 6

C 1 3

N o r m a l

5 1 1

2 5 / 4

/ 2 0 0 6

1 8 : 2 6 : 2 1 . 4

1 . 7

8

9 6 . 7

7

1

2 . 0

6 . 4

5 1

5 0

3

3 1 7

3 9

2 2 5

2 9 3

7

6 6

1 3 7

8 4

9 3

C 1 3

T h r u s t

5 1 2

9 / 5 / 2 0 0 6

3 : 2 5 : 2 6 . 2

1 . 0

2

9 7 . 0

5

2

4 . 0

4 . 9

6 7

1 1

1 0

1 2 6

2 0

2 2 0

3 2 8

2 6

1 1 4

1 2 2

6 6

7 9

C 1 3

T h r u s t

5 1 3

2 1 / 1 1 / 2 0 0 7

3 : 3 0 : 1 5

2 . 8

1

9 6 . 1

9

4

1 . 0

4 . 9

6 9

2

1 0

1 1 9

1 8

2 1 2

3 1 8

2 8

1 1 1

1 1 4

6 4

7 9

C 1 3

T h r u s t

5 1 4

2 1 / 1 1 / 2 0 0 7

3 : 3 0 : 1 5

2 . 8

1

9 6 . 1

9

4

1 . 0

4 . 9

6 9

2

1 0

1 1 9

1 8

2 1 2

3 1 8

2 8

1 1 1

1 1 4

6 4

7 9

C 1 3

T h r u s t

5 1 5

2 2 / 1 2 / 2 0 0 7

1 2 : 2 6 : 2 1 . 3

1 . 9

2

9 6 . 5

8

2

5 . 0

6 . 1

5 3

5 5

4

3 2 0

3 7

2 2 7

2 9 5

9

6 5

1 4 1

8 2

9 4

C 1 3

T h r u s t

5 1 6

2 2 / 1

/ 2 0 0 8

1 7 : 1 5 : 1 . 8

0 . 8

7

9 7 . 1

8

2

3 . 0

6 . 2

6 2

4 2

4

1 4 0

2 8

2 3 2

3 3 2

1 7

1 0 3

1 3 8

7 3

8 6

C 1 3

T h r u s t

5 1 7

2 4 / 1

/ 2 0 0 8

1 2 : 4 : 2 0 . 7

0 . 9

2

9 7 . 0

1

2

4 . 5

5 . 3

6 7

3 4

5

1 3 6

2 3

2 2 8

3 2 8

2 3

1 0 3

1 3 4

6 8

8 5

C 1 3

T h r u s t

5 1 8

2 0 / 2

/ 2 0 0 8

8 : 8 : 4 5 . 4

2 . 6

9

9 5 . 9

8

1

4 . 9

7 . 3

5 6

4 2

2

3 0 9

3 4

2 1 8

2 9 9

1 1

8 0

1 3 0

7 9

9 2

C 1 3

T h r u s t




and normal fault events have predominantly been generated by bending of the

subducting Indian Plate and occurred at the leading edge of the subduction zone and

the trench where the Indian plate descends. The concentration of strike-slip events in

the northern extremity of C4 cluster indicates the strike-slip movement along the

adjacent mantle penetrating hinge fault. Clusters C7 and C8 found below the ASR

exhibit predominantly normal and subordinate strike-slip events that illustrate the

basic tectonic pattern under the spreading arc. The strain partitioning in terms of

normal and strike-slip movement demonstrate dyke intrusion, spreading, rift

formation and collapse of rift wall, rift-related volcanism and generation of

earthquake swarms along ASR (Mukhopadhyay and Dasgupta 2008). The

composite CMT plot of C4, C5 and C6 clusters disposed parallel to the Andaman

trench shows thrust movement along N–S-oriented thrust planes dipping *308

easterly (figure 7(a)). Both compression and tensional directions are oriented E–W

where compression prevails at a shallower angle (248), while extension is taken up at

higher angle (648). This situation gets reversed at C7–C8 clusters along ASR where

overall strain partitioning is normal along ENE–WSW-oriented planes. Overallcompression is at a high angle (798) along NW–SE, whereas extension dominates at a

shallower angle (128) along a NNW–SSE direction (table 4). The tectonics along

clusters C7 and C8 is represented by penetration of crustal scale faults inside the hot

mantle along the upper plate, influence of branches of Kerguelen plume, magmatic

dyke intrusion, rifting and spreading that have been taken place for last 4 Ma. The

cluster has a shallow depth connotation up to 30 km, which by itself is suggestive of

the presence of an upwelling hot mantle underneath it.

4.3 South Andaman Sea Cluster C9

This large cluster has almost equal numbers of normal and strike-slip events. The

distribution of both strike-slip and normal fault events in this cluster zone actually

Table 4. Composite CMT solutions for the Burmese–Andaman and West Sunda Arc system(reference figures 6–8).

Cluster no

T axis P axisNodal fault

plane 1Nodal fault

plane 2

Plunge Azimuth Plunge Azimuth Strike

Dipand dipdirection Strike

Dipand dipdirection

Map (refer figs 6a–8a)C4, C5, C6 648 868 N 248 2668 N 88 N 298 E 1908 N 608 WC7, C8 128 3448 N 798 1518 N 738 N 338 SE 2528 N 558 NWC9 168 1098 N 248 68 N 548 N 868 SE 1468 N 628 SWC10, C11,

C12, C13758 468 N 258 2258 N 1328 N 648 SW 3198 N 258 NE

Section (reference figures 8(b) and 8(e))A 688 378 N 228 2298 N 3358 N 268 NE 1358 N 668 SWB 738 328 N 158 2268 N 3258 N 308 E 1318 N 618 SWC 658 468 N 248 2198 N 3038 N 198 NE 1328 N 718 SWD 668 408 N 248 2248 N 3208 N 228 NE 1318 N 698 SW




corresponds to a complex faulting episode: normal faulting for the rift zone and

strike-slip movements for its transgressive regional faults. This cluster is found in the

area where a swarm of events originated within a span of only 6 days between 26 and

31 January 2005, following the occurrence of the great Andaman earthquake (M w9.3) of 26 December 2004. The analysis and supporting evidence suggest that the

earthquakes in this cluster are generated by intruding magmatic dykes along theweak zones in the crust, followed by rifting, spreading and collapse of rift walls

(Mukhopadhyay and Dasgupta 2008). Magma injection in rifted areas commonly

invokes the injection of shallow, vertical, en-echelon dykes extending along a narrow

rift zone, and this injection accounts for the initial strike-slip motion followed by the

collapse of the closely spaced inner rift wall with earthquakes of a normal fault

source mechanism (Hill 1977). Similarly, CMT solutions for 2005 swarm activity

indicate that intrusion of magmatic dyke in the crustal weak zone is documented by

earthquakes showing a strike-slip solution. Subsequent events with a normal fault

mechanism corroborate the rift formation, collapse and its spread. The overall

composite CMT plot (figure 8(c)) indicates a major strike-slip motion with anappreciable normal component along a steep-dipping (868) NE–SW plane. Both

compressional and tensional axes make a shallow angle (24–168) along the N–S and

ESE–WNW directions respectively (table 4). The composite plot along the WAF

shows primarily a normal with right-lateral strike-slip component along a steep

dipping NE–SW trending surface in the overriding upper plate. Based on evidence

from seismology, bathymetry, gravity, time-dependent pore pressure perturbations,

rift-related volcanism and calculations on phases of rifting, we assume that a nascent

rift is in the process of formation at this location (Mukhopadhyay and Dasgupta

2008, Mukhopadhyay et al. 2010). The orientation of the nascent rift is perpen-

dicular to the regional trend of strike-slip faults of WAF and SFS.

4.4 Offshore Sumatra Clusters: C10–C13

This is by far the largest and most active seismic zone in the whole region. This zone

contains the aftershocks of two great earthquakes of recent times (Sumatra–

Andaman earthquake M w 9.3 of 26 December 2004 and Banyak Island earthquake

M w 8.7 of 28 March 2005). The zone (figure 9(a)) is almost exclusively dominated by

earthquakes of thrust mechanisms, related to the underthrusting of the Indian plate

below the Sunda Arc and also from the overriding SE Asian Plate. The overall

orientation of the thrust planes derived from the composite CMT plot is NW–SE

dipping 258 north-easterly (figure 9(a), table 4). P–T axes are orientated NE–SW inclose correspondence to the structural disposition of the arc geometry. The

concentrations of normal fault-related earthquakes in the longitudinal ends of

otherwise thrust-dominated cluster C13 are probably indicative of severe gravity

adjustments following the thrusting events. The association of earthquakes

belonging to both subducting and overriding plates to create the clusters will be

discussed in detail in the following section.

In this section, cluster C10 consists of two clusters A and B (figure 9(b), table 4).

These two clusters belong to the Indian and SE Asian plates. The strain partitioning

and stress distribution is similar in both these clusters depicting consistent thrust

motion along the NW–SE plane (parallel to arc disposition) dipping less than 308north-easterly. Such consistent thrust plane geometries along subducting and

overriding plates is also noticed in the depth section following the largest cluster C13










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Seismic Cluster Analysis From Burma-Andaman- West Sunda Arc

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