Journal of Seismology 7: 155–174, 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. Jana1 1 Geological

Survey of India, 27 J.L. Nehru Road, Calcutta – 700016, India; 2 Indian 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 about every 100–120 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 northeastern 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 typical 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 overall subducting 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 Andaman Sea are largely aseismic, suggesting that sub-

duction of the Indian plate in this region has stopped recently or occurs aseismically, and the hanging lithospheric 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 (Curray et al., 1979; Mukhopadhyay, 1984; Mukhopadhyay 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 (Brunnschweiler, 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

156 Table 1. Summary Statistics on numbers and magnitudes of 2135 earthquakes used in the study Magnitude∗

No. of events

≥8 7.0–7.9 6.0–6.9

1 11 30

5.0–5.9

18 13

4.0–4.9 3.5–3.9

460 1576 26

| | 48 | | | 473 |

Data coverage period+ A A A B A B B C

∗ M for A and M for B & C. s b + A:1897–1962; B:1963–1993; C: 1991–1993.

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 selective earthquakes from an ‘Earthquake Data Base File for 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 – unknown transverse faults which are developed in the downgoing lithosphere. Many moderate to large magnitude earthquakes relate to activity along such faults. To substantiate the deformation pattern in the subducting lithosphere we have also examined the results available from a large number of fault plane solutions.

Analysis of seismicity data a) BAAS Seismicity

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.

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 0–28◦N and longitudes 90–98◦E during the period 1897 to 1993. Out of these we select only 2202 earthquake events to study the seismicity pattern in plan view (Figure 2) while 2135 events with known focal depths were utilised to constrain the Benioff zone in sections (Figure 3). These 67 earthquakes (10 events of mag. 7.0–7.9; 28 events of mag. 6.0–6.9, and 29 events of mag. 5.0–5.9) whose focal depths are uncertain are otherwise well located events from the period

157 1897–1962 and are useful for correlating large earthquakes 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 hypocentral parameters are poorly determined (reported by only a few stations); (b) earthquakes whose focal depths 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 3.5–3.9 that occurred during 1991–1993, to better constrain the Benioff zone at lithospheric levels]. The map area is suitably segmented 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 seismically 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 Mukhopadhyay, 1984). Table 2 summarizes the spatial relationship of the large magnitude earthquakes associated 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 events are 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 relationship 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 seismicity 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. However, 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 events with magnitude ≥7.0 6.0–6.9

A1 A2 A3 B1 C1 D1 D2 E1 E2 E3 F1 G1 F2 H1 H2 H3 H4 H5 H6 I1 I2 J1 J2 J3 J4 K1 K2 L1 L2

2 – 1 1 2 – 2 1 – 1 –

3 1 1 5 1 4 8 – 3 – 1

1 1

– –

– – – 2 – – – – 1 – – 1 – 1 – 1 – 1 2 1

1 1 1 4 2 1 1 2 3 3 2 3 3 1 1 3 6 2 6 1

Tectonic domain

Interplate subduction & forearc

ShanAndaman Sagaing spreading fault ridge and Sumatra fault +

+ + + + + + + + + +

+ +

+ + + + + + + + + + + + + + + + + + + +

158

Figure 2. Seismotectonic map of the Burmese-Andaman Arc System (seismicity data for the period 1897–1993). 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.

Figure 3. Hypocentral depth sections across the Burmese-Andaman Arc System corresponding to 29 blocks (A1 through L2) sketched on Figure 2. Serial numbers in the Benioff zone refer to focal mechanism solutions(see Figure 4a and Table 5). SSF, Shan-Sagaing Fault; TA, Trench Axis; AOAR, Andaman Outer Arc Ridge; N, Narcondam Island; ASR; Andaman-Spreading Ridge; B, Barren Island; OAR, Outer Arc Ridge; RF, Renong Fault. Other abbreviations and symbols as in Figure 2.

159

Figure 3. Continued.

160

Figure 3. Continued.

161

Figure 3. Continued.

162

163 Table 3. Focal depth distribution for large interplate earthquakes of the BAAS Focal depth (km)

No. of earthquakes

Unknown 0–60 61–100 101–150 >150

24 21 12 13 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 (Figure 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 purpose. 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 of 1◦ width and the area occupied by them has demonstrably the similar tectonic trend which is the Burmese fold mountain belt. 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 plotted 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 earthquakes is found to be 52 occurring between blocks K2 and L1. However, between each block of the same block 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 sections 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 dip of the Benioff zone

Penetration depth (km) into the mantle

A1 A2 A3 B1 C1 D1 D2 E1 E2 E3 G1 F1 F2 H1 H2 H3 H4 H5 H6 I1 I2 J1 J2 J3 J4 K1 K2 L1 L2

50◦ 42◦ 45◦ 48◦ 50◦ 48◦ 35◦ 42◦ 32◦ 25◦ 30◦ 22◦ 30◦ 30◦ 30◦ 43◦ 53◦ 35◦ 45◦ 50◦ 45◦ 38◦ 43◦ 40◦ 53◦ 50◦ 36◦ 37◦ 42◦

150 180 200 160 190 190 140 140 110 110 110 70 70 80 90 130 160 170 220 220 180 180 200 240 280 270 230 240

Arc-Trench gap (km)

300 270 290 280 250 220 200 220 220 220 280 280 300 290 280 260 290 280 320 350 330

the block where the hypocenters are plotted according to their depths. The enveloping surface defining the subducting and overriding plates are manually adjusted 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) penetration depth of the subducting lithosphere, (c) the arc-trench gap, (d) the subduction zone geometry underlying the BAAS and (e) the probable contortions created therein due to plate deformations. The

164 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, inclination of the Benioff zone varies from 42–50◦ as subduction reaches down to 200 km depth. This region also houses the most intense seismicity of the entire BAAS. The Benioff zone dip gets gradually more moderate southward. In blocks D2 and E1, the dip of the Benioff zone varies from 35–42◦ with shallow penetration 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 22–32◦) where penetration 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 where the 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 H6–K1) varies again almost in this range (38–50◦) with an important distinction that seismically defined portion of the Indian lithosphere is much thinner below the Nicobar Islands. An inspection of Table 4 data also suggests that intrablock changes in dip angle are more than inter-block variation in the dip of the Bemoff zone. For example, the variation in dip angle between A3 and B1 (interblock), 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 a block 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 examination of the depth sections illustrated on Figure 3 suggests the followings: (a) Average dip of the Benioff zone varies significantly along the length of the BAAS. This has consequently produced a wide ranging configuration 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 layers, 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 continuation into 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 postulated by several hinge-faults whose throw decrease on the up-dip side (Figure 4). They are the discontinuities created on the upper surface of the Benioff 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 Figure 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

165 majority of them, the nodal planes do not corroborate to surficial features or general trend of the arc, rather they help substantiating the presence of the inferred transverse faults slicing the descending 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 threedimensional 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 contour 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 its mid-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 available 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’. Results from the 2-D sections discussed in the preceding section are utilized to constrain the contouring of the structure contour map. The 2D sections demonstrate that a large variation exists in the thickness of the seismically 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. Details of the map preparation and its main results are discussed below.

Figure 4. Structure Contour Map representing top surface of the subducting Indian lithosphere as imaged through the shallow foci earthquake distribution; details are in text. A total of 460 such shallowest 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.

166 a) Structure contour map Judging by the disposition of hypocentral data distribution in the 2-D sections and their respective blocks (Figures 2 and 3), it was found that some 1260 earthquake 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 lithosphere. A total of 460 shallowest hypocenters were thus sorted out and utilized to generate the structure contour map (Figure 4). The limitations in the technique 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 greater hypocentral depths compared with those from adjoining 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 contouring 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 discontinuities 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 workers 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 offset 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 segment between faults f4 and f5 represents another subsided block, on which, locates the Chindwin-Mt. Popa Volcanic 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 located. This part of the subducting lithosphere exhibits flattening of dip of the Benioff zone, thereby restricting 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 surface 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 immediate 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 1991–94 is developed (Dasgupta and Mukhopadhyay, 1997). Variable dip and penetration depth of the descending Indian plate below blocks H3 –H5 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 area to 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.

167 A set of three ENE oriented faults (f11 –f13) between the Little Andaman and Great Nicobar Islands 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 narrower but 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 converge 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 continental side of the BAAS are under the influence of extensional and compressional stress regimes respectively. 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 Frohlich and Apperson, 1992). Out of a total of 148 solutions 87 are from the HRVD catalogue. Of the remaining 61 P-wave solutions, 54 solutions are from Mukhopadhyay and Dasgupta (1988), Dasgupta (1992) and Dasgupta and Mukhopadhyay (1993) (these are carefully selected well constrained solutions with homogeneous distribution of stations from all the quadrants, 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 Solomon, 1985). These Benioff zone focal mechanism solutions are reviewed to study the faulting mechanism and stress pattern that characterise the BurmeseAndaman subducting plate particularly in relation to the geometry of the Benioff zone and to correlate with the lithospheric structural features that have been detected through the present study. Locations for such 88 Benioff zone earthquakes with their solutions are schematically depicted in Figure 5a; another 60 solutions whose nodal planes are obliquely oriented to the trend of the Benioff zone are shown in Figure 5b. Focal 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) characterise 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 (shallow) and 4 (deep) are downdip compressional (DDC) earthquakes. It may be noted that such compressive 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 bending inflexion point in D2. 6 shallow and 3 deeper DDT earthquakes are located in blocks E1, E3, H2 and J1J2, which are devoid of any compressive events. In block K2, one deeper and 4 shallow DDT events locate 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

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.

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.

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 solutions 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 examination 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 shallower section of the Benioff zone where another five shallow foci earthquakes (11–14 & 16) indicate left lateral shear along roughly E-W nodal plane (see also Mukhopadhyay and Dasgupta, 1988). Earthquake 32 is associated with f3 and gives a reverse fault solution along WNW nodal plane parallel to f3 . Similar solution 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 28–31 which indicate right lateral strike slip mechanism along NW trending nodal plane. At least four solutions (46, 50–52; all deeper events) can be correlated 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 reverse 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 solutions that 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 (97–98) 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 mechanism 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 known from 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 suggesting the presence of several transverse faults which dissect the subducting lithosphere into segments that undergo deformation. The top surface of the downgoing slab is imaged through foci distribution of the shallowest earthquake in each unit area of the Benioff zone below the BAAS. Such hypocentral values are next utilized to construct the structure contour map representing the top surface of the seismically active 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

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 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

04 11 08 04 11 07 04 06 03 08 05 12 04 08 08 02 07 07 04 03 01 08 06 03 05 04 12 12 05 05 05 04 08 05 05 07 02 10 05 07 12 07 04 06 10 08 10 02

96.34 96.37 96.06 96.08 96.12 95.37 95.20 95.69 95.71 95.13 94.21 94.73 94.66 94.67 95.15 94.21 95.22 95.31 95.34 95.25 95.26 95.12 94.78 94.62 94.74 92.43 92.85 93.12 93.52 93.53 93.68 94.93 94.93 94.06 94.09 94.86 93.00 93.33 94.20 94.19 94.27 94.49 94.46 93.95 94.28 94.41 94.70 94.70

1970 1972 1969 1985 1983 1970 1969 1964 1964 1988 1987 1971 1989 1983 1988 1965 1979 1964 1981 1992 1990 1983 1971 1984 1979 1989 1984 1991 1973 1984 1991 1992 1979 1970 1975 1973 1986 1977 1980 1986 1975 1973 1993 1964 1966 1987 1969 1978

06 01 29 24 16 29 28 03 27 13 18 29 03 30 06 18 13 12 25 25 09 23 26 05 29 13 30 20 31 06 11 15 11 29 21 04 08 13 20 26 13 27 01 13 22 24 17 23

05 21 10 06 00 10 12 02 04 19 01 22 19 10 00 04 23 20 11 22 18 12 02 21 00 07 23 02 23 15 02 01 20 10 03 21 00 11 13 20 22 20 16 17 03 09 01 23

07 53 02 47 54 16 50 49 30 59 53 27 39 39 36 26 20 15 32 32 51 12 16 26 39 25 33 06 39 19 15 32 32 33 16 04 28 32 19 24 35 23 30 35 03 24 25 18

59.8 45.8 49.6 45.2 11.4 20.4 17.2 17.2 36.1 51.0 51.3 03.5 31.5 27.2 25.5 34.7 08.8 58.8 23.0 34.2 29.2 17.5 36.9 42.6 52.1 33.0 35.0 05.2 52.4 11.3 22.2 11.3 07.9 58.6 18.3 46.2 54.0 09.3 52.2 49.6 44.2 48.6 09.8 58.3 24.4 40.0 11.5 34.0

26.45 26.44 26.35 26.18 26.16 26.02 25.93 25.88 25.82 25.29 25.23 25.17 25.15 25.04 25.13 24.97 24.88 24.88 24.89 24.82 24.74 24.55 24.60 24.52 24.50 24.40 24.66 24.69 24.31 24.22 24.26 24.27 24.20 23.96 23.86 23.60 23.87 23.47 23.72 23.71 23.62 23.27 23.21 23.00 23.04 23.05 23.09 23.08

5.2

5.9 4.8 7.2 4.3 5.0

4.6 5.1 4.9 5.7 5.8 4.5 3.9

5.0 5.4 4.9

4.8

5.0 5.2 5.2 5.3 5.0 6.4 5.0 5.4 5.3 5.0 5.7 5.6 5.3 5.7 6.6 5.4 4.9 5.5 5.7 5.2 6.1 5.2 5.0 5.2 5.2 5.0 5.5 5.3 5.8 5.7 5.0 5.5 5.0 5.1 5.3 5.0 5.2 5.2 4.8 5.2 5.2 5.4 5.3 5.2 5.1 5.1 6.1 5.0

98 93 72 42 139 68 68 121 115 87 55 46 69 64 108 45 108 152 146 106 118 126 74 70 82 29 02 41 1 54 64 116 113 49 51 126 38 61 83 35 62 60 105 60 72 94 124 113

18 19 13 10 3 40 57 50 16 12 25 7 12 9 5 17 17 28 2 22 20 5 14 30 21 43 4 7 8 2 3 4 22 1 43 16 29 37 4 2 40 33 37 31 22 14 63 21

309 307 308 256 51 239 257 288 276 232 27 32 27 23 217 58 308 240 52 216 16 229 280 343 348 265 238 206 48 25 25 214 288 22 196 105 186 354 204 83 314 250 309 278 259 9 276 274

72 71 77 68 44 18 21 40 72 73 6 21 1 33 65 17 72 49 83 59 58 67 72 54 60 47 67 45 14 27 16 66 67 15 13 72 10 27 14 23 50 56 25 24 15 54 25 59

142 135 143 139 144 126 133 102 62 98 294 124 297 118 117 153 114 8 308 85 142 123 100 127 120 97 338 109 139 116 116 114 116 112 95 310 90 107 114 352 145 56 59 24 356 119 120 144

6 4 4 19 45 45 23 3 10 12 64 68 78 55 24 66 4 27 7 21 24 22 12 17 20 6 22 44 73 62 74 24 3 75 44 7 59 41 75 67 6 8 43 49 63 33 9 22

40 38 39 350 318 26 33 194 184 325 192 286 202 279 310 288 217 145 143 315 276 321 6 242 250 1 146 304 287 290 285 306 19 294 350 196 344 223 300 179 48 156 174 144 118 270 26 12

30 32 32 324 177 11 24 14 23 307 67 260 71 155 284 286 44 16 135 272 140 297 350 114 109 291 350 258 274 157 159 281 11 247 331 20 224 145 251 130 5 10 99 63 39 135 22 331

28 26 32 38 58 76 70 84 30 35 68 80 81 60 45 90 28 30 44 30 32 44 26 21 30 6 45 54 86 69 77 45 22 80 70 60 62 41 78 70 9 12 44 50 63 42 70 31

223 220 222 182 286 265 257 171 178 152 163 166 163 255 148 196 215 129 329 144 267 158 190 238 241 181 128 150 183 254 251 145 200 155 225 185 321 228 160 36 230 156 1 329 308 254 228 201

64 64 58 58 62 50 31 6 62 58 77 70 82 74 54 66 62 78 47 70 69 54 56 78 69 88 53 66 74 73 81 54 68 80 48 30 77 84 82 76 84 78 83 86 85 67 22 69

MD MD MD HRV HRV MD MD MD MD HRV HRV MD HRV HRV HRV MD HRV MD HRV HRV HRV HRV MD HRV HRV HRV HRV HRV MD HRV HRV HRV MD MD MD MD HRV HRV MD HRV MD MD HRV MD MD HRV MD HRV

172 Table 5. Continued No Yr

Mo Dt Hr Mn Sec Lat

Long Ms Mb Depth Ppl Paz Tpl Taz Bpl Baz NP1st NP1dp NP2st NP2dp Source (km)

49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96

02 05 04 07 01 11 01 07 04 10 12 02 12 07 05 04 12 12 07 01 03 09 02 11 10 06 02 10 04 08 09 01 09 03 01 02 02 06 12 07 11 07 09 12 09 02 02 O6

94.70 94.56 94.51 94.54 92.40 93.92 93.58 94.26 94.36 94.38 94.47 94.40 94.43 94.62 92.96 93.70 93.82 93.80 93.68 93.69 94.59 94.95 93.99 94.32 94.41 94.83 95.07 94.80 94.85 94.67 93.55 93.12 93.39 93.56 93.59 93.04 93.00 92.50 93.62 93.94 92.87 92.88 92.58 92.99 93.65 92.32 93.53 93.83

1978 1981 1986 1989 1969 1980 1964 1988 1983 1983 1965 1964 1966 1975 1977 1974 1989 1989 1992 1979 1992 1989 1967 1992 1988 1965 1988 1979 1972 1980 1967 1968 1986 1984 1983 1978 1978 1941 1969 1991 1981 1979 1993 1982 1992 1974 1988 1980

03 01 26 15 25 20 22 03 17 21 15 27 15 08 12 05 02 08 08 01 27 24 15 22 23 01 19 03 28 27 06 12 20 22 24 07 07 26 04 10 02 05 30 16 16 16 28 01

23 04 00 00 23 18 15 08 23 08 04 15 02 12 12 03 19 00 10 18 00 10 05 11 11 04 23 11 11 04 07 04 10 05 23 20 12 11 00 09 21 15 17 08 04 01 03 23

46 08 25 09 34 14 58 19 16 44 43 10 08 04 20 46 44 04 09 51 05 55 57 42 43 32 17 35 30 30 30 17 05 36 09 31 30 52 34 49 10 39 04 56 23 51 19 11

42.4 10.0 58.4 14.9 28.4 11.4 43.7 18.6 33.8 47.3 47.4 47.8 03.1 38.0 00.6 29.7 26.8 26.7 47.8 10.9 18.4 20.2 30.5 45.4 09.4 48.5 14.1 14.1 18.1 16.7 10.8 43.1 01.3 37.4 21.7 54.6 40.4 03.0 58.6 31.0 25.5 41.7 48.0 35.3 17.3 10.8 36.2 24.0

23.02 22.94 22.85 22.79 22.98 22.74 22.33 22.07 22.03 22.00 22.00 21.65 21.51 21.42 21.68 21.33 21.21 21.19 21.06 20.89 20.87 20.69 20.33 20.33 20.30 20.13 18.41 18.11 16.99 15.83 14.65 13.27 13.02 12.93 12.91 12.89 12.81 12.50 12.45 12.59 12.18 11.98 11.84 11.70 11.64 11.47 11.07 10.70

5.1 4.1 4.8 4.9 5.4 5.2 5.1 5.2 6.3 5.2 5.1 5.3 5.2 6.0 5.4 5.9 5.4 5.0 4.6 5.2 4.5 5.6 4.7 5.4 4.7 5.3 5.3 5.2 5.4 4.4 5.3 5.1 5.2 4.4 5.3 4.9 5.6 5.3 4.9 5.4 5.5 5.5 4.8 5.0 6.1 5.6 5.6 5.3 5.5 7.7 8.0 5.2 5.0 5.5 5.7 4.5 5.0 4.8 5.3 5.4 5.2 5.2 5.0 4.1 4.9

92 98 116 98 49 30 60 88 100 93 109 91 84 112 39 47 51 47 42 60 97 135 51 69 71 81 66 54 28 29 36 33 56 92 85 17 03 60 93 138 24 45 23 60 149 19 119 138

6 34 21 19 29 30 54 63 49 47 66 35 25 25 10 19 63 51 75 16 57 1 20 58 64 47 56 58 2 5 59 35 25 41 7 9 10 40 65 27 28 14 20 9 14 38 19 2

81 210 324 337 280 85 254 244 229 259 254 258 274 251 172 85 187 199 237 23 242 317 176 197 198 240 250 235 320 27 286 190 266 74 235 68 69 326 254 333 50 332 260 216 271 238 354 28

37 49 59 43 60 57 25 27 31 43 23 55 65 63 15 28 9 19 13 69 33 45 30 23 21 39 33 31 41 84 31 17 62 49 52 81 60 50 25 58 59 60 70 46 74 50 50 45

170 70 95 86 118 239 123 54 95 76 60 73 108 50 79 185 79 83 90 248 65 48 74 63 54 86 52 70 230 234 106 88 114 254 136 230 321 146 74 119 259 216 90 117 62 42 109 120

52 21 21 40 8 12 24 4 24 2 3 2 6 8 72 55 25 32 8 14 1 45 53 21 14 13 8 7 49 3 0 50 12 0 37 3 28 0 0 16 13 27 3 43 7 9 34 45

347 314 226 230 14 348 22 146 349 167 152 166 6 159 295 326 344 340 358 117 334 226 295 324 319 344 148 336 52 117 16 338 1 344 330 338 164 56 344 235 147 70 351 315 179 142 251 296

312 247 87 111 348 208 14 133 236 122 334 0 352 0 216 316 196 213 191 93 159 83 218 187 168 343 114 181 12 114 16 322 332 344 291 162 129 56 344 97 110 30 344 268 11 10 125 154

66 22 30 44 18 19 74 18 26 2 68 10 21 22 72 84 42 38 33 32 12 59 54 29 27 85 14 16 62 40 76 78 23 86 50 36 43 5 70 23 21 38 25 52 32 10 39 58

208 137 217 217 196 345 254 327 345 347 142 166 189 156 125 223 328 328 353 305 334 192 124 317 313 232 329 334 264 299 196 222 185 164 174 336 2 236 166 231 331 263 173 158 175 140 238 264

62 82 69 75 74 76 30 72 81 88 22 80 70 70 87 56 59 71 58 62 78 61 84 71 68 14 78 76 63 50 15 52 71 4 62 54 61 85 20 74 74 64 65 67 60 84 72 62

MD HRV HRV HRV MD HRV MD HRV HRV HRV MD MD MD MD HRV MD HRV HRV HRV HRV HRV HRV MD HRV HRV MD HRV HRV D HRV F DM HRV DM HRV HRV HRV RV DM HRV HRV DM HRV HRV HRV DM HRV HRV

173 Table 5. Continued No Yr

Mo Dt Hr Mn Sec Lat

Long Ms Mb Depth Ppl Paz Tpl Taz Bpl Baz NP1st NP1dp NP2st NP2dp Source (km)

97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145

07 02 04 11 10 05 06 03 12 06 09 09 06 04 10 02 03 06 08 07 02 08 12 11 08 08 02 07 04 01 09 08 04 09 07 04 01 06 11 12 05 08 08 07 01 03 12 09 11

92.59 92.48 92.86 92.98 93.61 92.91 92.46 92.89 93.53 93.51 93.03 93.21 94.56 91.32 91.20 92.59 93.45 94.43 94.31 94.65 92.23 94.65 95.13 93.90 94.50 95.29 94.77 94.68 94.72 94.96 95.37 95.00 94.46 95.05 95.66 95.03 95.87 94.84 95.19 95.44 95.74 95.41 95.74 95.96 96.13 95.80 95.91 94.20 94.61

1973 1972 1976 1971 1968 1970 1971 1992 1992 1986 1964 1983 1986 1973 1979 1980 1991 1979 1976 1971 1989 1993 1986 1973 1937 1984 1982 1983 1983 1983 1981 1936 1988 1983 1989 1986 1974 1987 1976 1978 1977 1991 1967 1991 1990 1983 1977 1979 1969

09 22 21 05 06 06 05 17 08 02 15 17 19 07 16 19 08 08 05 17 10 28 07 09 04 11 13 02 04 30 10 23 03 17 20 29 01 10 03 18 25 06 21 23 22 16 03 29 21

16 18 19 22 07 15 01 02 07 17 15 04 18 03 22 17 01 20 13 05 16 20 05 23 23 11 19 09 02 01 14 21 14 05 06 13 14 16 09 08 14 02 07 13 17 09 13 18 02

19 43 09 11 42 21 38 14 08 51 29 40 12 00 51 27 42 36 37 32 59 14 40 26 35 56 56 34 51 26 17 12 27 56 27 59 07 03 54 26 55 17 33 25 26 13 41 37 05

46.8 42.0 59.6 15.5 25.2 55.1 10.9 48.8 42.1 56.1 32.2 36.8 30.4 58.8 23.0 36.5 00.5 40.6 16.7 42.9 15.0 43.0 39.5 39.0 18.0 51.7 13.2 05.1 34.5 06.2 44.2 13.0 10.0 56.7 26.4 22.1 40.1 55.7 38.2 20.1 45.0 33.0 00.6 48.9 12.3 11.9 20.9 12.5 35.3

10.66 10.42 10.29 10.18 09.98 09.81 09.38 09.13 09.27 09.12 08.90 07.94 07.81 07.00 06.37 06.73 07.27 07.30 07.00 06.98 06.25 06.50 06.81 05.98 06.00 06.05 05.75 05.71 05.71 05.47 05.50 05.00 04.71 04.76 05.07 04.48 04.64 04.18 04.22 04.20 04.21 03.86 03.72 03.81 03.92 03.51 03.52 01.16 01.94

4.5

5.2 5.5 4.5 5.0

5.4

5.6 5.4 5.3 5.7 5.0 5.3 5.3 4.9 5.9 5.6 6.3 5.2 5.8 5.8 5.2 5.1 5.2 5.1 5.7 5.6 5.3 5.7 5.3 5.1

6.0

7.1 5.8

5.0 5.3 5.1 5.5

5.9 4.6 6.8

5.3 5.1 5.6 6.5 5.2 5.1 7.3 5.8 5.7 5.8 5.2 5.1 5.5 5.5 5.3 5.7 5.9 6.1 5.8 6.0 5.3 5.8 6.2 6.4

44 4 52 55 124 32 25 71 94 101 89 54 191 39 38 32 54 120 112 138 39 122 204 44 120 136 77 78 95 67 97 40 32 57 93 39 74 61 54 66 67 21 40 52 59 22 21 30 20

12 27 20 25 39 55 55 13 12 16 17 58 60 10 15 19 31 32 15 8 16 5 8 33 19 21 4 8 1 59 8 5 14 45 8 17 45 45 18 61 54 2 33 52 18 48 30 2 2

182 336 287 284 206 240 240 183 3 346 204 186 66 350 331 254 180 336 296 292 154 159 230 38 113 100 320 325 143 173 254 170 23 157 118 11 189 150 90 191 237 33 203 158 223 187 302 149 147

22 10 64 64 23 35 35 14 33 46 52 11 23 1 5 56 31 58 56 54 11 50 8 51 9 52 21 61 61 19 50 42 63 17 43 49 39 16 42 22 32 87 57 36 72 12 24 0 20

86 70 157 120 98 60 60 90 101 94 88 77 204 78 239 134 73 163 173 192 247 255 139 248 205 222 52 69 51 47 155 78 143 49 20 261 46 44 198 53 28 277 23 359 44 292 47 239 236

63 61 18 7 43 0 0 71 55 39 33 29 18 82 74 27 43 3 30 35 70 40 79 19 69 30 68 28 29 23 39 47 22 40 46 36 19 41 43 17 14 3 0 10 0 40 50 88 69

305 182 29 19 343 330 333 314 256 243 308 341 302 168 131 354 309 68 27 29 10 65 5 139 322 356 219 230 234 308 352 264 287 304 216 114 300 300 344 316 127 123 293 261 313 36 170 332 56

135 22 355 0 335 330 333 227 137 118 141 199 263 34 14 308 219 55 340 350 291 283 274 321 158 348 94 83 207 169 309 116 141 182 169 61 299 176 330 173 78 120 293 132 313 342 87 284 282

86 80 30 20 81 80 80 71 58 45 70 43 27 84 76 35 43 13 40 48 71 53 79 82 83 74 72 44 51 32 50 65 36 45 54 43 86 46 74 27 19 44 12 13 27 45 50 88 74

223 116 215 200 234 150 153 317 235 227 258 324 128 124 106 186 309 249 220 230 200 38 4 80 249 232 188 211 78 298 194 224 275 289 62 308 200 284 222 309 310 306 113 260 133 232 354 194 15

65 64 66 70 42 10 10 89 76 72 38 62 70 86 83 69 90 77 70 62 87 61 90 20 70 34 78 59 53 68 64 59 63 73 68 71 20 72 44 70 78 47 78 82 63 70 87 89 76

DM DM DM DM DM DM DM HRV HRV HRV F HRV HRV BS HRV HRV HRV HRV DM DM HRV HRV HRV DM RV DM HRV HRV HRV HRV DM RV HRV HRV HRV HRV DM HRV DM HRV HRV HRV F HRV HRV DM HRV HRV DM

174 Table 5. Continued No Yr

Mo Dt Hr Mn Sec Lat

Long Ms Mb Depth Ppl Paz Tpl Taz Bpl Baz NP1st NP1dp NP2st NP2dp Source (km)

146 1982 10 31 02 48 11.8 02.93 96.06 5.1 5.5 48 147 1993 09 01 14 03 19.0 02.99 96.14 6.2 5.8 35 148 1984 05 29 04 36 09.7 03.64 97.14 5.7 72

51 145 7 244 38 340 299 36 221 54 43 1 311 306 65 199 24 30 4 299 130

51 9 21

184 132 297

63 81 70

HRV HRV 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 subduction 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 paper and 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 India Programme: 001/SEI/CHQ/GDB/1994–97 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, 72–92. Anon., 1999, Earthquake database for the Indian subcontinent (0–37◦ N/68–98◦ E) from 1897–1995. (database in db3plus maintained in GSI Geodata centre, Calcutta), Geol Surv. India, 133, 2, 24. Bergman, E.A. and Solomon, S.C., 1985, Earthquake source mechanisms from bodywaveform inversion and intraplate tectonics in the northern Indian Ocean, Phys. Earth Planet. Int. 40, 1–23. Bevis, M. and Isacks, B.L., 1984, Hypocentral trend surface analysis: Probing the geometry of Benioff zones, J. Geophys. Res. 89, 6153–6170. Burnnschweiler, R.O., 1974, Indoburman Ranges. In: Spencer, A.M. (ed.), Mesozoic-Cenozoic Orogenic Belts, Spec. Publ. Geol. Soc. London 4, 279–299. 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, 189–198. Curray, J.R., Emmel, F.J., Moore, D.G. and Raitt, R.W., 1982, Structure, tectonics and geological history of the northeastern Indian Ocean, In: Nairn, A.E.M. and Stehli, F.-G. (eds), The Ocean Basins and Margins, vol. 6, Plenum, New York, pp. 399–450.

Dasgupta, S., 1992, Seismotectonics and stress distribution in the Andaman plate, Mem. Geol. Soc. India 23, 319–334. 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 Andaman subduction zone and recent volcanism, J. Geol. Soc. India 49, 513–521. Dasgupta, S., Mukhopadhyay, M. and Nandy, D.R., 1990, Magmatism and tectonic setting in the Burmese-Andaman arc, Indian J. Geol. 62, 117–141. Eguchi, T., Uyeda, S. and Maki, T., 1979, Seismotectonics and tectonic history of the Andaman Sea, Tectonophysics 57, 35–51. Fitch, T.J., 1970, Earthquake mechanisms in the Himalayan, Burmese and Andaman regions and continental tectonics in central Asia, J. Geophy. Res. 75, 2699–2709. Fitch, T.J., 1972, Plate convergence, transcurrent faults, and internal deformation adjacent to southeast Asia and western Pacific, J. Geophys. Res. 77, 4432–4460. Frohlich, C. and Apperson, K.D., 1992, Earthquake focal mechanism, moment tensors and the consistency of seismic activity near plate boundaries, Tectonics 11, 279–296. 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, 453–472. Mitchell, A.H.G. and McKerrow, W.S., 1975, Analogous evolution of the Burma orogen and the Scottish Caledonides, Geol. Soc. Am. Bull. 86, 305–315. Mukhopadhyay, M., 1988, Gravity anomalies and deep structure of the Andaman arc, Marine Geophys. Res. 9, 197–210. Mukhopadhyay, M. and Dasgupta, S., 1988, Deep structure and tectonics of the Burmese arc: Constraints from earthquake and gravity data, Tectonophysics 149, 299–322. Mukhopadhyay, M. and Krishna, M.B.R., 1995, Gravity anomalies and deep structure of Ninetyeast Ridge north of the equator, Eastern Indian Ocean – A hot spot trace model, Marine Geophys. Res. 17, 201–216. 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 structural and magmatic features of northern Sumatra, J. Geol. Soc. London 136, 569–579. 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, 989–1005. Ritsema, A.R., 1956, The mechanism in the focus of 28 southeast Asian earthquakes, Lem. Metero. Dan Geofisik. 50, 1–72. Ritsema, A.R. and Veldkamp, J., 1960, Fault plane mechanism of southeast Asian earthquakes, Med. En Verhandelingen 76, 63– 85.

The geometry of the Burmese-Andaman subducting ...

ive earthquakes from an 'Earthquake Data Base File .... Seismotectonic map of the Burmese-Andaman Arc System (seismicity data for the period 1897–1993).

330KB Sizes 2 Downloads 34 Views

Recommend Documents

The geometry of the group of symplectic diffeomorphisms
Oct 15, 2007 - lems which come from the familiar finite dimensional geometry in the ...... It was Gromov's great insight [G1] that one can generalize some.

Teleseismic imaging of subducting lithosphere and ...
grid-search stacking of RRFs at each station [21]. For Ppps .... sess the resolution power of our data set, we thus performed ... scraped off the Tarim craton [17].

[PDF] Download The Geometry of Physics: An Introduction All Ebook ...
[PDF] Download The Geometry of Physics: An Introduction All ... ebooks Free Computer Science ebooks download computer science online ... undergraduate students of physics, engineering or mathematics as a course text or for self study.

eBook The Fractal Geometry of Nature PDF Free ...
... once and read it on your Kindle device PC phones or tablets Use features like fractal geometry of ... Download Ebook the fractal geometry of nature in PDF Format also available for mobile reader ... of art The Fractal Geometry of Nature pdf downl

Understanding the Geometry of Workspace Obstacles in Motion ...
Top (left): The Apollo dual-arm manipulation platform at the Max Planck Institute for ... International Conference on Robotics and Automation, May 2011.