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Significant trends related to the slab seismicity and tectonics in the Burmese arc region from Harvard CMT solutions
PHYSICS OFTHEEARTH AND PLANETARY INTERIORS
ELSEVIER
Physics of the Earth and Planetary Interiors 90 (1995) 75-80
Significant trends related to the slab seismicity and tectonics in the Burmese arc region from Harvard CMT solutions M. Ravi Kumar *, N. Purnachandra Rao National GeophysicalResearch Institute, Hyderabad 500 007, India Received 4 August 1994; accepted 9 December 1994
Abstract
A careful examination of 50 centroid-moment tensor (CMT) earthquake focal mechanisms in an east-west depth section in the Burmese arc region reveals some interesting trends. The P-axis azimuths of most of the mechanisms in the region of the east-dipping Indian lithospheric slab are, on average, oriented NNE. This indicates that the principal compressive stress direction in the Burmese arc region is commensurate with the direction of convergence of the Indian plate with respect to the Eurasian plate, rather than with respect to the Burmese plate. Another interesting observation is that the strike-slip and thrust type focal mechanisms along the slab do not coexist, but are distinctly segregated, the predominantly strike-slip type occurring in the upper half of the slab down to a depth of about 90 km, and the thrust type in the lower half, below that depth level. The former possibly indicates shearing of the Indian plate past the Burmese plate, along the zone of contact, which is inferred down to a depth of 90 km. The latter is suggestive of a NNE-directed stress on the lower portion of the subducted slab, as it is dragged against the asthenosphere by the Indian plate.
1. Introduction
The presence of the subducted slab of the Indian plate, dipping east along the Burmese arc, is well known and has been inferred from seismicity, gravity and seismic velocity studies (Chandra, 1975; Verma et al., 1976; Mukhopadhyay and Das Gupta, 1988; Ni et al., 1989; Gupta et al., 1990). It is interesting, however, that focal mechanism studies in this region indicate P-axis azimuths trending nearly n o r t h - s o u t h rather than perpendicular to the arc (LeDain et al., 1984; Ni et al.,
Corresponding author.
1989; Chen and Molnar, 1990). It has been suggested by LeDain et al. (1984) that there has been a major change in the tectonic pattern in this region, from subduction to collision and strike-slip motion during the late Cenozoic. Hence, India seems to be currently dragging its east-dipping lithospheric slab beneath the Indoburman ranges, northward through the asthenosphere. To decipher the style of current deformation in the Burmese arc region, we examine 50 Harvard centroid-moment tensor (CMT) solutions of earthquakes from 1977 to 1992 in this region (also reported regularly in Physics o f the Earth and Planetary Interiors, eg. Dziewonski et al.,
0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0031-9201(94)03012-X
76
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
1987). For a better understanding of the seismotectonics of the subducted Indian lithospheric slab in this region, an examination of the focal mechanisms and their P-axis trends in a depth section, rather than on a map, seems to be useful. The present data set appears fairly extensive and consistent. The P-axis trends, in general, are similar to those reported by the previous workers, mentioned above. Additionally, some very interesting trends are observed on the slab, which are discussed along with a plausible interpretation.
2. Significant trends inferred from focal mechanisms The focal mechanism solutions from the CMT data set in the Burmese arc region used in this study are listed in Table 1, and are indicated, along with the major tectonic features in Fig. 1. The data are presented using different symbols for thrust, strike-slip and normal type mechanisms, as this approach clearly brings out important seismicity trends (Kumar et al., 1995). It can
9¢
95 °
~oCE
Fig. 1. Seismo-tectonicmap of the Burmese arc region. The earthquakes (CMT data) predominantly thrust (o), strike-slip (e) and normal type ([]) mechanisms.
be seen that the number of thrust (19) and strike-slip (24) solutions is comparable: Interestingly, all the normal (7) solutions are clustered in the southern part of the arc, except for one in the east. In Fig. 2(a), the focal mechanisms in the Burmese arc region are plotted in an east-west depth section, as it provides a better perspective of their distribution along the subducted slab. It may be noted that although the locations of the solutions are as in a depth section, the 'beach-ball' representation, as would be seen in a map view, is retained. Fig. 2(a) shows that all the solutions, except those further east, form a dipping trend indicative of the subducted slab, and also shows some very interesting trends. For example, there is a clear segregation of strike-slip and thrust type solutions along the dipping slab, with the predominantly strike-slip solutions occurring exclusively in the upper half and the thrust solutions in the lower. Normal type solutions also are seen on the slab (Fig. 2(b)), down to a depth of about 90 km, like the strike-slip solutions. Shallow thrust and strike-slip solutions, occurring further east of the slab, show characteristics distinct from those of the solutions on the slab. This can be seen in Fig. 3, which shows P-axis azimuths plotted in a depth section. It may be noted that, in Fig. 3, the definition of azimuth as would be seen in a map view is retained, although the centre of each line corresponds to the position in depth of the solutions. An important observation is that the P-axis orientations of all the events associated with the slab (about 35), including thrust, strike-slip and normal type solutions, are predominantly NNE. Similar P-axis trends have been reported also by LeDain et al. (1984) and Ni et al. (1989). Solutions further east show a P-axis trend close to ENE, distinct from that seen on the slab. The depth level of 90 km appears significant, as it separates the regime of strike-slip solutions from the thrust solutions on the slab, and confines the normal solutions above it. We infer this depth range in terms of the zone of contact between the Indian and the Burmese lithospheres and propose the following as a plausible interpretation. The active tectonics in the Burmese arc region must be currently governed by the north-
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
77
Table 1 Focal m e c h a n i s m data from the Harvard C M T catalogue for earthquakes in the Burmese arc region (20-28°N, 90-99"E) occurring between 1977 and 1992 Serial no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2O 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 49 50
Date
Lat. (°N)
Long. (°E)
Depth (km)
st1
dl
sll
st2
d2
sl2
Mo
Ex
Ppl
Paz
Tpl
Taz
05/12/77 02/23/78 01/01/79 05/29/79 07/13/79 11/25/79 12/21/79 11/20/80 04/25/81 06/30/81 08/14/81 08/16/81 08/23/83 08/30/83 11/16/83 05/06/84 11/28/84 12/30/84 04/24/85 09/05/85 02/08/86 04/26/86 07/26/86 11/01/86 05/18/87 08/24/87 02/06/88 07/03/88 08/06/88 08/13/88 08/21/88 10/23/88 02/12/89 04/03/89 07/15/89 09/24/89 12/02/89 12/08/89 01/09/90 03/08/90 01/05/91 05/11/91 12/04/91 12/20/91 03/25/92 03/27/92 04/15/92 06/15/92 07/08/92 11/22/92
21.60 23.16 20.51 24.92 25.13 25.21 26.61 22.69 24.99 23.00 25.61 25.53 24.48 25.34 26.58 24.33 26.52 24.75 25.83 25.29 23.79 23.31 23.86 25.53 24.58 23.04 24.05 22.22 25.19 24.94 24.94 20.74 26.18 25.15 22.91 20.22 21.62 21.54 24.42 25.11 23.61 23.42 24.19 24.47 24.18 21.12 23.96 23.95 21.07 20.43
92.77 94.93 93.59 95.05 95.59 96.32 97.78 94.49 95.52 95.45 98.51 97.02 94.69 94.90 96.38 93.42 96.96 92.99 95.97 98.23 93.09 94.92 94.19 96.91 93.94 94.53 91.66 94.36 94.89 95.24 95.89 94.67 96.83 94.81 93.94 94.75 93.89 93.78 94.95 96.61 96.18 93.25 93.83 93.08 95.20 94.52 94.57 96.03 93.51 94.51
40.0 122.2 95.0 108.5 117.1 10.0 10.0 20.0 152.9 10.0 10.0 37.0 147.9 68.9 144.4 61.3 16.0 101.8 56.4 60.7 33.0 129.2 64.9 69.8 75.3 126.5 31.0 86.0 100.5 126.0 93.8 45.6 33.0 85.3 139.4 144.0 44.6 15.0 129.6 56.5 20.9 73.6 70.0 100.8 119.7 86.2 142.5 22.9 79.0 71.4
216 331 93 109 44 357 119 208 135 7 153 298 297 161 177 157 311 352 324 14 224 87 130 28 67 135 239 133 284 307 67 168 295 71 111 83 196 213 140 32 2 159 245 258 272 159 281 8 191 187
72 31 32 30 28 79 42 19 44 40 46 66 44 65 58 69 49 46 38 65 62 30 72 45 68 42 76 18 45 35 49 27 60 81 44 59 42 38 32 73 68 77 68 54 30 12 45 69 33 29
3 44 62 134 99 - 175 72 131 80 142 - 141 - 7 58 159 147 161 17 123 58 177 -15 136 15 132 - 14 144 9 - 104 55 69 28 -58 24 -8 159 145 -50 -31 139 173 166 171 11 30 43 -85 56 - 173 -75 -43
125 201 305 241 215 266 323 345 329 128 33 30 158 260 286 254 210 129 182 105 321 217 36 157 163 254 147 327 148 152 318 313 193 163 217 192 328 328 267 124 97 251 151 150 143 334 145 275 353 317
87 69 62 69 62 85 51 76 47 67 63 83 54 71 62 73 77 53 58 87 77 69 76 58 77 67 82 72 54 58 69 68 69 82 75 61 59 71 69 84 77 81 79 66 70 78 54 83 58 71
162 113 106 68 85 - 11 106 78 99 57 -52 - 156 118 26 37 22 138 61 113 25 -152 67 162 56 - 158 54 166 -85 120 104 136 - 105 148 - 171 48 36 - 120 - 124 64 18 23 13 157 140 113 -91 120 -21 - 100 - 112
9.91 5.37 1.12 1.31 3.88 9.16 1.13 7.79 3.89 2.76 9.00 4.95 6.76 4.24 13.47 11.90 3.78 1.33 8.75 2.15 1.46 4.75 1.74 6.96 2.76 9.76 6.70 4.45 8.90 4.56 6.78 5.18 9.53 2.73 2.54 11.97 1.14 1.66 3.05 7.68 3.11 1.43 2.00 11.95 6.53 2.76 4.09 3.14 8.00 7.61
24 23 24 24 23 23 24 23 24 23 23 23 23 24 23 24 24 25 23 24 24 23 24 23 25 23 24 23 26 23 23 23 23 24 24 23 24 24 25 23 26 24 24 23 23 24 24 25 23 23
10 21 16 21 17 11 4 30 2 15 55 22 5 4 3 2 18 4 10 16 29 21 2 7 25 14 4 63 5 12 12 64 6 12 19 1 63 51 20 8 6 3 8 7 22 57 4 20 75 58
172 274 23 348 308 221 42 85 52 242 352 257 229 29 51 25 267 239 256 237 186 324 83 270 27 9 194 244 217 232 16 198 246 27 337 317 187 199 16 256 228 25 200 206 216 242 214 229 237 197
15 59 69 60 72 5 77 57 83 55 10 ;I2 67 32 44 28 38 67 68 19 10 59 23 61 6 54 ;16 27 65 73 46 21 37 1 43 45 9 19 5;8 17 25 16 23 45 5;9 213 66 10 13 23
79 144 248 120 114 312 292 239 308 355 97 162 127 122 I44 116 162 339 139 333 90 95 352 13 294 119 103 54 117 98 274 54 151 297 86 48 79 83 142 349 321 116 106 109 85 65 114 323 90 63
(stl, dl, sll) and (st2, d2, s12) are the strike, dip, slip of the two planes, respectively. M o × 10 E× is the scalar seismic moment. (Ppl, Paz) and (Tpl, Taz) are the plunge and azimuth of the P and T axes, respectively.
78
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
northeastward motion of the Indian plate, as indicated by the consistent NNE trends of the P axes all along the slab. The Indian lithospheric slab, therefore, seems to be dragged NNE past (a)
the Burmese plate, resulting in large-scale shearing and strike-slip faulting all along the contact zone. This explains the presence of strike-slip solutions along the slab (Fig. 2(a)), down to a
95
~'"-\
~8
IO 0 E
~oJ,i&'-,odl--,, 17 ~-.~7
50--
100 --
150 --
22
9 55'~.-~ /
(kin)
(b)
sch
15
/
47 " x . . ~ ( " , ~ 13 ~ 3 6
95 I
tO0 E I
370032 28
IOC
(kin)
Fig. 2. (a) Thrust and strike-slip type focal mechanisms in an e a s t - w e s t depth section across the Burmese arc. It may be noted that although the locations of the solutions are as in a depth section, the 'beach-ball' representation, as would be seen in a m a p view, is retained. T h e dotted lirle indicates the inferred outer line of the subducted Indian slab. (b) Normal type focal mechanisms, as in
(a).
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80 90 °
95 °
79
I00 ° E
5C
100
1.50 (kml
Fig. 3. A plot of P-axis azimuths in an east-west depth section. The symbols have the same meaning as in Fig. 1. The definition of azimuth is as in a map view, and the centres of the lines indicate positions in depth.
depth of about 90 kin, where apparently the contact ends. The lower part of the slab is free of this shearing and is subjected, instead, to compressive stresses in the NNE direction as it penetrates through the asthenosphere. This is evidenced by the presence of thrust solutions exclusively in the lower portion, with P axes oriented NNE. The T axes of the normal solutions in the upper part of the slab are seen to be oriented predominantly in the east-west direction, possibly indicating down-dip extension of the subducted slab, as suggested also by Ni et al. (1989).
3. Conclusions (1) In the Burmese arc region, almost all the focal mechanisms on the subducted lithospheric slab show P-axis azimuths oriented NNE. This implies that the tectonics of the slab is governed mostly by a NNE-directed stress field owing to the Indian plate motion. (2) The focal mechanisms along the slab are distinctly segregated into predominantly strikeslip type, occurring in the upper half of the slab down to a depth of about 90 kin, and thrust type,
occurring exclusively in the lower half, below that depth level. This could be due to shearing of the Indian plate past the Burmese plate in the zone of contact, and compression of the lower portion of the slab against the asthenosphere in the NNE direction.
Acknowledgements We thank Dr. A.M. Dziewonski for providing the CMT data required for this study. We are grateful to Dr. Harsh K. Gupta for constant guidance, useful discussions and a critical review of the manuscript. We thank two anonymous reviewers for their useful comments.
References Chandra, U., 1975. Seismicity, earthquake mechanisms and tectonics of Burma 20-28 deg N. Geophys. J. R. Astron. Soc., 40: 367-381. Chen, W.P. and Molnar, P., 1990. Source parameters of earthquakes and intraplate deformation beneath the Shillong plateau and the northern Indoburman ranges. J. Geophys. Res., 95: 12527-12552.
80
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
Dziewonski, A,M., Ekstrom, G., Franzen, J.E. and Woodhouse, J.H., 1987. Global seismicity of 1977; centroid moment tensor solutions for 471 earthquakes. Phys. Earth Planet. Inter., 45: 11-36. Gupta, H.K., Fleitout, L. and Froidevaux, C., 1990. Lithospheric subduction beneath the Arakan Yoma fold belt: quantitative estimates using gravimetric and seismic data. J. Geol. Soc. India, 35: 235-250. Kumar, M.R., Rao, N.P. and Chalam, S.V., 1995. A seismotectonic study of the Burma and Andaman arc regions using centroid moment tensor data. Tectonophysics, in press. LeDain, A.Y., Tapponier, P. and Molnar, P., 1984. Active
faulting and tectonics of Burma and surrounding regions. J. Geophys. Res., 89: 453-472. Mukhopadhyay, M. and Das Gupta, S., 1988. Deep structure and tectonics of the Burmese arc: constraints from earthquake and gravity data. Tectonophysics, 149: 299-322. Ni, J.F., Speziale, M.G., Bevis, M., Holt, W.E., Wallace, T.C. and Seager, W.R., 1989. Accretionary tectonics of Burma and the three dimensional geometry of the Burma subduction. Geology, 17: 68-71. Verma, R.K,, Mukhopadhyay, M. and Ahluwalia, M.S., 1976. Earthquake mechanisms and tectonic features of northern Burma. Tectonophysics, 32: 387-399.
ELSEVIER
Physics of the Earth and Planetary Interiors 90 (1995) 75-80
Significant trends related to the slab seismicity and tectonics in the Burmese arc region from Harvard CMT solutions M. Ravi Kumar *, N. Purnachandra Rao National GeophysicalResearch Institute, Hyderabad 500 007, India Received 4 August 1994; accepted 9 December 1994
Abstract
A careful examination of 50 centroid-moment tensor (CMT) earthquake focal mechanisms in an east-west depth section in the Burmese arc region reveals some interesting trends. The P-axis azimuths of most of the mechanisms in the region of the east-dipping Indian lithospheric slab are, on average, oriented NNE. This indicates that the principal compressive stress direction in the Burmese arc region is commensurate with the direction of convergence of the Indian plate with respect to the Eurasian plate, rather than with respect to the Burmese plate. Another interesting observation is that the strike-slip and thrust type focal mechanisms along the slab do not coexist, but are distinctly segregated, the predominantly strike-slip type occurring in the upper half of the slab down to a depth of about 90 km, and the thrust type in the lower half, below that depth level. The former possibly indicates shearing of the Indian plate past the Burmese plate, along the zone of contact, which is inferred down to a depth of 90 km. The latter is suggestive of a NNE-directed stress on the lower portion of the subducted slab, as it is dragged against the asthenosphere by the Indian plate.
1. Introduction
The presence of the subducted slab of the Indian plate, dipping east along the Burmese arc, is well known and has been inferred from seismicity, gravity and seismic velocity studies (Chandra, 1975; Verma et al., 1976; Mukhopadhyay and Das Gupta, 1988; Ni et al., 1989; Gupta et al., 1990). It is interesting, however, that focal mechanism studies in this region indicate P-axis azimuths trending nearly n o r t h - s o u t h rather than perpendicular to the arc (LeDain et al., 1984; Ni et al.,
Corresponding author.
1989; Chen and Molnar, 1990). It has been suggested by LeDain et al. (1984) that there has been a major change in the tectonic pattern in this region, from subduction to collision and strike-slip motion during the late Cenozoic. Hence, India seems to be currently dragging its east-dipping lithospheric slab beneath the Indoburman ranges, northward through the asthenosphere. To decipher the style of current deformation in the Burmese arc region, we examine 50 Harvard centroid-moment tensor (CMT) solutions of earthquakes from 1977 to 1992 in this region (also reported regularly in Physics o f the Earth and Planetary Interiors, eg. Dziewonski et al.,
0031-9201/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0031-9201(94)03012-X
76
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
1987). For a better understanding of the seismotectonics of the subducted Indian lithospheric slab in this region, an examination of the focal mechanisms and their P-axis trends in a depth section, rather than on a map, seems to be useful. The present data set appears fairly extensive and consistent. The P-axis trends, in general, are similar to those reported by the previous workers, mentioned above. Additionally, some very interesting trends are observed on the slab, which are discussed along with a plausible interpretation.
2. Significant trends inferred from focal mechanisms The focal mechanism solutions from the CMT data set in the Burmese arc region used in this study are listed in Table 1, and are indicated, along with the major tectonic features in Fig. 1. The data are presented using different symbols for thrust, strike-slip and normal type mechanisms, as this approach clearly brings out important seismicity trends (Kumar et al., 1995). It can
9¢
95 °
~oCE
Fig. 1. Seismo-tectonicmap of the Burmese arc region. The earthquakes (CMT data) predominantly thrust (o), strike-slip (e) and normal type ([]) mechanisms.
be seen that the number of thrust (19) and strike-slip (24) solutions is comparable: Interestingly, all the normal (7) solutions are clustered in the southern part of the arc, except for one in the east. In Fig. 2(a), the focal mechanisms in the Burmese arc region are plotted in an east-west depth section, as it provides a better perspective of their distribution along the subducted slab. It may be noted that although the locations of the solutions are as in a depth section, the 'beach-ball' representation, as would be seen in a map view, is retained. Fig. 2(a) shows that all the solutions, except those further east, form a dipping trend indicative of the subducted slab, and also shows some very interesting trends. For example, there is a clear segregation of strike-slip and thrust type solutions along the dipping slab, with the predominantly strike-slip solutions occurring exclusively in the upper half and the thrust solutions in the lower. Normal type solutions also are seen on the slab (Fig. 2(b)), down to a depth of about 90 km, like the strike-slip solutions. Shallow thrust and strike-slip solutions, occurring further east of the slab, show characteristics distinct from those of the solutions on the slab. This can be seen in Fig. 3, which shows P-axis azimuths plotted in a depth section. It may be noted that, in Fig. 3, the definition of azimuth as would be seen in a map view is retained, although the centre of each line corresponds to the position in depth of the solutions. An important observation is that the P-axis orientations of all the events associated with the slab (about 35), including thrust, strike-slip and normal type solutions, are predominantly NNE. Similar P-axis trends have been reported also by LeDain et al. (1984) and Ni et al. (1989). Solutions further east show a P-axis trend close to ENE, distinct from that seen on the slab. The depth level of 90 km appears significant, as it separates the regime of strike-slip solutions from the thrust solutions on the slab, and confines the normal solutions above it. We infer this depth range in terms of the zone of contact between the Indian and the Burmese lithospheres and propose the following as a plausible interpretation. The active tectonics in the Burmese arc region must be currently governed by the north-
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
77
Table 1 Focal m e c h a n i s m data from the Harvard C M T catalogue for earthquakes in the Burmese arc region (20-28°N, 90-99"E) occurring between 1977 and 1992 Serial no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 2O 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 49 50
Date
Lat. (°N)
Long. (°E)
Depth (km)
st1
dl
sll
st2
d2
sl2
Mo
Ex
Ppl
Paz
Tpl
Taz
05/12/77 02/23/78 01/01/79 05/29/79 07/13/79 11/25/79 12/21/79 11/20/80 04/25/81 06/30/81 08/14/81 08/16/81 08/23/83 08/30/83 11/16/83 05/06/84 11/28/84 12/30/84 04/24/85 09/05/85 02/08/86 04/26/86 07/26/86 11/01/86 05/18/87 08/24/87 02/06/88 07/03/88 08/06/88 08/13/88 08/21/88 10/23/88 02/12/89 04/03/89 07/15/89 09/24/89 12/02/89 12/08/89 01/09/90 03/08/90 01/05/91 05/11/91 12/04/91 12/20/91 03/25/92 03/27/92 04/15/92 06/15/92 07/08/92 11/22/92
21.60 23.16 20.51 24.92 25.13 25.21 26.61 22.69 24.99 23.00 25.61 25.53 24.48 25.34 26.58 24.33 26.52 24.75 25.83 25.29 23.79 23.31 23.86 25.53 24.58 23.04 24.05 22.22 25.19 24.94 24.94 20.74 26.18 25.15 22.91 20.22 21.62 21.54 24.42 25.11 23.61 23.42 24.19 24.47 24.18 21.12 23.96 23.95 21.07 20.43
92.77 94.93 93.59 95.05 95.59 96.32 97.78 94.49 95.52 95.45 98.51 97.02 94.69 94.90 96.38 93.42 96.96 92.99 95.97 98.23 93.09 94.92 94.19 96.91 93.94 94.53 91.66 94.36 94.89 95.24 95.89 94.67 96.83 94.81 93.94 94.75 93.89 93.78 94.95 96.61 96.18 93.25 93.83 93.08 95.20 94.52 94.57 96.03 93.51 94.51
40.0 122.2 95.0 108.5 117.1 10.0 10.0 20.0 152.9 10.0 10.0 37.0 147.9 68.9 144.4 61.3 16.0 101.8 56.4 60.7 33.0 129.2 64.9 69.8 75.3 126.5 31.0 86.0 100.5 126.0 93.8 45.6 33.0 85.3 139.4 144.0 44.6 15.0 129.6 56.5 20.9 73.6 70.0 100.8 119.7 86.2 142.5 22.9 79.0 71.4
216 331 93 109 44 357 119 208 135 7 153 298 297 161 177 157 311 352 324 14 224 87 130 28 67 135 239 133 284 307 67 168 295 71 111 83 196 213 140 32 2 159 245 258 272 159 281 8 191 187
72 31 32 30 28 79 42 19 44 40 46 66 44 65 58 69 49 46 38 65 62 30 72 45 68 42 76 18 45 35 49 27 60 81 44 59 42 38 32 73 68 77 68 54 30 12 45 69 33 29
3 44 62 134 99 - 175 72 131 80 142 - 141 - 7 58 159 147 161 17 123 58 177 -15 136 15 132 - 14 144 9 - 104 55 69 28 -58 24 -8 159 145 -50 -31 139 173 166 171 11 30 43 -85 56 - 173 -75 -43
125 201 305 241 215 266 323 345 329 128 33 30 158 260 286 254 210 129 182 105 321 217 36 157 163 254 147 327 148 152 318 313 193 163 217 192 328 328 267 124 97 251 151 150 143 334 145 275 353 317
87 69 62 69 62 85 51 76 47 67 63 83 54 71 62 73 77 53 58 87 77 69 76 58 77 67 82 72 54 58 69 68 69 82 75 61 59 71 69 84 77 81 79 66 70 78 54 83 58 71
162 113 106 68 85 - 11 106 78 99 57 -52 - 156 118 26 37 22 138 61 113 25 -152 67 162 56 - 158 54 166 -85 120 104 136 - 105 148 - 171 48 36 - 120 - 124 64 18 23 13 157 140 113 -91 120 -21 - 100 - 112
9.91 5.37 1.12 1.31 3.88 9.16 1.13 7.79 3.89 2.76 9.00 4.95 6.76 4.24 13.47 11.90 3.78 1.33 8.75 2.15 1.46 4.75 1.74 6.96 2.76 9.76 6.70 4.45 8.90 4.56 6.78 5.18 9.53 2.73 2.54 11.97 1.14 1.66 3.05 7.68 3.11 1.43 2.00 11.95 6.53 2.76 4.09 3.14 8.00 7.61
24 23 24 24 23 23 24 23 24 23 23 23 23 24 23 24 24 25 23 24 24 23 24 23 25 23 24 23 26 23 23 23 23 24 24 23 24 24 25 23 26 24 24 23 23 24 24 25 23 23
10 21 16 21 17 11 4 30 2 15 55 22 5 4 3 2 18 4 10 16 29 21 2 7 25 14 4 63 5 12 12 64 6 12 19 1 63 51 20 8 6 3 8 7 22 57 4 20 75 58
172 274 23 348 308 221 42 85 52 242 352 257 229 29 51 25 267 239 256 237 186 324 83 270 27 9 194 244 217 232 16 198 246 27 337 317 187 199 16 256 228 25 200 206 216 242 214 229 237 197
15 59 69 60 72 5 77 57 83 55 10 ;I2 67 32 44 28 38 67 68 19 10 59 23 61 6 54 ;16 27 65 73 46 21 37 1 43 45 9 19 5;8 17 25 16 23 45 5;9 213 66 10 13 23
79 144 248 120 114 312 292 239 308 355 97 162 127 122 I44 116 162 339 139 333 90 95 352 13 294 119 103 54 117 98 274 54 151 297 86 48 79 83 142 349 321 116 106 109 85 65 114 323 90 63
(stl, dl, sll) and (st2, d2, s12) are the strike, dip, slip of the two planes, respectively. M o × 10 E× is the scalar seismic moment. (Ppl, Paz) and (Tpl, Taz) are the plunge and azimuth of the P and T axes, respectively.
78
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80
northeastward motion of the Indian plate, as indicated by the consistent NNE trends of the P axes all along the slab. The Indian lithospheric slab, therefore, seems to be dragged NNE past (a)
the Burmese plate, resulting in large-scale shearing and strike-slip faulting all along the contact zone. This explains the presence of strike-slip solutions along the slab (Fig. 2(a)), down to a
95
~'"-\
~8
IO 0 E
~oJ,i&'-,odl--,, 17 ~-.~7
50--
100 --
150 --
22
9 55'~.-~ /
(kin)
(b)
sch
15
/
47 " x . . ~ ( " , ~ 13 ~ 3 6
95 I
tO0 E I
370032 28
IOC
(kin)
Fig. 2. (a) Thrust and strike-slip type focal mechanisms in an e a s t - w e s t depth section across the Burmese arc. It may be noted that although the locations of the solutions are as in a depth section, the 'beach-ball' representation, as would be seen in a m a p view, is retained. T h e dotted lirle indicates the inferred outer line of the subducted Indian slab. (b) Normal type focal mechanisms, as in
(a).
M.R. Kumar, N.P. Rao / Physics of the Earth and Planetary Interiors 90 (1995) 75-80 90 °
95 °
79
I00 ° E
5C
100
1.50 (kml
Fig. 3. A plot of P-axis azimuths in an east-west depth section. The symbols have the same meaning as in Fig. 1. The definition of azimuth is as in a map view, and the centres of the lines indicate positions in depth.
depth of about 90 kin, where apparently the contact ends. The lower part of the slab is free of this shearing and is subjected, instead, to compressive stresses in the NNE direction as it penetrates through the asthenosphere. This is evidenced by the presence of thrust solutions exclusively in the lower portion, with P axes oriented NNE. The T axes of the normal solutions in the upper part of the slab are seen to be oriented predominantly in the east-west direction, possibly indicating down-dip extension of the subducted slab, as suggested also by Ni et al. (1989).
3. Conclusions (1) In the Burmese arc region, almost all the focal mechanisms on the subducted lithospheric slab show P-axis azimuths oriented NNE. This implies that the tectonics of the slab is governed mostly by a NNE-directed stress field owing to the Indian plate motion. (2) The focal mechanisms along the slab are distinctly segregated into predominantly strikeslip type, occurring in the upper half of the slab down to a depth of about 90 kin, and thrust type,
occurring exclusively in the lower half, below that depth level. This could be due to shearing of the Indian plate past the Burmese plate in the zone of contact, and compression of the lower portion of the slab against the asthenosphere in the NNE direction.
Acknowledgements We thank Dr. A.M. Dziewonski for providing the CMT data required for this study. We are grateful to Dr. Harsh K. Gupta for constant guidance, useful discussions and a critical review of the manuscript. We thank two anonymous reviewers for their useful comments.
References Chandra, U., 1975. Seismicity, earthquake mechanisms and tectonics of Burma 20-28 deg N. Geophys. J. R. Astron. Soc., 40: 367-381. Chen, W.P. and Molnar, P., 1990. Source parameters of earthquakes and intraplate deformation beneath the Shillong plateau and the northern Indoburman ranges. J. Geophys. Res., 95: 12527-12552.
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