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Earthquake mechanisms and tectonic features of Northern Burma
Z’ectonophysics, 32 (1976) 387-399 0 Elsevier Scientific Publishing Company,
Amsterdam
- Printed
in The Netherlands
EARTHQUAKE MECHANISMS AND TECTONIC FEATURES OF NORTHERN BURMA
R.K. VERMA,
MANOJ
MUKHOPADHYAY
and M.S. AHLUWALIA
Indian School of Mines, Dhanbad, Bihar (India) (Submitted
May 30, 1975; revised
version
accepted
January
6, 1976)
ABSTRACT Verma, R.K., Mukhopadhyay, M. and Ahiuwalia, MS., 1976. Earthquake and tectonic features of northern Burma. Tectonophysics, 32: 387-399.
mechanisms
A seismicity map of northeast India and Burma for the period 1906-1970 is given. A zone of intermediate-depth foci dipping to the east of Arakan Yoma is found to be present underneath the Burmese plains. The seismic zone appears to be of V shape. Six new focal-mechanism solutions of earthquakes in Burma have been determined. These mechanisms, in conjunction with earlier studies of Fitch (1970), Rastogi et al. (1973) and Tandon and Srivastava (1975), indicate that normal as well as thrust fauiting takes place in Burma. The compressive stress axis is found to be more nearly vertical than horizontal. The results are discussed in the light of plate-tectonic theory as applied to the IndoBurma regions
INTRODUCTION
The northern part of Burma and the adjoining areas of the Indian Peninsula is one of the most interesting areas of southeast Asia from the point of view of tectonics. Here large-scale thrust movements have taken place from the east towards the west, culminating in the Naga and Ha~ong-Disang thrusts in northern Assam (Mathur and Evans, 1964), the frontal folded belt of Tripura and the Arakan Yoma mountain system. Also large-scale vertical movements have taken place resulting in uplift of the Shan Plateau in Burma and the Shillong Plateau in Assam during Tertiary times (Krishnan, 1953). The vertical movements have also played their part in the deposition of large thicknesses of sediments belonging to the Irrawaddy system of Burma (estimated to be 3000 m thick, Vishnu, 1960) and Bengal basin in India (maximum thickness estimated to be 13,000 meters according to Evans, 1964). This region has been formed as a result of collision of the Indian and Burmese or Eurasian continental plates (Desikachar, 1974). Northern Burma forms a part of the Burmese arc which sweeps in a broad curve through Arakan and the Andaman islands into Sumatra and beyond. It is convex towards India. The region is characterized by high seismic& with
shallow- and intermeciiate-focus earthquakes, the seismic zone leaving a northerly to northeasterly strike. Seismically, this region forms a part of the Alpide-Himalaya belt towarus the north and of the Java--Sumatra seismic belt towards the south. ‘Sea-floor spreading’ and ‘plate tectonic’ theory offer a reasonably good explanation for the seismicity of the region (Le Pichon, 1968; Fitch, 1970). In the Burma region, thrust faulting has been suggested by the focalmechanism solutions (Fitch, 1970; Rastogi et al., 1973). The underthrusting of the Indian plate towards the east beneath the Burmese arc has also been indicated by these studies and supported by seismological studies of Santo (1969). Rastogi et al. (1973) have indicated that the axis of maximum compressive stress is more nearly horizontal than vertical and it trends perpendicular to the strike of the mountain arcs in Burma. Isacks and Molnar (1971), reviewing the fault-plane solutions in the area, have shown that down-dip extensional stress prevails in the inclined seismic zone of P*n-ma. In the present study six new focal-mechanism solutions for Burmese earthquakes have been obtained. The present results, together with those of earlier workers, have been reviewed. The nature of faulting and seismicity in the area is discussed in the light of plate-tectonic theory. GEOLOGICAL
HISTORY
OF THE AREA
The area under discussion lies between 20-28” N and 92-98” E. Geologically the entire area has evolved during Mesozoic to Tertiary times. The Arakan Yoma constitutes the major mountain range of Burma. These are composed of folded Mesozoic and Tertiary rocks and intruded by granitic and ultrabasic rocks. In the core of the belt are the Triassic and the Cretaceous rocks which were folded during the Late Cretaceous and subsequently in the Tertiary. In this arc and parallel to it, is the main volcanic zone with Late Tertiary to Recent volcanic activity continuing into the volcanic zone of Sumatra, Java and other islands of the Indonesian archipelago. The Tertiary rocks have been faulted against the Precambrian rocks of Shan Plateau belt in the east. The Shanbelt shows Precambrian, Paleozoic and some Mesozoic rocks intruded into by Precambrian and Mesozoic granites. The Shan belt is geologically allied to SW China and Indo-China (Krishnan, 1953). Major mountain building in Burma took place towards the end of Miocene times along with the uplift of the Himalaya and the formation of the Baluchistan arc. Large thicknesses of sediments composing the Pegu and the Irrawaddy systems were deposited during Miocene and Early Pleistocene times. A geological map of the area after Krishnan (1960) is shown in Fig. 1. Northwest of Burma is the Naga-Disang thrust belt, consisting of several thrusts with a NE-SW strike direction. The movements along this thrust have been in a SE-NW direction. Thrusting along the Naga Hills has brought
389
Fig. 1. Geological
map of Burma
after
Krishnan,
1960.
the geosynclinal facies of the Burma region and the shelf facies of the Indian shield close together. West of the central part of Burma is the Bengal basin in which large thicknesses of sediments have been deposited during Cretaceous to Tertiary times. The tectonics of the area has been discussed in the light of a plate-tectonic model by Desikachar (1974). According to him, it appears that the western subsiding part of the Bengal basin formed a part of the Indian plate, while the eastern part formed a distinct part of the Burmese plate which has moved westward over the Indian plate during the Miocene period. The subsiding tract of the Bengal basin, approximately east of 90”E, marks the subduction zone between the two plates. The tectonic framework of eastern India and Burma according to the plate-tectonic model is shown in Fig. 2.
Fig. 2. Tectonic
SEISMICITY
map of eastern
India and adjoining
areas after Ilesikachar
(1974).
MAP
A seismicity map of the area for the period 1900-1970, prepared recently using seismic data obtained from the International Seismology Summary Bulletin; the Internation~ Seismological Research Center, United States Coast and Geodetic Survey reports and from the India Meteorological Department catalogue of earthquakes is shown in Fig. 3. The figure shows the locations of epicenters of earthquakes and their magnitudes. It is evident from the figure that the whole of northern Burma is a region of high seismicity. The zone is relatively narrow, aligned in a northsouth direction and spread over the Burmese plains, being bordered by the Arakan Yoma ranges in the west and Shan Plateau in the east. The depth of foci of earthquakes varies from near-surface to about 200 km. A profile
391
Fig. 3. Seismicity map of northeast India for the period 1900-1970. upon India Meteorological Department Bulletins, I.S.S. and Bulletins Seismological Center, U.S.C.G.S. earthquake data reports.
The map is based of International
(A *A,) drawn in a NW-SE direction across the central part of Burma shows the distribution of earthquakes with depth (Fig. 4). For this study, the earthquakes occurring upto a distance of about 200 km from the profile were projected on the profile. The profile shows clearly that the depth of foci increases towards Burma and the deepest earthquakes are located underneath the Burmese plains, east of Arakan Yoma. ~GCA~-MECHANISM
SOLUTIONS
All earthquakes of U.S.C.G.S. magnitude ( mb ) = 5 or greater occurring between 22--27”N and 93-Q7”E during the period January 1961 to July 1970 were considered. Focal-mech~ism solutions were determined using
PROFILE A, A2 -ARAKAN-YWA-
50
o%o”
.
5
100 \
I
k 0” 150
.”
A
POSTULATED THRUST FAULT
0
.
0 0
.
\
t
m\
o
J&,00”
0
0 200
0
8 %*o
0
0
.
\
El 0
MAGC6.5
l HAG76.5
m
ALLUVIUM EOCENE
1=-_1
IRRAWADY
MOLIGOCENE
CRETAC EOIJS
m
H,OCENE
CRYSTALLINES
Fig. 4. Depths of foci of earthquakes projected along profile AlAz of Fig. 3. The region of intermediate-depth foci is seen to be dipping t,owards the east of Arakan Yoma. The seismic zone appears to be of V shape.
short-period, P-wave first-motion directions reported in the International Seismological Summary (Edinburgh) for six selected earthquakes. The locations of these are shown in Fig. 5. The U.S.C.G.S. earthquake parameters for these earthquakes and also for those of other workers whose fault-plane solutions have been determined earlier are given in Table I. Ichikawa et al. (1972) determined quite a good number of focal-mechanism solutions for the earthquakes occurring in and around the Himalayan and Burmese mountain belts. But all of these are located far away from the present area and hence they have not been con-
393
Fig. 5. Locations of earthquakes whose focal-mechanism solutions hlave been studied discussed in the text. Numbers correspond to events listed in Tables I and II.
and
sidered here. First-motion directions of these six earthquakes were plotted on an equal-angle projection of the lower hemisphere of the focal sphere. The focal-mechanism solutions are shown in Fig. 6. A double-couple source model was assumed. The orientation of nodal planes, P- and T-axes for earthquakes listed in Table I are given in Table II. INTERPRETATION
OF FOCAL
MECHANISMS
The six events considered in the present study indicate predominantly normal faulting. Solutions for events 1,3 and 6 are comparatively well determined. Event 1 is located on the convex side of the Arakan Yoma ranges, while event 3 is located only about 80 km east of this. Events 2, 5 and 6 correspond to earthquakes located in northern Burma. Events 5 and 6 are located within a distance of 30 km from each other, while event 2 is about 45 km away from event 6. A number of inconsistent observations for event 6 may indicate the multiple nature of the event. Event 4 corresponds to an interm~iate focal-depth e~thqu~e located in Central Burma, The P-,l'and B-axes of fifteen focal-mechanism solutions are plotted on
I
Dec. Jan. July Jan. Feb. Sept. June
Dec. June Nov. Aug. Dec.
4 5 6 7 8 9 10
11 12 13 14 15
15, 1966 1, 1965 19,1966 29,1969 29,1971
15,1965 30, 1967 29, 1970 22,1964 27,1964 22, 1962 3.1964
Feb. 18,1965
3 22.0 26.1 26.0 22.4 21.7 26.5 25.9 21.5 20.3 18.4 26.3 25.2
04:43:47.4 21:01:30.0 10:16:20.4 15:58:46.5 15:10:48.8 06:51:28.0 02:49:14:9
02:08:48.0 04~32142.8 07 ~42127.8 10:02:50.0 22~27~24.0
24.97
25.88
02:49:17.2
June 3, 1964
2
04 ~26~34.7
24.86
08:51:50.0
Feb. 4, 1961
1
94.4 95.0 95.3 96.1 94.7
94.47 96.14 95.37 93.6 94.4 96.8 95.8
94.21
95.69
93.34
Long. (OE)
(ON)
Event. No. __-
Lat.
India-Burma border India-Burma border India-Burma border Burma Burma Burma Burma Burma Burma Burma-India border Burma Burma Burma Burma Burma-India border
Region
as given in I.S.C. bulletins
Origin time (G.M.T.)
of earthquakes
____
parameters
Date
U.S.C.G.S.
TABLE
98 53 53 72 46
present study present study present study Fitch, 1970 Fitch, 1970 Rastogi et al., 1973 Rastogi et al., 1973
5.2 5.4 6.4 6.1 6.4
5.6 5.5 5.3 5.2 5.1
5.5
present
5.4
Rastogi Rastogi Rastogi Tandon Tandon
et al., 1973 et al., 1973 et al., 1973 and Srivastava, and Srivastava.
study
study
45
present
5.4
study
121
present
Reference
7.6
Magnitude
141
(km)
Depth
1975 1975
Fig. 6. A-F. Focal-mechanism solutions of six earthquakes determined in the present study. l = compressions, 0 = tensions, @ = pressure axis, FJ = tension axis.
II
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Event No.
of nodal
270 89 209 135 343 56 56 293 293 252 190 176 273 298
5
azimuth
31 28 40 6 30 10 4 4 74 84 70 24 18 56 56
plunge
and plunge
123 90 308 38 314 150 150 153 93 113 72 10 58 50 60
azimuth 40 62 40 84 60 80 45 54 16 6 20 66 56 46 56
plunge
..~
59 270 22 208 143 344 204 208 273 293 255 190 138 256 90
azimuth
P-axis
in degrees)
Pole of 2nd nodal plane ~___ ~~~
planes (azimuth
Pole of 1st nodal plane
Orientation
TABLE
54 73 67 50 85 56 26 30 29 39 25 69 52 3 88
plunge 157 90 288 28 316 160 94 90 93 113 64 10 19 56 180
azimuth
T-axis
4 17 1 40 13 34 33 40 61 51 65 21 22 66 55
plunge present study present study present study present study present study present study Fitch, 1970 Fitch, 1970 Rastogi et al., 1973 Rastogi et al., 1973 Rastogi et al., 1973 Rastogi et al., 1973 Rastogi et al., 1973 Tandon and Srivastava, Tandon and Srivastava,
Reference
1975. 1975
397 DIP so
80
TO
oNF oNF
60
(DEG.) 50
40
20
30
l
oNF
owrF
p
ow
NF JF
IO
.NF
0
TF. OF
.TF ,v
TF
TF
a
90
o*
100
‘TF
.N
NF”
ONF
110
NF J 150
Fig. 7. Orientations graphic
of P, T- and B-axes for fifteen
o-
P AXES
NF-NORMAL
FAULT
l -
T AXES
T F - THRUST
FAULT
earthquakes
plotted
on Wulff
stereo-
projection.
Fig. 8. Plot of P- and T-axes, vs. depth for earthquakes listed in Table II. The figure shows that the majority of P-axes have dips larger that 50’ and dip of T-axes increases with depth.
an equal-area stereographic projection in Fig. 7. This figure shows that for most of the events, the axis of maximum compressive stress is more nearly vertical than horizontal. The down-dip orientation of stresses (P- andTaxes) corresponding to different focal depths is plotted and shown in Fig. 8 for fifteen events listed in Table I. DISCUSSION
OF RESULTS
From Fig. 8 it is seen that at shallow depths (less than 70 km) normal faulting characterized by steep dip of pressure axes predominates. Between depths of 72 and 105 km, thrust faulting is predominant. Again at larger depths (more than 110 km), normal faulting is predominant. The normal faulting at shallow depths may be largely due to vertical forces acting in the area trying to bring about adjustments in the geological strata as well as in the crust and the upper mantle. The thrust faulting at intermediate depths
39H
could be ascribed to relative movements between the Indian and the Asian plates, the movements taking place in the upper part of the lithosphere. Normal faulting at shallow depths supports the views of Malahoff (1970) that a descending lithospheric block can cause normal faulting near the surface as observed in the Japan trench (Ludwig et al., 1966). However, the conditions prevailing under Burma are somewhat different from those prevailing in typical island arcs (Isacks and Molnar, 1971). In island arcs, mostly thrust faulting takes place along the dip of the Benioff zone, however in the case of Burma, strain release at intermediate depths takes place both by thrust faulting as well as by normal faulting. A characteristic feature of this area appears to be that the dip of tensional stress axes increases with depth. SUMMARY
OF RESULTS
The northern and the central parts of Burma form a part of a very active seismic belt which lies at the junction of the Alpide-Himalaya seismic belt to the north and the Indonesian arc to the south. The area is characterized by shallow- as well as intermediate-focus earthquakes forming an approximately V-shaped seismic zone. The seismic activity is a result of interaction of the Indian with the Burmese or Eurasian plates. Both normal as well as thrust faulting takes place in the region. Normal faulting appears to be predominant at depths shallower than 70 km and again at depths larger than 105 km. Between 72 and 105 km, the predominant mode of strain release is by thrust faulting. The axes of tensional stress become more nearly vertical with increasing depth. These results tend to support the ideas of convergence of the Indian plate with the Eurasian plate. ACKNOWLEDGEMENT
We are thankful to Dr. H.M. Chowdhury, Director of Seismology, Indian Meteorological Department for use of the catalogue of earthquakes. The manuscript was reviewed by Dr. K.N. Khatri and we are thankful to him for his critical comments and suggestions. Thanks are due to Shri SK. Gangopadhyay for typing the manuscript and Shri B.N.P. Singh for drafting the figures.
REFERENCES Desikachar, S.K., 1974. A review of tectonic and geological history of Eastern India in terms of plate-tectonic theory. J. Geol. Sot. India, 15: 137-149. Evans, P., 1964. The tectonic framework of Assam. J. Geol. Sot. India, 5: 80-96. Fitch, T.J., 1970. Earthquake mechanisms in the Himalayan Burmese and Andaman regions and continental tectonics in Central Asia. J. Geophys. Res., 75: 2699-2709.
399 Ichikawa, quakes
M., Srivastava, H.N. and Drakopolous, J., 1972. Focal mechanisms of earthoccurring in and around the Himalayan and Burmese mountain belts. Pap.
Meteorol. Geophys. Tokyo, 23: 149-162. Isacks, B. and Molnar, B., 1971. Distribution of stresses in the descending lithosphere from a global survey of focal-mechanism solutions of mantle earthquakes. Rev. Geophys. Space Phys., 9: 103-174. Krishnan, M.S., 1953. The structure and tectonics of India. Geol. Surv. India, Mem., 81. Govt. of India Press, Calcutta, 38 pp. Krishnan, M.S., 1960. Geology of India and Burma. Higginbothams, Madras, 553 pp. Le Pichon, X., 1968. Sea-floor spreading and continental drift. J. Geophys. Res., 73: 3661-3697. Ludwig, W.J., Ewing, J.I., Ewing, M., Muranchi, S., Dev, N., Asano, S., Hotta, H., Hayakawa, M., Asanuma, T., Ichikawa, K. and Naguchi, I., 1966. Sediments and structure of the Japan trench. J. Geophys. Res., 71(8): 2121-2135. Malahoff, A., 1970. Some possible mechanisms for gravity and thrust faults under oceanic trenches. J. Geophys. Res., 75: 1992-2000. Mathur, L.P. and Evans, P., 1964. Oil in India. Proc. 22nd Int. Geol. Congr., India. Print Craft. Private, Digboi, Assam, 85 pp. Rastogi, B.K., Singh, J. and Verma, R.K., 1973. Earthquake mechanisms and tectonics in the Assam-Burma region. Tectonophysics, 18: 355-366. Santo, T., 1969. On the characteristic seismicity in south Asia from Hindukush to Burma. Bull. Int. Inst. Seismol. Earthquake Eng., 6: 81-93. Tandon, A.N. and Srivastava, H.N., 1975. Focal mechanisms of some recent Himalayan earthquakes and regional plate tectonics. Bull. Seismol. Sot. Am., 65: 963-969.
Amsterdam
- Printed
in The Netherlands
EARTHQUAKE MECHANISMS AND TECTONIC FEATURES OF NORTHERN BURMA
R.K. VERMA,
MANOJ
MUKHOPADHYAY
and M.S. AHLUWALIA
Indian School of Mines, Dhanbad, Bihar (India) (Submitted
May 30, 1975; revised
version
accepted
January
6, 1976)
ABSTRACT Verma, R.K., Mukhopadhyay, M. and Ahiuwalia, MS., 1976. Earthquake and tectonic features of northern Burma. Tectonophysics, 32: 387-399.
mechanisms
A seismicity map of northeast India and Burma for the period 1906-1970 is given. A zone of intermediate-depth foci dipping to the east of Arakan Yoma is found to be present underneath the Burmese plains. The seismic zone appears to be of V shape. Six new focal-mechanism solutions of earthquakes in Burma have been determined. These mechanisms, in conjunction with earlier studies of Fitch (1970), Rastogi et al. (1973) and Tandon and Srivastava (1975), indicate that normal as well as thrust fauiting takes place in Burma. The compressive stress axis is found to be more nearly vertical than horizontal. The results are discussed in the light of plate-tectonic theory as applied to the IndoBurma regions
INTRODUCTION
The northern part of Burma and the adjoining areas of the Indian Peninsula is one of the most interesting areas of southeast Asia from the point of view of tectonics. Here large-scale thrust movements have taken place from the east towards the west, culminating in the Naga and Ha~ong-Disang thrusts in northern Assam (Mathur and Evans, 1964), the frontal folded belt of Tripura and the Arakan Yoma mountain system. Also large-scale vertical movements have taken place resulting in uplift of the Shan Plateau in Burma and the Shillong Plateau in Assam during Tertiary times (Krishnan, 1953). The vertical movements have also played their part in the deposition of large thicknesses of sediments belonging to the Irrawaddy system of Burma (estimated to be 3000 m thick, Vishnu, 1960) and Bengal basin in India (maximum thickness estimated to be 13,000 meters according to Evans, 1964). This region has been formed as a result of collision of the Indian and Burmese or Eurasian continental plates (Desikachar, 1974). Northern Burma forms a part of the Burmese arc which sweeps in a broad curve through Arakan and the Andaman islands into Sumatra and beyond. It is convex towards India. The region is characterized by high seismic& with
shallow- and intermeciiate-focus earthquakes, the seismic zone leaving a northerly to northeasterly strike. Seismically, this region forms a part of the Alpide-Himalaya belt towarus the north and of the Java--Sumatra seismic belt towards the south. ‘Sea-floor spreading’ and ‘plate tectonic’ theory offer a reasonably good explanation for the seismicity of the region (Le Pichon, 1968; Fitch, 1970). In the Burma region, thrust faulting has been suggested by the focalmechanism solutions (Fitch, 1970; Rastogi et al., 1973). The underthrusting of the Indian plate towards the east beneath the Burmese arc has also been indicated by these studies and supported by seismological studies of Santo (1969). Rastogi et al. (1973) have indicated that the axis of maximum compressive stress is more nearly horizontal than vertical and it trends perpendicular to the strike of the mountain arcs in Burma. Isacks and Molnar (1971), reviewing the fault-plane solutions in the area, have shown that down-dip extensional stress prevails in the inclined seismic zone of P*n-ma. In the present study six new focal-mechanism solutions for Burmese earthquakes have been obtained. The present results, together with those of earlier workers, have been reviewed. The nature of faulting and seismicity in the area is discussed in the light of plate-tectonic theory. GEOLOGICAL
HISTORY
OF THE AREA
The area under discussion lies between 20-28” N and 92-98” E. Geologically the entire area has evolved during Mesozoic to Tertiary times. The Arakan Yoma constitutes the major mountain range of Burma. These are composed of folded Mesozoic and Tertiary rocks and intruded by granitic and ultrabasic rocks. In the core of the belt are the Triassic and the Cretaceous rocks which were folded during the Late Cretaceous and subsequently in the Tertiary. In this arc and parallel to it, is the main volcanic zone with Late Tertiary to Recent volcanic activity continuing into the volcanic zone of Sumatra, Java and other islands of the Indonesian archipelago. The Tertiary rocks have been faulted against the Precambrian rocks of Shan Plateau belt in the east. The Shanbelt shows Precambrian, Paleozoic and some Mesozoic rocks intruded into by Precambrian and Mesozoic granites. The Shan belt is geologically allied to SW China and Indo-China (Krishnan, 1953). Major mountain building in Burma took place towards the end of Miocene times along with the uplift of the Himalaya and the formation of the Baluchistan arc. Large thicknesses of sediments composing the Pegu and the Irrawaddy systems were deposited during Miocene and Early Pleistocene times. A geological map of the area after Krishnan (1960) is shown in Fig. 1. Northwest of Burma is the Naga-Disang thrust belt, consisting of several thrusts with a NE-SW strike direction. The movements along this thrust have been in a SE-NW direction. Thrusting along the Naga Hills has brought
389
Fig. 1. Geological
map of Burma
after
Krishnan,
1960.
the geosynclinal facies of the Burma region and the shelf facies of the Indian shield close together. West of the central part of Burma is the Bengal basin in which large thicknesses of sediments have been deposited during Cretaceous to Tertiary times. The tectonics of the area has been discussed in the light of a plate-tectonic model by Desikachar (1974). According to him, it appears that the western subsiding part of the Bengal basin formed a part of the Indian plate, while the eastern part formed a distinct part of the Burmese plate which has moved westward over the Indian plate during the Miocene period. The subsiding tract of the Bengal basin, approximately east of 90”E, marks the subduction zone between the two plates. The tectonic framework of eastern India and Burma according to the plate-tectonic model is shown in Fig. 2.
Fig. 2. Tectonic
SEISMICITY
map of eastern
India and adjoining
areas after Ilesikachar
(1974).
MAP
A seismicity map of the area for the period 1900-1970, prepared recently using seismic data obtained from the International Seismology Summary Bulletin; the Internation~ Seismological Research Center, United States Coast and Geodetic Survey reports and from the India Meteorological Department catalogue of earthquakes is shown in Fig. 3. The figure shows the locations of epicenters of earthquakes and their magnitudes. It is evident from the figure that the whole of northern Burma is a region of high seismicity. The zone is relatively narrow, aligned in a northsouth direction and spread over the Burmese plains, being bordered by the Arakan Yoma ranges in the west and Shan Plateau in the east. The depth of foci of earthquakes varies from near-surface to about 200 km. A profile
391
Fig. 3. Seismicity map of northeast India for the period 1900-1970. upon India Meteorological Department Bulletins, I.S.S. and Bulletins Seismological Center, U.S.C.G.S. earthquake data reports.
The map is based of International
(A *A,) drawn in a NW-SE direction across the central part of Burma shows the distribution of earthquakes with depth (Fig. 4). For this study, the earthquakes occurring upto a distance of about 200 km from the profile were projected on the profile. The profile shows clearly that the depth of foci increases towards Burma and the deepest earthquakes are located underneath the Burmese plains, east of Arakan Yoma. ~GCA~-MECHANISM
SOLUTIONS
All earthquakes of U.S.C.G.S. magnitude ( mb ) = 5 or greater occurring between 22--27”N and 93-Q7”E during the period January 1961 to July 1970 were considered. Focal-mech~ism solutions were determined using
PROFILE A, A2 -ARAKAN-YWA-
50
o%o”
.
5
100 \
I
k 0” 150
.”
A
POSTULATED THRUST FAULT
0
.
0 0
.
\
t
m\
o
J&,00”
0
0 200
0
8 %*o
0
0
.
\
El 0
MAGC6.5
l HAG76.5
m
ALLUVIUM EOCENE
1=-_1
IRRAWADY
MOLIGOCENE
CRETAC EOIJS
m
H,OCENE
CRYSTALLINES
Fig. 4. Depths of foci of earthquakes projected along profile AlAz of Fig. 3. The region of intermediate-depth foci is seen to be dipping t,owards the east of Arakan Yoma. The seismic zone appears to be of V shape.
short-period, P-wave first-motion directions reported in the International Seismological Summary (Edinburgh) for six selected earthquakes. The locations of these are shown in Fig. 5. The U.S.C.G.S. earthquake parameters for these earthquakes and also for those of other workers whose fault-plane solutions have been determined earlier are given in Table I. Ichikawa et al. (1972) determined quite a good number of focal-mechanism solutions for the earthquakes occurring in and around the Himalayan and Burmese mountain belts. But all of these are located far away from the present area and hence they have not been con-
393
Fig. 5. Locations of earthquakes whose focal-mechanism solutions hlave been studied discussed in the text. Numbers correspond to events listed in Tables I and II.
and
sidered here. First-motion directions of these six earthquakes were plotted on an equal-angle projection of the lower hemisphere of the focal sphere. The focal-mechanism solutions are shown in Fig. 6. A double-couple source model was assumed. The orientation of nodal planes, P- and T-axes for earthquakes listed in Table I are given in Table II. INTERPRETATION
OF FOCAL
MECHANISMS
The six events considered in the present study indicate predominantly normal faulting. Solutions for events 1,3 and 6 are comparatively well determined. Event 1 is located on the convex side of the Arakan Yoma ranges, while event 3 is located only about 80 km east of this. Events 2, 5 and 6 correspond to earthquakes located in northern Burma. Events 5 and 6 are located within a distance of 30 km from each other, while event 2 is about 45 km away from event 6. A number of inconsistent observations for event 6 may indicate the multiple nature of the event. Event 4 corresponds to an interm~iate focal-depth e~thqu~e located in Central Burma, The P-,l'and B-axes of fifteen focal-mechanism solutions are plotted on
I
Dec. Jan. July Jan. Feb. Sept. June
Dec. June Nov. Aug. Dec.
4 5 6 7 8 9 10
11 12 13 14 15
15, 1966 1, 1965 19,1966 29,1969 29,1971
15,1965 30, 1967 29, 1970 22,1964 27,1964 22, 1962 3.1964
Feb. 18,1965
3 22.0 26.1 26.0 22.4 21.7 26.5 25.9 21.5 20.3 18.4 26.3 25.2
04:43:47.4 21:01:30.0 10:16:20.4 15:58:46.5 15:10:48.8 06:51:28.0 02:49:14:9
02:08:48.0 04~32142.8 07 ~42127.8 10:02:50.0 22~27~24.0
24.97
25.88
02:49:17.2
June 3, 1964
2
04 ~26~34.7
24.86
08:51:50.0
Feb. 4, 1961
1
94.4 95.0 95.3 96.1 94.7
94.47 96.14 95.37 93.6 94.4 96.8 95.8
94.21
95.69
93.34
Long. (OE)
(ON)
Event. No. __-
Lat.
India-Burma border India-Burma border India-Burma border Burma Burma Burma Burma Burma Burma Burma-India border Burma Burma Burma Burma Burma-India border
Region
as given in I.S.C. bulletins
Origin time (G.M.T.)
of earthquakes
____
parameters
Date
U.S.C.G.S.
TABLE
98 53 53 72 46
present study present study present study Fitch, 1970 Fitch, 1970 Rastogi et al., 1973 Rastogi et al., 1973
5.2 5.4 6.4 6.1 6.4
5.6 5.5 5.3 5.2 5.1
5.5
present
5.4
Rastogi Rastogi Rastogi Tandon Tandon
et al., 1973 et al., 1973 et al., 1973 and Srivastava, and Srivastava.
study
study
45
present
5.4
study
121
present
Reference
7.6
Magnitude
141
(km)
Depth
1975 1975
Fig. 6. A-F. Focal-mechanism solutions of six earthquakes determined in the present study. l = compressions, 0 = tensions, @ = pressure axis, FJ = tension axis.
II
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Event No.
of nodal
270 89 209 135 343 56 56 293 293 252 190 176 273 298
5
azimuth
31 28 40 6 30 10 4 4 74 84 70 24 18 56 56
plunge
and plunge
123 90 308 38 314 150 150 153 93 113 72 10 58 50 60
azimuth 40 62 40 84 60 80 45 54 16 6 20 66 56 46 56
plunge
..~
59 270 22 208 143 344 204 208 273 293 255 190 138 256 90
azimuth
P-axis
in degrees)
Pole of 2nd nodal plane ~___ ~~~
planes (azimuth
Pole of 1st nodal plane
Orientation
TABLE
54 73 67 50 85 56 26 30 29 39 25 69 52 3 88
plunge 157 90 288 28 316 160 94 90 93 113 64 10 19 56 180
azimuth
T-axis
4 17 1 40 13 34 33 40 61 51 65 21 22 66 55
plunge present study present study present study present study present study present study Fitch, 1970 Fitch, 1970 Rastogi et al., 1973 Rastogi et al., 1973 Rastogi et al., 1973 Rastogi et al., 1973 Rastogi et al., 1973 Tandon and Srivastava, Tandon and Srivastava,
Reference
1975. 1975
397 DIP so
80
TO
oNF oNF
60
(DEG.) 50
40
20
30
l
oNF
owrF
p
ow
NF JF
IO
.NF
0
TF. OF
.TF ,v
TF
TF
a
90
o*
100
‘TF
.N
NF”
ONF
110
NF J 150
Fig. 7. Orientations graphic
of P, T- and B-axes for fifteen
o-
P AXES
NF-NORMAL
FAULT
l -
T AXES
T F - THRUST
FAULT
earthquakes
plotted
on Wulff
stereo-
projection.
Fig. 8. Plot of P- and T-axes, vs. depth for earthquakes listed in Table II. The figure shows that the majority of P-axes have dips larger that 50’ and dip of T-axes increases with depth.
an equal-area stereographic projection in Fig. 7. This figure shows that for most of the events, the axis of maximum compressive stress is more nearly vertical than horizontal. The down-dip orientation of stresses (P- andTaxes) corresponding to different focal depths is plotted and shown in Fig. 8 for fifteen events listed in Table I. DISCUSSION
OF RESULTS
From Fig. 8 it is seen that at shallow depths (less than 70 km) normal faulting characterized by steep dip of pressure axes predominates. Between depths of 72 and 105 km, thrust faulting is predominant. Again at larger depths (more than 110 km), normal faulting is predominant. The normal faulting at shallow depths may be largely due to vertical forces acting in the area trying to bring about adjustments in the geological strata as well as in the crust and the upper mantle. The thrust faulting at intermediate depths
39H
could be ascribed to relative movements between the Indian and the Asian plates, the movements taking place in the upper part of the lithosphere. Normal faulting at shallow depths supports the views of Malahoff (1970) that a descending lithospheric block can cause normal faulting near the surface as observed in the Japan trench (Ludwig et al., 1966). However, the conditions prevailing under Burma are somewhat different from those prevailing in typical island arcs (Isacks and Molnar, 1971). In island arcs, mostly thrust faulting takes place along the dip of the Benioff zone, however in the case of Burma, strain release at intermediate depths takes place both by thrust faulting as well as by normal faulting. A characteristic feature of this area appears to be that the dip of tensional stress axes increases with depth. SUMMARY
OF RESULTS
The northern and the central parts of Burma form a part of a very active seismic belt which lies at the junction of the Alpide-Himalaya seismic belt to the north and the Indonesian arc to the south. The area is characterized by shallow- as well as intermediate-focus earthquakes forming an approximately V-shaped seismic zone. The seismic activity is a result of interaction of the Indian with the Burmese or Eurasian plates. Both normal as well as thrust faulting takes place in the region. Normal faulting appears to be predominant at depths shallower than 70 km and again at depths larger than 105 km. Between 72 and 105 km, the predominant mode of strain release is by thrust faulting. The axes of tensional stress become more nearly vertical with increasing depth. These results tend to support the ideas of convergence of the Indian plate with the Eurasian plate. ACKNOWLEDGEMENT
We are thankful to Dr. H.M. Chowdhury, Director of Seismology, Indian Meteorological Department for use of the catalogue of earthquakes. The manuscript was reviewed by Dr. K.N. Khatri and we are thankful to him for his critical comments and suggestions. Thanks are due to Shri SK. Gangopadhyay for typing the manuscript and Shri B.N.P. Singh for drafting the figures.
REFERENCES Desikachar, S.K., 1974. A review of tectonic and geological history of Eastern India in terms of plate-tectonic theory. J. Geol. Sot. India, 15: 137-149. Evans, P., 1964. The tectonic framework of Assam. J. Geol. Sot. India, 5: 80-96. Fitch, T.J., 1970. Earthquake mechanisms in the Himalayan Burmese and Andaman regions and continental tectonics in Central Asia. J. Geophys. Res., 75: 2699-2709.
399 Ichikawa, quakes
M., Srivastava, H.N. and Drakopolous, J., 1972. Focal mechanisms of earthoccurring in and around the Himalayan and Burmese mountain belts. Pap.
Meteorol. Geophys. Tokyo, 23: 149-162. Isacks, B. and Molnar, B., 1971. Distribution of stresses in the descending lithosphere from a global survey of focal-mechanism solutions of mantle earthquakes. Rev. Geophys. Space Phys., 9: 103-174. Krishnan, M.S., 1953. The structure and tectonics of India. Geol. Surv. India, Mem., 81. Govt. of India Press, Calcutta, 38 pp. Krishnan, M.S., 1960. Geology of India and Burma. Higginbothams, Madras, 553 pp. Le Pichon, X., 1968. Sea-floor spreading and continental drift. J. Geophys. Res., 73: 3661-3697. Ludwig, W.J., Ewing, J.I., Ewing, M., Muranchi, S., Dev, N., Asano, S., Hotta, H., Hayakawa, M., Asanuma, T., Ichikawa, K. and Naguchi, I., 1966. Sediments and structure of the Japan trench. J. Geophys. Res., 71(8): 2121-2135. Malahoff, A., 1970. Some possible mechanisms for gravity and thrust faults under oceanic trenches. J. Geophys. Res., 75: 1992-2000. Mathur, L.P. and Evans, P., 1964. Oil in India. Proc. 22nd Int. Geol. Congr., India. Print Craft. Private, Digboi, Assam, 85 pp. Rastogi, B.K., Singh, J. and Verma, R.K., 1973. Earthquake mechanisms and tectonics in the Assam-Burma region. Tectonophysics, 18: 355-366. Santo, T., 1969. On the characteristic seismicity in south Asia from Hindukush to Burma. Bull. Int. Inst. Seismol. Earthquake Eng., 6: 81-93. Tandon, A.N. and Srivastava, H.N., 1975. Focal mechanisms of some recent Himalayan earthquakes and regional plate tectonics. Bull. Seismol. Sot. Am., 65: 963-969.