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The characteristics of the seismic activity in the Qinghai-Xizang (Tibet) Plateau of China
Tecronophysics, Elsevier
129
134 (1987) 129-144
Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
The characteristics in the Qinghai-Xizang
of the seismic activity (Tibet) Plateau of China
TENG JIWEN, WE1 SIYU, SUN KEZHANG Institute of Geophysics, Academia (Received
June 21,1985;
and XUE CHANGSHUN
Sinica, BeiJmg (P.R. of China) accepted
.lanuary
1,1986)
Abstract Teng, J.W., Wei, S.Y., Sun, K.Z. and Xue, C.S., 1987. The characteristics (Tibet)
Plateau
of China,
In: H.K. Gupta
(Editor),
Deep Seated
of the seismic activity
Processes in Collision
in the Qinghai-Xizang
Zones.
Tectonophysics.
134:
129-144. Based
on the analysis
Qinghai-Xizang plate,
Plateau
this paper
between
the Indian
studies
of the spatial and its marginal the characteristics
and Eurasian
plates,
distribution region,
of many
of this plateau and the relations
and between
Introduction
The Qinghai-Xizang (Tibet) Plateau is adjacent to the Yunnan, Szechuan, Gansu Provinces and the Sinking Uigher autonomous region of China on the north and the east. Its western, southern and southwestern parts border on India, Nepal, Sikkim and Burma. The area of this plateau is over 1.2 million square kilometres, nevertheless, the area concerned in the present study is more extensive, covering a vast area between 15”-42”N and 60”-180”E. The appearance of the Himalayan mountain system and the rise of the Qinghai-Xizang Plateau are the greatest events in the history of the earth in the Cenozoic in Asia. At that time, when the Indian plate migrated northward and the ancient trench was thrust down into the southern side of the Eurasian plate, the Indian subcontinent was slowly approaching the Eurasian plate, and finally they collided with each other, thus forming a long-lasting compression, causing a repeat of folding, rifting, overthrusting and overlapping, and a 0040-1951/87/$03.50
shallow-
their fault plane solutions
0 1987 Blsevier Science Publishers
B.V.
and intermediate-depth
earthquakes
and the seismo-tectonics
the transitional this Plateau
zone of collision and continental
in the
in the continental and
compression
plate movement.
regular distribution of ophiolite suites, melange and g;laucophane-schists on the boundary between these two plates. As a result, the present Himalayan mountain system was formed (Chang et al.! 1973; Teng et al., 1980). Th.e Qinghai-Xizang Plateau is a giant Cenozoic mountain-building area and the youngest active zone of recent tectonic movement, as well as one of the earthquake-prone areas in the west of China. The tectonic deformation in this area is not only complicated but also has its regional characteristics. The destructive earthquakes within the plateau were not the same as those which occurred in other parts of China. Their geographic distribution is in belts with a certain density. In these belts, the large and small earthquakes showed a similarity of occurrence, intensity and space and had a relationship with the geologic structures of the plateau. For these reasons, the study of seismic activky, fault-plane solution, characteristics of wave propagation and the relationship with the structural motion and plate tectonic movement in this area is of great significance in the study of
130
continental plate-tectonics and seismic marks in the boundary between plates and inner factors of plateau-uplift. On the basis of a large amount of seismic data collected since 1902, in accordance with the seismic parameters, fault-mechanism solutions, stress field and characteristics of wave propagation determined in recent years by the special regional network of seismic stations, in combination with the research into geological structure and crustal and upper mantle physics, we have carried out a study into seismic activity, distribution of stress fields, plateau uplift, formation of a large and thick crust (Teng et al., 1981; Ye et al., 1981) and special seismic patterns. In addition we have analyzed the collision of the Indian plate with the Eurasian plate and the characteristics of transitional belts formed by this collision. The focal distribution within the Qingha-Xizang Plateau and its marginal zones In the distribution of epicentres throughout the world there are two characteristics: (1) the concentration of epicentres on the boundary between two plates; (2) the concentration of epicentres in special tectonic active regions within the plate. It indicates that seismic activity is associated with the block movement in the plate or on the plate boundaries. We would like to point out that the focal activity and its distribution are not uniform, so the collision and deformation of these two large plates along the compression zone were different in their extent, and the eastern and western upper parts of the arcuate mountain system were extremely deformed. The focal distribution (Huan et al., 1980) within the Qinghai-Xizang Plateau and its surrounding regions indicates that it is still an active seismic zone. The plateau includes the area north of the Ganges Plateau, south of the Alkin mountains, west of the Szechuan-Yunnan district and east of the Hindu Kush and Pamir Plateau. Within the plate the foci are distributed along four arcuate mountain systems running parallel to each other and running from the east to the southwest. As is seen in Fig. 1, in this area are not only distributed the shallow-focus earthquakes with depths of no
more than 70 km in the earth’s crust, but also the intermediate focus earthquakes originating from the middle of the upper mantle are concentrated in the eastern and western upper parts of the Himalayan arcuate mountain system. The Distribution
of shallow-focus
earthquakes
In the Himalayan region within the QinghaiXizang Plateau seismic activity is more intensive than in the northern part of the plateau, and the foci are located more deeply than those in East China. The Pamirs, Mt. Kunlun, Mt. Transversal, Mt. Himalaya and India-Burma mountain areas are mainly classified as the zones prone to shallow-focus earthquakes (with depths of 35-70 km). The whole of the plateau is affected by large earthquakes (it4 2 7) along its edges. It presents a special pattern, as if the M > 7 earthquakes are distributed along the margins of the plateau, and gives us an outline of the seismic zones in this area. According to incomplete statistics, during recorded history (Yan et al., 1980; Ye et al., 1981) within the plateau and its surrounding areas there have been 20 earthquakes with M > 8 (Table 1) and many more earthquakes with M = 6-6.9 or 7-7.9. They are characterized by features of distinct zonation and regionalization. The ground surface of the Himalayan region was uplifted highest of all. There the crustal thickness changes greatly, but the foci are shallow in places far from the intermediate focus earthquake regions. For instance, the focal depths in the northern part of the plateau, the Szechuan-Yunnan district and the Ganges plain are usually between 0 and 35 km. The northern marginal zone of the Ganges plain
This marginal zone, is seismically still a very active region. It strikes roughly parallel to the Himalayan arcuate mountain system. The observations of 1961-1981 show that most of the earthquakes were shallow shocks with focal depths of 70 km. This seismically active zone is located between the northern margin of the Indian shield and the southern foot of the Himalaya range with a width of about 200 km. The most intensive
.
Hetan
New
Focal
depths
Delhi \
(km)
II
Fig
“Y
1. A map of the distribution
TABLE
of earthquake
J”
foci in the Qinghai-Xizang
Plateau
and its vicinity.
1
Large earthquakes No.
I .
in the Qinghai-Xizang
Regions
in which the
earthquakes
took place
Plateau
and adjacent
regions
Total No. of
Magnitudes
earthquakes
of earthquakes
M = 64.9 Southern
for each region
the data are used
l-l.3
8-8.4
8.5
foot
of the Himalayas Southern
Years from which
and frequencies
59
40
14
4
1
1753
184
152
21
4
1
1512
117
97
19
1
part
of the QinghaiXizang Plateau and adjacent Central
areas
part
and eastern margin
of the Qing-
hai-Xizang Northern
Plateau
814
part
of the QinghaiXizang Plateau
74
51
19
3
region
14
60
16
5
Kush region
97
82
15
1907
24
21
3
1895
Tianshan 6
Hindu
7
West Kunlun
1
193 B.C. 1765
and
Pamirs region
132
tectonic movement at present is closely associated with the M > 5 earthquakes, which agrees with the evidence of movement of the Himalaya main central fault (MCT) and the Himalaya main boundary fault (MBT). Since these earthquakes occurred at depths of 20-40 km, their activity may have a relationship with the underlying and extending of the imbricate structure, MCT and MBT, as well as with their slow dipping toward the north. Along the reverse faults distributed in the foreland of the Himalayas and their surroundings the seismicity seems to be more stable. From 1961 to 1980 the land between 73.5OE and 75”E was a moderately seismic zone and around 75.5”E was a highly seismic zone. The strong activity of earthquake swarms usually appeared at the intersection of several major rifts, at a distance of 100 km to the south of the peak value, exactly beneath or in the thick-bedded sedimentary rocks (T) in the basin of the Indian river. Satellite images give a picture of folds since the Middle Cenozoic. The Himalayan region
This region was located at the front margin during the collision of the Indian and Eurasian continents. If we cut a section perpendicular to the central part of the Himalayan arcuate mountain system and make an observation upon the focal distribution in a range of 200 km on both sides of the southern foot of Himalaya, it can be seen that a shallow-focus earthquake belt lies at the southern slope of the eastern Himalaya, to the east of 80”E. From sections located to the west of Nepal and at the eastern end of the Himalayas (Fig. 2), we can see that the earthquakes are closely concentrated in the area between Nepal and the Himalaya where the geographic height suddenly increases (Huan et al., 1980; Shi et al., 1982) and the focal plane dips to the north with a low angle. In the area within western Nepal the focal plane dips at an angle of 30” and its maximum depth reaches about 70 km. Therefore, a shallow focal plane dipping toward the north with a low angle may exist beneath the southern slope of the Himalaya.
Main
A profile
fault
of
Himalayas
Himalayan
Mts.
Himalayan
Mts.
of earthquake
foci
across
a’
b’
the
southern
Himalayas
uted in schistose rocks. They are mainly shallow focus earthquakes with depths less than 70 km and most of the foci are shallower than 35 km; nevertheless, the data recorded by the network of stations in the Damxung region (Yang et al., 1981), which was set up in 1977, show that there are two groups of shallow focal planes slightly dipping toward the south at high angles in the Lhasa and Damxung regions (Fig. 3) with maximum depths of about 50 km. In addition, some shallow-focus earthquakes were distributed in the eastern and western parts of the Qinghai-Xizang Plateau. Generally speaking, most of the earthquakes which have occurred on the plateau were shallow ones, but the distribution of their focal depths was not uniform. In the Himalayan region and its southern margin the focal depths usually ranged from 40 to 70 km. On the western side of the northern and central parts of the plateau they
50i
9
Htkml
The Zangbei Plateau
Fig. 3. Distribution
The earthquakes
the Damxung
in this Plateau were distrib-
Boundary
region.
of the depths
of the earthquakes
0
18.5 km
of 1977 in
133
were mostly from 30 to 45 km. In the Szechuan-Yunnan rhombic block within the eastern part of the plateau and in the eastern part of the seismic region of the northern part of the plateau the foci were shallower than 20 km, and most of them were about 10 km. Intermediate-focus
100
200
300
A’(N)
400
A
300
earthquakes
(km)
A(S)
600
\ \
B’(N)
B(S)
It is worth noting that in the Qinghai-Xizang plateau the distribution of the intermediate focus earthquakes is related to deep large faults in the active tectonic zones having a special topographic expression (Figs. 4 and 5). The Western tip of the arc; the Hindu Kwh Pamirs region
500
and
In this region the shallow earthquakes with focal depths of less than 50 km (H-C 50 km) mainly occurred in the southern and eastern bases of several folded chains and along a line parallel to these chains. The intermediate earthquakes (H > 100 km) closely gathered in a limited region within the Xizang Plateau. It is difficult to subdivide the shallow and intermediate earthquakes into belts. The western tip of the arc seems to have met an obstruction and developed from NE to SW. Many of the intermediate focal planes in this region dipped towards the north with an angle of 45” (Fig. 4a) nevertheless, in the Pamirs region they dipped toward the SE at 60”. Their maximum depths were between 250-300 km, thus, a V-shaped distribution of focal planes was formed (Jeminicikaya, 1975; Tetsuo Santo, 1979). The maximum focal depth was 300 km, but they were mainly concentrated above 200 km. These intermediate focus earthquakes have no relationship with the present continental margins and trenches. Then what is their relationship with the stress which caused them? It is well known that the Hindu Kush is an active seismic zone of great significance in the world. Since many earthquakes took place in this limited range between 70”-72”E and 36”-37”N, it is clear that the occurrences of earthquakes in this region are related to the structure in the deeper crust. The surface geology around the active seismic zone
o-
I
..:
5 ,oo_’ 0 u
*.-, “.,
.
..* 1
500
600
. .
200-
300-
400(a) (km)
NW 100
200
300
400
\
200_)
India
plate Alagan
Eurasian shan
of depths
of intermediate
plate
y=zq$I=I
Fig. 4. Distribution the Himalayas
and adjacent
areas. b. India-Burma
regions.
earthquakes
a. Hindu-Kush
in
and Pamir
regions.
indicates that a Tertiary sedimentary folded zone on the north and a Palaeozoic strcture on the south were separated by a NE-SW striking fractured zone.
134
MBF
Ylrlun# Zanabo
Aitun
River Tanggula Shari
Fig. 5. Distribution
of intermediate
The seismic evidence demonstrates that the intermediate-focus earthquakes along the Hindu Kush seismic zone might have occurred due to bending of the two Mediterranean plates (Fig. 6; Jeminicikaya, 1975). One block might have subsided into the mantle due to its higher density. The intersection of these two structural units represents a clear place where the geological structure is sharply bent. In these places the earthquake frequency was rather high and the geological structure seems to be very complicated. For instance, the Hindu Kush complex seismic structure, the Yasman fractured zone, the Chaman fault and the Himalaya converged at the above-mentioned place in this region. The seismic activity has a distinct expression in the Lar region which lies at a sharp bend of the Zagros thrust fault. Whole of the seismic zone formed a distinctive mountain range; the ridge runs ENE with a dip toward the northwest and extends southward tuming gradually to nearly E-W. Compared to the velocity of seismic waves in the mantle within the ordinary continental regions, a significant increase Karakorum
Fig. 6. Simplified Kush and Pamir
Fault
map of the plate area.
subduction
in the Hindu
Shan
Kunlu,,
earthquakes
in the Himalayas.
of the velocities of P and S-waves could be observed, especially in the range of 100-200 km (Jeminicikaya, 1975). However, the low-velocity zone has not been found at the depth of 100 km or more, so we can infer that the mantle mass at this depth is probably very hard, earthquakes occur readily in the process of interaction. On the other hand, before the Indian and the Asian plates joined together, the fronts of these two plates had extremely projecting topographies, so that they first touched each other by collision and, then, their continental crust mass was pressed into the mantle. The continental crust formed a V-shaped hard mass belt in the mantle. Within the belt a N-S striking horizontal compression between these two plates and continuing upward migration of the material still causes constant earthquakes (Jeminicikaya, 1976). The eastern top part of the arcarea
the India-Burma
This area and the Hindu Kush region are composed of two symmetrical arc tops of the arcuate mountain system which sandwich the Himalaya between them. Here the northern part of the Burma arc and the eastern section of the arcuate mountain system intersect and form a zone where the stresses are even more concentrated on the top of the NE striking arc. However, the zone is not isolated, because its focal distribution extends slightly to the north and south, and connects in the south with that of a long seismic belt running along the island-arc system from the Andaman Islands through Sumatra to Java. The earthquakes with different depths were distributed nearly parallel to the western boundary of Burma and the arcuate mountain system, and then became deeper in the east (Fig. 4; Chang et al., 1973; Yan et al.,
135
1980, 1981). The intermediate
focus earthquakes
being
of this kind
under
medium
were
distributed
foots of Mt. Naga and Mt. Alsgan Their
maximum
not connect Tsangpo
was 200 km, but they did
with the seismic zone along the Yalu
River.
Burma quakes
depth
the eastern
in an arc shape.
is the only
of island-arc
of a continent. seismically this zone pressure
place
in which
type occur within
The appearance
the earththe interior
of this particular
active zone may be due to the fact that has not caused
been
subjected
by a northward
to the frontal movement
of the
established itself
by
and
the
the differently
tures. The fault plane the
south,
then
a V-shaped
tell because zone
distribution
earthquakes earthquakes
to a small number.
penetrates
the
mantle.
in the
and
of the
was caused, in this area
It is difficult
Further
to
if a Benioff study
and
(Teng et al., 1980).
It could be seen from the fractured appeared
frac-
the north
of the lack of evidence
proof are needed
in the
trending
tilted toward
shallow and intermediate though the intermediate only amount
inhomogeneity
focal
region,
that
zone, which the zone
is
Indian subcontinent. When the Indian subcontinent moved northwards, its pressure to the east was small. As is seen in Fig. 6, the giant plates did
composed of two groups of conjugate planes. In brief, in the western part of Hindu Kush the maximum focal depth is 320 km, while in the
not fit closely thus still retaining perfectly the characteristics of the earthquake which originated
eastern and central parts of the India-Burma mountain area it is 180-200 km and 140 km
in the island-arc
respectively
oceanic
system
due to the sinking
of the
plates.
The 1950 Mato earthquake (M = 8.5) and the 1897 Assam (India) earthquake (M = 8.7) occurred nearby at the turning point of the Himalayan arcuate mountain system, where the structure is considerably complicated and the stress
(Huan
et al., 1984; Shu et al., 1984)
which clearly reflects the time differences in stress conditions in the process of collision and compression between
Indian
Characteristics
and Eurasian
of seismicity
plates.
in the Qinghai-Xizang
Plateau and its marginal zone
is highly concentrated. Time-space The central part of the Himalaya arcuate mountain system, the Yalu Tsangpo River area In this area intermediate earthquakes are rarely seen. They are only scattered
in some places where
distribution
of shallow earthquake
tivity on the Qinghai-Xizang
ac-
Plateau
Long-standing information regarding historical earthquakes in Asia shows that in different earth-
the focal plane trends toward the south (Fig. 5). To the north of the Ganges River there were some intermediate earthquakes whose focal planes dipped northward. In the vicinities of Kashmir,
quake zones the seismicities vary almost neously. In some time intervals during
Kathmandu and Gauhati (India) the focal depths range between 70 and 100 km. To the north of the Ganges and Yalu Tsangpo rivers intermediate focus shocks occurred, their depths changing from 82 km to 180 km. Intermediate focus earthquakes were also distributed on both sides of the
intervals during an active state they occurred with higher frequency and higher magnitudes; the seismically quiet and seismically active stages repeated periodically. The time interval in which the
Himalayan
belt but the focal depths
on the south-
ern side were shallower than those on the northern side; they dipped to the north and the south respectively (Teng et al., 1980). It is clear from the data above that the Eurasian and Indian plates have been colliding for a long time. The faults in the earth’s crust formed one after another, the orientation of their dips in the transitional belt
state the earthquakes quency and smaller
occurred magnitudes;
simultaa quiet
with lower frein some time
seismicity of an earthquake zone develops from the relatively quiet stage to the relatively active stage can be regarded as a seismically active period, and the mean value of time duration of all the seismically active periods can be approximately considered as a repetition period of the seismicity in this zone. Figure 7 shows a time-space correlation of the large shallow earthquakes in various seismic zones within the plateau (Shi et al., 1982). Obviously, the
136
Periodicity, repeatability and mobi&
of seismic
activity on the Qinghai-Xizang Plateau
1500
1600
m:
1700
1800
1900
2ooc
36" 34O 32' 309 2 a" 2C 24' 1500
1600
1700m
1800
1900
2000
It can be seen from Table 2 that the repetition intervals in different regions within the QinghaiXizang Plateau are not the same. On both sides of the Yalu Tsangpo River and in the Himalayan region it is 40 years (6 < M < 8), in the central and northern parts of the Plateau it is 40 and 100 years, respectively. It is quite evident that the periodicity of seismic activities increased regularly from the collision-compressed transitional zone towards the north, i.e. along an extension into the plate.
1
Characteristics of the b-value
;-_ L
o;__ 1890
III
190019101920193019401950196019m19eo
Relotlve
qulescenc&
stage Remarkably
octfve
stage
The b-value shows the different types of seismicities in different earthquake zones. It may be related to the medium properties and the stress state in the localities and changes from a high b-value zone to a low b-value zone on the plate boundary as follows: (1) The b-values in the Hindu Kush-Tian Shan-Mongolian Altai-Baikal area were 0.73, 0.6, 0.54, 0.7. (2) In the southern, northern and central parts of the Qinghai-Xizang Plateau the b-values are 0.84, 0.75, 0.58 respectively. This demonstrates a transition from a high b-value in the collision compressed transitional zone to a low b-value within the plate. Characteristics of earthquakes (M 3 8)
0
8-8.4
0 0
7-74
0
6-6.9
Fig. 7. Time-space in different
75-7.9
distributions
seismic regions
of strong
shallow
of the Qinghai-Xizang
earthquakes Plateau.
earthquakes occurring in the relatively quiet stage and the significantly active stage are distributed in a regular pattern, but they are different from each other in their intensity and frequency.
In 1950 an earthquake of magnitude 8.5 took place in Medog county. Tracing back through the years before it occurred, one can see that there have been eight earthquakes (M > 7) in the range of 300-500 km with the Zayu county as a centre. All these earthquakes happened around the epicentre of the Zayu-Medog earthquake (Fig. 8). It should be pointed out that the occurrences of the 1950 M= 8.5 Medog earthquake and the 1951 M = 8 Danxung earthquake coincide with the statistical data which indicate that the M - 8 earthquakes of recent years happened in couples throughout China (data from Huang Shengmu, Lu
137
TABLE
2
Periodicity Region
of seismic activity Quiescent
on the Qinghai-Xizang Seismically
period
1
Plateau
(after Shi et al., 1982; Huan
active period 2
Frequency 6-6.9
3
(in times)
I-7.9
7.8
et al., 1980) Duration (yrs)
Repe-
Remark
tition interval
Southern
1897
part of
1918-1930
1897-1916
the Xizang
(three earth-
Plateau
quakes of magnitude
7
3
18
Long@ earthquakes
1916-1936
18 after 1957
3
19
40
2
(M = 7.3 or 7.4)
7
were recorded) Central part of the Xizang 100
Plateau Northern
1353-1560
part of
(three earth-
the Xizang
quakes of
Plateau
magnitude
9
1353-1718
5
1
355
6
were recorded) 1719-1876
1876-1979
300
(four earthquakes of magnitude 6 were recorded)
35
30
Jia Jing et al., 1982). In addition, acterized by a repeated motion
they were chartoward the east
and west, as well as by alternating
evolution.
The Stress distribution in the Plateau and its marginal zones
Qinghai-Xizang
The stress field direction obtained from focal mechanism solutions represents the tectonic stress field in this area. In the vast area from the Indian platform up to the Zangbei (northern Tibet) Plateau including the Hindu Kush and the top part of the eastern arc, the axis of principal compressive stress is nearly horizontal. The direction of the main compressive stress axis is between
25
Fig. 8. The phenomenon
of earthquakes
the 9 years before
the Medog
County
and the epicentres
of which
surrounded
(M = 7) occurring earthquake
in
of 15-8-1950
the Medog
epicentre.
NW and NE and perpendicular to the strike of the arcuate structure on the whole. The results from fault plane solutions show that faulting in the Himalayan seismic region is of the thrust type. Normal faults have been found on the Ganges plain near Delhi corresponding to the
138
TABLE
3
Frequency
of earthquakes
Earthquake
zones
and types of faults in the earthquake Frequency (in times)
The arcuate
Frequency
belt of the Himalayas
of earthquakes
thrust
in different
types of faults
strike-slip
normal
strike-slip
strike-slip
fault
fault
and thrust
normal
and
fault
mountain
system of the IndiaBurma regions Himalayan
belt
The arcuate Total amount Per cent
of Baluch
16
4
5
6
20
15
1
3
1 1
42
12
22
5
3
78
31
25
14
4
1
39.7
35.9
17.9
5.1
1.3
normal fault on the outer side of the island-arc and trench systems. A strike-slip fault feature can be clearly seen at the northern and eastern margins and in the central part of the Qinghai-Xizang Plateau either from satellite images or from fault plane solutions (Yan et al., 1980). These roughly parallel arcuate tectonics within the plateau are left-lateral strike-slip faults. As may be seen from Table 3, thrust and strike-slip type focal mechanism earthquakes dominate in the Himalaya, although other combinations of faulting are also observed. Discussion on the fault plane solution of the shallow and intermediate earthquakes Fault plane solution of the shallow earthquakes
The axis direction of the principal compressive stress of the shallow earthquakes with focal depth of 20-70 km changed at about the same time as the change in the direction of strikes of the arcuate structure. The axes of the principal compressive stresses in the Himalayan arcuate mountain system are mostly N-S and perpendicular to the strikes of the arcuate structure, in the eastern section of the system they point to the NNW, and in the western section they point to the NNE, in the eastern part of the India-Burma area they lie W-E (Yan et al., 1981), in the northern section of the Burma arc they point toward the WNW, in the southern of the Burma arc they point to the NNE (Fig. 9). Thus, a nearly E-W striking horizontal compression characterizes the structure of the Burma arc. The Irrawaddy basin, however, may be
the result of the normal faulting caused by an approximately W-E striking tension. In the Hindu Kush region the directions of the principal compressive stress axes are perpendicular to the strikes of surface structures (a NNW compression). The same focal mechanism solutions between the Burma and Himalaya arcs show that the directions of principal compressive stresses are also perpendicular to the strikes of the arcuate structure, that is to the ENE. The principal compressive stresses in the outside of the Baluch area act in NW direction. Most of the faults belong to the thrust or hinge type but some are of normal type. In the Kida fault zone there has been largescale left-lateral strike-slip movement since the Cretaceous. The focal mechanisms of earthquakes at depths of more than 70 km have the same regularity as those in the Himalayan and Burma arcs and correspond well with the topographic and geologic features of the localities. It is seen that the vertical movement still exists in the Xizang Plateau and most of the shallow earthquakes are more or less like those at depths of 20-70 km. Fault plane solution of earthquakes in the Mantle
In the area from Hindu Kush to Himalaya and further east to Burma, most principal compressive stress axes are of the focal dislocation type with an approximately N-S or nearly NNE compression, with normal or strike-slip faults. A few of the axes show reverse or thrust faults. The intermediate focus earthquake of August 13, 1976 oc-
139
Fig. 9. Horizontal
projections
of principal
compressive
curring in the vicinity of Lhasa (Tibet) was due to the activity of a normal fault (Yan et al., 1980) containing a strike-slip component, which indicates that in the upper mantle there is at present a tectonic stress field represented by a NNE striking horizontal compressive stress and a tensile stress perpendicular to the former (the axes of principal compressive stress is toward N4*E and the axis of principal stress is toward SlOSOE). Since this intermediate earthquake occurred on the northern side of the Yalu Tsangpo river, where the E-W striking Ganges Shan and NE striking Nyainqentanglha Shan (mountain ranges) intersect and the NE striking fault occurs, the neo-tectonic movement in this area is extremely active and earthquakes occur frequently. In 1951 an M = 8 earthquake shocked Damxung county. Considering the above data and in comb~ation with the earthquakes seen in the eastern and western parts of the Himalaya arcuate mountain system, we can suggest that the occurrence of these earthquakes may be controlled by a common factor. As to the earthquakes with foci deeper than 200 km, they are mainly characterized by horizontal compression and tension in the Hindu Kush region (Nowroozi, 1972).
axes of shallow
earthquakes
in the Qinghai-Xizang
Plateau.
It can also be pointed out that the occurrence of these earthquakes was due to lithosphere subduction, as the principal compressive stress axes of intermediate earthquakes were roughly parallel to the trend of the focal zone. The multiplicity of fault plane solutions for earthquakes in China, indicate that deformation in the mantle media at the contact zone is extremely complicated. ~~tEr~~rrnation~~~d~~l~ between crustaf and mantle media in the Xizang Plateau region In the Himalaya region the stress distribution shows that, when h > 70 km and h < 70 km (Ye et al., 1981), the directions of the principal compressive stress axes differ in their distribution (Fig. 10). Since the crustal thickness in plateau regions is up to 70 km (Teng et al., 1980), there must be a large ~scont~uity at about this depth. The arcuate structure is only formed in the crustal medium, and the upper mantle medium restricted by mass migration in depth is not involved in formation of the arcuate structure, thus the latter expresses a S-N striking horizontal compression. The directions of principal compressive stresses in the earth’s crust continuously vary with the strikes of
140 73
78 I”
83
88
93
98
1
1
1
I
I
1
73
78
73
/
78 1
I
1
83
88
83
93
88
93
1
98
98 1
1
25
p
73
Fig. 10. Fault plane solutions
78
83
of the P wave in the Himalaya
80
arc mountains
the arcuate structure which controls the stress distribution in this tectonic zone, and on both sides of the arcuate system the directions of stress agree with the surface structure. In the region of the Himalaya arcuate mountain system all the fault plane solutions gave a pattern of shallow sliding ( < 300), which could be seen even on the main boundary fault. Such data indicates that the shallow slipping and underthrusting are the main type.
93
and their vicinity.
98
a. h > 70 km. b. h i 70 km
Characteristics of seismic wave propagation in the Qinghai-Xizang Plateau Determination
of magnitude
characteristics
The data of the Lhasa seismic station show that for earthquakes outside the plateau the magnitudes observed by this station are usually lower than those recorded by other stations beyond the plateau. It means that the media of the plateau are
141
different in their structure and property. plateau is an independent block.
The
Stress drop determined from the P wave of Q small
TABLE
4
Q values for different No.
hind of earthquake
Region
Wave kind for
Q value
calculation
earthquake
Among the 77 M = 1.5-4.5 small earthquakes there were 38 earthquakes which occurred in the Damxung-Yangbajian-Zhigung-Zetang-Quxu district. The seismic data and observations of the P waves of small ear~qu~es show that the stress drop of small earthquakes in the Zhamo region is lo-30 bar (Yang et al., 1981) and in the Damxung region 5-15 bar, the latter being obviously lower than that of former. Meanwhile, within the epicentre distance of O-200 km the average velocity of a P wave calculated from its travel time in the upper crust is 5.65 km/s. The velocity value of a short-period Rayleigh wave in the Xizang Plateau obtained from seismic surface waves is lower than that in adjacent areas. Obviously, the crustal media of the plateau are not rigid.
waves
of Q value 1
Xizang Plateau
P wave of small earthquake
et,=35
2
Xizang Plateau
seismic S wave
es=25
3
Xizang Plateau
seismic surface
4
Haicheng
P wave of small
5
Haicheng
P wave of small
6
Shimian
P wave of small
wave
Qa = 30-40
earthquake
QP = 220-1047
earthquake
QP = 510
earthquake
QP = 560
Q values of seismic waves The Q value observed within the Xizang Plateau shows that all the Q values determined from surface waves, S waves and P waves of small earthquakes are smaller than those obtained in other regions; Qs = 25, Qr = 35 (Chang et al., 1980; Feng et al., 1980; Yang et al., 1981). The Q values gamed from long-period surface waves are 30-40. However, in the Haicheng and Shimian regions the Qp values obtained from P waves of small earthquakes are 200-1047, 510 and 560 (Molnar et al., 1975; Zhu et al., 1977; Wang, 1981), which shows that the absorbtion capability of mantle media in the Xizang Plateau is much strong than that in other regions (Table 4).
P IO
Propagation of seismic Lg waues
Figure 11 shows a distribution of the characteristics of Lg wave propagation in the QinghaiXizang Plateau. The Lg wave observed by seismic stations on and surrounding the plateau shows that the energy of Lg waves penetrating through
(b)
Fig. 11. Characteristics the Xizang through
Plateau
the plateau
of an Lg wave (l-9) but
not
(Ruzaikin
through
penetrating
it, (10-19)
et al., 1977).
into
penetrating
142
the Xizang had
only
Plateau
was considerably
a low frequency
decreased,
or even
disappeared
plane
solutions
usually
have a very clear config-
uration
and a simple
(Wei et al., 1981) while the energy of Lg waves not
relative
movement
penetrating
was rela-
continental
interior
the crustal
boundaries
of continent
through
the Xizang
tively
increased.
media
in this plateau
sorb
the
It demonstrates
seismic
above-mentioned
the
waves.
This
agrees
by Chinese
to ab-
with
the
boundary
clear and reliable,
component
of
the Xizang
and the characteristics quite complicated.
to the
a large
at
Soviet
of Fig. 11 is energy
high(Fig.
related
plate in
or
in
the
The pattern of deformation
of
seem to be
movement
in a wide belt
movement,
be the product
motions.
of repeated the
case
of motion
Thus, we can say, it is not only
to the horizontal
and
The
either in
and the related strong vertical Indian
and
in the
and continental
collision
stations
with a developed
and
case
faults
However,
to define precisely;
underthrust-convergence.
through
adjacent
acting
this case are difficult the
of narrow
the case is quite different.
seismic
and shown in the seismograms
frequency
ability
low Q value and low stress drop.
obtained
northern
Union
that
has a strong
The Lg wave not penetrating Plateau
Plateau
model
(slip vector).
Eurasian
but may After
subcontinents
the ap-
Ila). The seismograms in Fig. lib give us another picture, in which the the energy of the Lg wave
proached each other, the western top part of the arc, the Hindu Kush region, first came into colli-
decreased as it penetrated through the Xizang Plateau. With the extension of the route, the wave even disappeared. The characteristics of Lg waves
sion in the Paleocene Epoch (about 70 m.y. ago). Then the collision between the plates took place at
show that in the Sizang Plateau,
the Assam region
(India) and the extended zone of the western upper part of the Himalayan arcuate mountain system, the energy disappeared.
was much weakened
and even
the eastern part of the arc in the India-Burma mountain region in the Oligocene Epoch (50 m.y. ago) and these two giant plates bordered fully on each other in the Late Oligocene (about 30 m.y. ago). This led to an intense compression and a severe deformation in the earth’s crust, caused a
Such a feature can also be seen from the characteristics of Lg wave propagation in various regions in China, including the eastern part of China,
progressive rise of the Himalayan mountain system and the strong folding of the lithosphere in a horizontal direction (about 2000 km) and formed
though this part is quite unlike the Xizang region (Wei et al., 1981). The time period of Lg wave
a criss-cross network of faults, a topographic lift and a thick crust (Teng et al., 1981). Therefore, a pattern of intensive seismicity
penetrating through the path in the QinghaiXizang Plateau is twice as long as that of the Lg wave penetrating through the various regions in China, (Wang, 1981, table 5) and the range of the time period is much wider.
upand
a complicated stress field was established. The crustal mass did not only bend with the arcuate structure, but also flowed transversely, which intensified the seismic activity and terrestrial heat flow. Meanwhile, the above processes extended to
Characteristics of seismicity and plate tectonics
the east through Afghanistan and the Xizang Plateau. The crustal shortening and the mass
To summarize Himalayan region
it could be seen that the is one of the most important
migration exerted an influence on the stress field distribution in vast areas of China (Fig. 12).
seismically active zones throughout the world, as well as a place where shallow focus earthquakes are widely distributed, but the characteristics of seismicity in this region are different from those in most other parts of the world. The seismicity all over the world, including the location and type of motion, mostly agrees with the theory of plate tectonics. It is well known that the seismicity on the boundaries of oceanic plates and the fault
What type of collision occurred between the Indian plate and the Eurasian plate? The versions are different. Someone regards it as an underthrust (Li et al., 1980) someone considers it to be an upthrust and others believe it to be a collisioncompression and an opposite underthrust in the crustal media (Huan et al., 1980). But, from the above-mentioned special patterns of seismicity, focal distribution, seismic wave propagation and
143
focal
mechanism
solutions
after the disintegration
it could
be seen that
of Gondwanaland
the In-
dian plate was (Zhu et al., 1984) divided
into three
dian plate was subjected Eurasian,
and
Qinghai-Xizang
then
to an obstruction
was
Plateau.
by the
thrust
underneath
Owing
to the collision
the
blocks A, B, C and further drifted to the north and collided with the Eurasian plate. Early in the
and the long-period of compression between these two plates, the loading on the collision-compres-
Triassic
sion transitional
period
the Indian
plate were separated
by vast oceans, between
lay a Tethys
Sea. During
Indian
started
plate
plate and the Eurasian the Middle
to disintegrate
them
Triassic and
the
divided
zone (Teng et al., 1980) between
two plates was very strong, graphic uplift and formation
same time forcing the mass down into a “bending”
into three blocks. In the Middle
and Late Jurassic,
from
the boundaries
block
block)
over,
in this portion,
A (the present
contact
Tanggula
with Eurasia.
Block B (the present
dise block) was drifting with Eurasia
made
first
Gang-
deformation
intensified
continuously
causing
in the Middle
Cretaceous.
These two
forming the configuration of present continents. After the collision of these two plates, the Indian plate continuously acted upon the Eurasian plate with a very strong force. However, the In-
01
fault,
projections
3 = compressive
stresses,
of principal
.3
compressive
4 = tensile stresses,
--4
and the faults grew thickly, earthquakes
along the stress
of plate,
110
120 --5
stress axes for shallow earthquakes
5 = boundary
More-
concentrated,
roughly parallel to the trend of the focal plane. For these reasons, on the both sides of the collision-compression transitional zone between two plates a pattern of opposite under thrusts was formed and the V-shaped structure of the focal
110
.2
the stresses
plane. This is proof that the principal compressive stress direction of the intermediate earthquakes is
100
9”
Fig. 12. Horizontal
of these two plates.
in the ocean, then collided
plates continued to collide until Eocene or Oligocene times and the Tethys Sea disappeared, thus
thus causing a topoof the plateau, at the
6 = transition
--__
in China. belt.
6
1 = normal
120
17
fault, 2 = strike-slip
144
plane was initiated in the Hindu Kush region, the central part of Himalayas and the eastern top part of the arc. The Himalayan region is not only a distribution zone of the geophysical anomaly field, but also a place where hydrothermal activity occured (Teng et al., 1980; Wei et al., 1983) the ophiolite belts regularly appeared and the crustal structure suddenly changed. On this basis a pattern of plate collision and interplate seismicity and a model of collision of the type of opposite “underthrust” between two giant plates were formed.
in the Central
and Eastern
Asian continent.
Scientia
Sinica,
motion
in the
13: 840-849. Shu, P.Y. et al., 1984. Stress Karakoram
tor), International University Teng,
field and
and surrounding Karakoram
J.W.
et al., 1979. China
“Root
of the world”
neighbouring
Acta Geophys.
J.W. et al., 1981. Explosion
distribution
and structure
from Damxung
of the geophysical
of the Qinghai-Xizang
regions.
field
plateau
and its
Sin., 23 (3): 254-296.
seismic
study
of the crust
to Yadong
still moving
28 (16): 64-65.
Teng, J.W. et al., 1980. Characteristics
Teng,
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pp. 185-199.
Recontructs,
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mantle
Acta
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129
134 (1987) 129-144
Science Publishers
B.V., Amsterdam
- Printed
in The Netherlands
The characteristics in the Qinghai-Xizang
of the seismic activity (Tibet) Plateau of China
TENG JIWEN, WE1 SIYU, SUN KEZHANG Institute of Geophysics, Academia (Received
June 21,1985;
and XUE CHANGSHUN
Sinica, BeiJmg (P.R. of China) accepted
.lanuary
1,1986)
Abstract Teng, J.W., Wei, S.Y., Sun, K.Z. and Xue, C.S., 1987. The characteristics (Tibet)
Plateau
of China,
In: H.K. Gupta
(Editor),
Deep Seated
of the seismic activity
Processes in Collision
in the Qinghai-Xizang
Zones.
Tectonophysics.
134:
129-144. Based
on the analysis
Qinghai-Xizang plate,
Plateau
this paper
between
the Indian
studies
of the spatial and its marginal the characteristics
and Eurasian
plates,
distribution region,
of many
of this plateau and the relations
and between
Introduction
The Qinghai-Xizang (Tibet) Plateau is adjacent to the Yunnan, Szechuan, Gansu Provinces and the Sinking Uigher autonomous region of China on the north and the east. Its western, southern and southwestern parts border on India, Nepal, Sikkim and Burma. The area of this plateau is over 1.2 million square kilometres, nevertheless, the area concerned in the present study is more extensive, covering a vast area between 15”-42”N and 60”-180”E. The appearance of the Himalayan mountain system and the rise of the Qinghai-Xizang Plateau are the greatest events in the history of the earth in the Cenozoic in Asia. At that time, when the Indian plate migrated northward and the ancient trench was thrust down into the southern side of the Eurasian plate, the Indian subcontinent was slowly approaching the Eurasian plate, and finally they collided with each other, thus forming a long-lasting compression, causing a repeat of folding, rifting, overthrusting and overlapping, and a 0040-1951/87/$03.50
shallow-
their fault plane solutions
0 1987 Blsevier Science Publishers
B.V.
and intermediate-depth
earthquakes
and the seismo-tectonics
the transitional this Plateau
zone of collision and continental
in the
in the continental and
compression
plate movement.
regular distribution of ophiolite suites, melange and g;laucophane-schists on the boundary between these two plates. As a result, the present Himalayan mountain system was formed (Chang et al.! 1973; Teng et al., 1980). Th.e Qinghai-Xizang Plateau is a giant Cenozoic mountain-building area and the youngest active zone of recent tectonic movement, as well as one of the earthquake-prone areas in the west of China. The tectonic deformation in this area is not only complicated but also has its regional characteristics. The destructive earthquakes within the plateau were not the same as those which occurred in other parts of China. Their geographic distribution is in belts with a certain density. In these belts, the large and small earthquakes showed a similarity of occurrence, intensity and space and had a relationship with the geologic structures of the plateau. For these reasons, the study of seismic activky, fault-plane solution, characteristics of wave propagation and the relationship with the structural motion and plate tectonic movement in this area is of great significance in the study of
130
continental plate-tectonics and seismic marks in the boundary between plates and inner factors of plateau-uplift. On the basis of a large amount of seismic data collected since 1902, in accordance with the seismic parameters, fault-mechanism solutions, stress field and characteristics of wave propagation determined in recent years by the special regional network of seismic stations, in combination with the research into geological structure and crustal and upper mantle physics, we have carried out a study into seismic activity, distribution of stress fields, plateau uplift, formation of a large and thick crust (Teng et al., 1981; Ye et al., 1981) and special seismic patterns. In addition we have analyzed the collision of the Indian plate with the Eurasian plate and the characteristics of transitional belts formed by this collision. The focal distribution within the Qingha-Xizang Plateau and its marginal zones In the distribution of epicentres throughout the world there are two characteristics: (1) the concentration of epicentres on the boundary between two plates; (2) the concentration of epicentres in special tectonic active regions within the plate. It indicates that seismic activity is associated with the block movement in the plate or on the plate boundaries. We would like to point out that the focal activity and its distribution are not uniform, so the collision and deformation of these two large plates along the compression zone were different in their extent, and the eastern and western upper parts of the arcuate mountain system were extremely deformed. The focal distribution (Huan et al., 1980) within the Qinghai-Xizang Plateau and its surrounding regions indicates that it is still an active seismic zone. The plateau includes the area north of the Ganges Plateau, south of the Alkin mountains, west of the Szechuan-Yunnan district and east of the Hindu Kush and Pamir Plateau. Within the plate the foci are distributed along four arcuate mountain systems running parallel to each other and running from the east to the southwest. As is seen in Fig. 1, in this area are not only distributed the shallow-focus earthquakes with depths of no
more than 70 km in the earth’s crust, but also the intermediate focus earthquakes originating from the middle of the upper mantle are concentrated in the eastern and western upper parts of the Himalayan arcuate mountain system. The Distribution
of shallow-focus
earthquakes
In the Himalayan region within the QinghaiXizang Plateau seismic activity is more intensive than in the northern part of the plateau, and the foci are located more deeply than those in East China. The Pamirs, Mt. Kunlun, Mt. Transversal, Mt. Himalaya and India-Burma mountain areas are mainly classified as the zones prone to shallow-focus earthquakes (with depths of 35-70 km). The whole of the plateau is affected by large earthquakes (it4 2 7) along its edges. It presents a special pattern, as if the M > 7 earthquakes are distributed along the margins of the plateau, and gives us an outline of the seismic zones in this area. According to incomplete statistics, during recorded history (Yan et al., 1980; Ye et al., 1981) within the plateau and its surrounding areas there have been 20 earthquakes with M > 8 (Table 1) and many more earthquakes with M = 6-6.9 or 7-7.9. They are characterized by features of distinct zonation and regionalization. The ground surface of the Himalayan region was uplifted highest of all. There the crustal thickness changes greatly, but the foci are shallow in places far from the intermediate focus earthquake regions. For instance, the focal depths in the northern part of the plateau, the Szechuan-Yunnan district and the Ganges plain are usually between 0 and 35 km. The northern marginal zone of the Ganges plain
This marginal zone, is seismically still a very active region. It strikes roughly parallel to the Himalayan arcuate mountain system. The observations of 1961-1981 show that most of the earthquakes were shallow shocks with focal depths of 70 km. This seismically active zone is located between the northern margin of the Indian shield and the southern foot of the Himalaya range with a width of about 200 km. The most intensive
.
Hetan
New
Focal
depths
Delhi \
(km)
II
Fig
“Y
1. A map of the distribution
TABLE
of earthquake
J”
foci in the Qinghai-Xizang
Plateau
and its vicinity.
1
Large earthquakes No.
I .
in the Qinghai-Xizang
Regions
in which the
earthquakes
took place
Plateau
and adjacent
regions
Total No. of
Magnitudes
earthquakes
of earthquakes
M = 64.9 Southern
for each region
the data are used
l-l.3
8-8.4
8.5
foot
of the Himalayas Southern
Years from which
and frequencies
59
40
14
4
1
1753
184
152
21
4
1
1512
117
97
19
1
part
of the QinghaiXizang Plateau and adjacent Central
areas
part
and eastern margin
of the Qing-
hai-Xizang Northern
Plateau
814
part
of the QinghaiXizang Plateau
74
51
19
3
region
14
60
16
5
Kush region
97
82
15
1907
24
21
3
1895
Tianshan 6
Hindu
7
West Kunlun
1
193 B.C. 1765
and
Pamirs region
132
tectonic movement at present is closely associated with the M > 5 earthquakes, which agrees with the evidence of movement of the Himalaya main central fault (MCT) and the Himalaya main boundary fault (MBT). Since these earthquakes occurred at depths of 20-40 km, their activity may have a relationship with the underlying and extending of the imbricate structure, MCT and MBT, as well as with their slow dipping toward the north. Along the reverse faults distributed in the foreland of the Himalayas and their surroundings the seismicity seems to be more stable. From 1961 to 1980 the land between 73.5OE and 75”E was a moderately seismic zone and around 75.5”E was a highly seismic zone. The strong activity of earthquake swarms usually appeared at the intersection of several major rifts, at a distance of 100 km to the south of the peak value, exactly beneath or in the thick-bedded sedimentary rocks (T) in the basin of the Indian river. Satellite images give a picture of folds since the Middle Cenozoic. The Himalayan region
This region was located at the front margin during the collision of the Indian and Eurasian continents. If we cut a section perpendicular to the central part of the Himalayan arcuate mountain system and make an observation upon the focal distribution in a range of 200 km on both sides of the southern foot of Himalaya, it can be seen that a shallow-focus earthquake belt lies at the southern slope of the eastern Himalaya, to the east of 80”E. From sections located to the west of Nepal and at the eastern end of the Himalayas (Fig. 2), we can see that the earthquakes are closely concentrated in the area between Nepal and the Himalaya where the geographic height suddenly increases (Huan et al., 1980; Shi et al., 1982) and the focal plane dips to the north with a low angle. In the area within western Nepal the focal plane dips at an angle of 30” and its maximum depth reaches about 70 km. Therefore, a shallow focal plane dipping toward the north with a low angle may exist beneath the southern slope of the Himalaya.
Main
A profile
fault
of
Himalayas
Himalayan
Mts.
Himalayan
Mts.
of earthquake
foci
across
a’
b’
the
southern
Himalayas
uted in schistose rocks. They are mainly shallow focus earthquakes with depths less than 70 km and most of the foci are shallower than 35 km; nevertheless, the data recorded by the network of stations in the Damxung region (Yang et al., 1981), which was set up in 1977, show that there are two groups of shallow focal planes slightly dipping toward the south at high angles in the Lhasa and Damxung regions (Fig. 3) with maximum depths of about 50 km. In addition, some shallow-focus earthquakes were distributed in the eastern and western parts of the Qinghai-Xizang Plateau. Generally speaking, most of the earthquakes which have occurred on the plateau were shallow ones, but the distribution of their focal depths was not uniform. In the Himalayan region and its southern margin the focal depths usually ranged from 40 to 70 km. On the western side of the northern and central parts of the plateau they
50i
9
Htkml
The Zangbei Plateau
Fig. 3. Distribution
The earthquakes
the Damxung
in this Plateau were distrib-
Boundary
region.
of the depths
of the earthquakes
0
18.5 km
of 1977 in
133
were mostly from 30 to 45 km. In the Szechuan-Yunnan rhombic block within the eastern part of the plateau and in the eastern part of the seismic region of the northern part of the plateau the foci were shallower than 20 km, and most of them were about 10 km. Intermediate-focus
100
200
300
A’(N)
400
A
300
earthquakes
(km)
A(S)
600
\ \
B’(N)
B(S)
It is worth noting that in the Qinghai-Xizang plateau the distribution of the intermediate focus earthquakes is related to deep large faults in the active tectonic zones having a special topographic expression (Figs. 4 and 5). The Western tip of the arc; the Hindu Kwh Pamirs region
500
and
In this region the shallow earthquakes with focal depths of less than 50 km (H-C 50 km) mainly occurred in the southern and eastern bases of several folded chains and along a line parallel to these chains. The intermediate earthquakes (H > 100 km) closely gathered in a limited region within the Xizang Plateau. It is difficult to subdivide the shallow and intermediate earthquakes into belts. The western tip of the arc seems to have met an obstruction and developed from NE to SW. Many of the intermediate focal planes in this region dipped towards the north with an angle of 45” (Fig. 4a) nevertheless, in the Pamirs region they dipped toward the SE at 60”. Their maximum depths were between 250-300 km, thus, a V-shaped distribution of focal planes was formed (Jeminicikaya, 1975; Tetsuo Santo, 1979). The maximum focal depth was 300 km, but they were mainly concentrated above 200 km. These intermediate focus earthquakes have no relationship with the present continental margins and trenches. Then what is their relationship with the stress which caused them? It is well known that the Hindu Kush is an active seismic zone of great significance in the world. Since many earthquakes took place in this limited range between 70”-72”E and 36”-37”N, it is clear that the occurrences of earthquakes in this region are related to the structure in the deeper crust. The surface geology around the active seismic zone
o-
I
..:
5 ,oo_’ 0 u
*.-, “.,
.
..* 1
500
600
. .
200-
300-
400(a) (km)
NW 100
200
300
400
\
200_)
India
plate Alagan
Eurasian shan
of depths
of intermediate
plate
y=zq$I=I
Fig. 4. Distribution the Himalayas
and adjacent
areas. b. India-Burma
regions.
earthquakes
a. Hindu-Kush
in
and Pamir
regions.
indicates that a Tertiary sedimentary folded zone on the north and a Palaeozoic strcture on the south were separated by a NE-SW striking fractured zone.
134
MBF
Ylrlun# Zanabo
Aitun
River Tanggula Shari
Fig. 5. Distribution
of intermediate
The seismic evidence demonstrates that the intermediate-focus earthquakes along the Hindu Kush seismic zone might have occurred due to bending of the two Mediterranean plates (Fig. 6; Jeminicikaya, 1975). One block might have subsided into the mantle due to its higher density. The intersection of these two structural units represents a clear place where the geological structure is sharply bent. In these places the earthquake frequency was rather high and the geological structure seems to be very complicated. For instance, the Hindu Kush complex seismic structure, the Yasman fractured zone, the Chaman fault and the Himalaya converged at the above-mentioned place in this region. The seismic activity has a distinct expression in the Lar region which lies at a sharp bend of the Zagros thrust fault. Whole of the seismic zone formed a distinctive mountain range; the ridge runs ENE with a dip toward the northwest and extends southward tuming gradually to nearly E-W. Compared to the velocity of seismic waves in the mantle within the ordinary continental regions, a significant increase Karakorum
Fig. 6. Simplified Kush and Pamir
Fault
map of the plate area.
subduction
in the Hindu
Shan
Kunlu,,
earthquakes
in the Himalayas.
of the velocities of P and S-waves could be observed, especially in the range of 100-200 km (Jeminicikaya, 1975). However, the low-velocity zone has not been found at the depth of 100 km or more, so we can infer that the mantle mass at this depth is probably very hard, earthquakes occur readily in the process of interaction. On the other hand, before the Indian and the Asian plates joined together, the fronts of these two plates had extremely projecting topographies, so that they first touched each other by collision and, then, their continental crust mass was pressed into the mantle. The continental crust formed a V-shaped hard mass belt in the mantle. Within the belt a N-S striking horizontal compression between these two plates and continuing upward migration of the material still causes constant earthquakes (Jeminicikaya, 1976). The eastern top part of the arcarea
the India-Burma
This area and the Hindu Kush region are composed of two symmetrical arc tops of the arcuate mountain system which sandwich the Himalaya between them. Here the northern part of the Burma arc and the eastern section of the arcuate mountain system intersect and form a zone where the stresses are even more concentrated on the top of the NE striking arc. However, the zone is not isolated, because its focal distribution extends slightly to the north and south, and connects in the south with that of a long seismic belt running along the island-arc system from the Andaman Islands through Sumatra to Java. The earthquakes with different depths were distributed nearly parallel to the western boundary of Burma and the arcuate mountain system, and then became deeper in the east (Fig. 4; Chang et al., 1973; Yan et al.,
135
1980, 1981). The intermediate
focus earthquakes
being
of this kind
under
medium
were
distributed
foots of Mt. Naga and Mt. Alsgan Their
maximum
not connect Tsangpo
was 200 km, but they did
with the seismic zone along the Yalu
River.
Burma quakes
depth
the eastern
in an arc shape.
is the only
of island-arc
of a continent. seismically this zone pressure
place
in which
type occur within
The appearance
the earththe interior
of this particular
active zone may be due to the fact that has not caused
been
subjected
by a northward
to the frontal movement
of the
established itself
by
and
the
the differently
tures. The fault plane the
south,
then
a V-shaped
tell because zone
distribution
earthquakes earthquakes
to a small number.
penetrates
the
mantle.
in the
and
of the
was caused, in this area
It is difficult
Further
to
if a Benioff study
and
(Teng et al., 1980).
It could be seen from the fractured appeared
frac-
the north
of the lack of evidence
proof are needed
in the
trending
tilted toward
shallow and intermediate though the intermediate only amount
inhomogeneity
focal
region,
that
zone, which the zone
is
Indian subcontinent. When the Indian subcontinent moved northwards, its pressure to the east was small. As is seen in Fig. 6, the giant plates did
composed of two groups of conjugate planes. In brief, in the western part of Hindu Kush the maximum focal depth is 320 km, while in the
not fit closely thus still retaining perfectly the characteristics of the earthquake which originated
eastern and central parts of the India-Burma mountain area it is 180-200 km and 140 km
in the island-arc
respectively
oceanic
system
due to the sinking
of the
plates.
The 1950 Mato earthquake (M = 8.5) and the 1897 Assam (India) earthquake (M = 8.7) occurred nearby at the turning point of the Himalayan arcuate mountain system, where the structure is considerably complicated and the stress
(Huan
et al., 1984; Shu et al., 1984)
which clearly reflects the time differences in stress conditions in the process of collision and compression between
Indian
Characteristics
and Eurasian
of seismicity
plates.
in the Qinghai-Xizang
Plateau and its marginal zone
is highly concentrated. Time-space The central part of the Himalaya arcuate mountain system, the Yalu Tsangpo River area In this area intermediate earthquakes are rarely seen. They are only scattered
in some places where
distribution
of shallow earthquake
tivity on the Qinghai-Xizang
ac-
Plateau
Long-standing information regarding historical earthquakes in Asia shows that in different earth-
the focal plane trends toward the south (Fig. 5). To the north of the Ganges River there were some intermediate earthquakes whose focal planes dipped northward. In the vicinities of Kashmir,
quake zones the seismicities vary almost neously. In some time intervals during
Kathmandu and Gauhati (India) the focal depths range between 70 and 100 km. To the north of the Ganges and Yalu Tsangpo rivers intermediate focus shocks occurred, their depths changing from 82 km to 180 km. Intermediate focus earthquakes were also distributed on both sides of the
intervals during an active state they occurred with higher frequency and higher magnitudes; the seismically quiet and seismically active stages repeated periodically. The time interval in which the
Himalayan
belt but the focal depths
on the south-
ern side were shallower than those on the northern side; they dipped to the north and the south respectively (Teng et al., 1980). It is clear from the data above that the Eurasian and Indian plates have been colliding for a long time. The faults in the earth’s crust formed one after another, the orientation of their dips in the transitional belt
state the earthquakes quency and smaller
occurred magnitudes;
simultaa quiet
with lower frein some time
seismicity of an earthquake zone develops from the relatively quiet stage to the relatively active stage can be regarded as a seismically active period, and the mean value of time duration of all the seismically active periods can be approximately considered as a repetition period of the seismicity in this zone. Figure 7 shows a time-space correlation of the large shallow earthquakes in various seismic zones within the plateau (Shi et al., 1982). Obviously, the
136
Periodicity, repeatability and mobi&
of seismic
activity on the Qinghai-Xizang Plateau
1500
1600
m:
1700
1800
1900
2ooc
36" 34O 32' 309 2 a" 2C 24' 1500
1600
1700m
1800
1900
2000
It can be seen from Table 2 that the repetition intervals in different regions within the QinghaiXizang Plateau are not the same. On both sides of the Yalu Tsangpo River and in the Himalayan region it is 40 years (6 < M < 8), in the central and northern parts of the Plateau it is 40 and 100 years, respectively. It is quite evident that the periodicity of seismic activities increased regularly from the collision-compressed transitional zone towards the north, i.e. along an extension into the plate.
1
Characteristics of the b-value
;-_ L
o;__ 1890
III
190019101920193019401950196019m19eo
Relotlve
qulescenc&
stage Remarkably
octfve
stage
The b-value shows the different types of seismicities in different earthquake zones. It may be related to the medium properties and the stress state in the localities and changes from a high b-value zone to a low b-value zone on the plate boundary as follows: (1) The b-values in the Hindu Kush-Tian Shan-Mongolian Altai-Baikal area were 0.73, 0.6, 0.54, 0.7. (2) In the southern, northern and central parts of the Qinghai-Xizang Plateau the b-values are 0.84, 0.75, 0.58 respectively. This demonstrates a transition from a high b-value in the collision compressed transitional zone to a low b-value within the plate. Characteristics of earthquakes (M 3 8)
0
8-8.4
0 0
7-74
0
6-6.9
Fig. 7. Time-space in different
75-7.9
distributions
seismic regions
of strong
shallow
of the Qinghai-Xizang
earthquakes Plateau.
earthquakes occurring in the relatively quiet stage and the significantly active stage are distributed in a regular pattern, but they are different from each other in their intensity and frequency.
In 1950 an earthquake of magnitude 8.5 took place in Medog county. Tracing back through the years before it occurred, one can see that there have been eight earthquakes (M > 7) in the range of 300-500 km with the Zayu county as a centre. All these earthquakes happened around the epicentre of the Zayu-Medog earthquake (Fig. 8). It should be pointed out that the occurrences of the 1950 M= 8.5 Medog earthquake and the 1951 M = 8 Danxung earthquake coincide with the statistical data which indicate that the M - 8 earthquakes of recent years happened in couples throughout China (data from Huang Shengmu, Lu
137
TABLE
2
Periodicity Region
of seismic activity Quiescent
on the Qinghai-Xizang Seismically
period
1
Plateau
(after Shi et al., 1982; Huan
active period 2
Frequency 6-6.9
3
(in times)
I-7.9
7.8
et al., 1980) Duration (yrs)
Repe-
Remark
tition interval
Southern
1897
part of
1918-1930
1897-1916
the Xizang
(three earth-
Plateau
quakes of magnitude
7
3
18
Long@ earthquakes
1916-1936
18 after 1957
3
19
40
2
(M = 7.3 or 7.4)
7
were recorded) Central part of the Xizang 100
Plateau Northern
1353-1560
part of
(three earth-
the Xizang
quakes of
Plateau
magnitude
9
1353-1718
5
1
355
6
were recorded) 1719-1876
1876-1979
300
(four earthquakes of magnitude 6 were recorded)
35
30
Jia Jing et al., 1982). In addition, acterized by a repeated motion
they were chartoward the east
and west, as well as by alternating
evolution.
The Stress distribution in the Plateau and its marginal zones
Qinghai-Xizang
The stress field direction obtained from focal mechanism solutions represents the tectonic stress field in this area. In the vast area from the Indian platform up to the Zangbei (northern Tibet) Plateau including the Hindu Kush and the top part of the eastern arc, the axis of principal compressive stress is nearly horizontal. The direction of the main compressive stress axis is between
25
Fig. 8. The phenomenon
of earthquakes
the 9 years before
the Medog
County
and the epicentres
of which
surrounded
(M = 7) occurring earthquake
in
of 15-8-1950
the Medog
epicentre.
NW and NE and perpendicular to the strike of the arcuate structure on the whole. The results from fault plane solutions show that faulting in the Himalayan seismic region is of the thrust type. Normal faults have been found on the Ganges plain near Delhi corresponding to the
138
TABLE
3
Frequency
of earthquakes
Earthquake
zones
and types of faults in the earthquake Frequency (in times)
The arcuate
Frequency
belt of the Himalayas
of earthquakes
thrust
in different
types of faults
strike-slip
normal
strike-slip
strike-slip
fault
fault
and thrust
normal
and
fault
mountain
system of the IndiaBurma regions Himalayan
belt
The arcuate Total amount Per cent
of Baluch
16
4
5
6
20
15
1
3
1 1
42
12
22
5
3
78
31
25
14
4
1
39.7
35.9
17.9
5.1
1.3
normal fault on the outer side of the island-arc and trench systems. A strike-slip fault feature can be clearly seen at the northern and eastern margins and in the central part of the Qinghai-Xizang Plateau either from satellite images or from fault plane solutions (Yan et al., 1980). These roughly parallel arcuate tectonics within the plateau are left-lateral strike-slip faults. As may be seen from Table 3, thrust and strike-slip type focal mechanism earthquakes dominate in the Himalaya, although other combinations of faulting are also observed. Discussion on the fault plane solution of the shallow and intermediate earthquakes Fault plane solution of the shallow earthquakes
The axis direction of the principal compressive stress of the shallow earthquakes with focal depth of 20-70 km changed at about the same time as the change in the direction of strikes of the arcuate structure. The axes of the principal compressive stresses in the Himalayan arcuate mountain system are mostly N-S and perpendicular to the strikes of the arcuate structure, in the eastern section of the system they point to the NNW, and in the western section they point to the NNE, in the eastern part of the India-Burma area they lie W-E (Yan et al., 1981), in the northern section of the Burma arc they point toward the WNW, in the southern of the Burma arc they point to the NNE (Fig. 9). Thus, a nearly E-W striking horizontal compression characterizes the structure of the Burma arc. The Irrawaddy basin, however, may be
the result of the normal faulting caused by an approximately W-E striking tension. In the Hindu Kush region the directions of the principal compressive stress axes are perpendicular to the strikes of surface structures (a NNW compression). The same focal mechanism solutions between the Burma and Himalaya arcs show that the directions of principal compressive stresses are also perpendicular to the strikes of the arcuate structure, that is to the ENE. The principal compressive stresses in the outside of the Baluch area act in NW direction. Most of the faults belong to the thrust or hinge type but some are of normal type. In the Kida fault zone there has been largescale left-lateral strike-slip movement since the Cretaceous. The focal mechanisms of earthquakes at depths of more than 70 km have the same regularity as those in the Himalayan and Burma arcs and correspond well with the topographic and geologic features of the localities. It is seen that the vertical movement still exists in the Xizang Plateau and most of the shallow earthquakes are more or less like those at depths of 20-70 km. Fault plane solution of earthquakes in the Mantle
In the area from Hindu Kush to Himalaya and further east to Burma, most principal compressive stress axes are of the focal dislocation type with an approximately N-S or nearly NNE compression, with normal or strike-slip faults. A few of the axes show reverse or thrust faults. The intermediate focus earthquake of August 13, 1976 oc-
139
Fig. 9. Horizontal
projections
of principal
compressive
curring in the vicinity of Lhasa (Tibet) was due to the activity of a normal fault (Yan et al., 1980) containing a strike-slip component, which indicates that in the upper mantle there is at present a tectonic stress field represented by a NNE striking horizontal compressive stress and a tensile stress perpendicular to the former (the axes of principal compressive stress is toward N4*E and the axis of principal stress is toward SlOSOE). Since this intermediate earthquake occurred on the northern side of the Yalu Tsangpo river, where the E-W striking Ganges Shan and NE striking Nyainqentanglha Shan (mountain ranges) intersect and the NE striking fault occurs, the neo-tectonic movement in this area is extremely active and earthquakes occur frequently. In 1951 an M = 8 earthquake shocked Damxung county. Considering the above data and in comb~ation with the earthquakes seen in the eastern and western parts of the Himalaya arcuate mountain system, we can suggest that the occurrence of these earthquakes may be controlled by a common factor. As to the earthquakes with foci deeper than 200 km, they are mainly characterized by horizontal compression and tension in the Hindu Kush region (Nowroozi, 1972).
axes of shallow
earthquakes
in the Qinghai-Xizang
Plateau.
It can also be pointed out that the occurrence of these earthquakes was due to lithosphere subduction, as the principal compressive stress axes of intermediate earthquakes were roughly parallel to the trend of the focal zone. The multiplicity of fault plane solutions for earthquakes in China, indicate that deformation in the mantle media at the contact zone is extremely complicated. ~~tEr~~rrnation~~~d~~l~ between crustaf and mantle media in the Xizang Plateau region In the Himalaya region the stress distribution shows that, when h > 70 km and h < 70 km (Ye et al., 1981), the directions of the principal compressive stress axes differ in their distribution (Fig. 10). Since the crustal thickness in plateau regions is up to 70 km (Teng et al., 1980), there must be a large ~scont~uity at about this depth. The arcuate structure is only formed in the crustal medium, and the upper mantle medium restricted by mass migration in depth is not involved in formation of the arcuate structure, thus the latter expresses a S-N striking horizontal compression. The directions of principal compressive stresses in the earth’s crust continuously vary with the strikes of
140 73
78 I”
83
88
93
98
1
1
1
I
I
1
73
78
73
/
78 1
I
1
83
88
83
93
88
93
1
98
98 1
1
25
p
73
Fig. 10. Fault plane solutions
78
83
of the P wave in the Himalaya
80
arc mountains
the arcuate structure which controls the stress distribution in this tectonic zone, and on both sides of the arcuate system the directions of stress agree with the surface structure. In the region of the Himalaya arcuate mountain system all the fault plane solutions gave a pattern of shallow sliding ( < 300), which could be seen even on the main boundary fault. Such data indicates that the shallow slipping and underthrusting are the main type.
93
and their vicinity.
98
a. h > 70 km. b. h i 70 km
Characteristics of seismic wave propagation in the Qinghai-Xizang Plateau Determination
of magnitude
characteristics
The data of the Lhasa seismic station show that for earthquakes outside the plateau the magnitudes observed by this station are usually lower than those recorded by other stations beyond the plateau. It means that the media of the plateau are
141
different in their structure and property. plateau is an independent block.
The
Stress drop determined from the P wave of Q small
TABLE
4
Q values for different No.
hind of earthquake
Region
Wave kind for
Q value
calculation
earthquake
Among the 77 M = 1.5-4.5 small earthquakes there were 38 earthquakes which occurred in the Damxung-Yangbajian-Zhigung-Zetang-Quxu district. The seismic data and observations of the P waves of small ear~qu~es show that the stress drop of small earthquakes in the Zhamo region is lo-30 bar (Yang et al., 1981) and in the Damxung region 5-15 bar, the latter being obviously lower than that of former. Meanwhile, within the epicentre distance of O-200 km the average velocity of a P wave calculated from its travel time in the upper crust is 5.65 km/s. The velocity value of a short-period Rayleigh wave in the Xizang Plateau obtained from seismic surface waves is lower than that in adjacent areas. Obviously, the crustal media of the plateau are not rigid.
waves
of Q value 1
Xizang Plateau
P wave of small earthquake
et,=35
2
Xizang Plateau
seismic S wave
es=25
3
Xizang Plateau
seismic surface
4
Haicheng
P wave of small
5
Haicheng
P wave of small
6
Shimian
P wave of small
wave
Qa = 30-40
earthquake
QP = 220-1047
earthquake
QP = 510
earthquake
QP = 560
Q values of seismic waves The Q value observed within the Xizang Plateau shows that all the Q values determined from surface waves, S waves and P waves of small earthquakes are smaller than those obtained in other regions; Qs = 25, Qr = 35 (Chang et al., 1980; Feng et al., 1980; Yang et al., 1981). The Q values gamed from long-period surface waves are 30-40. However, in the Haicheng and Shimian regions the Qp values obtained from P waves of small earthquakes are 200-1047, 510 and 560 (Molnar et al., 1975; Zhu et al., 1977; Wang, 1981), which shows that the absorbtion capability of mantle media in the Xizang Plateau is much strong than that in other regions (Table 4).
P IO
Propagation of seismic Lg waues
Figure 11 shows a distribution of the characteristics of Lg wave propagation in the QinghaiXizang Plateau. The Lg wave observed by seismic stations on and surrounding the plateau shows that the energy of Lg waves penetrating through
(b)
Fig. 11. Characteristics the Xizang through
Plateau
the plateau
of an Lg wave (l-9) but
not
(Ruzaikin
through
penetrating
it, (10-19)
et al., 1977).
into
penetrating
142
the Xizang had
only
Plateau
was considerably
a low frequency
decreased,
or even
disappeared
plane
solutions
usually
have a very clear config-
uration
and a simple
(Wei et al., 1981) while the energy of Lg waves not
relative
movement
penetrating
was rela-
continental
interior
the crustal
boundaries
of continent
through
the Xizang
tively
increased.
media
in this plateau
sorb
the
It demonstrates
seismic
above-mentioned
the
waves.
This
agrees
by Chinese
to ab-
with
the
boundary
clear and reliable,
component
of
the Xizang
and the characteristics quite complicated.
to the
a large
at
Soviet
of Fig. 11 is energy
high(Fig.
related
plate in
or
in
the
The pattern of deformation
of
seem to be
movement
in a wide belt
movement,
be the product
motions.
of repeated the
case
of motion
Thus, we can say, it is not only
to the horizontal
and
The
either in
and the related strong vertical Indian
and
in the
and continental
collision
stations
with a developed
and
case
faults
However,
to define precisely;
underthrust-convergence.
through
adjacent
acting
this case are difficult the
of narrow
the case is quite different.
seismic
and shown in the seismograms
frequency
ability
low Q value and low stress drop.
obtained
northern
Union
that
has a strong
The Lg wave not penetrating Plateau
Plateau
model
(slip vector).
Eurasian
but may After
subcontinents
the ap-
Ila). The seismograms in Fig. lib give us another picture, in which the the energy of the Lg wave
proached each other, the western top part of the arc, the Hindu Kush region, first came into colli-
decreased as it penetrated through the Xizang Plateau. With the extension of the route, the wave even disappeared. The characteristics of Lg waves
sion in the Paleocene Epoch (about 70 m.y. ago). Then the collision between the plates took place at
show that in the Sizang Plateau,
the Assam region
(India) and the extended zone of the western upper part of the Himalayan arcuate mountain system, the energy disappeared.
was much weakened
and even
the eastern part of the arc in the India-Burma mountain region in the Oligocene Epoch (50 m.y. ago) and these two giant plates bordered fully on each other in the Late Oligocene (about 30 m.y. ago). This led to an intense compression and a severe deformation in the earth’s crust, caused a
Such a feature can also be seen from the characteristics of Lg wave propagation in various regions in China, including the eastern part of China,
progressive rise of the Himalayan mountain system and the strong folding of the lithosphere in a horizontal direction (about 2000 km) and formed
though this part is quite unlike the Xizang region (Wei et al., 1981). The time period of Lg wave
a criss-cross network of faults, a topographic lift and a thick crust (Teng et al., 1981). Therefore, a pattern of intensive seismicity
penetrating through the path in the QinghaiXizang Plateau is twice as long as that of the Lg wave penetrating through the various regions in China, (Wang, 1981, table 5) and the range of the time period is much wider.
upand
a complicated stress field was established. The crustal mass did not only bend with the arcuate structure, but also flowed transversely, which intensified the seismic activity and terrestrial heat flow. Meanwhile, the above processes extended to
Characteristics of seismicity and plate tectonics
the east through Afghanistan and the Xizang Plateau. The crustal shortening and the mass
To summarize Himalayan region
it could be seen that the is one of the most important
migration exerted an influence on the stress field distribution in vast areas of China (Fig. 12).
seismically active zones throughout the world, as well as a place where shallow focus earthquakes are widely distributed, but the characteristics of seismicity in this region are different from those in most other parts of the world. The seismicity all over the world, including the location and type of motion, mostly agrees with the theory of plate tectonics. It is well known that the seismicity on the boundaries of oceanic plates and the fault
What type of collision occurred between the Indian plate and the Eurasian plate? The versions are different. Someone regards it as an underthrust (Li et al., 1980) someone considers it to be an upthrust and others believe it to be a collisioncompression and an opposite underthrust in the crustal media (Huan et al., 1980). But, from the above-mentioned special patterns of seismicity, focal distribution, seismic wave propagation and
143
focal
mechanism
solutions
after the disintegration
it could
be seen that
of Gondwanaland
the In-
dian plate was (Zhu et al., 1984) divided
into three
dian plate was subjected Eurasian,
and
Qinghai-Xizang
then
to an obstruction
was
Plateau.
by the
thrust
underneath
Owing
to the collision
the
blocks A, B, C and further drifted to the north and collided with the Eurasian plate. Early in the
and the long-period of compression between these two plates, the loading on the collision-compres-
Triassic
sion transitional
period
the Indian
plate were separated
by vast oceans, between
lay a Tethys
Sea. During
Indian
started
plate
plate and the Eurasian the Middle
to disintegrate
them
Triassic and
the
divided
zone (Teng et al., 1980) between
two plates was very strong, graphic uplift and formation
same time forcing the mass down into a “bending”
into three blocks. In the Middle
and Late Jurassic,
from
the boundaries
block
block)
over,
in this portion,
A (the present
contact
Tanggula
with Eurasia.
Block B (the present
dise block) was drifting with Eurasia
made
first
Gang-
deformation
intensified
continuously
causing
in the Middle
Cretaceous.
These two
forming the configuration of present continents. After the collision of these two plates, the Indian plate continuously acted upon the Eurasian plate with a very strong force. However, the In-
01
fault,
projections
3 = compressive
stresses,
of principal
.3
compressive
4 = tensile stresses,
--4
and the faults grew thickly, earthquakes
along the stress
of plate,
110
120 --5
stress axes for shallow earthquakes
5 = boundary
More-
concentrated,
roughly parallel to the trend of the focal plane. For these reasons, on the both sides of the collision-compression transitional zone between two plates a pattern of opposite under thrusts was formed and the V-shaped structure of the focal
110
.2
the stresses
plane. This is proof that the principal compressive stress direction of the intermediate earthquakes is
100
9”
Fig. 12. Horizontal
of these two plates.
in the ocean, then collided
plates continued to collide until Eocene or Oligocene times and the Tethys Sea disappeared, thus
thus causing a topoof the plateau, at the
6 = transition
--__
in China. belt.
6
1 = normal
120
17
fault, 2 = strike-slip
144
plane was initiated in the Hindu Kush region, the central part of Himalayas and the eastern top part of the arc. The Himalayan region is not only a distribution zone of the geophysical anomaly field, but also a place where hydrothermal activity occured (Teng et al., 1980; Wei et al., 1983) the ophiolite belts regularly appeared and the crustal structure suddenly changed. On this basis a pattern of plate collision and interplate seismicity and a model of collision of the type of opposite “underthrust” between two giant plates were formed.
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