Download P-wave crustal velocity structure in western Sichuan and eastern

Vol. 46 Supp.
SCIENCE IN CHINA (Series D)
October 2003
P-wave crustal velocity structure in western Sichuan and
eastern Tibetan region
WANG Chunyong (), WU Jianping (), LOU Hai (
ZHOU Mindu (
) & BAI Zhiming ()
),
Institute of Geophysics, China Seismological Bureau, Beijing 100081, China
Received January 28, 2003
Abstract A deep seismic sounding profile located in the western Sichuan and eastern Tibetan
region extends from Batang (Zhubalong) to Zizhong, Sichuan. It passes through the Songpan-Garzê Fold System and the Longmenshan Tectonic Zone, and ends in the Yangtze Craton.
Based on the travel times of phases on the profile, incorporating information on the relevant amplitudes, we determined 2-D P-wave crustal velocity structure along the profile, analyzed the principle differences between the crustal and upper mantle structure in the Western Sichuan Plateau
and Sichuan Basin, discussed the deep feature of the major faults on the profile, the tectonic relation between the Yangtze Craton and the Tibetan Plateau and the deep structural environment
where strong earthquakes occurred.
Keywords: Tibetan Plateau, deep seismic sounding, crustal and upper mantle structure, low-velocity zone.
DOI: 10.1360/03dz0020
The Tibetan Plateau is a region on the earth where the modern tectonic movement is the most
intensive. The collision between the Indian Plate and Eurasian Plate, the uplifting of the Tibetan
Plateau and its influence on the tectonic movement of Eastern Asia Continent are the hot spots of
study in the field of international geosciences. The collision between the two plates leads to the
sharp upheaval of the Tibetan Plateau and the obvious thickening of its underlying crust[1]. As the
transient zone from Tibetan Plateau to the Yangtze Craton, the eastern margin of the plateau has
been experienced drastic crustal deformation and faulting to adapt the upheaval. Till now, such a
movement is going on. The measurements of GPS show that the present horizontal displacement
on the surface of the eastern part of the plateau rises up to 15 mm/a[2, 3].
In the eastern margin of Tibetan Plateau, some explorations and researches[4
ü7]
have ever
been conducted. However, our knowledge of the crustal structure of the whole eastern margin is
still absent. From August to October of 2000, we deployed a seismic sounding profile along the
altitude of 30eN in the Western Sichuan and Eastern Tibetan region, and achieved new recognition of the crustal structure. In this paper, we present the main results obtained in the investigation.
1
1.1
Regional geological setting and previous research on deep structure
Regional geological setting
Longmenshan fault zone is a boundary of the geologic setting in the Western Sichuan and
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Eastern Tibetan region, with the Songpan-Garzm Fold System to the west, and the Yangtze Craton
to the east (fig. 1). A series of low-grade metamorphic layers of Paleozoic and Triassic of Mesozoic in the Songpan-Garzm Fold System had been overthusted over the shallow-oceanic and continental deposition of stable craton of the Paleozoic and Mesozoic. Since the late Paleozoic, the
depositional environment of Yangtze Craton was stable, and had thick non-metamorphic sedimentary blanket while fold movements occurred in different degrees during the Yanshan Movement
and the Himalayan Movement[8]. Xu et al.[9] call the Songpan-Garzm Fold System and Longmenshan fault zone by a joint name as Songpan-Garzm Orogenic Belt, and divided the orogen into
west Collision Belt, main block and Longmenshan Piedmont Thrust Zone.
Fig. 1. Regional geological setting and location of seismic profile. 1, Longmenshan Fault; 2, Xianshuihe Fault; 3, Anninghe
Fault; 4, Garzm-Litang Fault; 5, Jinshajiang Fault; 6, Lancangjiang Fault; 7, Lujiang Fault; A, Zhubalong-Zizhong seismic
sounding profile.
Besides the Longmenshan Fault Zone, the major fault zones in the study region include the
Xianshuihe Fault Zone, Anninghe Fault Zone, Garzm-Litang Fault Zone and Jinshajiang Fault
Zone (fig. 1). The Xianshuihe Fault Zone is a left-lateral fault zone, which is very active during
the Quaternary[10, 11]. Recently, strong earthquakes frequently occurred within the study region[12,13], which is hence one of the main strong earthquake regions in China.
1.2
Previous deep geophysical research
Some deep seismic sounding profiles have been carried out. The Liangzhong-Tangke Line
and the Tangke-Pujiang Line of the Longmenshan triangle profile[4] were deployed over the
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Longmenshan Fault Zone and Mingshan Fault Zone, respectively. The cross section indicates that
the thickness of the crust increases from about 40 km in the Yangtze Craton up to 53 km in the
Songpan-Garzm Fold Zone. The Huashixia-Shaoyang profile[5], extending from Eastern Kunlun
Orogenic Belt, passes through the Songpan-Garzm Fold Zone to the Yangtze Craton (Sichuan Basin). The section shows that the crustal thickness in the Songpan-Garzm Fold Zone is about 60 km.
The Lijiang-Panzhihua-Zhehai profile[14] passes through the Panxi Tectonic Zone, and the result
reveals that there exist low-velocity-zones in the middle crust (5.6ü6.0 km/s) and in the uppermost mantle (7.7ü7.9 km/s). The Lijiang-Xinshizhen profile and Lazha-Xichang-Changheba profile[15] as well, show that there exist low-velocity-zones in the middle crust and uppermost mantle
of the Panxi Tectonic Zone.
Longmenshan Fault Zone is featured as a large-scale gravity gradient belt striking NNE with
the width of 150 km and the length of 900 km. This is the main feature of Burger gravity anomaly
in this region[16]. The gravity field changes slowly in the area between the Longmenshan Fault
Zone and Jinshajiang Fault Zone. The Ninglang-Luzhou and Aba-Luzhou telluric electromagnetic
sounding profiles passes through the Panxi Tectonic Zone and the Longmenshan Tectonic Zone,
respectively. There exists a low-resistance layer in the crust at the depth of 11ü37 km with the
thickness of 5.5ü11 km and the resistivity of several to tens Ωgm, and another one in the upper
mantle at the depth of 80ü145 km with the resistivity of more than ten to tens Ωgm[17].
2
Seismic sounding profile in the Western Sichuan and Eastern Tibetan region
The deep seismic sounding profile starts from Zhubalong nearby the Jinshajiang River,
through Litang, Yajiang, Kangding and Ya’an, to Zizhong nearby the Mingjiang River, with the
whole length of 552 km (fig. 1). This profile is linked to the Lazha-Xichang-Changheba profile[15]
at Kangding. The positions of 6 shots are listed in table 1, in which the shots were fired under water except that the Litang shot was fired in borehole. Meanwhile, there are 200 short-periodic
3-component digital seismometers deployed on the line, with the intervals of 2ü4 km. The observation was recorded on the whole line, i.e. all the 6 shots were recorded by all the seismometers
on line.
Table 1 Location of shot points and the charges
No.
Shot point
Code
Latitude/(e)
Longitude/(e)
Altitude/m
Charge/kg
1
2
3
4
5
6
Zhubalong
Litang
Yajiang
Luding
Hongya
Zizhong
ZBL
LT
YJ
LD
HY
ZZ
29N52.15
29N57.68
30N10.56
29N50.57
29N55.97
29N53.07
99E02.22
100E17.38
101E00.84
102E12.34
103E07.40
104E45.09
2500
3940
2630
1270
530
310
2500
3000
2500
2000
2500
2500
On the record section of each shot along the profile, Pg, as the first arrival phase, can be
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clearly identified. It is labeled as the head wave or diving wave, propagating within the crystalline
basement. PmP is the wave reflected in the Moho discontinuity. Pn, as the refraction wave in the
uppermost mantle, can be identified beyond 200 km. Besides, there are 4 reflective phases, which
are labeled as P1, P2, P3, P4 following their arrival order and can be recognized in the Western
Sichuan Plateau, but only 3 phases (without P2) in the Sichuan Basin. The traceable distances and
average (or apparent) velocities of all phases on the record sections are listed in table 2, wherein
the first row of each phase is the traceable distance, and the second is the average (or apparent)
velocity.
Table 2 Traceable distance (km) and average (apparent) velocity (kmgs−1) on the record section of each shot
Shot
Pg
P1
P2
P3
P4
PmP
Pn
ZBL
east
~70
5.90
60ü100
5.94
60ü120
5.98
130ü200
6.18
160ü220
6.28
160ü250
6.32
LT
west
east
~50
~100
5.85
5.85
40ü100 60ü130
5.88
5.90
40ü125 80ü130
5.93
5.92
70ü125 100ü180
6.15
6.15
170ü220
6.30
165ü260
6.33
190ü
7.75
YJ
west
~80
6.00
50ü100
5.95
50ü120
5.90
60ü160
6.08
140ü190
6.34
east
~80
5.95
70ü140
6.12
90ü160
6.10
120ü210
6.19
180ü240
6.37
160ü250
6.36
LD
west
east
~120
~60
5.95
6.05
60ü140 50ü90
5.95
6.00
120ü180
6.02
140ü200 60ü140
6.20
6.10
160ü210 100ü180
6.39
6.25
140ü250
6.50
160ü
7.95
HY
west
east
~100
~60
6.05
5.75
60ü120 65ü130
6.05
6.05
ZZ
west
~100
5.75
30ü110
5.90
90ü160 65ü160 60ü160
6.13
6.08
6.15
140ü210 80ü160 80ü180
6.27
6.25
6.25
200ü290 100ü160 120ü260
6.42
6.48
6.50
120ü
8.15
On the section of ZBL shot, P1 is weak, P2 strong, P3 clear, PmP weak, and Pn not clear. To
the west of shot point LT, the observation distance was short. To the east of LT, P3 is clear, PmP
and Pn can be traced, and the apparent velocity of Pn is relatively low. To the west of shot point YJ,
P3 is clear and the tracing distance of PmP is small; and to the east of YJ, Pn is not clear. To the
west of shot point LD, it is the Western Sichuan Plateau, and to the east of LD, there are the
Longmenshan Mountains and Sichuan Basin. The record section (fig. 2) illustrates that the phases
on the west and east sides of the shot point have different characteristics. And in the records beyond 160 km to the west side, there exists a group of special phases with low frequency, high energy and high apparent velocity, which shield PmP and Pn. To the west of shot point HY, PmP is
weaker than that to the east. On the record section of ZZ shot, PmP has strong energy and higher
average velocity while Pn has relatively apparent velocity. Among the record sections of all shots,
we selected the section of LD shot and show it in fig. 2.
3 2-D velocity structure of seismic profile
3.1
Interpretation methods
The seismic records were interpreted using the asymptotic ray method[18]. The travel times of
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Fig. 2. Trace-normalized vertical-component record section of shot point LD with reduced velocity 6.0 km/s.
phases on the record sections of each shot were picked out, and topographic correction was made
with a reference altitude of 3000 m. The interpretation of the first arrival in the upper crust, i.e. Pg,
could be realized with the tomography method of refracted wave (or diving wave), such as the
ü21]
finite difference travel time inversion[19
. And 2-D velocity structure of the upper crust was ob-
tained. In the algorithm, the relation between travel time perturbation and slowness perturbation
was considered as linear, so an iterative process was used. The final spatial resolution was determined by the geometric location of the field observation, i.e. it was estimated by distribution of
the ray number covering each unit[22]. The inverse model was parameterized as rectangle unit of 4
km (horizontal) × 2 km (vertical). To maintain the calculating accuracy, a uniform 1 km square
grid was used for the travel time calculation. Based on the inversion of travel time of Pg, a 2-D
initial crustal velocity model was built up. And then, the ray tracing method was used to fit the
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P-WAVE CRUSTAL STRUCTURE IN WEST SICHUAN AND EAST TIBET
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travel time of refractive and reflective phases, and the ray synthetic seismogram method[23] was
used to constrain the amplitudes and improve the initial crustal velocity structure. The calculation
was repeated till the final 2-D crustal model was satisfied[24].
3.2 2-D P-wave crustal velocity structure along the profile
3.2.1 2-D structure in the upper crust. Along the profile, we totally collected 470 first arrival
data from 6 shots for the finite difference inversion[21]. The final velocity model was obtained after
5 iterations. RMS of travel time decreases from the initial 0.69 s down to 0.19 s. In fig. 3, the final
2-D velocity model in the upper crust and the 2-D distribution of the ray number along the profile
is illustrated by velocity contour. Except the western and eastern ends of the profile and beneath
the shot of YJ where the rays penetrate shallowly (5ü7 km), the rays on the most of the profile
penetrate down to 10 km, e.g. the penetrating depth reaches 13 km between HY and LD, and between LT and ZBL. In most parts at these depths, the ray number of each cell is above 10, partly
from 5 to 10. Nevertheless, the result of inversion has a high confidence.
The 2-D structure of the upper crust (fig. 3) along the profile implicates the difference between different tectonic units and the existence and shape of Garzm-Litang Fault, Xianshuihe Fault,
and Longmenshan Fault. It also shows that there respectively exists a low-velocity zone near the
surface east to HY, east to LD and west to LT, and the contour of low velocity concaves downward.
The crystalline basement nearby YJ is uplifted. Garzm-Litang Fault and Longmenshan Fault extend into deep crust in listric shape. Garzm-Litang Fault is the boundary between the high-velocity
zone and low-velocity zone. There exists a low-velocity zone which stretches down to the depth of
10 km and deeper, in the surrounding areas of Longmenshan Fault. All those characteristics are in
good agreement with the geological tectonics on the surface.
3.2.2 2-D crustal velocity structure. In order to get 2-D crustal structure along the profile, we
use the ray tracing method to fit the travel times of all of refractive and reflective phases, wherein
the part above the depth of 10 km is from the inverse of Pg travel times. In the 2-D crustal model
(fig. 4), the bottom interface of the ith layer is constrained by Pi (and hereby is called Pi interface,
i=1, 2, 3, 4). Consequently, we found that the average depth of the first layer is about 13 km in the
Western Sichuan Plateau and 17 km in the Sichuan Basin. Between P1 interface and P2 interface,
there is a low-velocity (5.80 km/s) layer with the thickness of 8ü10 km in the plateau but none in
the basin. The average depths of P3 and P4 interfaces are 30 and 40 km in the plateau, respectively, and 28 and 39 km in the basin. The average depth of Moho discontinuity is 61 km in the
plateau and 45 km in the basin. Therefore, in the Western Sichuan Plateau, the crust becomes
thicker and thicker from west to east, and reaches its maximum between YJ and LD. In the east
segment of the profile, i.e. the segment from LD to ZZ, the thickness of the crust increases a little
from west to east, but decreases greatly from LD to YJ, i.e. from 63 km down to 44 km. Near
Kangding, the velocity structure of the profile is in good accordance with the result of
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Lazha-Xichang-Changhe profile; that is, there exists low-velocity anomaly in the middle part of
the crust and in the uppermost mantle.
Fig. 3. 2-D velocity structure of the upper crust (a) and its corresponding ray number distribution (b) by inversing with the
method of finite difference of travel time.
Fig. 4. 2-D velocity structure of the crust along the profile.
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For the final 2-D model shown in fig. 4, the fitting result of each shot’s records is satisfied,
and the fitting error is 0.05 s. Fig. 5 illustrates the fitting of synthetic seismograms with respect to
the record section. We found that, besides P2 and P3, it is a good fitting in the arrival times of
PmP, whose amplitude is a little higher. This phenomenon will be further discussed later. It must
be noted that the final model is not concerned in the special phase with low frequency, high energy
and high apparent velocity recorded west to LD shot (fig. 2).
3.3 The difference in the crustal structure between the Western Sichuan Plateau and Sichuan
Basin
The velocity model in the western Sichuan-eastern Tibetan region is bounded by Longmenshan Fault, as same as in geologic setting. Between the Western Sichuan Plateau of west segment
of profile and Sichuan Basin of east segment of profile, there exist obvious differences in crustal
thickness, the average velocity of the crust and the velocity of Pn of the uppermost mantle (table
3). The Western Sichuan Plateau’s crustal structure belongs to orogenic-belt-type while the Sichuan Basin’s crustal structure is of craton-type[25]. The Yangtze block (Sichuan Basin) has higher
average velocity of crust, so the crustal rock is harder, while it is weaker in the Western Sichuan
Plateau. The relative low average velocity of Pn in the Western Sichuan Plateau might be related to
the marked thermal process through the whole Cenozoic[26].
Table 3 Basic parameters of the crustal structure along the profile
Crustal parameter
Crustal thickness
Average crustal velocity
P-wave velocity of uppermost mantle
Low-velocity zone in crust
Dominant discontinuity in crust
Western Sichuan Plateau
60ü65 km
6.25ü6.30 km/s
7.60ü7.80 km/s
about 10 km thick in upper crust
at depth of 30ü40 km
Sichuan Basin
45ü50 km
6.45ü6.50 km/s
8.00ü8.20 km/s
none
not clear
There exists not only the low-velocity zone, but also the low-resistivity zone in the Western
Sichuan Plateau[27]. The buried depth is in basic accordance with that of the low-velocity zone. It
is argued that the low-velocity zone in the crust is related to pore pressure and hydrological characteristics of the rock, or is due to the change of medium’s thermal state, or result from special
mineral compositions[28]. It is suggested that the low-resistance zone in crust is usually composed
of partly melting medium, therefore, the low-velocity, low-resistivity zone might be related to the
thermal state, and it is the mark of active tectonic structure[25].
In the field observation, both the explosion with more dynamite in the Western Sichuan Plateau, e.g. LT shot, and that with less dynamite, e.g. ZZ shot, were recorded at the same station.
Under the condition of the same shot-detector distance, the latter shot usually excites more energy
of the reflection on Moho discontinuity than the former. The underplating of magma may cause
abnormal low-velocity zone in the lower crust, and obscure the Moho discontinuity. Hence, we
suggest that it might lead to the smaller energy of PmP in the Western Sichuan Plateau.
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Fig. 5. Fitting results of the record section of LT shot by using 2-D crustal velocity structure. (a) Seismic record section; (b)
synthetic seismograms; (c) fitting of travel time; (d) ray path diagram. h, Observational travel time; +, theoretical travel time.
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4 Discussion and conclusion
4.1
Deep structural relation
between the Yangtze Craton and
Tibetan Plateau
The Longmenshan Fault
Zone is an overthrust fault zone
in tectonics. According to the
2-D velocity structure obtained in
this paper, we infer that beneath
Kangding-Luding, there exist a
series of listric faults, which are
connected to the low-velocity
zone of the upper crust, incline
westwards, extend upwards and
are connected to the Piedmont Fig. 6. Dynamic model inferred from the 2-D crustal velocity structure. The
overthrust fault zone on the sur- geologic background was modified from ref. [9].
face. The low-velocity and low-resistivity zone in crust may play an important role in the process
of detachment and overthrust shear, and this view is in good accordance with the previous conclusion[29] that there exists a ductile shear zone with high temperature in deep in the Songpan-Garzê
area. Due to the influence of overthrust fault zones, Longmenshan Moun- tain is shifted over the
Sichuan Basin. Due to the collision between the Yangtze Craton with harder media and Songpan-Garzm block with softer media, the vehement deformation appears in the relatively soft block
so as to thicken the crust, reduce crustal velocity, and produce a series of thrust fault zones (fig. 6).
Such a collision pattern is named “Crocodile Structure”[30]. However, on the seismic records along
the profile, there is no evidence that there exist eastward-inclining faults in the lower crust.
Xianshuihe Fault Zone is a huge left-lateral strike-slip fault zone, which came into being in
the postorogenic phase. It intersects with the seismic profile near Kangding, where the velocities
of both lower crust and uppermost mantle are relatively low, being 6.6 and 7.6ü7.7 km/s, respectively (Fig. 4). This result is nearly in agreement with the 3-D distribution of velocity anomaly
obtained by seismological tomography[31]. About the deep characteristics of Xianshuihe Fault
Zone, we will discuss it clearly in another paper[32].
4.2 Seismotectonics
The Western Sichuan Plateau is located in the middle of North-South Seismic Zone, which is
one of the major seismically active areas in the Chinese Continent. Along this profile, there are
Batang Seismic Zone, Litang Seismic Zone and Kangding Seismic Zone, which are related to Jinshajiang Fault Zone, Garzm-Litang Fault Zone, and Xianshuihe Fault Zone, respectively. In the
Western Sichuan and Eastern Tibetan region, the modern seismotectonics is controlled by the re-
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gional tectonic stress field with the direction of nearly north-south and the local tectonic stress
field with the direction of east-west[33]. Regionally, there exists a deep structural environment
where strong earthquakes occurred in the Western Sichuan Plateau. According to the historic
earthquake catalogs and recent observation reports of networks, most of the shocks in the Western
Sichuan Plateau occurred at the depth between 5 and 25 km, which is within the upper and middle
crust with brittle medium. The low-velocity anomaly’s distribution in the lower crust and uppermost mantle is in favor of centralizing the stress in the brittle crust above itself, and can be regarded as the deep medium condition in which strong earthquakes frequently occur. In this paper,
we state that there are lots of seismological evidences related to the thermal state of medium
within this area. Hence, the crustal and upper mantle structure is related to definite thermal process, which is also another important factor leading to strong shocks in this area.
Acknowledgements We devote our sublime salute to Li Ping and Zhang Xinzhong who sacrificed in the field scientific
investigation, and gratefully thank the Main Station and Batang Station of Sichuan and the Tibetan Army Station of PLA, the
Geophysical Exploration Center of China Seismological Bureau, Seismological Bureau of Sichuan Province, for their cooperation and support in field works. This work was supported by the State Key Basic Research Development and Programming Project (Grant No. G1998040700, Contribution No. 03AC1008), the Institute of Geophysics, China Seismological Bureau.
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