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S-wave velocity structure of the crust and upper mantle under southeastern China by surface wave dispersion analysis
Journal of Asian Earth Sciences 18 (2000) 255±265
S-wave velocity structure of the crust and upper mantle under southeastern China by surface wave dispersion analysis Yi-Ben Tsai, Hsin-Hung Wu* Institute of Geophysics, National Central University, Chung-Li, Taiwan Received 4 January 1998; received in revised form 13 November 1998
Abstract Group velocity dispersion data of fundamental-mode Rayleigh and Love waves for 12 wave paths within southeastern China have been measured by applying the multiple-®lter technique to the properly rotated three-component digital seismograms from two Seismic Research Observatory stations, TATO and CHTO. The generalized surface wave inversion technique was applied to these group velocity dispersion data to determine the S-wave velocity structures of the crust and upper mantle for various regions of southeastern China. The results clearly demonstrate that the crust and upper mantle under southeastern China are laterally heterogeneous. The southern China region south of 258N and the eastern China region both have a crustal thickness of 30 km. The eastern Tibet plateau along the 1008E meridian has a crustal thickness of 60 km. Central China, consisting mainly of the Yangtze and Sino-Korean platforms, has a crustal thickness of 40 km. A distinct S-wave low-velocity layer at 10±20 km depth in the middle crust was found under wave paths in southeastern China. On the other hand, no such crustal low-velocity layer is evident under the eastern Tibet plateau. This low-velocity layer in the middle crust appears to re¯ect the presence of a sialic low-velocity layer perhaps consisting of intruded granitic laccoliths, or possibly the remnant of the source zone of widespread magmatic activities known to have taken place in these regions since the late Carboniferous. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The purpose of this study is to determine the S-wave velocity structure of the crust and upper mantle under southeastern China by surface wave dispersion analysis and to expound the related tectonic signi®cance. The present study covers a rectangular area spanning approximately from 958E to 1208E in longitude and 208N to 408N in latitude. Analysis of the S-wave velocity structure of the crust and upper mantle under southeastern China is particularly interesting because of its apparent tectonic heterogeneity and widespread magmatism since late Carboniferous. Fig. 1 shows that the present study area covers four tectonic provinces (Beckers et al., 1994): The South China fold system in the south, the Yangtze platform in the center, the * Corresponding author.
Sino-Korean platform in the north, and the Tibet plateau in the west. The average crustal thickness under the study area was found previously to increase from 30 km in the east, to 45 km in the west, and to more than 60 km farther to the west (Chinese Academy of Sciences, 1974; Feng and Teng, 1983; Mooney et al., 1998). It is interesting to provide additional independent evidence for such regional variations in crustal thickness. Sengor et al. (1993) recently presented a comprehensive analysis of the space±time evolution of magmatism along the Tethysides. The present study area is located at the eastern end of Tethysides. According to Sengor et al. (1993), wide areas of southeastern China have experienced varying degrees of collision-related or subduction-related magmatic activities during the long geologic history from late Carboniferous±Permian (320±248 Ma) to late Cenozoic (24±0 Ma). It is of considerable interest to determine, whether there is any
1367-9120/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 5 8 - 9
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discernable S-wave low-velocity layer which can be identi®ed as a remnant of the magma source zone in the crust or upper mantle. The S-wave velocity structures of the crust and upper mantle under southeastern China and the adjacent Tibet plateau have been studied by many authors (e.g. Tung, 1974; Chen and Molnar, 1975; Chun and Yoshi, 1977; Rosenthal and Teng, 1977; Patton, 1980; Pines et al., 1980; Romanowicz, 1982, 1984; Wier, 1982; Feng and Teng, 1983; Jobert et al. 1985; Brandon and Romanowicz, 1986; Wang and Yao, 1989; Zhao et al., 1991; Bourjot and Romanowicz, 1992; Wu and Levshin, 1994; Mitchell et al., 1997; Wu et al., 1997; Zhang, 1998). Recently, Ritzwoller and Levshin (1998) gave a comprehensive review of surface wave tomography of Eurasia, including our present study area. The present study analyzes the group velocity dispersion of surface waves using more recent data with improved spatial coverage and over shorter paths.
Both Rayleigh and Love wave group velocity dispersion curves over a total of 12 paths traversing the study area from 14 earthquakes to two Seismic Research Observatory (SRO) stations, CHTO in Chiangmai, Thailand and TATO in Taipei, Taiwan (Peterson et al., 1980) are analyzed. 2. Data Fourteen earthquakes (Table 1) were chosen for the present study, because their epicenters fall in the eastern and western border zones. Moreover, the wave paths from these earthquake epicenters to the two stations, TATO and CHTO, are con®ned within the study area (Fig. 1). The path lengths range from approx. 1800 to 2850 km. When available, seismograms from multiple earthquakes are used for the same wave paths in order to ensure data repeatability.
Fig. 1. The wave paths and path ID. The solid lines show the paths from epicenters of sources to station TATO, and dashed lines to station CHTO. Thin curved line segments are boundaries of the four tectonic structures traversed. The locations of stations are shown by solid triangles and epicenters by solid circles. The thickness of the crust along each path is shown on the side of path.
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In total, twelve wave paths traverse the study area, forming two sets of approx. 908 fan-shaped coverage. The ®rst set includes six wave paths, T1±T6 (Fig. 1), arriving at TATO on the southeastern corner. The second set also includes six wave paths, C1±C6 (Fig. 1), arriving at CHTO on the southwestern corner of the present study area. The SRO data in digital form is convenient for computer processing. Since the SRO seismographs operate at high sensitivity levels, they provide excellent records useful for surface wave dispersion analysis from moderate-magnitude earthquakes (M 0 5.5) which can be considered as point source at the wavelengths concerned. The long-period digital seismograms used for this study were provided by the Incorporated Research Institute for Seismology (IRIS). Instrumental response was removed from each recorded trace using the method of Luh (1977). Pure Rayleigh and Love wave components are obtained by rotating the two original EW and NS components into the radial and transverse components. Normally, three components, namely, the vertical (V) and radial (R) components for Rayleigh waves and the transverse (T) component for Love waves, are available at each station. Unfortunately, due to complication of the radiation patterns and propagation path eects, sometimes the qualities of one or two components may not be so good. Poor data from such cases were not used for further interpretation. 3. Method A multiple-®ltering analysis (Dziewonski and Hales, 1972; Herrmann, 1973) was applied to each surface wave train to obtain the fundamental-mode group-vel-
257
ocity curve. Typical contoured plots of relative amplitude of wave energy arrivals at TATO over Path T4 from Yunnan are shown in Fig. 2 for the vertical component of Rayleigh waves, in Fig. 3 for the radial component of Rayleigh waves, and in Fig. 4 for the transverse component of Love waves, respectively. In these ®gures the open circles, marking the local maxima, represent the average group velocities obtained from the multiple-®ltering analysis. The contour lines sometimes give more than one local maxima which make a clear-cut determination of group velocity values dicult. We also use a phase-matched ®lter (Herrin and Goforth, 1977; Goforth and Herrin, 1979) to identify and remove multipathing arrivals to improve the quality of the determined dispersion curves. It is remarkable that both vertical and radial components of Rayleigh waves give almost identical group velocity values for all periods above 15 s. At shorter periods an ambiguity often arises in drawing contours due to interference by S waves. Thus, we exclude the shortperiod dispersion data in subsequent inversion to obtain the S-wave velocity structure. Finally, simultaneous inversion of both Rayleigh and Love wave dispersion data measured over similar paths is made by the generalized inversion method (Backus and Gilbert, 1967, 1968, 1970) to determine the S-wave velocity structure. Detailed descriptions of the algorithm to implement this method can be found elsewhere (Jackson, 1972; Wiggins, 1972; Crossen, 1976). In applying the inversion algorithm, we use a layered half-space velocity model for all wave paths. In the model the upper 160 km are divided into 16 layers with a uniform thickness of 10 km. Below 160 km lies an in®nite half space. The same initial model with a uniform S-wave velocity of 5 km sÿ1 for
Table 1 Pertinent earthquakes information used in this study Earthquake
TATO
No.
Date
Epicenter
Depth
Magnitude
1 2 3 4 5 6 7 8 9 10 11 12 13 14
19900911140902.50 19910118013626.60 19910312060405.00 19910325180241.50 19910902110550.40 19910920111611.50 19920112001227.10 19920122214125.90 19930126203206.90 19931026113821.90 19940103055227.60 19940111005156.30 19960203111422.00 19960204165809.00
22.6920N120.9090E 23.6920N121.3460E 23.1610N120.0530E 39.8870N113.9230E 37.4400N 95.4020E 36.1910N100.0630E 39.6710N98.3000E 35.3510N121.1090E 23.0270N101.0620E 38.4770N98.6550E 36.0280N100.1040E 25.2310N97.2030E 27.2000N100.5000E 27.0000N100.5000E
33.0 11.0 17.0 10.0 10.0 13.0 22.0 33.0 33.0 8.0 8.0 10.0 33.0 33.0
5.8 5.9 5.6 5.5 5.5 5.5 5.6 5.5 5.6 5.9 5.8 6.1 6.4 5.6
D (km)
1801
2085 2618 2381 2443 2109 2108
V
R
CHTO T
ID
T1
T6 T2 T3 T5 T4 T4
D (km)
V
R
2320 2379 2240 2746 2100 1935 2320 2849
2187 1918
T
ID
C1 C1 C1 C3 C6 C4 C5 C2
C5 C4
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all layers is used in the inversion for all wave paths. We use the same initial model in order to obtain ®nal velocity structures re¯ecting the dierences in the dispersion curves of dierent path groups but free from bias from initial models. In doing the above analysis, we have assumed that the medium is isotropic and we only do simple one-dimensional modeling. The phase and group velocities of Rayleigh waves are more sensitive to deeper shear velocity changes than Love waves (Ritzwoller and Levshin, 1998). On the other hand, for phase velocity, the crustal eect for Love waves is more than twice that of Rayleigh waves. For example, Love waves at 40 s are most sensitive to the S wave velocity changes in the uppermost 60 km. In contrast, Rayleigh waves at these periods are primarily sensitive to the uppermost mantle structure (Mooney et al., 1998). In other words, at short periods, Love waves are more sensitive to shallower S wave velocity changes than Rayleigh waves. Conversely, at long periods, Rayleigh waves are more sensitive to deeper S wave velocity changes than Love
waves. Thus, simultaneous inversion of both Love and Rayleigh waves can complement each other in the results. Fig. 5 shows an example of the ®nal S-wave velocity structure obtained from inversion of dispersion data over Path T4 from Yunnan to Taipei. The upper part of the ®gure shows remarkably close matching between the calculated and observed group velocity dispersion curves for both Rayleigh and Love waves. In this case the observed Rayleigh wave data include both radial and vertical components from two earthquakes (No. 13 and No. 14 in Table 1). The observed Love wave data include the transverse component from No. 13 earthquake (Table 1). The lower part of Fig. 5 shows several remarkable features in the ®nal velocity structure. First, it has a distinct upper crust at depths of from 0 to 20 km, with a low velocity zone (LVZ) at depth from 10 to 20 km, which shows only a small decrease in seismic velocities, and a lower crust from 20 to 40 km. Second, the Swave velocity increases gradually with depth in the
Fig. 2. Typical contoured plots of relative amplitude of wave energy arrivals at TATO over Path T4 from Yunnan for the vertical component of Rayleigh waves. The open circles show the picked group velocities.
Yi-Ben Tsai, Hsin-Hung Wu / Journal of Asian Earth Sciences 18 (2000) 255±265
upper mantle to reach a maximum value of 4.3 km sÿ1 at depths between 70 and 90 km. This means that there exists a 90-km thick rigid lithosphere lid under the wave path. A mild LVZ appears to extend from 90 km down to at least 160 km depth, where the upper boundary of in®nite half space in the model is designated. Finally, the crust has a thickness of 40 km if we take 4.1 km sÿ1 as the upper limit for S-wave velocity for the crustal layer just above the Moho discontinuity, as was done by Feng and Teng (1983). 4. Results and discussion The wave paths are divided into four groups according to the similarities of their dispersion data, in order to highlight regional variations in the S-wave velocity structure of the crust and the upper mantle in southeastern China. Group A includes three EW-trending paths (i.e. C1, T5 and T6) which traverse southern China and northern Indochina in the southern part of
259
the present study area. Group B includes two wave paths (i.e. C2 and T1) which traverse central and eastern China, respectively. Group C includes four wave paths (i.e. C3, T2, T3 and T4) which traverse diagonally through central China. Group D includes three NS-trending wave paths (i.e. C4, C5 and C6) which traverse the eastern Tibet plateau in the western part of the study area. An average S-wave velocity structure under each group of wave paths is obtained from inversion of the group velocity dispersion data of surface waves. The results are summarized in Fig. 6. It can be seen clearly that the S-wave velocity structures under the four group of paths vary signi®cantly from the surface down to a depth 100 km. Generally speaking, southern and eastern China have thinner crust and higher Vs values in the upper mantle. The eastern Tibet plateau has thicker crust and lower Vs values. Central China has intermediate crustal thickness and Vs values. Detailed descriptions of individual path groups are given below.
Fig. 3. The same as Fig. 2, but for the radial component of Rayleigh waves.
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4.1. Group A: paths C1, T5 and T6 across Southern China The group velocity dispersion curves of fundamental-mode Rayleigh (LR) and Love (LQ) waves are shown in Fig. 7 for the three paths in Group A (i.e. C1, T5 and T6) which traverse southern China and northern Indochina. The ®gure shows that the group velocity dispersion curves of both Rayleigh and Love waves for the two paths follow each other very closely for periods longer than 15 s. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.7 km sÿ1 at period 40 s. It then increases slightly to about 3.8 km sÿ1 at period 110 s. The LQ group velocity increases linearly from 3.3 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 40 s. It then increases slowly to 4.3 km sÿ1 at period 110 s. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of the LR and LQ group velocity dispersion curves shown in Fig. 7
for Group-A paths. From the ®gure we can see a pronounced LVZ at depths 10±20 km with a Vs of 3.3 km sÿ1. These paths have an average crustal thickness of 30 km. The Vs in the upper mantle reaches a maximum of 4.5 km sÿ1 at depths 40±50 km. This high velocity lid also shows up clearly on the tomographic images of Wu and Levshin (1994). Here the thin Archean crust is probably the main controlling factor. Below 50 km Vs decreases gradually to 4.2 km sÿ1 at depths 120±130 km. It appears that southern China has a relatively thin lithosphere lid. This is consistent with the result of Rosenthal and Teng (1977). On the South China block, Rosenthal and Teng have inverted group velocities of Love and Rayleigh waves over the period range 10±60 s to obtain the crust and upper mantle structure. Their model shows the Sn velocity is 4.5 2 0.1 km sÿ1, and the lid is believed to be thin, between 20 and 30 km. The pronounced LVZ in the middle crust appears to re¯ect the presence of a sialic low-velocity layer due to the intruded granitic lacco-
Fig. 4. The same as Fig. 2, but for the traverse component of Love waves.
Yi-Ben Tsai, Hsin-Hung Wu / Journal of Asian Earth Sciences 18 (2000) 255±265
261
liths as shown by Mueller (1977). It also could be the remnant of a crustal magma source layer feeding widespread shallow magmatic activities, which are known to have taken place in the region since late the Carboniferous (320 Ma) (Sengor et al., 1993). 4.2. Group B: paths C2 and T1 across Eastern China The group velocity dispersion curves of fundamental-mode LR and LQ waves are shown in Fig. 8 for the two paths in Group B (i.e. C2 and T1) which traverse eastern China. The ®gure shows that the group velocity dispersion curves of both Rayleigh and Love waves for the two paths follow each other very closely
Fig. 6. Shear wave velocity models for group A, group B, group C, and group D.
for periods longer than 15 s. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.7 km sÿ1 at period 40 s. It then increases slightly to about 3.9 km sÿ1 at period 110 s. The LQ group velocity increases linearly from 3.3 km sÿ1 at
Fig. 5. The observed and calculated dispersion curves (top ®gure), and the corresponding shear wave velocity model derived (bottom ®gure).
Fig. 7. The group velocity dispersion curves for group A (paths C1, T5, and T6 across Southern China).
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period 15 s to about 4.0 km sÿ1 at period 40 s. It then increases slowly to 4.5 km sÿ1 at period 110 s. The data quantity for this group is rather limited, so the result for this group is preliminary. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of LR and LQ group velocity dispersion curves shown in Fig. 8 for Group-B paths. From the ®gure we can see, as before, a pronounced LVZ at depth 10±20 km with a Vs of 3.1 km sÿ1. These paths have an average crustal thickness of 30 km. The Vs in the upper mantle reaches a maximum of 4.3 km sÿ1 at depths of 50±60 km. Below 60 km the Vs decreases only a little to 4.2 km sÿ1 at depth 120±130 km. It appears that eastern China also has a relatively thin lithosphere lid. The pronounced LVZ in the middle crust appears to re¯ect the presence of a sialic low-velocity layer resulting from intruded granitic laccoliths as shown by Mueller (1977). It also could be the remnant of a crustal magma source layer of the widespread shallow magmatic activities, known to have taken place in the region since the late Carboniferous (320 Ma) (Sengor et al., 1993). 4.3. Group C: paths C3, T2, T3 and T4 across Central China Fig. 9 shows the group velocity dispersion curves of fundamental-mode LR and LQ waves for the four paths in Group C (i.e. C3, T2, T3, and T4). The main portions of these paths traverse the Yangtze and SinoKorean platforms in central China. The ®gure shows that the LR and LQ group velocity dispersion curves
Fig. 8. The group velocity dispersion curves for group B (paths C2 and T1 across Eastern China).
follow closely each other for periods longer than 15 s. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.5 km sÿ1 at period 40 s. It then increases slowly to about 3.9 km sÿ1 at period 110 s. The LQ group velocity increases linearly from 3.3 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 60 s. It then increases slowly to 4.6 km sÿ1 at period 110 s. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of the group velocity dispersion curves shown in Fig. 9. The result is almost identical to that of T4 shown in Fig. 5. A very mild LVZ is present at depth 10±20 km under these paths. This group of four paths have a crustal thickness of 40 km. The Vs in the upper mantle increases gradually to a maximum of 4.4 km sÿ1 at depth 80± 90 km. Below 90 km there exists a very mild low velocity zone. These results suggest that central China has a regular lithosphere lid. 4.4. Group D: the eastern Tibet plateau The group velocity dispersion curves of fundamental-mode LR and LQ waves are shown in Fig. 10 for the three paths in Group D (i.e. C4, C5 and C6) which traverse the eastern Tibet Plateau. The ®gure shows that both LR and LQ group velocity dispersion curves for the three paths are almost identical to each other for periods longer than 15 seconds. The LR group velocity stays constant at 2.9 km sÿ1 for periods from 15 to 30 seconds. It then increases gradually to about 3.7 km sÿ1 at period 110 s. The LQ group velocity increases slowly from 3.3 km sÿ1 at period 15 s to
Fig. 9. The group velocity dispersion curves for group C (paths C3, T2, T3, and T4 across Central China).
Yi-Ben Tsai, Hsin-Hung Wu / Journal of Asian Earth Sciences 18 (2000) 255±265
Fig. 10. The group velocity dispersion curves for group D (paths C4, C5, and C6 across the eastern Tibet plateau).
about 4.2 km sÿ1 at period 110 s. These wave paths propagate through the areas at the eastern boundary of Tibet plateau (1058E) and they go from north to south. The average velocities calculated are signi®cantly lower than those obtained in previous studies, most of which used wave paths going in the east-west direction. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of LR and LQ group velocity dispersion curves shown in Fig. 10. From the ®gure we see no LVZ in the crust. The crustal thickness is 60 km under this group of paths. This result is very consistent with many previous studies cited above. The S-wave velocity increases slightly to 4.2 km sÿ1 at depths 90±100 km. The low-velocity zone below is hardly recognizable. 4.5. Anomalous LR and LQ group velocity dispersion curves Among the 12 surface wave paths two are found to have anomalous LR or LQ group velocity dispersion curves. Fig. 11 shows the fundamental mode group
263
Fig. 11. The group velocity dispersion curves for wave paths T2 and T5.
velocity dispersion curves of LR and LQ waves for the two paths, T2 and T5. Path T2 traverses diagonally through the study area from the source in NW corner to TATO at the SE corner. As shown in Fig. 1, it crosses the boundary zones separating the Tibet plateau, Yangtze platform and South China fold system provinces. Fig. 11 shows that the group velocity dispersion curves of Rayleigh and Love waves are not very well separated. The LR group velocity increases steeply from 3.0 km sÿ1 at period 15 s to about 3.6 km sÿ1 at period 40 s. It then increases slowly to about 3.9 km sÿ1 at period 90 s. This is quite similar to the corresponding LR dispersion curve for the neighboring T3. On the other hand, the LQ group velocity increases slowly from 3.4 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 80 s. This is signi®cantly smaller than the corresponding LQ dispersion curve for the neighboring T3. Thus, T2 seems to have anomalously low LQ group velocities at long periods. This is probably caused by the complicated lateral variations in the crustal structure under the path. Path T5 traverses eastward from Myanmar to TATO along the 258N parallel. Fig. 11 shows that the
Table 2 Summary of characteristics of the S-wave velocity structure of the crust and upper mantle under Southeastern China Path ID
H (km)
Vsmax (km sÿ1)
Crustal LVZ
Region
Group Group Group Group
30 30 40 60
4.5 4.3 4.4 4.2
Yes Yes Yes No
Southern China Eastern China Central China E. Tibet plateau
A (C1, T5, T6) B (C2, T1) C (C3, T2, T3, T4) D (C4, C5, C6)
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group velocity dispersion curves of Rayleigh and Love waves are unusually widely separated. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.5 km sÿ1 at period 40 s. It then increases very little to about 3.6 km sÿ1 at period 110 s. This is signi®cantly smaller than the corresponding LR dispersion curve for the neighboring T6. On the other hand, the LQ group velocity increases rapidly from 3.4 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 40 s. It then increases linearly to about 4.7 km sÿ1 at period 110 s. This is very similar to the corresponding LQ dispersion curve for the neighboring T6. Thus, T5 seems to have anomalously low LR group velocities at long periods. There are two probable causes for the anomalous behavior in the LR dispersion curve over T5. First, T5 has crossed the complicated tectonic structures in western Yunnan, where active magmatic activities are still in progress. Second, it appears that the crustal thickness changes abruptly from 30 km over T6 south of 258N to over 40 km T4 to the north. This issue deserves further investigations. 5. Conclusions From the diversi®ed dispersion curves obtained over dierent path groups and the corresponding velocity structures inverted, it is clearly seen that the crust and upper mantle structure in southeastern China is laterally heterogeneous. In southeastern China, the shear wave velocities of 4.5 km sÿ1 or less in the uppermost mantle are relatively low for a continent. This is consistent with the results of Rosenthal and Teng (1977), Chun and Yoshi (1977), and Wier (1982). In the uppermost mantle, southern China has the highest shear velocity, 4.5 km sÿ1, the Tibet plateau has the lowest shear velocity, 4.2 km sÿ1, and central and eastern China have intermediate shear velocities in between 4.2 and 4.5 km sÿ1. We have also obtained a low velocity zone at depth 10±20 km in the middle crust of southeastern China. This is possibly the so called sialic low-velocity layer (Mueller, 1977), which may be the remnant of a source layer of widespread magmatism in the geologic past (Table 2). Acknowledgements This study was supported by the National Science Council of the Republic of China under Grant No. NSC 86-2116-M-008-007. Discussions with Drs GueyKuen Yu, Ching-Hwa Lo, Tung-Yi Lee and Sun-Lin Chung have been very helpful. We thank Dr Walter D. Mooney and a anonymous reviewer for their valuable comments and suggestions on this paper. Thanks
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patterns of magmatism along the Tethysides: a preliminary study. The Journal of Geology 101, 51±84. Tung, J.P., 1974. The surface wave study of crustal and upper mantle of mainland China. Ph.D. dissertation, University of Southern California. Wang, K., Yao, Z.X., 1989. Preliminary study of upper mantle shear velocity structure of China. Chinese Journal of Geophysics 2, 621±626. Wier, S., 1982. Surface wave dispersion and Earth structure in south-eastern China. Geophysical Journal of the Royal Astronomical Society 69, 33±47. Wiggins, R.A., 1972. The general linear inverse problems: implications of surface waves and free oscillations for earth structure. Reviews of Geophysics and Space Physics 10, 251±285. Wu, F.T., Levshin, A.L., 1994. Surface-wave group velocity tomography of East Asia. Physics of the Earth and Planetary Interiors 84, 59±77. Wu, F.T., Levshin, A.L., Kozhevnikov, V.M., 1997. Rayleigh wave group velocity tomography of Siberia, China, and the vicinity. Pure and Applied Geophysics 149, 447±473. Zhang, Y.-S., 1998. Three-dimensional velocity structure beneath East Asia and its tectonic implication. In: Flower, M.F.J., et al. (Eds.), Mantle Dynamics and Plate Interactions in East Asia. American Geophysical Union, pp. 11±23. Zhao, L.S., Helmberger, D.V., Harkrider, D.G., 1991. Shear velocity structure of the crust and upper mantle beneath the Tibetan plateau and southeastern China. Geophysical Journal International 105, 713±730.
S-wave velocity structure of the crust and upper mantle under southeastern China by surface wave dispersion analysis Yi-Ben Tsai, Hsin-Hung Wu* Institute of Geophysics, National Central University, Chung-Li, Taiwan Received 4 January 1998; received in revised form 13 November 1998
Abstract Group velocity dispersion data of fundamental-mode Rayleigh and Love waves for 12 wave paths within southeastern China have been measured by applying the multiple-®lter technique to the properly rotated three-component digital seismograms from two Seismic Research Observatory stations, TATO and CHTO. The generalized surface wave inversion technique was applied to these group velocity dispersion data to determine the S-wave velocity structures of the crust and upper mantle for various regions of southeastern China. The results clearly demonstrate that the crust and upper mantle under southeastern China are laterally heterogeneous. The southern China region south of 258N and the eastern China region both have a crustal thickness of 30 km. The eastern Tibet plateau along the 1008E meridian has a crustal thickness of 60 km. Central China, consisting mainly of the Yangtze and Sino-Korean platforms, has a crustal thickness of 40 km. A distinct S-wave low-velocity layer at 10±20 km depth in the middle crust was found under wave paths in southeastern China. On the other hand, no such crustal low-velocity layer is evident under the eastern Tibet plateau. This low-velocity layer in the middle crust appears to re¯ect the presence of a sialic low-velocity layer perhaps consisting of intruded granitic laccoliths, or possibly the remnant of the source zone of widespread magmatic activities known to have taken place in these regions since the late Carboniferous. # 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction The purpose of this study is to determine the S-wave velocity structure of the crust and upper mantle under southeastern China by surface wave dispersion analysis and to expound the related tectonic signi®cance. The present study covers a rectangular area spanning approximately from 958E to 1208E in longitude and 208N to 408N in latitude. Analysis of the S-wave velocity structure of the crust and upper mantle under southeastern China is particularly interesting because of its apparent tectonic heterogeneity and widespread magmatism since late Carboniferous. Fig. 1 shows that the present study area covers four tectonic provinces (Beckers et al., 1994): The South China fold system in the south, the Yangtze platform in the center, the * Corresponding author.
Sino-Korean platform in the north, and the Tibet plateau in the west. The average crustal thickness under the study area was found previously to increase from 30 km in the east, to 45 km in the west, and to more than 60 km farther to the west (Chinese Academy of Sciences, 1974; Feng and Teng, 1983; Mooney et al., 1998). It is interesting to provide additional independent evidence for such regional variations in crustal thickness. Sengor et al. (1993) recently presented a comprehensive analysis of the space±time evolution of magmatism along the Tethysides. The present study area is located at the eastern end of Tethysides. According to Sengor et al. (1993), wide areas of southeastern China have experienced varying degrees of collision-related or subduction-related magmatic activities during the long geologic history from late Carboniferous±Permian (320±248 Ma) to late Cenozoic (24±0 Ma). It is of considerable interest to determine, whether there is any
1367-9120/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 7 - 9 1 2 0 ( 9 9 ) 0 0 0 5 8 - 9
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discernable S-wave low-velocity layer which can be identi®ed as a remnant of the magma source zone in the crust or upper mantle. The S-wave velocity structures of the crust and upper mantle under southeastern China and the adjacent Tibet plateau have been studied by many authors (e.g. Tung, 1974; Chen and Molnar, 1975; Chun and Yoshi, 1977; Rosenthal and Teng, 1977; Patton, 1980; Pines et al., 1980; Romanowicz, 1982, 1984; Wier, 1982; Feng and Teng, 1983; Jobert et al. 1985; Brandon and Romanowicz, 1986; Wang and Yao, 1989; Zhao et al., 1991; Bourjot and Romanowicz, 1992; Wu and Levshin, 1994; Mitchell et al., 1997; Wu et al., 1997; Zhang, 1998). Recently, Ritzwoller and Levshin (1998) gave a comprehensive review of surface wave tomography of Eurasia, including our present study area. The present study analyzes the group velocity dispersion of surface waves using more recent data with improved spatial coverage and over shorter paths.
Both Rayleigh and Love wave group velocity dispersion curves over a total of 12 paths traversing the study area from 14 earthquakes to two Seismic Research Observatory (SRO) stations, CHTO in Chiangmai, Thailand and TATO in Taipei, Taiwan (Peterson et al., 1980) are analyzed. 2. Data Fourteen earthquakes (Table 1) were chosen for the present study, because their epicenters fall in the eastern and western border zones. Moreover, the wave paths from these earthquake epicenters to the two stations, TATO and CHTO, are con®ned within the study area (Fig. 1). The path lengths range from approx. 1800 to 2850 km. When available, seismograms from multiple earthquakes are used for the same wave paths in order to ensure data repeatability.
Fig. 1. The wave paths and path ID. The solid lines show the paths from epicenters of sources to station TATO, and dashed lines to station CHTO. Thin curved line segments are boundaries of the four tectonic structures traversed. The locations of stations are shown by solid triangles and epicenters by solid circles. The thickness of the crust along each path is shown on the side of path.
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In total, twelve wave paths traverse the study area, forming two sets of approx. 908 fan-shaped coverage. The ®rst set includes six wave paths, T1±T6 (Fig. 1), arriving at TATO on the southeastern corner. The second set also includes six wave paths, C1±C6 (Fig. 1), arriving at CHTO on the southwestern corner of the present study area. The SRO data in digital form is convenient for computer processing. Since the SRO seismographs operate at high sensitivity levels, they provide excellent records useful for surface wave dispersion analysis from moderate-magnitude earthquakes (M 0 5.5) which can be considered as point source at the wavelengths concerned. The long-period digital seismograms used for this study were provided by the Incorporated Research Institute for Seismology (IRIS). Instrumental response was removed from each recorded trace using the method of Luh (1977). Pure Rayleigh and Love wave components are obtained by rotating the two original EW and NS components into the radial and transverse components. Normally, three components, namely, the vertical (V) and radial (R) components for Rayleigh waves and the transverse (T) component for Love waves, are available at each station. Unfortunately, due to complication of the radiation patterns and propagation path eects, sometimes the qualities of one or two components may not be so good. Poor data from such cases were not used for further interpretation. 3. Method A multiple-®ltering analysis (Dziewonski and Hales, 1972; Herrmann, 1973) was applied to each surface wave train to obtain the fundamental-mode group-vel-
257
ocity curve. Typical contoured plots of relative amplitude of wave energy arrivals at TATO over Path T4 from Yunnan are shown in Fig. 2 for the vertical component of Rayleigh waves, in Fig. 3 for the radial component of Rayleigh waves, and in Fig. 4 for the transverse component of Love waves, respectively. In these ®gures the open circles, marking the local maxima, represent the average group velocities obtained from the multiple-®ltering analysis. The contour lines sometimes give more than one local maxima which make a clear-cut determination of group velocity values dicult. We also use a phase-matched ®lter (Herrin and Goforth, 1977; Goforth and Herrin, 1979) to identify and remove multipathing arrivals to improve the quality of the determined dispersion curves. It is remarkable that both vertical and radial components of Rayleigh waves give almost identical group velocity values for all periods above 15 s. At shorter periods an ambiguity often arises in drawing contours due to interference by S waves. Thus, we exclude the shortperiod dispersion data in subsequent inversion to obtain the S-wave velocity structure. Finally, simultaneous inversion of both Rayleigh and Love wave dispersion data measured over similar paths is made by the generalized inversion method (Backus and Gilbert, 1967, 1968, 1970) to determine the S-wave velocity structure. Detailed descriptions of the algorithm to implement this method can be found elsewhere (Jackson, 1972; Wiggins, 1972; Crossen, 1976). In applying the inversion algorithm, we use a layered half-space velocity model for all wave paths. In the model the upper 160 km are divided into 16 layers with a uniform thickness of 10 km. Below 160 km lies an in®nite half space. The same initial model with a uniform S-wave velocity of 5 km sÿ1 for
Table 1 Pertinent earthquakes information used in this study Earthquake
TATO
No.
Date
Epicenter
Depth
Magnitude
1 2 3 4 5 6 7 8 9 10 11 12 13 14
19900911140902.50 19910118013626.60 19910312060405.00 19910325180241.50 19910902110550.40 19910920111611.50 19920112001227.10 19920122214125.90 19930126203206.90 19931026113821.90 19940103055227.60 19940111005156.30 19960203111422.00 19960204165809.00
22.6920N120.9090E 23.6920N121.3460E 23.1610N120.0530E 39.8870N113.9230E 37.4400N 95.4020E 36.1910N100.0630E 39.6710N98.3000E 35.3510N121.1090E 23.0270N101.0620E 38.4770N98.6550E 36.0280N100.1040E 25.2310N97.2030E 27.2000N100.5000E 27.0000N100.5000E
33.0 11.0 17.0 10.0 10.0 13.0 22.0 33.0 33.0 8.0 8.0 10.0 33.0 33.0
5.8 5.9 5.6 5.5 5.5 5.5 5.6 5.5 5.6 5.9 5.8 6.1 6.4 5.6
D (km)
1801
2085 2618 2381 2443 2109 2108
V
R
CHTO T
ID
T1
T6 T2 T3 T5 T4 T4
D (km)
V
R
2320 2379 2240 2746 2100 1935 2320 2849
2187 1918
T
ID
C1 C1 C1 C3 C6 C4 C5 C2
C5 C4
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all layers is used in the inversion for all wave paths. We use the same initial model in order to obtain ®nal velocity structures re¯ecting the dierences in the dispersion curves of dierent path groups but free from bias from initial models. In doing the above analysis, we have assumed that the medium is isotropic and we only do simple one-dimensional modeling. The phase and group velocities of Rayleigh waves are more sensitive to deeper shear velocity changes than Love waves (Ritzwoller and Levshin, 1998). On the other hand, for phase velocity, the crustal eect for Love waves is more than twice that of Rayleigh waves. For example, Love waves at 40 s are most sensitive to the S wave velocity changes in the uppermost 60 km. In contrast, Rayleigh waves at these periods are primarily sensitive to the uppermost mantle structure (Mooney et al., 1998). In other words, at short periods, Love waves are more sensitive to shallower S wave velocity changes than Rayleigh waves. Conversely, at long periods, Rayleigh waves are more sensitive to deeper S wave velocity changes than Love
waves. Thus, simultaneous inversion of both Love and Rayleigh waves can complement each other in the results. Fig. 5 shows an example of the ®nal S-wave velocity structure obtained from inversion of dispersion data over Path T4 from Yunnan to Taipei. The upper part of the ®gure shows remarkably close matching between the calculated and observed group velocity dispersion curves for both Rayleigh and Love waves. In this case the observed Rayleigh wave data include both radial and vertical components from two earthquakes (No. 13 and No. 14 in Table 1). The observed Love wave data include the transverse component from No. 13 earthquake (Table 1). The lower part of Fig. 5 shows several remarkable features in the ®nal velocity structure. First, it has a distinct upper crust at depths of from 0 to 20 km, with a low velocity zone (LVZ) at depth from 10 to 20 km, which shows only a small decrease in seismic velocities, and a lower crust from 20 to 40 km. Second, the Swave velocity increases gradually with depth in the
Fig. 2. Typical contoured plots of relative amplitude of wave energy arrivals at TATO over Path T4 from Yunnan for the vertical component of Rayleigh waves. The open circles show the picked group velocities.
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upper mantle to reach a maximum value of 4.3 km sÿ1 at depths between 70 and 90 km. This means that there exists a 90-km thick rigid lithosphere lid under the wave path. A mild LVZ appears to extend from 90 km down to at least 160 km depth, where the upper boundary of in®nite half space in the model is designated. Finally, the crust has a thickness of 40 km if we take 4.1 km sÿ1 as the upper limit for S-wave velocity for the crustal layer just above the Moho discontinuity, as was done by Feng and Teng (1983). 4. Results and discussion The wave paths are divided into four groups according to the similarities of their dispersion data, in order to highlight regional variations in the S-wave velocity structure of the crust and the upper mantle in southeastern China. Group A includes three EW-trending paths (i.e. C1, T5 and T6) which traverse southern China and northern Indochina in the southern part of
259
the present study area. Group B includes two wave paths (i.e. C2 and T1) which traverse central and eastern China, respectively. Group C includes four wave paths (i.e. C3, T2, T3 and T4) which traverse diagonally through central China. Group D includes three NS-trending wave paths (i.e. C4, C5 and C6) which traverse the eastern Tibet plateau in the western part of the study area. An average S-wave velocity structure under each group of wave paths is obtained from inversion of the group velocity dispersion data of surface waves. The results are summarized in Fig. 6. It can be seen clearly that the S-wave velocity structures under the four group of paths vary signi®cantly from the surface down to a depth 100 km. Generally speaking, southern and eastern China have thinner crust and higher Vs values in the upper mantle. The eastern Tibet plateau has thicker crust and lower Vs values. Central China has intermediate crustal thickness and Vs values. Detailed descriptions of individual path groups are given below.
Fig. 3. The same as Fig. 2, but for the radial component of Rayleigh waves.
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4.1. Group A: paths C1, T5 and T6 across Southern China The group velocity dispersion curves of fundamental-mode Rayleigh (LR) and Love (LQ) waves are shown in Fig. 7 for the three paths in Group A (i.e. C1, T5 and T6) which traverse southern China and northern Indochina. The ®gure shows that the group velocity dispersion curves of both Rayleigh and Love waves for the two paths follow each other very closely for periods longer than 15 s. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.7 km sÿ1 at period 40 s. It then increases slightly to about 3.8 km sÿ1 at period 110 s. The LQ group velocity increases linearly from 3.3 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 40 s. It then increases slowly to 4.3 km sÿ1 at period 110 s. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of the LR and LQ group velocity dispersion curves shown in Fig. 7
for Group-A paths. From the ®gure we can see a pronounced LVZ at depths 10±20 km with a Vs of 3.3 km sÿ1. These paths have an average crustal thickness of 30 km. The Vs in the upper mantle reaches a maximum of 4.5 km sÿ1 at depths 40±50 km. This high velocity lid also shows up clearly on the tomographic images of Wu and Levshin (1994). Here the thin Archean crust is probably the main controlling factor. Below 50 km Vs decreases gradually to 4.2 km sÿ1 at depths 120±130 km. It appears that southern China has a relatively thin lithosphere lid. This is consistent with the result of Rosenthal and Teng (1977). On the South China block, Rosenthal and Teng have inverted group velocities of Love and Rayleigh waves over the period range 10±60 s to obtain the crust and upper mantle structure. Their model shows the Sn velocity is 4.5 2 0.1 km sÿ1, and the lid is believed to be thin, between 20 and 30 km. The pronounced LVZ in the middle crust appears to re¯ect the presence of a sialic low-velocity layer due to the intruded granitic lacco-
Fig. 4. The same as Fig. 2, but for the traverse component of Love waves.
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261
liths as shown by Mueller (1977). It also could be the remnant of a crustal magma source layer feeding widespread shallow magmatic activities, which are known to have taken place in the region since late the Carboniferous (320 Ma) (Sengor et al., 1993). 4.2. Group B: paths C2 and T1 across Eastern China The group velocity dispersion curves of fundamental-mode LR and LQ waves are shown in Fig. 8 for the two paths in Group B (i.e. C2 and T1) which traverse eastern China. The ®gure shows that the group velocity dispersion curves of both Rayleigh and Love waves for the two paths follow each other very closely
Fig. 6. Shear wave velocity models for group A, group B, group C, and group D.
for periods longer than 15 s. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.7 km sÿ1 at period 40 s. It then increases slightly to about 3.9 km sÿ1 at period 110 s. The LQ group velocity increases linearly from 3.3 km sÿ1 at
Fig. 5. The observed and calculated dispersion curves (top ®gure), and the corresponding shear wave velocity model derived (bottom ®gure).
Fig. 7. The group velocity dispersion curves for group A (paths C1, T5, and T6 across Southern China).
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period 15 s to about 4.0 km sÿ1 at period 40 s. It then increases slowly to 4.5 km sÿ1 at period 110 s. The data quantity for this group is rather limited, so the result for this group is preliminary. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of LR and LQ group velocity dispersion curves shown in Fig. 8 for Group-B paths. From the ®gure we can see, as before, a pronounced LVZ at depth 10±20 km with a Vs of 3.1 km sÿ1. These paths have an average crustal thickness of 30 km. The Vs in the upper mantle reaches a maximum of 4.3 km sÿ1 at depths of 50±60 km. Below 60 km the Vs decreases only a little to 4.2 km sÿ1 at depth 120±130 km. It appears that eastern China also has a relatively thin lithosphere lid. The pronounced LVZ in the middle crust appears to re¯ect the presence of a sialic low-velocity layer resulting from intruded granitic laccoliths as shown by Mueller (1977). It also could be the remnant of a crustal magma source layer of the widespread shallow magmatic activities, known to have taken place in the region since the late Carboniferous (320 Ma) (Sengor et al., 1993). 4.3. Group C: paths C3, T2, T3 and T4 across Central China Fig. 9 shows the group velocity dispersion curves of fundamental-mode LR and LQ waves for the four paths in Group C (i.e. C3, T2, T3, and T4). The main portions of these paths traverse the Yangtze and SinoKorean platforms in central China. The ®gure shows that the LR and LQ group velocity dispersion curves
Fig. 8. The group velocity dispersion curves for group B (paths C2 and T1 across Eastern China).
follow closely each other for periods longer than 15 s. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.5 km sÿ1 at period 40 s. It then increases slowly to about 3.9 km sÿ1 at period 110 s. The LQ group velocity increases linearly from 3.3 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 60 s. It then increases slowly to 4.6 km sÿ1 at period 110 s. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of the group velocity dispersion curves shown in Fig. 9. The result is almost identical to that of T4 shown in Fig. 5. A very mild LVZ is present at depth 10±20 km under these paths. This group of four paths have a crustal thickness of 40 km. The Vs in the upper mantle increases gradually to a maximum of 4.4 km sÿ1 at depth 80± 90 km. Below 90 km there exists a very mild low velocity zone. These results suggest that central China has a regular lithosphere lid. 4.4. Group D: the eastern Tibet plateau The group velocity dispersion curves of fundamental-mode LR and LQ waves are shown in Fig. 10 for the three paths in Group D (i.e. C4, C5 and C6) which traverse the eastern Tibet Plateau. The ®gure shows that both LR and LQ group velocity dispersion curves for the three paths are almost identical to each other for periods longer than 15 seconds. The LR group velocity stays constant at 2.9 km sÿ1 for periods from 15 to 30 seconds. It then increases gradually to about 3.7 km sÿ1 at period 110 s. The LQ group velocity increases slowly from 3.3 km sÿ1 at period 15 s to
Fig. 9. The group velocity dispersion curves for group C (paths C3, T2, T3, and T4 across Central China).
Yi-Ben Tsai, Hsin-Hung Wu / Journal of Asian Earth Sciences 18 (2000) 255±265
Fig. 10. The group velocity dispersion curves for group D (paths C4, C5, and C6 across the eastern Tibet plateau).
about 4.2 km sÿ1 at period 110 s. These wave paths propagate through the areas at the eastern boundary of Tibet plateau (1058E) and they go from north to south. The average velocities calculated are signi®cantly lower than those obtained in previous studies, most of which used wave paths going in the east-west direction. Fig. 6 shows the average S-wave velocity structure obtained from simultaneous inversion of LR and LQ group velocity dispersion curves shown in Fig. 10. From the ®gure we see no LVZ in the crust. The crustal thickness is 60 km under this group of paths. This result is very consistent with many previous studies cited above. The S-wave velocity increases slightly to 4.2 km sÿ1 at depths 90±100 km. The low-velocity zone below is hardly recognizable. 4.5. Anomalous LR and LQ group velocity dispersion curves Among the 12 surface wave paths two are found to have anomalous LR or LQ group velocity dispersion curves. Fig. 11 shows the fundamental mode group
263
Fig. 11. The group velocity dispersion curves for wave paths T2 and T5.
velocity dispersion curves of LR and LQ waves for the two paths, T2 and T5. Path T2 traverses diagonally through the study area from the source in NW corner to TATO at the SE corner. As shown in Fig. 1, it crosses the boundary zones separating the Tibet plateau, Yangtze platform and South China fold system provinces. Fig. 11 shows that the group velocity dispersion curves of Rayleigh and Love waves are not very well separated. The LR group velocity increases steeply from 3.0 km sÿ1 at period 15 s to about 3.6 km sÿ1 at period 40 s. It then increases slowly to about 3.9 km sÿ1 at period 90 s. This is quite similar to the corresponding LR dispersion curve for the neighboring T3. On the other hand, the LQ group velocity increases slowly from 3.4 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 80 s. This is signi®cantly smaller than the corresponding LQ dispersion curve for the neighboring T3. Thus, T2 seems to have anomalously low LQ group velocities at long periods. This is probably caused by the complicated lateral variations in the crustal structure under the path. Path T5 traverses eastward from Myanmar to TATO along the 258N parallel. Fig. 11 shows that the
Table 2 Summary of characteristics of the S-wave velocity structure of the crust and upper mantle under Southeastern China Path ID
H (km)
Vsmax (km sÿ1)
Crustal LVZ
Region
Group Group Group Group
30 30 40 60
4.5 4.3 4.4 4.2
Yes Yes Yes No
Southern China Eastern China Central China E. Tibet plateau
A (C1, T5, T6) B (C2, T1) C (C3, T2, T3, T4) D (C4, C5, C6)
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group velocity dispersion curves of Rayleigh and Love waves are unusually widely separated. The LR group velocity increases steeply from 2.9 km sÿ1 at period 15 s to about 3.5 km sÿ1 at period 40 s. It then increases very little to about 3.6 km sÿ1 at period 110 s. This is signi®cantly smaller than the corresponding LR dispersion curve for the neighboring T6. On the other hand, the LQ group velocity increases rapidly from 3.4 km sÿ1 at period 15 s to about 4.0 km sÿ1 at period 40 s. It then increases linearly to about 4.7 km sÿ1 at period 110 s. This is very similar to the corresponding LQ dispersion curve for the neighboring T6. Thus, T5 seems to have anomalously low LR group velocities at long periods. There are two probable causes for the anomalous behavior in the LR dispersion curve over T5. First, T5 has crossed the complicated tectonic structures in western Yunnan, where active magmatic activities are still in progress. Second, it appears that the crustal thickness changes abruptly from 30 km over T6 south of 258N to over 40 km T4 to the north. This issue deserves further investigations. 5. Conclusions From the diversi®ed dispersion curves obtained over dierent path groups and the corresponding velocity structures inverted, it is clearly seen that the crust and upper mantle structure in southeastern China is laterally heterogeneous. In southeastern China, the shear wave velocities of 4.5 km sÿ1 or less in the uppermost mantle are relatively low for a continent. This is consistent with the results of Rosenthal and Teng (1977), Chun and Yoshi (1977), and Wier (1982). In the uppermost mantle, southern China has the highest shear velocity, 4.5 km sÿ1, the Tibet plateau has the lowest shear velocity, 4.2 km sÿ1, and central and eastern China have intermediate shear velocities in between 4.2 and 4.5 km sÿ1. We have also obtained a low velocity zone at depth 10±20 km in the middle crust of southeastern China. This is possibly the so called sialic low-velocity layer (Mueller, 1977), which may be the remnant of a source layer of widespread magmatism in the geologic past (Table 2). Acknowledgements This study was supported by the National Science Council of the Republic of China under Grant No. NSC 86-2116-M-008-007. Discussions with Drs GueyKuen Yu, Ching-Hwa Lo, Tung-Yi Lee and Sun-Lin Chung have been very helpful. We thank Dr Walter D. Mooney and a anonymous reviewer for their valuable comments and suggestions on this paper. Thanks
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