SUMMARY We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Green’s function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others they may appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity.

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Surface wave array tomography in SE Tibet from ambient seismic noise and two-station analysis - II. Crustal and upper-mantle structure

[1] Understanding the geotectonic evolution of the southeastern Tibetan plateau requires knowledge about the structure of the lithosphere. Using data from 77 broadband stations in SW China, we invert Rayleigh wave phase velocity dispersion curves from ambient noise interferometry (T =1 0–40 s) and teleseismic surface waves (T =2 0–150 s) for 3‐D heterogeneity and azimuthal anisotropy in the lithosphere to ∼150 km depth. Our surface wave array tomography reveals (1) deep crustal zones of anomalously low shear wave speed and (2) substantial variations with depth of the pattern of azimuthal anisotropy. Upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern Himalayan syntaxis, with fast directions generally parallel to the main strike slip faults. The mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south. The tomographically inferred 3‐D variation in azimuthal anisotropy helps constrain the source region of shear wave splitting. South of ∼26°N (off the high plateau) most of the observed splitting can be accounted for by upper mantle anisotropy, but for stations on the plateau proper (with thick crust) crustal anisotropy cannot be ignored. On long wavelengths, the pattern of azimuthal anisotropy in the crust differs from that in the mantle. This is easiest explained if deformation varies with depth. The deep crustal zones of low shear wave speed (and, presumably, mechanical strength) may represent loci of ductile deformation. But their lateral variation suggests that in SE Tibet (localized) crustal channel flow and motion along the major strike slip faults are both important.

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Heterogeneity and anisotropy of the lithosphere of SE Tibet from surface wave array tomography

Structure of the crust beneath the southeastern Tibetan Plateau from teleseismic receiver functions

http://www.igg.cas.cn/xwzx/yjcg/201502/W020150209405366175654.pdf

Tearing of the Indian lithospheric slab beneath southern Tibet revealed by SKS-wave splitting measurements

3D imaging of subducting and fragmenting Indian continental lithosphere beneath southern and central Tibet using body-wave finite-frequency tomography

S receiver function analysis of the crustal and lithospheric structures beneath eastern Tibet

Surface wave data were initially collected from events of magnitude Ms ≥ 5.0 and shallow or moderate focal depth occurred between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities. After inverting the path-averaged group times by means of a damped least-squares approach, we have retrieved location-dependent group velocities on a 2° × 2°-sized grid and constructed Rayleigh- and Love-wave group velocity maps at periods 10.4–105.0 s. Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results. Afterwards, depth-dependent SV- and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method. The results provide: (a) Rayleigh- and Love-wave group velocities at various periods; (b) SV- and SH-wave differential velocity maps at different depths; (c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles; (d) regionalized dispersion curves and velocity-depth models related to the main geological formations. The lithospheric root presents a depth that can be substantiated at ~140 km (Qiangtang Block) and exceptionally at ~180 km in some places (Lhasa Block), and which exhibits laterally varying fast velocity very close to that of some shields that even reaches ~4.8 km/s under the northern Lhasa Block and the Qiangtang Block. Slow-velocity anomalies of 7–10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet.

Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas

Receiver function of body wave under the 23 stations in Yunnan was extracted from 3-component broadband digital recording of teleseismic event. Thus, the S-wave velocity structure and distribution characteristics of Poisson’s ratio in crust of Yunnan are obtained by inversion. The results show that the crustal thickness is gradually thinned from north to south. The crustal thickness in Zhongdian of northwest reaches as many as 62.0 km and the one in Jinghong of further south end is only 30.2 km. What should be especially noted is that there exists a Moho upheaval running in NS in the Chuxiong region and a Moho concave is generally parallel to it in Dongchuan. In addition, there exists an obvious transversal inhomogeneity for the S-wave velocity structure in upper mantle and crust in the Yunnan region. The low velocity layer exists not only in 10.0-15.0 km in upper crust in some regions, but also in 30.0-40.0 km in lower crust. Generally, the Poisson’s ratio is on the high side, however it has a better corresponding relation to the crustal velocity structure. An obvious block distribution feature is still shown on such a high background of Poisson’s ratio. It is discovered by synthetically analyzing the velocity structure and Poisson’s ratio distribution that there are high Poisson’s ratio and complicated crust-mantle velocity structure feature in the Sichuan-Yunnan Diamond Block with Xiaojiang fault to be the east boundary and Yulong Snow Mountain fault to be the west boundary besides the frequent seismicity. This feature differs obviously from that of surrounding areas, which would provide geophysical evidence to deeply study the eastwardly flowage of lithospheric substances in the Qinghai-Tibet Plateau.

S-wave velocity and Poisson’s ratio structure of crust in Yunnan and its implication

SUMMARY Crustal thickness and Poisson’s ratio are two key parameters for investigating tectonic setting and evolution. The H-k technique has become popular to determinate their values in recent years.However,ifacomplexstructureexistsinthecrust,thereverberatedphasesfromdifferent depths may interfere with each other, resulting in reduction or even absence of the PpPs phase from the Moho, such that the H-k algorithm may not reveal unambiguous estimates of these two parameters. In this paper, the H-k technique is applied to process a synthetic receiver function for a test case, the result of which is compared with that from direct picking of the time delays of the converted and reverberated phases. Then we process the observational data recorded at two stations in eastern Tibet, determine the crustal thickness and the velocity ratio using the two methods, compare the result with that obtained by Xu et al .a t the same location, and analyse the potential causes for discrepancy as well as the reliability of the two methods. Finally, the crustal thickness and the Poisson’s ratio in eastern Tibet and Sichuan Basin are determined from the receiver functions recorded at 51 broad-band stations, using the two different methods. The results indicate that the crustal thickness from the time delays is thicker than that from the H-k algorithm near the eastern Himalayan syntaxis, and the crustal Poisson’s ratio in the rigid Sichuan Basin from the time delays ranges from 0.26 to 0.28, which are more reasonable than that from the H-k (0.28 to 0.34). Additionally, the Poisson’s ratio under the faults from the time delays is found to be higher than those on the two sides, which is consistent with the tectonic background and previous study.

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Analysis of the crustal thickness and Poisson's ratio in eastern Tibet from teleseismic receiver functions

The receiver functions of body waves of distant earthquakes obtained for the regions beneath 41 digital stations (Lhasa and GANZ in Tibet, Mandalay and Rangoon in Myanmar, SHIO in India, CHTO in Thailand, and station network in Sichuan and Yunnan) were used to invert for S-wave structure in the crust and upper mantle in Sichuan, Yunnan, and their surrounding areas. Meanwhile the distribution characteristics of the Poisson’s ration and the crustal thickness in Sichuan and Yunnan areas were also obtained. Results indicate that the depth of Moho beneath the eastern side of Qinghai-Tibetan plateau varies strikingly. It is obvious that the greatest changes in crustal thickness occur in a north-south direction. The crustal thickness decreases from north to south, being as thick as 70 km in eastern Tibet, the northern portion of our area of interest, and less than 30 km in Chaing Mai and Rangoon, the southern portion of our area. There are, however, exceptions regarding the trend. The thickness exhibits an east-west variation trend in the area from Ma’erkong-Kongding in Sichuan to Lijiang in Yunnan. In general the Jinpingshan-Longmenshan fault and Anninghe fault can be taken as the boundaries of this exception area. The thickness in Kongding in the west is 68 km, while it is only 39 km in Yongchuan in the east. Moreover the Poisson’s ratio values in the blocks of central Sichuan and Sichuan-Yunnan Diamond are high, and a low velocity layer in the crust of this area can be obviously detected. The distribution characteristics of the high Poisson’s ratio and the low velocity of the crust in this block correspond to the tectonic structure, being in contrast with the surrounding areas. Combining with the distribution features of the modern tectonic stress field, it is deduced that the Sichuan-Yunnan area is probably the channel through which the materials of the lithosphere flow eastward.

Jiafu Hu

Papers

S receiver function analysis of the crustal and lithospheric structures beneath eastern Tibet

Love and Rayleigh Wave Tomography of the Qinghai-Tibet Plateau and Surrounding Areas

S-wave velocity and Poisson’s ratio structure of crust in Yunnan and its implication

Analysis of the crustal thickness and Poisson's ratio in eastern Tibet from teleseismic receiver functions

Structure and Significance of S-wave Velocity and Poisson's Ratio in the Crust beneath the Eastern Side of the Qinghai-Tibet Plateau