Study On The Phase Velocity And Azimuthal Anisotropy From Rayleigh Surface-Wave In The Crust And Upper-Mantle Beneath The Continental China And Its Adjacent Regions

Based on the Rayleigh surface-wave dispersion characteristics, this paper will provide the Rayleigh-wave phase velocity dispersion curves by utilizing a two-station cross-correlation of narrow band-pass filter and an image analysis technique. The dispersion data were then used to invert phase velocity and azimuthal anisotropy distribution in the crust and upper-mantle beneath the continental China and its adjacent regions (70°-135°E, 18°-55°N). Several main results obtained are as follows:1. This paper will present a large amount of teleseismic surface waveform data, which had been collected from both permanent and portable digital seismic networks distributed in continental China and its adjacent areas. Further according to the waveform data quality, the vertical-component records of 409 earthquakes during 1990-2006 from 204 stations were processed to obtain inter-station phase velocity dispersions of fundamental-mode Rayleigh waves at periods between 20 and 120s along 4961 paths (including multiple paths) or 2610 independent paths.2. This paper will first provide relatively high resolution Rayleigh-wave phase velocity distributions at periods 20-120s in the continental China and its adjacent regions. Checkerboard tests show that the lateral resolution is about 5°in the western China and adjacent regions, about 3°in the central-eastern China, and even up to 2°for some local areas.The significant lateral variation of phase velocity distribution indicates various phase velocity structures in the studied area. The North-South Seismic Belt in China has relatively low phase velocity at all periods, and therefore becomes a natural boundary between eastern continental China and western portion with different lithospheric phase velocity features.3. It's also the first time that relatively high resolution azimuthal anisotropy distributions of Rayleigh-wave phase velocity at various periods in the continental China and its adjacent regions have been obtained, and this knowledge is valuable for understanding the dynamics of plate-motion model and material migration pattern during deformation. The azimuthal anisotropy distributions in the study area also display spatial heterogeneity.The amplitude of azimuthal anisotropy beneath the central Qinghai-Tibet plateau is higher than that both in the western and eastern parts, and the maximum amplitude of azimuthal anisotropy is up to 4% at periods 40-80s; 3% appears near the eastern Himalayan syntaxis at periods 40-60s. It is also indicated that the anisotropy distribution beneath Songpan-Garze orogenic belt has significant spatial variation, and the maximum amplitude of anisotropy up to 3% is observed near the northern boundary at the east end of the east Kunlun fault zone at periods 35-40s. By considering the correlated variation between fast propagation direction, the anisotropy amplitude and different periods, it can be deduced that strong deformation has been occurred especially in the lower crust and the upper-mantle lid beneath Qinghai-Tibet block and its eastern margin. It is also recognized that fast propagation direction at short periods basically coincides with directions at mid-long periods beneath Qinghai-Tibet block, which indicates that the crust and upper-mantle lid are coupled there. Clockwise rotation is presented around the eastern Himalayan syntaxis from the west to the east within the block, which may indicate the direction of material flow (or escaping) in the block after the Eurasian and Indian collision. Azimuthal anisotropy maps at long periods show that clockwise rotation is disappeared and the dominant direction of fast propagation in Qinghai-Tibet block correlates well with the direction of the Indian plate movement. Since mantle convection mainly drives plate motion, the dominant direction maybe is indicative of the mantle flow direction beneath the Qinghai-Tibet block. Azimuthal anisotropy maps also indicate that the anisotropy amplitude of the eastern and western parts of the Tarim basin in front of the northwest of Qinghai-Tibet block is greater than that of the central part and the maximum amplitude is up to 3%. The maps at short periods show that the dominant directions of fast propagation in middle crust in the western, central and eastern parts are NE-SW, E-W and NW-SE, respectively, and present a clockwise rotation from west to east. This finding is in great accordance with Qinghai-Tibet block deformation and they may result from the same source property. Notable changes of fast propagation direction can be detected in western basin at periods above 30s; therefore, it becomes an indicator that the Eurasia-India collision effect on lower crust and upper mantle beneath the western Tarim basin is weakened clearly. It is recognized that fast propagation direction in the middle crust is clearly different from those both in the lower crust and upper mantle underlying the Sichuan basin adjacent to the eastern plateau margin. This finding suggests that decoupled deformation processes between middle crust and lower crust beneath the basin have been occurred. But further analysis of the maps at periods equal to or longer than 30s indicates that the lower crust and upper mantle are strongly coupled. The relatively large amplitude of azimuthal anisotropy in Sichuan basin may be the'fossil'anisotropy frozen in the lithosphere, and this'fossil'anisotropy could have been from more recent large scale deformation.By analyzing phase velocity and azimuthal anisotropy maps at various periods, the present paper indicates that the tectonic deformation of the crust and upper-mantle east of approximately 104°E is weaker than that of the west. But local exceptions are also found near the intersection of the southwest Ordos, eastern segment of Qilian fold belt and the western segment of Qinling-Dabie orogenic belt. The maximum amplitude of anisotropy is up to 2% at periods 55-85s, and is representative of strong deformation in geological history. It is observed that weak azimuthal anisotropy exists in the crust and upper mantle beneath the eastern continental China. This might result from the synthetic effects from the collision between India-Australia and Eurasian plates, as well as the Pacific plate and Philippine Sea plate subduction under the Eurasia plate along the margins of the continental China. The overall effect on the eastern flank is obviously weaker than that on the western continental China.The lateral heterogeneity of phase velocity structure and amplitude of azimuthal anisotropy in the upper mantle beneath continental China decreases as period increases, except for some local areas. However, anisotropy amplitudes underlying the Sea of Japan, Korean peninsula and Indo-China block located in the west of Hainan Island increase apparently as the periods equal to or greater than 65s. Increasing of upper-mantle anisotropy amplitude along southeast China coast, where the lithosphere is experiencing extension and thinning processes, may be related to mantle convection in asthenosphere, and the fast propagation direction is an indicator of crystal lattice orientation of olivine representing the mantle flow direction.According to the phase velocity distribution and the azimuthal anisotropy amplitude, the Longmen Shan tectonic zone can be divided into southwestern and northeastern sections at about 103°E. The southwestern section has relatively lower phase velocity, while the northeastern area has higher phase velocity and stronger anisotropy with NE-SW Rayleigh-wave fast-propagation direction at periods above 30s. This conclusion suggests that the NE striking unilateral rupture propagation of the Wenchuan Ms8.0 earthquake on May 12, 2008, which occurred at the central segment of the Longmen Shan tectonic zone, may be related not only to the cumulated high stress of the northeastern section, but also to the underlying medium property along the segment where high phase velocity (suitable for energy accumulation and concentrated release) and NE-SW fast-propagation direction (suitable for seismic energy propagation) are expressed.By synthesizing all the data, it is concluded that the fast anisotropy direction at any period isn't consistent with the strike of the large-scale geological structures in the studied area, and further this cognition maybe means that large-scale geological structures have been formed and evolved in different tectonic episodes, and therefore the activities along large-scale structures behave obvious regionalization and segmentation.4. Distribution of inter-station paths has great advantage of determining reliable phase-velocity and azimuthal anisotropy maps. Because most paths are located within the continental China, this ray path distribution therefore can consequently minimize the influence from the structures outside of the research area on the inversion results. Additionally, many measurements of phase velocity dispersions are along relatively short paths, and then they can further constrain the phase velocity structure within the local area, and finally this advantage makes the distribution of the phase velocity and azimuthal anisotropy more reliable in the studied area. Consequently, the result is of important significance for further inverting reliable shear wave velocity structure of the studied region.