A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity mea- surements of periods between 10 and 70 s. Rayleigh-wave group-vel...A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity mea- surements of periods between 10 and 70 s. Rayleigh-wave group-velocity dispersions along more than 3,000 inter- station paths were measured based on analysis of telese- ismic wavelbrm data recorded by temporary seismic stations. These observations were then utilized to construct 2D group-velocity maps in the period range of 10-70 s. Tile new group-velocity maps have an enhanced resolution compared with previous global and regional group-velocity models in this region because of the denser and more uniform data coverage. The lateral resolution across the region is about 0.5° for the periods used in this study. Local dispersion curves were then inverted for a 3D shear-wave velocity model of the region by applying a linear inversion scheme. Our 3D shear-wave model confirms the presence of low-velocity zones （LVZs） in the crust beneath the northern part of this region. Our irnaging shows that the upper-middle crustal LVZ beneath the Tengchong region is isolated from these LVZs beneath the eastern and northern part of this region. The upper-middle crustal LVZ may be regarded as evidence of a rnagma chamber in the crust beneath the Tengchong Volcanoes. Our model also reveals a slow lithospheric structure beneath Tengchong and a fast shield-like mantle beneath the stable Yangtze block.展开更多
High-frequency （〉2 Hz） Rayleigh-wave data acquired with a multichannel recording system have been utilized to determine shear （S）-wave velocities in near-surface geophysics since the early 1980s. This overview ar...High-frequency （〉2 Hz） Rayleigh-wave data acquired with a multichannel recording system have been utilized to determine shear （S）-wave velocities in near-surface geophysics since the early 1980s. This overview article discusses the main research results of high-frequency surface-wave techniques achieved by research groups at the Kansas Geological Survey and China University of Geosciences in the last 15 years. The multichannel analysis of surface wave （MASW） method is a non-invasive acoustic approach to estimate near-surface S-wave velocity. The differences between MASW results and direct borehole measurements are approximately 15% or less and random. Studies show that simultaneous inversion with higher modes and the fundamental mode can increase model resolution and an investigation depth. The other important seismic property, quality factor （Q）, can also be estimated with the MASW method by inverting attenuation coefficients of Rayleigh waves. An inverted model （S-wave velocity or Q） obtained using a damped least-squares method can be assessed by an optimal damping vector in a vicinity of the inverted model determined by an objective function, which is the trace of a weighted sum of model-resolution and model-covariance matrices. Current developments include modeUng high-frequency Rayleigh-waves in near-surface media, which builds a foundation for shallow seismic or Rayleigh-wave inversion in the time-offset domain; imaging dispersive energy with high resolution in the frequency-velocity domain and possibly with data in an arbitrary acquisition geometry, which opens a door for 3D surface-wave techniques; and successfully separating surface-wave modes, which provides a valuable tool to perform S-wave velocity profiling with high-horizontal resolution.展开更多
基金supported by the China National Special Fund for Earthquake Scientific Research in Public Interest(201008001)NSFC(41074067)
文摘A shear-wave velocity model of the crust and uppermost mantle beneath the SE Tibetan plateau was derived by inverting Rayleigh-wave group-velocity mea- surements of periods between 10 and 70 s. Rayleigh-wave group-velocity dispersions along more than 3,000 inter- station paths were measured based on analysis of telese- ismic wavelbrm data recorded by temporary seismic stations. These observations were then utilized to construct 2D group-velocity maps in the period range of 10-70 s. Tile new group-velocity maps have an enhanced resolution compared with previous global and regional group-velocity models in this region because of the denser and more uniform data coverage. The lateral resolution across the region is about 0.5° for the periods used in this study. Local dispersion curves were then inverted for a 3D shear-wave velocity model of the region by applying a linear inversion scheme. Our 3D shear-wave model confirms the presence of low-velocity zones （LVZs） in the crust beneath the northern part of this region. Our irnaging shows that the upper-middle crustal LVZ beneath the Tengchong region is isolated from these LVZs beneath the eastern and northern part of this region. The upper-middle crustal LVZ may be regarded as evidence of a rnagma chamber in the crust beneath the Tengchong Volcanoes. Our model also reveals a slow lithospheric structure beneath Tengchong and a fast shield-like mantle beneath the stable Yangtze block.
基金supported by Kansas Geological Survey, The University of Kansas and China University of Geosciences
文摘High-frequency （〉2 Hz） Rayleigh-wave data acquired with a multichannel recording system have been utilized to determine shear （S）-wave velocities in near-surface geophysics since the early 1980s. This overview article discusses the main research results of high-frequency surface-wave techniques achieved by research groups at the Kansas Geological Survey and China University of Geosciences in the last 15 years. The multichannel analysis of surface wave （MASW） method is a non-invasive acoustic approach to estimate near-surface S-wave velocity. The differences between MASW results and direct borehole measurements are approximately 15% or less and random. Studies show that simultaneous inversion with higher modes and the fundamental mode can increase model resolution and an investigation depth. The other important seismic property, quality factor （Q）, can also be estimated with the MASW method by inverting attenuation coefficients of Rayleigh waves. An inverted model （S-wave velocity or Q） obtained using a damped least-squares method can be assessed by an optimal damping vector in a vicinity of the inverted model determined by an objective function, which is the trace of a weighted sum of model-resolution and model-covariance matrices. Current developments include modeUng high-frequency Rayleigh-waves in near-surface media, which builds a foundation for shallow seismic or Rayleigh-wave inversion in the time-offset domain; imaging dispersive energy with high resolution in the frequency-velocity domain and possibly with data in an arbitrary acquisition geometry, which opens a door for 3D surface-wave techniques; and successfully separating surface-wave modes, which provides a valuable tool to perform S-wave velocity profiling with high-horizontal resolution.
