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Long-period surface waves and mantle boundary undulations 总被引：2，自引：0，他引：2
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Three dimensional shear wave velocity structure of the crust and upper mantle beneath China from ambient noise surface wave tomography 
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下载免费PDF全文 Xinlei Sun Xiaodong Song Sihua Zheng Yingjie Yang Michael H. Ritzwoller 《地震科学（英文版）》2010,23(5):449-463
We determine the three-dimensional shear wave velocity structure of the crust and upper mantle in China using Green's functions obtained from seismic ambient noise cross-correlation.The data we use are from the China National Seismic Network,global and regional networks and PASSCAL stations in the region.We first acquire cross-correlation seismograms between all possible station pairs.We then measure the Rayleigh wave group and phase dispersion curves using a frequency-time analysis method from 8 s to 60 s.After that,Rayleigh wave group and phase velocity dispersion maps on 1° by 1° spatial grids are obtained at different periods.Finally,we invert these maps for the 3-D shear wave velocity structure of the crust and upper mantle beneath China at each grid node.The inversion results show large-scale structures that correlate well with surface geology.Near the surface,velocities in major basins are anomalously slow,consistent with the thick sediments.East-west contrasts are striking in Moho depth.There is also a fast mid-to-lower crust and mantle lithosphere beneath the major basins surrounding the Tibetan plateau (TP) and Tianshan (Junggar,Tarim,Ordos,and Sichuan).These strong blocks,therefore,appear to play an important role in confining the deformation of the TP and constraining its geometry to form its current triangular shape.In northwest TP in Qiangtang,slow anomalies extend from the crust to the mantle lithosphere.Meanwhile,widespread,a prominent low-velocity zone is observed in the middle crust beneath most of the central,eastern and southeastern Tibetan plateau,consistent with a weak (and perhaps mobile) middle crust. 相似文献
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B. G. Bukchin A. Z. Mostinsky A. A. Egorkin A. L. Levshin M. H. Ritzwoller 《Pure and Applied Geophysics》2001,158(8):1497-1515
—?We test the hypothesis that the existence of an observable non-zero isotropic component of seismic moment can be used as a discriminant to distinguish nuclear explosions from shallow earthquakes. We do this by applying the method described herein to a small set of data recorded between 1990 and 1996 following events (seven nuclear explosions, three earthquakes) that occurred on the Lop Nor test site in Western China. We represent each source as a sum of an isotropic component at the surface and a nonisotropic, double-couple component at an estimated depth. The explosions all possess a significant non-zero isotropic component and the estimated depth of the double-couple component of the moment tensor, presumably the result of tectonic release, lies between about 0 and 3?km. For the earthquakes studied, the isotropic component is indistinguishable from zero and the depths of the sources are estimated at 3, 17 and 31?km. The data set we have studied, although still very small, suggests that certain source characteristics (namely, double-couple depth and the ratio of the isotropic to nonisotropic components of seismic moment) may prove useful in discriminating explosions from shallow earthquakes. Further work is needed to determine whether these observations hold for explosions at other test sites, to investigate a much larger set of shallow earthquakes located in regions of interest, and to study the robustness of the estimated source parameters as source magnitude and the number of observing stations decrease. 相似文献
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Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements 总被引：28，自引：0，他引：28
G. D. Bensen M. H. Ritzwoller M. P. Barmin A. L. Levshin F. Lin M. P. Moschetti N. M. Shapiro Y. Yang 《Geophysical Journal International》2007,169(3):1239-1260
Ambient noise tomography is a rapidly emerging field of seismological research. This paper presents the current status of ambient noise data processing as it has developed over the past several years and is intended to explain and justify this development through salient examples. The ambient noise data processing procedure divides into four principal phases: (1) single station data preparation, (2) cross-correlation and temporal stacking, (3) measurement of dispersion curves (performed with frequency–time analysis for both group and phase speeds) and (4) quality control, including error analysis and selection of the acceptable measurements. The procedures that are described herein have been designed not only to deliver reliable measurements, but to be flexible, applicable to a wide variety of observational settings, as well as being fully automated. For an automated data processing procedure, data quality control measures are particularly important to identify and reject bad measurements and compute quality assurance statistics for the accepted measurements. The principal metric on which to base a judgment of quality is stability, the robustness of the measurement to perturbations in the conditions under which it is obtained. Temporal repeatability, in particular, is a significant indicator of reliability and is elevated to a high position in our assessment, as we equate seasonal repeatability with measurement uncertainty. Proxy curves relating observed signal-to-noise ratios to average measurement uncertainties show promise to provide useful expected measurement error estimates in the absence of the long time-series needed for temporal subsetting. 相似文献
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Surface wave tomography of the western United States from ambient seismic noise: Rayleigh and Love wave phase velocity maps 总被引：18，自引：0，他引：18
Fan-Chi Lin Morgan P. Moschetti Michael H. Ritzwoller 《Geophysical Journal International》2008,173(1):281-298
We present the results of Rayleigh wave and Love wave phase velocity tomography in the western United States using ambient seismic noise observed at over 250 broad-band stations from the EarthScope/USArray Transportable Array and regional networks. All available three-component time-series for the 12-month span between 2005 November 1 and 2006 October 31 have been cross-correlated to yield estimated empirical Rayleigh and Love wave Green's functions. The Love wave signals were observed with higher average signal-to-noise ratio (SNR) than Rayleigh wave signals and hence cannot be fully explained by the scattering of Rayleigh waves. Phase velocity dispersion curves for both Rayleigh and Love waves between 5 and 40 speriod were measured for each interstation path by applying frequency–time analysis. The average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station-triplets. We find that empirical Green's functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross-correlation without explicit knowledge of the source distribution. The average traveltime uncertainty is less than 1 s at periods shorter than 24 s. We present Rayleigh and Love wave phase speed maps at periods of 8, 12, 16,and 20 s. The maps show clear correlations with major geological structures and qualitative agreement with previous results based on Rayleigh wave group speeds. 相似文献
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