We propose a method to invert surface wave dispersion data directly for 3-D variations of shear wave speed, that is, without the intermediate step of phase or group velocity maps, using frequency-dependent ray tracing and a wavelet-based sparsity-constrained tomographic inversion. A fast marching method is used to compute, at each period, surface wave traveltimes and ray paths between sources and receivers. This avoids the assumption of great-circle propagation that is used in most surface wave tomographic studies, but which is not appropriate in complex media. To simplify the problem we consider quasi-stratified media with smoothly varying seismic properties. We represent the 3-D shear wave speed model by means of 1-D profiles beneath grid points, which are determined from all dispersion data simultaneously using a wavelet-based sparsity-constrained tomographic method. The wavelet coefficients of the wave speed model are estimated with an iteratively reweighted least squares algorithm, and upon iteration the surface wave ray paths and the data sensitivity matrix are updated using the newly obtained wave speed model. To demonstrate its feasibility, we apply the method to determine the 3-D shallow crustal shear wave speed variations in the Taipei basin of Taiwan using short period interstation Rayleigh wave phase velocity dispersion measurements extracted from the ambient noise cross-correlation method. The results are consistent with previous studies and reveal strong shallow crustal heterogeneity that correlates with surface geology.

Southwest China, located at the southeastern margin of the Tibetan plateau, plays an important role for the plateau growth and its material extrusion. It has complicated tectonic environment and strong seismic activities including the 2008 Wenchuan great earthquake. Numerous geophysical studies have been conducted in southwest China. However, a community velocity model (CVM) in this region is still not available, which makes it difficult to have a consistent catalog of earthquake locations and focal mechanisms and a consistent velocity model for simulating strong ground motions and evaluating earthquake hazards. In this study, we aim at building a high‐resolution CVM (both $VP$ and $VS$⁠) of the crust and uppermost mantle in southwest China along with earthquake locations by joint inversion of body‐ and surface‐wave travel‐time data. In total, we have assembled 386,958 P‐ and 372,662 S‐wave first arrival times and nearly 8100 Rayleigh‐wave dispersion curves in the period band of 5–50 s. A multigrid strategy is adopted in the joint inversion. A coarser horizontal grid interval of 0.5° is first used and then a finer grid interval of 0.25° is used with initial models interpolated from the coarser‐grid inverted velocity models. The spatial resolution of both $VP$ and $VS$ models can reach up to 0.5° horizontally and 10 km vertically according to the checkerboard tests. The comparisons of our inverted $VP$ and $VS$ models with those from other studies show general consistency in large‐scale features. The inverted models are further validated by P‐wave arrival times from active sources and Rayleigh‐wave data. In general, our velocity models show two low‐velocity zones in the middle‐lower crust and a prominent high‐velocity region in between them. Our new models have been served as the first version of the CVM in southwest China (SWChinaCVM‐1.0) for future studies.

Azimuthal anisotropy retrieved from surface waves is important for constraining depth‐varying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the three‐dimensional shear wave speed azimuthal anisotropy based on mixed‐path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3‐D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3‐D Vsv azimuthal anisotropy and additional 3‐D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade‐off between strong heterogeneity and anisotropy. With frequency‐dependent ray tracing based on 2‐D isotropic phase speed maps, the new method takes into account the ray‐bending effect on surface wave propagation. We apply the new method to a regional array in Yunnan, southwestern China. Using Rayleigh wave phase velocity dispersion data in the period band of 5–40 s extracted from ambient noise interferometry, we obtain a 3‐D model of shear wave speed and azimuthal anisotropy in the crust and uppermost mantle in Yunnan. This model reveals that two midcrust low‐velocity zones are possible weak channels, and the azimuthal anisotropy at a depth of 5 to 30 km is mainly controlled by nearby strike‐slip faults, some of which also approximately coincide with the lateral boundaries of the crustal low‐velocity zones. Approximately south of 26°N, the upper crustal azimuthal anisotropy from our model is significantly different from the upper mantle anisotropy inferred by shear wave splitting, indicating different deformation styles between the crust and upper mantle in southern Yunnan.

Imaging near‐surface structure with active sources, such as dynamite or vibrators, is usually difficult in urban areas because of dense population and safety issues. However, ambient‐noise tomography can provide an alternative way to investigate near‐surface structures without such difficulties. Here, we conduct ambient‐noise tomography to investigate the near‐surface shear‐velocity structure in the urban area of the city of Hefei (about a 5 km×7 km region) in eastern China, using two weeks of continuous ambient‐noise data. The direct surface‐wave tomographic method with period‐dependent raytracing is used to invert all surface‐wave dispersion data in the period band 0.5–2 s simultaneously for 3D variations of shear‐velocity structures. To constrain the top 30‐m velocity structure in the inversion, we utilize the borehole data in the city of Hefei to obtain a better 3D reference wavespeed model in the top 30 m. Our tomographic results from surface to about 400 m depth suggest that the northern part of the urban region presents higher shear velocity than the southern part. This result is consistent with regional geology; that is, the southern part of the study region is closer to Chao Lake, the fifth largest freshwater lake in China, which has much thicker sediments and lower shear wavespeed. The sharp velocity variation in our obtained model matches the location of the Shushan fault, which is associated with large gravity variations. Our near‐surface velocity model of the Hefei urban area provides useful information for city planning as well as for earthquake strong ground motion prediction, which may bring strong engineering interests in the future.