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SUMMARYThe Qinling belt is a transitional zone lying among three units: the North China block (NCB), the South China block (SCB) and the northeastern Tibetan Plateau (NETP). Owing to the interaction of these units, complex deformation has occurred in the Qinling belt. Although many studies have been conducted to understand the deformation mechanism in the Qinling belt, some key issues are still under debate, such as whether middle-lower crustal flow exists beneath the western Qinling belt (WQB). High-resolution images of subsurface structures are essential to shed light on the deformation mechanism. In this paper, high-resolution images of the velocity structure and azimuthal anisotropy beneath the Qinling belt are obtained by using an eikonal equation-based traveltime tomography method. Our seismic tomography inverts 38 719 high-quality P-wave first arrivals from 1697 regional earthquakes recorded by 387 broad-band seismic stations. In the WQB, our tomography results show low-velocity anomalies but relatively weak anisotropy in the middle-lower crust. These features suggest that middle-lower crustal flow may not exist in this area. In the central Qinling belt (CQB), we find low-velocity anomalies in the middle-lower crust; however, the fast velocity directions no longer trend E–W but vary from NNE–SSW to N–S. These characteristics can be ascribed to the convergence and collision between the NCB and the SCB. In addition, we find strong low-velocity anomalies in the uppermost mantle beneath the CQB, which may indicate delamination of the lower crust. In the southern Qinling belt, we observe significant high-velocity anomalies in the upper crust beneath the Hannan–Micang and Shennong–Huangling domes. These high-velocity anomalies indicate a mechanically strong upper crust, which is responsible for the arc-shaped deformation process of the Dabashan fold. Based on the P-wave velocity and azimuthal anisotropic structures revealed by the inversion of high-quality seismic data, the deformation of the Qinling belt is affected mainly by the convergence between the NCB and the SCB rather than by the middle-lower crustal flow from the Tibetan Plateau.

SUMMARY The Tengchong–Baoshan region (TBR) is located at the margin of the southeastern Tibetan Plateau (SETP) and is subject to the complex interaction among multiple tectonic blocks. Therefore, the TBR is approximate for investigating the mechanism responsible for the interaction among the SETP and its surrounding blocks. Based on eikonal equation-based seismic tomography, we obtain high-resolution P-wave velocity and azimuthal anisotropy at depths of 0–40 km beneath the TBR. We discover a large magma reservoir (with horizontal scale of ∼50 km and vertical scale of ∼15 km) in the middle-lower crust beneath the Tengchong volcanic zone, which is directly fed by hot materials from the uppermost mantle. We also detect a shallow magma chamber in the uppermost crust above the large reservoir. These magma reservoirs at different depths constitute a multiscale magma system. Furthermore, we observe a low-velocity belt beneath the Weixi–Qiaohou–Weishan and Nantinghe faults and postulate that it corresponds to lower crustal flow and hot mantle upwelling. Additionally, we infer that two M > 7.0 earthquakes that have occurred in Baoshan block nucleated in the transition zone between low- and high-velocity anomalies. Based on the azimuthal anisotropy tomography results, we consider that incoherent deformation in the crust causes the uneven accumulation of stress, which facilitated the occurrence of these two events. The 2021 Ms 6.4 Yangbi earthquake also occurred in the transition zone between low- and high-velocity anomalies. Based on the P-wave velocity tomography results, we suggest that hot material decreases the fault strength and leads to the instability of the fault.

SUMMARY In this paper, we introduce the so-called symplectic discrete singular convolution differentiator (SDSCD) method for structure-preserving modelling of elastic waves. In the method presented, physical space is discretized by the DSCD, whereas an explicit third-order symplectic scheme is used for the time discretization. This approach uses optimization and truncation to form a localized operator. This preserves the fine structure of the wavefield in complex media and avoids non-causal interaction when parameter discontinuities are present in the medium. Theoretically, the approach presented is a structure-preserving algorithm. Also, some numerical experiments are shown in this paper. Elastic wavefield modelling experiments on a laterally heterogeneous medium with high parameter contrasts demonstrate the superior performance of the SDSCD for suppression of numerical dispersion. Long-term computational experiments exhibit the remarkable capability of the approach presented for long-time simulations. Promising numerical results suggest the SDSCD is suitable for high-precision and long-time numerical simulations, as it has structure-preserving property and it can suppress effectively numerical dispersion when coarse grids are used.

We have developed a mixed-grid finite element method (MGFEM) to simulate seismic wave propagation in 2D structurally complex media. This method divides the physical domain into two subdomains. One subdomain covering the major part of the physical domain is divided by regular quadrilateral elements, while the other subdomain uses triangular elements to correctly fit a rugged free surface topography. The local stiffness matrix of any quadrilateral element is identical and matrix-vector production is calculated using an element-by-element technique, which avoids assembling a huge global stiffness matrix. As only a few triangular elements exist in the subdomain containing the rugged free surface topography, the memory requirements for storing the assembled subdomain global stiffness matrix are significantly reduced. To eliminate artificial boundary reflections, the MGFEM is also implemented to solve the system equations of PML absorbing boundary conditions (PML ABC). The accuracy and efficiency of the MGFEM is tested in numerical experiments by comparing it with conventional methods, and numerical comparisons also indicate its tremendous ability to describe rugged surfaces.