中国地震科学实验场位于青藏高原、华南块体和印度板块的交汇地带,地质构造复杂、地壳变形强烈,是我国大陆强震最活跃的地区.高分辨率的壳幔三维速度结构是认识强震孕育环境、壳幔变形机制和深部动力学过程的重要基础.本研究基于接收函数与面波联合反演方法构建高垂向分辨率三维初始速度模型,采用双差层析成像方法获得了中国地震科学实验场最高横向分辨率为0.25°的三维P波和S波速度模型CSES-VM1.0.受Sn观测资料的限制, S波速度模型在下地壳和上地幔顶部的分辨率降低,使其更接近接联合反演给出的初始模型.基于人工爆破观测数据的检验表明,与以往的速度模型相比,新模型的初至P波理论走时更接近观测值.横跨2022年芦山6.1级地震和泸定6.8级地震震源区的速度剖面揭示,前者发生在青藏高原与四川盆地相接触的软弱带附近,后者位于花岗岩分布区,北段受强度较大的高速体阻挡.扬子板块西缘存在断续分布的显著高速异常带,揭示了新元古代中基性侵入岩的空间分布,并在很大程度上控制了断裂带的分布形态,甚至影响大地震的破裂过程;攀枝花附近存在沿北东-南西和南北向分布的两个高速异常,它们可能与新元古代和中晚二叠世的中基性侵入岩分布有关.在实验场西南地区,成像结果清晰地揭示出临沧花岗岩带、思茅盆地“中轴”断裂带隆起区以及哀牢山杂岩带的空间分布,展现出比以往成像模型更高的空间分辨能力.本项工作建立的速度模型可为地震科学实验场深部构造研究、高精度地震定位和强地面运动模拟等提供重要基础.

The interior of the Ordos block, located northeast of the Qinghai-Xizang Plateau, is not entirely stable or homogeneous and may have been subjected to tectonic deformation. However, there has been no definite conclusion regarding the extent and mechanism of this deformation. Here, we obtained a high-resolution P-wave velocity (Vp) model for the northeastern Qinghai-Xizang (Tibetan) Plateau, Ordos block and surrounding areas using traveltime data involving over 1200 stations from a newly deployed dense temporary array and permanent seismic network. Our results showed that the northern part of Ordos has a relatively high Vp with minimal lateral change. In contrast, the Vp structure in the southern Ordos block varied in the lateral direction, with low-velocity anomalies at a depth of 15 km. The lower crust with Vp from 6.8 to 7.3 km s-1 is thicker in the west and gradually thins towards the east. The northern part of the Ordos block is relatively stable, whereas the southern part has undergone crustal deformation. This deformation may be related to the eastward compressive forces from the Longxi block, which could be associated with the change in the Haiyuan fault from thrust to strike-slip. In the Bayan Har block, a thick low-velocity anomaly exists in the middle and upper crusts, whereas the lower crust is relatively thin. The crust in this area likely experienced crustal shortening and delamination. Beneath the Longxi and Alxa blocks, a low-velocity layer appears in the middle crust, which may be related to ductile shear in the crustal brittle-ductile transition zone caused by plateau expansion. The range of this low-velocity layer indicates that the influence of the plateau expansion exceeded that of the Haiyuan-Tianjingshan fault zone.

The China Seismic Experimental Site (CSES) is located at the intersection of the Tibetan Plateau, South China Block, and Indian Plate and has complex geological settings and intense crustal deformation, making it one of the most seismically active areas in Chinese mainland. A high-resolution, three-dimensional (3D) crust-mantle velocity structure is crucial for understanding seismotectonic environments, lithospheric deformation mechanisms, and deep dynamic processes. We first constructed a high-vertical-resolution 3D initial velocity model using the joint inversion of receiver functions and surface waves and then obtained a 3D P- and S-wave velocity model (CSES-VM1.0) with the highest lateral resolution of 0.25° for the CSES using double-difference tomography. Owing to the limitations of the Sn observation data, the resolution of the S-wave velocity model in the lower crust and upper mantle was reduced, making it closer to the initial model provided by joint inversion. A comparison with explosive-source seismic data showed that the synthetic P-wave first-arrival travel times of the new model were closer to the observations than those of the previous velocity models. The velocity cross-sections across the source areas of the 2022 Lushan MS6.1 and Ludian MS6.8 earthquakes reveal that the former earthquake occurred near a weak contact zone between the Tibetan Plateau and Sichuan Basin, and the rupture of the latter earthquake occurred in a granitic area, with the northern end blocked by rigid high-velocity bodies. A clear high-velocity anomaly zone is distributed along the western margin of the Yangtze Block, revealing the spatial distribution of Neoproterozoic intermediate-basic intrusions. This high-velocity zone significantly controls the morphology of fault zones and influences the rupture processes of major earthquakes. Two northeast-southwest and north-south trending high-velocity anomalies were found near Panzhihua, potentially related to Neoproterozoic and Middle-Late Permian intermediate-basic intrusions. The imaging results revealed the spatial distribution of the Lincang granitoid batholith, the uplifted zone of the central axis fault in the Simao Basin, and the Ailaoshan complex belt in the southwestern CSES, demonstrating a higher spatial resolution compared to previous results. Our velocity model provides an essential foundation for deep structural studies, high-precision earthquake locations, and strong ground motion simulations in the CSES.

Large earthquakes frequently occur along complex fault systems. Understanding seismic rupture and long-term fault evolution requires constraining the geometric and material properties of fault zone structures. We provide a comprehensive overview of recent advancements in seismological methods used to study fault zone structures, including seismic tomography, fault zone seismic wave analysis, and seismicity analysis. Observational conditions limit our current ability to fully characterize fault zones, for example, insufficient imaging resolution to discern small-scale anomalies, incomplete capture of crucial fault zone seismic waves, and limited precision in event location accuracy. Dense seismic arrays can overcome these limitations and enable more detailed investigations of fault zone structures. Moreover, we present new insights into the structure of the Anninghe-Xiaojiang fault zone in the southeastern margin of the Qinghai-Xizang Plateau based on data collected from a dense seismic array. We found that utilizing a dense seismic array can identify small-scale features within fault zones, aiding in the interpretation of fault zone geometry and material properties.

Ordos Block has undergone rapid uplift, and a series of rift basins have been formed around the block since the Cenozoic, but the formation mechanisms remain controversial. High-resolution 3D velocity structure of crust and mantle is important for understanding lithospheric deformation and deep dynamic process Here we present a 3D S-wave velocity structure of the crust and upper mantle in the Ordos Block and surrounding regions by joint inversion of receiver functions and surface wave data from a dense broadband seismic deployment. The lithosphere of the Ordos Block exhibits an obvious high-velocity anomaly. In the east and north of the Ordos and the southwestern part of the Tibetan Plateau, obvious low-velocity anomalies are detected in the upper mantle and extend into the Ordos The lithosphere of the Ordos Block is thick in the center and thin in the edge, while the crust is relatively thin in the center and thick in the southwest and northeast. The crustal thickness of the tensional basin in the north is greater than that in the central Ordos. We suggest that the outward expansion of the mantle thermal materials in eastern Tibet and the upper mantle thermal upwelling in the eastern part of the North China Craton lead to the non-uniform lithospheric thinning, temperature rise and density reduction of the Ordos Block. The additional buoyancy and thermodynamic effects provided by them contributed to the continuous uplift of the Ordos Block since the Cenozoic. Influenced by the extrusion of Tibetan Plateau, the crustal thickening and rapid uplift occur in the southwestern and northern parts of the Ordos Block. The lithospheric structures of the Alxa and Ordos Blocks are different, and they may belong to different independent blocks before the Mesozoic.

新生代以来,鄂尔多斯块体发生了快速隆升并在周边形成了一系列拉张断陷盆地,但其形成机制仍然存在争议.高分辨率壳幔三维速度结构是研究岩石圈变形和深部动力学过程的重要依据.文章利用鄂尔多斯块体及周边地区密集宽频带地震台站的观测资料,通过接收函数与面波联合反演获得了研究区地壳和上地幔三维S波速度结构.鄂尔多斯块体岩石圈整体上表现出明显的高速异常.在块体东部、北部以及西南部的青藏高原地区,上地幔存在明显的低速异常,并延伸至块体内部.鄂尔多斯块体岩石圈呈中部厚、外围薄的特征,地壳厚度则表现为中部薄、西南部和东北部厚,且北缘张拉盆地的地壳厚度大于块体中部地区.文章认为,青藏高原东部地幔热物质向外扩张以及华北克拉通东部地区上地幔热物质上涌,导致鄂尔多斯块体岩石圈发生了非均匀减薄、升温和密度降低,其产生的附加浮力和热动力作用,促使鄂尔多斯块体新生代以来发生持续隆升,同时受青藏高原隆升扩张产生的挤压作用等影响,鄂尔多斯西南部及北部地区发生了地壳增厚和快速隆升.阿拉善块体和鄂尔多斯块体的岩石圈速度结构存在明显差异,中生代之前可能属于不同的独立块体.

The Ordos block is located on the west side of the North China Carton, adjacent to the northeastern part of the Tibetan Plateau. Affected by two tectonic movements, Ordos block internal structure remains relatively stable structure, but surrounded by active tectonic belts. With the development of the second and third part of the “China Seismological Science Array”, the distribution of seismic observation stations in Ordos region has been greatly improved. This study will use the new seismic observation data of "Array III", and combined with the phase observation data of "Array II" to form a more complete seismic phase travel time data set. The regional seismic body-wave travel time tomography will figure out a more reliable three-dimensional velocity structure of P waves in Ordos.Our study area spans from 32°N to 42°N and 108°E to 114°E , which includes the Ordos block and its adjacent structures . The seismic data we used for inversion were recorded by 1244 stations including: 198 permanent stations and 1043 temporary stations (ChinArray II and III), from November 2013 to August 2017. After manual labeled the seismic phase, we select events with more than ten phase records of individual seismic events. The epicentral distance is less than 200km. Finally, we obtained about 22,500 phase records of 1882 local seismic events.The preliminary results are consistent with previous studies and surface structures of a wide range of velocity distributions. However, in the middle-upper crust under the Liupan Mountain west, the low-speed anomaly extending downward is shown, which may be caused by the shallow crustal damage caused with the continuous eastward compression of asthenosphere in the northeastern margin of the Qinghai-Tibet Plateau during the Cenozoic. It is worth noting that there is an EW-trending low-velocity zone under the Dingbian-Suide fault beneath the Ordos Basin, with a depth form lower crust to 50 km in upper mantel. This low-velocity anomaly divides the high-speed disturbance in the Ordos block into two parts，indicate the depth of the fault can reach the upper mantel. In the Taihang Mountains in the west of the study area, low-velocity anomalies extending to the upper layer of the mantle are shown. We initially believe that this anomaly is related to the volcanic thermal motion that once existed on the area.