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Some evidences for earthquake occurrence in the shallow crust in some areas of North China and Southwest China

Acta Seismologica Sinica(English Edition) 2001年第4期14卷 456-465页

作者：胡新亮刁桂苓高景春张四昌张学民李钦祖秦清娟朱振兴张彦清 Seismological Bureau of Hebei Province Shijiazhuang China

The parameters of hypocentral location are important fundamental data for the study of seismology and the earth interior physics; among them, the focal depth is a very important one but can hardly be measured with higher precision. With the increase of seismic stations in number, the heightening of observation quality and the improvement of determination method, such a situation has been changed much. In this paper, the results of hypocentral location and re-location by 7 small-aperture digital seismic station networks at Tangshan, Zhangbei and Huailai of Hebei Province, Datong of Shanxi Province and Jianchuan, Luquan and Yao'an of Yunnan Province are used. Using these results together with those of focal depth inversion by far-field data of some individual large shocks in corresponding areas and those re-determined by the Linfen telemetered seismic network of Shanxi Province, a comparison with the results of focal depth determination in the past earthquake catalogues is made. It is found out that the focal depths determined by the small-aperture seismic networks are basically distributed in the range from the earth surface to ten-odd kilometers underground. In contrast, the focal depths determined in the past are mostly distributed at the depth range from the earth surface to 30 km underground. Besides, there is a difference of 50% to 100% between the average values of the two cases, which is quite an obvious one. From the result of analysis, it is considered that the results determined by the small-aperture seismic networks may be closer to the reality of focal depths distribution. That is to say, earthquakes in the above-mentioned areas should be distributed in the shallow part of the crust. The causes that lead to the above situation are discussed preliminarily, and some suggestions and measures for improving the precision of focal depth determination are put forward.

关键词： North China region Southwest China region focal depth shallow crust

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shallow Structure of the crust in the Sulu-Dabie Region,China and its Seismotectonic Implication

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Acta Geologica Sinica(English Edition) 2021年第3期95卷 988-995页

作者： XU Jiren LI Haibing QI Xuexiang ZHAO Zhixin Jiangsu Donghai-Sichuan Longmenshan crustal Activity National Observation and Research Station DonghaiJiangsu 222300China Key Laboratory of Deep-Earth Dynamics Ministry of Natural ResourcesBeijing 100037China Institute of Geology Chinese Academy of Geological SciencesBeijing 100037China

The P-wave velocity structure in the shallow crust is investigated in and around the Sulu-Dabie region by using seismic reflection data for deep soundings in 48 survey profiles and from rock velocity determinations.The observed velocity distributions show obvious heterogeneities in this region.The low velocity anomalies are observed mainly in the west of the Dabie region and the East Sea regions.The high velocity anomalies emerge in the shallow crust of the Sulu and Dabie orogeny.These high-velocity anomalies can be attributed to the ultra-high pressure metamorphosed(UHPM)rock formed by exhumation motion of mantle materials during the orogeny.The high-velocity anomalies in the different shallow layers beneath the Sulu region are located to the northeast of the Tan-Lu fault.The high-velocity anomalies beneath the Dabie region are located southwest of the Tan-Lu fault.Such a distribution pattern of velocity anomaly zones may reveal historical motion of a left-lateral strike-slip for the Tan-Lu fault,which differs from the result of a right-lateral strike-slip motion regime known from modern seismology,indicating a more complex tectonic motion along the Tan-Lu fault.

关键词： high seismic velocity anomaly shallow crust strike-slip motion Tan-Lu fault Sulu-Dabie orogen

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shallow crustal velocity structures revealed by active source tomography and fault activities of the Mianning–Xichang segment of the Anninghe fault zone, Southwest China

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Earth and Planetary Physics 2022年第2期6卷 204-212页

作者： XiHui Shao HuaJian Yao Ying Liu HongFeng Yang BaoFeng Tian LiHua Fang Laboratory of Seismology and Physics of Earth’s Interior School of Earth and Space SciencesUniversity of Science and Technology of ChinaHefei 230026China Mengcheng National Geophysical Observatory University of Science and Technology of ChinaMengcheng Hefei 233500China CAS Center for Excellence in Comparative Planetology University of Science and Technology of ChinaHefei 230026China Earth System Science Programme Faculty of ScienceThe Chinese University of Hong KongHong Kong 999077China Shenzhen Research Institute The Chinese University of Hong KongShenzhen 518057China Institute of Geophysics China Earthquake AdministrationBeijing 100089China

The Anninghe fault is a large left-lateral strike-slip fault in southwestern China. It has controlled deposition and magmatic activities since the Proterozoic, and seismic activity occurs frequently. The Mianning-Xichang segment of the Anninghe fault is a seismic gap that has been locked by high stress. Many studies suggest that this segment has great potential for large earthquakes(magnitude >7). We obtained three vertical velocity profiles of the Anninghe fault(between Mianning and Xichang) based on the inversion of P-wave first arrival times. The travel time data were picked from seismograms generated by methane gaseous sources and recorded by three linearly distributed across-fault dense arrays. The inversion results show that the P-wave velocity structures at depths of 0-2 km corresponds well with the local lithology. The Quaternary sediments have low seismic velocities, whereas the igneous rocks,metamorphic rocks, and bedrock have high seismic velocities. We then further discuss the fault activities of the two fault branches of the Anninghe fault in the study region based on small earthquakes(magnitudes between ML 0.5 and ML 2.5) detected by the Xichang array.The eastern fault branch is more active than the western branch and that the fault activities in the eastern branch are different in the northern and southern segments at the border of 28°21′N. The high-resolution models obtained are essential for future earthquake rupture simulations and hazard assessments of the Anninghe fault zone. Future studies of velocity models at greater depths may further explain the complex fault activities in the study region.

关键词： Anninghe fault zone shallow crust P-wave velocity methane gaseous source fault activity

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Middle and upper crust shear-wave velocity structure of the Chinese mainland

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Acta Seismologica Sinica(English Edition) 2007年第4期20卷 359-369页

作者：冯梅安美建 Institute of Geomechanics Chinese Academy of Geological Sciences

In order to give a more reliable shallow crust model for the Chinese mainland, the present study collected many short-period surface wave data which are better sensitive to shallow earth structures. Different from traditional two-step surface wave tomography, we developed a new linearized surface wave dispersion inversion method to directly get a 3D S-wave velocity model in the second step instead of inverting for 1D S-velocity profile cell by cell. We convert all the regionalized dispersions into linear constraints for a 3D S-velocity model. Checkerboard tests show that this method can give reasonable results. The distribution of the middle- and upper-crust shear-wave velocity of the Chinese mainland in our model is strongly heterogeneous and related to different geotectonic terrains. Low-velocity anomalies delineated very well most of the major sedimentary basins of China. And the variation of velocities at different depths gives an indication of basement depth of the basins. The western Tethyan tectonic domain （on the west of the 95°E longitude） is characterized by low velocity, while the eastern Tethyan domain does not show obvious low velocity. Since petroleum resources often distribute in sedimentary basins where low-velocity anomaly appears, the low velocity anomalies in the western Tethyan domain may indicate a better petroleum prospect than in its eastern counterpart. Besides, low velocity anomaly in the western Tethyan domain and around the Xing＇an orogenic belt may be partly caused by high crustal temperature. The weak low-velocity belt along -105°E longitude corresponds to the N-S strong seismic belt of central China.

关键词： surface-wave tomography shear-wave velocity sedimentary basins shallow crust China

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