Shear-wave velocity structure of the crust and uppermost mantle in the Shanxi rift zone

The Shanxi rift zone is one of the largest and active Cenozoic grabens in the world, studying the velocity structure of the crust and upper mantle in this region may help us to understand the mechanisms of rift processes and the seismogenic environment of active seismicity in continental rifts. In this work, using the broadband seismic data of Shanxi, Hebei, Henan, Shaanxi provinces, and the Inner Mongolia Autonomous Region from February 2009 to November 2011, we have picked out 350 high-quality phase velocity dispersion curves of fundamental mode Rayleigh waves at periods from 8 to 75 s, and Rayleigh wave phase velocity maps have been constructed from 8 to 75 s period with horizontal resolution ranging from 40 to 50 km by two-station surface-wave tomography. Then, using a genetic algorithm, a 3D shear-wave speed model of the crust and uppermost mantle have been derived from these maps with a spatial resolution of 0.4° × 0.4°. Four characteristics can be outlined from the results： （1） Except in the Datong volcanic zone, in the depth range of 11-30 km, the location of a transition zone between the highand low-velocity regions is in agreement with the seismicity pattern in the study region, and the earthquakes are mostly concentrated near this transition zone; （2） In the depth range of 31-40 km, shear-wave velocities are higher to the south of the Taiyuan Basin and lower to the north, which is similar to the distribution pattern of Moho depth variations in the Shanxi region; （3） The shear-wave velocity pattern of higher velocities to the south of 38×N and lower velocities to the north is found to be consistent with that from the upper crustal levels to depth of 70 km. At the deeper depths, the spatial scale of the low-velocity anomalies zone in the north is gradually shrinking with depth increasing, the low-velocity anomalies are gradually disappearing beneath the Datong volcanic zone at the depth of 151-200 km. We proposed that the root of the Datong volcano may reach to a depth around 150 km; （4） Along the N-S vertical profile at 112.8°E, the 38°N latitude is the boundary between high and low velocities, arguing the tectonic difference between the Shanxi rift zone and its flanks, in the rift zone the seismic velocity is dominated by low-velocity anomalies while in the flanks it is high.

Regional stress field around the Taigu fault zone in Shanxi Province,China

A comprehensive study on regional stress field around the Taigu fault zone in Shanxi Province, China, was performed in this study. To get a better understanding of the present-day stress status in this area, 31 focal mecha- nisms of ML 〉3 earthquakes since 1965 were compiled, and the best stress tensor was then inverted based on the database. Additionally, magnetic fabrics along the Taigu fault zone were investigated to get an indication of the regional stress field in the past. Our results show that the present-day stress field around the Taigu fault zone is characterized by astable NW-SE extension with a strike- slip component, consistent with the geological surveys and recent GPS data. Results from magnetic fabrics indicate that the orientations of principal stress axes from magnetic fabrics of sedimentary rocks in Neogene coincide to the orientations of principal stress axes from focal mecha- nisms. The south segment of the Taigu fault displays more complicated magnetic fabrics and more activity of mod- erate earthquakes. It is connected with the Mianshan west fault and intersects with NW-SE striking Fenyang fault and the north fault of the Lingshi uplift at the south edge of Taiyuan basin. This may be the area needing more atten- tion in terms of seismic risk along the Taigu fault.

正断层阶区现今连接模式综合研究——以山西裂谷系口泉断裂禅房阶区为例

正断层阶区有软连接(soft-link)和硬连接(hard-link)2种端元连接模式。判定正断层阶区现今的连接模式不但有助于理解正断层的生长演化过程,也能指示未来正断层上发生的地震是否会在阶区内形成地表破裂,这对于评价阶区内工程场地的地震安全性具有重要意义。判定正断层阶区内是否存在活动的连接断层是区别正断层阶区现今连接模式的直接方法。然而,在多数情况下,由于正断层阶区内的变形量相对小,加上人类活动对阶区内地貌的改造,导致在本以硬连接模式生长的正断层阶区内也很难直接看到断层陡坎,这为判定正断层阶区内是否存在活动的连接断层带来了困难。山西裂谷系是东亚大陆显著的活动裂谷带,也是中国重要的强震带。口泉断裂位于山西裂谷系北部,是大同盆地的西缘边界断裂。口泉断裂在禅房村附近发育一个正断层阶区,文中称之为禅房阶区。文中以口泉断裂禅房阶区为例,通过构造地貌野外调查、地貌数据分析和地质雷达探测研究禅房阶区的连接模式。结果显示,口泉断裂禅房阶区内部存在活动的连接断层,说明禅房阶区的连接方式为硬连接模式。文中研究基于构造地貌野外调查、地貌数据分析和地质雷达探测的综合方法,可有效定位弱活动断层的空间展布,降低单一方法的不确定性。

山西地区现代地壳垂直运动研究

对山西地区1980-2010年的水准观测资料计算，获得垂直形变速率，并按照地质构造单元分块拟合，得到块体运动速率和块体边界断裂带的活动速率。在此基础上研究了山西地区近30年来的地壳垂直变化和构造活动。结果表明，山西地区西部上升，东部下降。吕梁山块体掀斜抬升，太行山块体为掀斜活动，山西断陷带为掀斜下降。地壳垂直形变揭示出山西断陷带现代构造活动为裂陷运动，裂陷速率为2．68mm／a，边界断裂带活动速率为1．14～4．33mm／a，均为正断活动。

五台山北麓断裂繁峙段晚第四纪活动性研究

五台山北麓断裂位于山西地堑系北部。本文以五台山北麓断裂繁峙段的地质地貌为研究对象,分别在繁峙县的大峪村和岗里村两地断裂沿线进行了无人机测量。利用三维结构的运动重建技术(Structure from Motion,SfM)进行影像数据处理,得到高精度点云数据,并通过进一步处理获得了分辨率达0.5m的高清断错地貌正射影像(DOM)和数字高程模型(DEM)。通过对典型地区的详细野外调查和挖掘探槽等手段对该段晚第四纪的活动性进行研究,发现断层晚第四纪以来的活动主要是以正倾滑运动为主。同时在五台山北麓断裂沿线的大峪村、岗里村等地进行了断错地貌分析和晚第四纪滑动速率计算,得到约20ka以来的断层垂向滑动速率为0.4—0.6mm/a,近18ka以来该段发生过至少两次古地震事件。古地震事件和滑动速率分析表明,五台山北麓断裂晚第四纪,尤其是全新世以来活动强烈,且不同段落存在明显的活动性差异。

山西裂谷系的深部地球物理探测

综述了山西裂谷系近年来通过重力、工程爆破及测震、大地电磁测深、航磁资料反演、深地震测深等研究该地区地壳与上地馒结构的结果与进展。文中还提出了在本区继续开展地壳与上地幔细结构研究的任务与设想。

High-resolution crustal velocity structure in the Shanxi rift zone and its tectonic implications

The Shanxi rift zone,located in the Trans-North China Orogen(TNCO)of the North China Craton(NCC),is wellknown for hosting large intraplate earthquakes in continental China.The TNCO is a suture zone formed by the amalgamation of the eastern and the western blocks of the NCC.After its formation,it was reactived and deformed by later tectonic activities,which result in complex lithospheric heterogeneities.Thus,the detailed crustal structure of the Shanxi rift zone is critical for understanding the tectonics and seismogenic mechanism in this area,which will shed new lights on the formation and dynamic evolution of the NCC.In this study,we applied ambient noise tomography based on 18 months continuous records from 108 seismic stations located in Shanxi and its surroundings,in order to constrain its detailed crustal structure.We measured 4437 Rayleigh wave phase velocity dispersion curves in the period of 5–45 s from the cross-correlation functions.Next,a surface wave direct inversion algorithm based on surface-wave ray tracing was used to resolve a 3-D S-wave velocity model in the upper 60 km with lateral resolution of~50–80 km.The tomographic images show that the sedimentary thickness of the Taiyuan Basin is less than 5 km.At depth of 0–10 km,we observe a good correlation between the imaged structural variations with geological and topographic features at the surface.For example,the center of rift shows low-velocity anomalies and the uplifting areas on both sides are characterized by high velocity anomalies.The western and eastern boundaries of the slow materials coincide with the faults that control the basin.The slow material extends from the shallow surface to depth of about 15 km but it getting smaller in shape at deeper depth.For the Taiyuan Basin,Linfen Basin,and Yuncheng Basin in the central and southern parts,the structure is dominant by slow materials in the upper crust but changes to strong high-velocity anomalies in the lower crust and the uppermost mantle at depth deeper than 25 km.We interprete these high-velocity anomalies to be associated with the cold remnant of the underplated basalt in the lower crust that were formed in early Tertiary before the basin was stretched.We also observe the low-velocity anomaly beneath the Datong volcanic area,which extends from the uppermost mantle to a depth of 20 km vertically and migrates from west to east laterally.It may reflect the upwelling channel of the magmatic material in Datong.Moreover,the strong low-velocity anomalies presented north of 38°N could be related to the heated crustal materials with paritial melting as a result of the intensive magmatic activities of the Datong Volcano since the Cenozoic.In our study region,seismicity mainly concentrates in the depth range of 5–20 km and we find that most earthquakes appear to occur in places where velocity changes from high to low rapidly,with slight higher concentration in the faster material areas.In summary,our high-resolution 3-D crustal velocity model provides important seismological constraints to understand the tectonic evolution and seismicity across the Shanxi rift zone.