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 proces...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.展开更多
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 amalg...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.展开更多
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 ...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.展开更多
基金supported by Open Grant from State key Laboratory of Geodesy and Earth’s Dynamics (Grant No. SKLGED2014-4-4-E)Office of Science and Technology in Shanxi province based on research Projects (2012011029)+1 种基金Scientific and Technological Research Projects in Shanxi province (20100311129-2, 20090311084)the China Earthquake Administration spark Project (XH15007)
文摘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.
基金supported by the Earthquake Science and Technology Spark Project of China Earthquake Administration(Grant No.XH20009Y)the National Natural Science Foundation of China(Grant Nos.41790464,41574034,41704040)the LU JIAXI International Team Program supported by the KC Wong Education Foundation and Chinese Academy of Sciences(Grant No.GJTD-2018-12)。
文摘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.
基金partly funded by Open Fund of State Key Laboratory of Earthquake Dynamics,CEA(LED2011B05)the Foundation for the Returned Overseas of Shanxi Province(121)the National Science Fund of Shanxi Province(Youth,2010021005)
文摘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.
