The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China.Investigations of the structures of the fault zone and its surrounding areas have attracted much attention.In this study,we used dense-array am...The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China.Investigations of the structures of the fault zone and its surrounding areas have attracted much attention.In this study,we used dense-array ambient noise tomography to construct a threedimensional shear wave velocity model of shallow crust in an area about 80km×70km in Lujiang,Anhui Province,eastern China.For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region,and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method;we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves.Based on the direct surface wave tomography method,we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km.The results revealed important geological structural features of the study area.In the north region,the sedimentary center of the Hefei Basin—the southwestern part of the Chaohu Lake—shows a significant low-velocity anomaly to a depth of at least 5 km.The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks,respectively,and both show obvious high-speed anomalies;the sedimentary area within the Tan-Lu fault zone(about 10 km wide)shows low-velocity anomalies.However,the volcanic rock intrusion area in the fault zone is shown as high velocity.Our shallow crustal imaging results reflect the characteristics of different structures in the study area,especially the high-speed intrusive rocks in the Tan-Lu fault zone,which were probably partially derived from the magmatic activity of Luzong volcanic basin.From the Late Cretaceous to Early Tertiary,the Tan-Lu fault zone was in a period of extensional activity;the special stress environment and the fractured fault zone morphology展开更多
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-Xich...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.展开更多
A new method is developed to constrain S-wave velocity structures of the shallow crust based on frequencydependent amplitudes of direct P-waves in P-wave receiver functions(P-RFs). This method involves the following t...A new method is developed to constrain S-wave velocity structures of the shallow crust based on frequencydependent amplitudes of direct P-waves in P-wave receiver functions(P-RFs). This method involves the following two steps:first, the high-frequency approximate amplitude formula of direct P-waves in P-RFs of individual stations is used to fit the observed amplitude distribution against the ray parameters at different frequencies, and second, the S-wave velocity depth profile beneath each station is constrained according to an empirical correlation between frequency and depth. Unlike traditional inversion techniques, the newly developed method is not dependent on initial velocity models, and the lateral and vertical resolutions of the results are controlled by the interstation distance and the data frequency, respectively. The effectiveness of the method is verified by synthetic tests on various models. The method is then applied to teleseismic P-RF data from a NW-SEtrending linear seismic array extending from the northeastern Tibetan Plateau to the central Sichuan Basin to construct an S-wave velocity image of the shallow crust along the array. The imaged velocity structure is further analysed and compared with the regional geology. In particular, the structural differences of sedimentary basins in the cratonic area of the stable Sichuan Basin and tectonically active belts in northeastern Tibet are investigated. By combining our results with previous observations, the relationship between the surficial geology and deep processes in the study region is also discussed.展开更多
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.Th...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.展开更多
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 tra...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.展开更多
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 hig...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.展开更多
Mantle heat flow is an important geophysical parameter in investigation of deep interior of the Earth, which may avoid and/or reduce the effects of various geological and environmental factors at shallow depth. It is ...Mantle heat flow is an important geophysical parameter in investigation of deep interior of the Earth, which may avoid and/or reduce the effects of various geological and environmental factors at shallow depth. It is believed that compared with the heat flow observed on the Earth’s surface, the mantle heat flow gives more information from the depth. In recent years, along with systematical geothermal studies, the mantle heat flow in the North China Basin has been calculated and reported.展开更多
According to the ＂Netlike Plastic-Flow （NPF）＂ continental dynamics model, the transition of the deformation regime from brittle in shallow layers to ductile in deep layers in the lithosphere, and the controlling e...According to the ＂Netlike Plastic-Flow （NPF）＂ continental dynamics model, the transition of the deformation regime from brittle in shallow layers to ductile in deep layers in the lithosphere, and the controlling effect of NPF in the lower lithosphere result in intraplate multilayer tectonic deformation. NPF is a viscous （plastic） flow accompanied by shear strain localization, forming a plastic-flow network in the lower lithosphere. The strain rates in the seismogeulc layer can be estimated using the ＂earthquake-recurrence-interval＂ method, in which the strain rate is calculated in terms of the recurrence interval of two sequential earthquakes and the seismic probability of the second earthquake. The strains in the lower lithosphere are estimated using the ＂conjugate-angle＂ method, which takes the relationship between the conjugate angles and the compressive strains of the network, and calculates the characteristic strain rates in this layer from the strains and the durations of deformation inferred. The contour map of characteristic maximum principal compressive strain rates in the lower lithosphere in central-eastern Asia given in the paper shows strain rates with magnitudes on the order of 10^-15 - 10^-14/s in this region. The strain rates within the plastic-flow belts, which control seismic activities in the seismogeulc layer, are greater than the characteristic strain rates of the network and, in addition, the strain rates and seismic activities in the seismogeulc layer are also influenced by other factors, including the directive action of driving boundary along the upper crust, the effects of plastic-flow waves and the existence of the transitional weak layer distributed discontinuously between the upper and lower layers. The comparison between the strain rates in the seismogeulc layer and the characteristic strain rates in the lower lithosphere for 11 potential hypocenter areas in the region from the Qinghai-Xizang （Tibet） plateau to the North China plain indicates that, except for 展开更多
The 1:6,000,000 Epicenter Distribution Map of China compiled by the Center for Analysis and Prediction,SSB,has given more than 100,000 epicenters of large and small earthquakes.The images composed of these dots contai...The 1:6,000,000 Epicenter Distribution Map of China compiled by the Center for Analysis and Prediction,SSB,has given more than 100,000 epicenters of large and small earthquakes.The images composed of these dots contain plenty of information concerning the structures in the middle-upper crust.Using the so-called visual seismic line method,a tectonic interpretation of the epicenter distribution map of shallow focus earthquakes in China mainland is given in this paper,and the seismotectonic patterns and dynamic characteristics of the Tianshan,AIxa-Qilianshan and Qinghai-Xizang(Tibet)seismic belts,the S-N central axis belt as well as Northeast China,North China,the middle and lower reaches of the Yangtze River and the southeast coastal areas are analyzed and interpreted.展开更多
In this paper,the focal mechanisms of the fore-main-after shocks of the 1989 Datong-Yanggao earthquake are determined by fitting Pnl and SH waveforms,and the earthquake processes and seismotectonics are inferred.The r...In this paper,the focal mechanisms of the fore-main-after shocks of the 1989 Datong-Yanggao earthquake are determined by fitting Pnl and SH waveforms,and the earthquake processes and seismotectonics are inferred.The results show that the fore-main-after shocks occurred on a composite fault plane,The fault motion at the shallow part is right-lateral strike-slip with mechanisms:strike 201,dip 75,rake 191; at the deeper part,the fault movement is strike 201,dip 58,rake 222.The epicentral area is subject to a ENE-WSW horizontal compression and a NNW-SSE extension which is consistent with the tectonic stress orientation of North China.The P-axis at greater depth has an azimuth of 54.5 and a plunge of 51.The relatively large P-axis plunge in depth corresponds with the great depressive deformation in this area.In addition,the similarities and differences between the Datong-Yanggao earthquakes and general graben-associated earthquakes are discusses.展开更多
Shallow layer method(SLM)based on the definition of the geoid can determine the gravity field inside the shallow layer.In this study,the orthometric height of Mount Everest(HME)is calculated based on SLM,in which the ...Shallow layer method(SLM)based on the definition of the geoid can determine the gravity field inside the shallow layer.In this study,the orthometric height of Mount Everest(HME)is calculated based on SLM,in which the key is to construct the shallow layer model.The top and bottom boundaries of the shallow layer model are the natural surface of the Earth and the surface at a certain depth below the reference geoid,respectively.The model-combined strategies to determine the geoid undulation(N)based on SLM are applied to calculate the HME by two approaches:(1)direct calculation by combining N and geodetic height(h);(2)calculation by the segment summation approach(SSA)using the gravity field inside the shallow layer.On December 8,2020,the Chinese and Nepalese governments announced an authoritative value of 8848.86 m,which is referred to a geoid determined by the International Height Reference System(IHRS)(i.e.,the geopotential is 62636853.4 m^(2) s^(-2)).Here,our results(combined strategies(1)EGM2008 and CRUST1.0,(2)EGM2008 and CRUST2.0,(3)EIGEN-6 C4 and CRUST1.0,and(4)EIGEN-6 C4 and CRUST2.0)are referred to the geoid defined by WGS84(i.e.,the geopotential is 62636851.7 m^(2) s^(-2)).The differences between our results and the authoritative value(8848.86 m)are 0.448 m,-0.009 m,-0.295 m,and -0.741 m by the first approach,and 0.539 m,0.083 m,-0.214 m,and -0.647 m by the second approach.When the reference surface WGS84 geoid is converted to the IHRS geoid,the differences are 0.620 m,0.163 m,-0.123 m,and -0.569 m by the first approach,and0.711 m,0.225 m,-0.042 m,and -0.475 m by the second approach.展开更多
基金the National Natural Science Foundation of China(project 41790464)the China Postdoctoral Fund(BH2080000099).
文摘The Tan-Lu fault zone is a large NNE-trending fault zone in eastern China.Investigations of the structures of the fault zone and its surrounding areas have attracted much attention.In this study,we used dense-array ambient noise tomography to construct a threedimensional shear wave velocity model of shallow crust in an area about 80km×70km in Lujiang,Anhui Province,eastern China.For approximately one month we collected continuous ambient noise signals recorded by 90 short-period seismographs in the region,and obtained the short-period Rayleigh wave empirical Green's functions between stations by the cross-correlation method;we also extracted 0.5–8 s fundamental mode Rayleigh wave group velocity and phase velocity dispersion curves.Based on the direct surface wave tomography method,we jointly inverted the group velocity and phase velocity dispersion data of all paths and obtained the 3-D shear wave velocity structure in the depth range of 0–5 km.The results revealed important geological structural features of the study area.In the north region,the sedimentary center of the Hefei Basin—the southwestern part of the Chaohu Lake—shows a significant low-velocity anomaly to a depth of at least 5 km.The southwestern and southeastern regions of the array are the eastern margin of the Dabie orogenic belt and the intrusion area of Luzong volcanic rocks,respectively,and both show obvious high-speed anomalies;the sedimentary area within the Tan-Lu fault zone(about 10 km wide)shows low-velocity anomalies.However,the volcanic rock intrusion area in the fault zone is shown as high velocity.Our shallow crustal imaging results reflect the characteristics of different structures in the study area,especially the high-speed intrusive rocks in the Tan-Lu fault zone,which were probably partially derived from the magmatic activity of Luzong volcanic basin.From the Late Cretaceous to Early Tertiary,the Tan-Lu fault zone was in a period of extensional activity;the special stress environment and the fractured fault zone morphology
基金supported by the Key Research and Development Project of the Ministry of Science and Technology(Grant No.2018YFC1503400)。
文摘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.
基金supported by the National Natural Science Foundation of China (Grant No. 41688103)the Strategic Priority Research Program (A) of the Chinese Academy of Sciences (Grant No. XDA20070302)+1 种基金the Independent Project of the State Key Laboratory of the Lithospheric Evolution, IGGCAS (SKL-Z201704-11712180)The field work for seismic data collection was financially supported by the Projects (Grant Nos. SinoProbe-02-03, 2011ZX05008-001)
文摘A new method is developed to constrain S-wave velocity structures of the shallow crust based on frequencydependent amplitudes of direct P-waves in P-wave receiver functions(P-RFs). This method involves the following two steps:first, the high-frequency approximate amplitude formula of direct P-waves in P-RFs of individual stations is used to fit the observed amplitude distribution against the ray parameters at different frequencies, and second, the S-wave velocity depth profile beneath each station is constrained according to an empirical correlation between frequency and depth. Unlike traditional inversion techniques, the newly developed method is not dependent on initial velocity models, and the lateral and vertical resolutions of the results are controlled by the interstation distance and the data frequency, respectively. The effectiveness of the method is verified by synthetic tests on various models. The method is then applied to teleseismic P-RF data from a NW-SEtrending linear seismic array extending from the northeastern Tibetan Plateau to the central Sichuan Basin to construct an S-wave velocity image of the shallow crust along the array. The imaged velocity structure is further analysed and compared with the regional geology. In particular, the structural differences of sedimentary basins in the cratonic area of the stable Sichuan Basin and tectonically active belts in northeastern Tibet are investigated. By combining our results with previous observations, the relationship between the surficial geology and deep processes in the study region is also discussed.
基金This study was supported financially by the Natural Science Foundation of China(Grant No.41374052)the Science Foundation of China Geological Survey(No.J1901)the project of Regional Geological Survey(No.D1911).
文摘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.
基金National Natural Science Foundation of China (40504011, 40674058)State Special Project of Oil-Gas of the Minis-try of Land and Resources (XQ-2004-01)
文摘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.
基金State Natural Science Foundation (49974006) National Key Basic Research Project (G199804070401).
文摘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.
文摘Mantle heat flow is an important geophysical parameter in investigation of deep interior of the Earth, which may avoid and/or reduce the effects of various geological and environmental factors at shallow depth. It is believed that compared with the heat flow observed on the Earth’s surface, the mantle heat flow gives more information from the depth. In recent years, along with systematical geothermal studies, the mantle heat flow in the North China Basin has been calculated and reported.
基金Supported bythe Joint Earthquake Science Foundation of China(grant 199061) Contribution No.2005B0011 of the Institute of Geology,China Earthquake Administration.
文摘According to the ＂Netlike Plastic-Flow （NPF）＂ continental dynamics model, the transition of the deformation regime from brittle in shallow layers to ductile in deep layers in the lithosphere, and the controlling effect of NPF in the lower lithosphere result in intraplate multilayer tectonic deformation. NPF is a viscous （plastic） flow accompanied by shear strain localization, forming a plastic-flow network in the lower lithosphere. The strain rates in the seismogeulc layer can be estimated using the ＂earthquake-recurrence-interval＂ method, in which the strain rate is calculated in terms of the recurrence interval of two sequential earthquakes and the seismic probability of the second earthquake. The strains in the lower lithosphere are estimated using the ＂conjugate-angle＂ method, which takes the relationship between the conjugate angles and the compressive strains of the network, and calculates the characteristic strain rates in this layer from the strains and the durations of deformation inferred. The contour map of characteristic maximum principal compressive strain rates in the lower lithosphere in central-eastern Asia given in the paper shows strain rates with magnitudes on the order of 10^-15 - 10^-14/s in this region. The strain rates within the plastic-flow belts, which control seismic activities in the seismogeulc layer, are greater than the characteristic strain rates of the network and, in addition, the strain rates and seismic activities in the seismogeulc layer are also influenced by other factors, including the directive action of driving boundary along the upper crust, the effects of plastic-flow waves and the existence of the transitional weak layer distributed discontinuously between the upper and lower layers. The comparison between the strain rates in the seismogeulc layer and the characteristic strain rates in the lower lithosphere for 11 potential hypocenter areas in the region from the Qinghai-Xizang （Tibet） plateau to the North China plain indicates that, except for
文摘The 1:6,000,000 Epicenter Distribution Map of China compiled by the Center for Analysis and Prediction,SSB,has given more than 100,000 epicenters of large and small earthquakes.The images composed of these dots contain plenty of information concerning the structures in the middle-upper crust.Using the so-called visual seismic line method,a tectonic interpretation of the epicenter distribution map of shallow focus earthquakes in China mainland is given in this paper,and the seismotectonic patterns and dynamic characteristics of the Tianshan,AIxa-Qilianshan and Qinghai-Xizang(Tibet)seismic belts,the S-N central axis belt as well as Northeast China,North China,the middle and lower reaches of the Yangtze River and the southeast coastal areas are analyzed and interpreted.
文摘In this paper,the focal mechanisms of the fore-main-after shocks of the 1989 Datong-Yanggao earthquake are determined by fitting Pnl and SH waveforms,and the earthquake processes and seismotectonics are inferred.The results show that the fore-main-after shocks occurred on a composite fault plane,The fault motion at the shallow part is right-lateral strike-slip with mechanisms:strike 201,dip 75,rake 191; at the deeper part,the fault movement is strike 201,dip 58,rake 222.The epicentral area is subject to a ENE-WSW horizontal compression and a NNW-SSE extension which is consistent with the tectonic stress orientation of North China.The P-axis at greater depth has an azimuth of 54.5 and a plunge of 51.The relatively large P-axis plunge in depth corresponds with the great depressive deformation in this area.In addition,the similarities and differences between the Datong-Yanggao earthquakes and general graben-associated earthquakes are discusses.
基金supported in part by the National Natural Science Foundations of China under Grants Nos.41631072,42030105,41721003,41804012,and 41874023。
文摘Shallow layer method(SLM)based on the definition of the geoid can determine the gravity field inside the shallow layer.In this study,the orthometric height of Mount Everest(HME)is calculated based on SLM,in which the key is to construct the shallow layer model.The top and bottom boundaries of the shallow layer model are the natural surface of the Earth and the surface at a certain depth below the reference geoid,respectively.The model-combined strategies to determine the geoid undulation(N)based on SLM are applied to calculate the HME by two approaches:(1)direct calculation by combining N and geodetic height(h);(2)calculation by the segment summation approach(SSA)using the gravity field inside the shallow layer.On December 8,2020,the Chinese and Nepalese governments announced an authoritative value of 8848.86 m,which is referred to a geoid determined by the International Height Reference System(IHRS)(i.e.,the geopotential is 62636853.4 m^(2) s^(-2)).Here,our results(combined strategies(1)EGM2008 and CRUST1.0,(2)EGM2008 and CRUST2.0,(3)EIGEN-6 C4 and CRUST1.0,and(4)EIGEN-6 C4 and CRUST2.0)are referred to the geoid defined by WGS84(i.e.,the geopotential is 62636851.7 m^(2) s^(-2)).The differences between our results and the authoritative value(8848.86 m)are 0.448 m,-0.009 m,-0.295 m,and -0.741 m by the first approach,and 0.539 m,0.083 m,-0.214 m,and -0.647 m by the second approach.When the reference surface WGS84 geoid is converted to the IHRS geoid,the differences are 0.620 m,0.163 m,-0.123 m,and -0.569 m by the first approach,and0.711 m,0.225 m,-0.042 m,and -0.475 m by the second approach.
