The assessment of the completeness of earthquake catalogs is a prerequisite for studying the patterns of seismic activity.In traditional approaches,the minimum magnitude of completeness(MC)is employed to evaluate cata...The assessment of the completeness of earthquake catalogs is a prerequisite for studying the patterns of seismic activity.In traditional approaches,the minimum magnitude of completeness(MC)is employed to evaluate catalog completeness,with events below MC being discarded,leading to the underutilization of the data.Detection probability is a more detailed measure of the catalog's completeness than MC;its use results in better model compatibility with data in seismic activity modeling and allows for more comprehensive utilization of seismic observation data across temporal,spatial,and magnitude dimensions.Using the magnitude-rank method and Maximum Curvature(MAXC)methods,we analyzed temporal variations in earthquake catalog completeness,finding that MC stabilized after 2010,which closely coincides with improvements in monitoring capabilities and the densification of seismic networks.Employing the probability-based magnitude of completeness(PMC)and entire magnitude range(EMR)methods,grounded in distinct foundational assumptions and computational principles,we analyzed the 2010-2023 earthquake catalog for the northern margin of the Ordos Block,aiming to assess the detection probability of earthquakes and the completeness of the earthquake catalog.The PMC method yielded the detection probability distribution for 76 stations in the distance-magnitude space.A scoring metric was designed based on station detection capabilities for small earthquakes in the near field.From the detection probabilities of stations,we inferred detection probabilities of the network for diff erent magnitude ranges and mapped the spatial distribution of the probability-based completeness magnitude.In the EMR method,we employed a segmented model fitted to the observed data to determine the detection probability and completeness magnitude for every grid point in the study region.We discussed the sample dependency and low-magnitude failure phenomena of the PMC method,noting the potential overestimation of detection probabilities for lower magnitudes and the underestimation of MC in areas with weaker monitoring capabilities.The results obtained via the two methods support these hypotheses.The assessment results indicate better monitoring capabilities on the eastern side of the study area but worse on the northwest side.The spatial distribution of network monitoring capabilities is uneven,correlating with the distribution of stations and showing significant diff erences in detection capabilities among diff erent stations.The truncation eff ects of data and station selection aff ected the evaluation results at the edges of the study area.Overall,both methods yielded detailed descriptions of the earthquake catalog,but careful selection of calculation parameters or adjustments based on the strengths of diff erent methods is necessary to correct potential biases.展开更多
The Ordos Basin in the western part of the North China Craton is commonly believed to be a multi-controlled oil- bearing basin. It is bounded by the Xing＇an--Mongolian Orogen to the north, the Qingling Orogen to the ...The Ordos Basin in the western part of the North China Craton is commonly believed to be a multi-controlled oil- bearing basin. It is bounded by the Xing＇an--Mongolian Orogen to the north, the Qingling Orogen to the south, the Lüliang mountain to the east and the Helanshan--Liupanshan mountain belt to the west. The interpretation of geophysical data reveals a latitudinal （38°） fault belt in the centre of the Ordos Basin, which controls the hydrocarbon generation, migration and accumulation in the basin. This study attempts to investigate this belt from outcrops and indicates a structurally controlled system of migration fairway within the fault belt.展开更多
In recent years,the Fuxian area in the southeastern Ordos Basin has undergone significant exploration,with industrial gas flow tested in wells drilled into the Ordovician marine carbonates.Despite this,the gas accumul...In recent years,the Fuxian area in the southeastern Ordos Basin has undergone significant exploration,with industrial gas flow tested in wells drilled into the Ordovician marine carbonates.Despite this,the gas accumulation patterns of this area are not fully understood,posing challenges for further exploration.Our analysis of geological conditions indicates that the Ordovician Majiagou Formation in this area hosts two gas plays:one found in weathering crusts and the other found in interior of the formation.We investigated various typical gas reservoirs in the area,focusing on differentiating the geological conditions and factors controlling gas accumulation in the weathering-crust and interior gas reservoirs.The results suggest three primary gas accumulation patterns in the Majiagou Formation in the Fuxian area:(1)upper gas accumulation in weathering crusts,present in the high parts of landforms such as residual paleo-hills or buried paleo-platform(Pattern I);(2)the stereoscopic pattern with gas accumulation in both weathering crusts and strata interior,arising in high parts of landforms such as residual paleo-hills or buried paleo-platforms(Pattern II);(3)lower gas accumulation in strata interior,occurring in the upper reaches and on both sides of paleo-trenches(Pattern III).This study will serve as a geological basis for future exploration deployment in the Fuxian area.展开更多
A set of low-grade clastic metamorphic and carbonate rocks,and greenschists outcropping in the southwestern(SW)margin of the North China Craton(NCC),was originally classified as the Paleoproterozoic Xiong’er Group ac...A set of low-grade clastic metamorphic and carbonate rocks,and greenschists outcropping in the southwestern(SW)margin of the North China Craton(NCC),was originally classified as the Paleoproterozoic Xiong’er Group according to stratigraphic correlation.To verify the age,this paper carried out detrital zircon U–Pb LA-ICP-MS dating of low-grade clastic metamorphic rocks exposed in the Changqing area at the SW margin of the Ordos Block in the SW part of the NCC.Results from detrital zircon dating indicate that the metamorphic and carbonate rocks can be classified into the Neoproterozoic Nanhua System,which is the only Nanhua System stratum in this block so far,and it probably could provide new clues to Rodinia break-up and Snowball Earth of the NCC.The nine peak ages of the low-grade clastic metamorphic rocks reflected its relatively complex provenance,and almost all major geological events experienced by the NCC basement since the Neoarchean,but some age peaks were difficult to correspond to that of the NCC,indicating that the southwestern part of the Ordos Block was also affected by the Qinling and Qiliang orogenic belts during Nanhua System of Neoproterozoic.Combined with provenance analysis,it was revealed that the current southwest boundary of the Ordos Block was the previous southwest boundary of the Ordos Block during the Qingbaikou-Nanhua Period of the Neoproterozoic.展开更多
In the eastern edge of the Ordos Basin,the coalbed methane(CBM)development has not made substantial progress in the past20 years,and the origin of gas can be used to guide the CBM block-selecting and development.Based...In the eastern edge of the Ordos Basin,the coalbed methane(CBM)development has not made substantial progress in the past20 years,and the origin of gas can be used to guide the CBM block-selecting and development.Based on the 37 sets of carbon isotope data,the origin of the gas was determined and the origin mechanism was studied in this work.The 13CPDB of methane ranges from 70.5‰to 36.19‰in the eastern edge in the Ordos Basin and the value becomes heavier from the north to the south.The secondary biogenic gas and the thermogenic gas are mixed in the shallow area and the thermogenic gas occurs in the medium and deep levels.The phenomenon is controlled mainly by the distribution of coal rank and hydrodynamics.Firstly,based on the relationship between China coal rank and methane 13CPDB,the medium rank coal is dominant in the eastern edge of the Ordos Basin,and the mixture of the secondary biogenic gas and the thermogenic gas is formed in the coal of vitrinite reflectant ratio(Rmax)between 0.5%and 2.0%if there is appropriate hydrodynamics;at the same time,because of the shallow burial depth,and the well-developed coal outcrop,meteoric water and other surface water carrying bacteria recharge the coal reservoir,metabolize the organic compounds at a relatively low temperature,and generate methane and carbon dioxide.Wherever the trapping mechanisms occur in the coal,such as Liulin and Hancheng,modern gas content should be high.展开更多
The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block, also being the northern segment of the North-South Seismic Belt, is characterized by considerably high ...The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block, also being the northern segment of the North-South Seismic Belt, is characterized by considerably high seismicity level and high risk of strong earthquakes. In view of the special tectonic environment and deep tectonic setting in this area, this study used two seismic wide-angle reflection/refraction cross profiles for double constraining, so as to more reliably obtain the fine-scale velocity structure characteristics in both the shallow and deep crust of individual blocks and their boundaries in the study area, and further discuss the seismogenic environment in seismic zones with strong historical earthquakes. In this paper, the P-wave data from the two profiles are processed and interpreted, and two-dimensional crustal velocity structure models along the two profiles are constructed by travel time forward modeling. The results show that there are great differences in velocity structure, shape of intra-crustal interfaces and crustal thickness among different blocks sampled by the two seismic profiles. The crustal thickness along the Lanzhou-Huianbu-Yulin seismic sounding profile （L1） increases from -43 km in the western margin of Ordos Block to -56 km in the Qilian Block to the west. In the Ordos Block, the velocity contours vary gently, and the average velocity of the crust is about 6.30 km s^-1; On the other hand, the velocity structures in the crust of the Qilian Block and the arc-like tectonic zone vary dramatically, and the average crustal velocities in these areas are about 0.10 km s^-1 lower than that of the Ordos Block. In addition, discontinuous low-velocity bodies （LVZ1 and LVZ2） are identified in the crust of the Qilian Block and the arc-like tectonic zone, the velocity of which is 0.10-0.20 krn s^-1 lower than that of the surroundings. The average crustal thickness of the Ordos Block is consistently estimated to be around 43 km along both Profile L2 （Tongchuan-Huianbu-Alashan left banner seismic sounding profile） and Profile L1. In contrast to the gently varying intra-crustal interfaces and velocity contours in the Ordos Block along Profile L 1, which is a typical structure characteristic of stable cratons, the crustal structure in the Ordos Block along Profile L2 exhibits rather complex variations. This indicates the presence of significant structural differences in the crust within the Ordos Block. The crustal structure of the Helan Mountain Qilian Block and the Yinchuan Basin is featured by ＂uplift and depression＂ undulations, showing the characteristics of localized compressional deformation. Moreover, there are low-velocity zones with altemative high and low velocities in the middle and lower crust beneath the Helan Mountain, where the velocity is about 0.15-0.25 km s^-1 lower than that of the surrounding areas. The crustal thickness of the Alxa Block is about 49 kin, and the velocity contours in the upper and middle-lower crust of the block vary significantly. The complex crustal velocity structure images along the two seismic sounding profiles L1 and L2 reveal considerable structural differences among different tectonic blocks, their coupling relationships and velocity structural features in the seismic zones where strong historical earthquakes occurred. The imaging result of this study provides fine-scale crustal structure information for further understanding the seismogenic environment and mechanism in the study area.展开更多
The area along the Grear Wall in northern Shaanxi between the Loess Plateau and the Ordos Desert is one of the major agrarian-pastoral regions of northwest China. Historically, the land was fought over by the nomadic ...The area along the Grear Wall in northern Shaanxi between the Loess Plateau and the Ordos Desert is one of the major agrarian-pastoral regions of northwest China. Historically, the land was fought over by the nomadic and the agrarian peoples of the region. The Yansui section of the Ming Great Wall and thirty-nine fortified encampments along it were built during fighting between the Mongols and the Han people. As all of them were located along communication lines vital to economic development, they played an important role in politics, the economy and transport over three hundred years of Ming and Qing rule. However, they fell into disuse in the late Qing and the Republican period and are now in ruins. The main reason underlying their decline was that the sites had been chosen for their defensive value, so the subsequent northern expansion of China’s borders and structural changes in the border economy hastened their decline. Thus the rise and fall of these fortified towns at the intersection of the agrarian and the pastoral regions was closely related to imperial political and military activity and was in line with ethnic and tribal movements and migrations and the evolution of civilizations. In sum, the reasons behind the demise of these fortified towns and camps were highly complex and usually involved multiple factors.展开更多
基金funded by Director Fund of the Inner Mongolia Autonomous Region Seismological Bureau(No.2023GG02,2023MS05)the Inner Mongolia Natural Science Foundation(No.2024MS04021)。
文摘The assessment of the completeness of earthquake catalogs is a prerequisite for studying the patterns of seismic activity.In traditional approaches,the minimum magnitude of completeness(MC)is employed to evaluate catalog completeness,with events below MC being discarded,leading to the underutilization of the data.Detection probability is a more detailed measure of the catalog's completeness than MC;its use results in better model compatibility with data in seismic activity modeling and allows for more comprehensive utilization of seismic observation data across temporal,spatial,and magnitude dimensions.Using the magnitude-rank method and Maximum Curvature(MAXC)methods,we analyzed temporal variations in earthquake catalog completeness,finding that MC stabilized after 2010,which closely coincides with improvements in monitoring capabilities and the densification of seismic networks.Employing the probability-based magnitude of completeness(PMC)and entire magnitude range(EMR)methods,grounded in distinct foundational assumptions and computational principles,we analyzed the 2010-2023 earthquake catalog for the northern margin of the Ordos Block,aiming to assess the detection probability of earthquakes and the completeness of the earthquake catalog.The PMC method yielded the detection probability distribution for 76 stations in the distance-magnitude space.A scoring metric was designed based on station detection capabilities for small earthquakes in the near field.From the detection probabilities of stations,we inferred detection probabilities of the network for diff erent magnitude ranges and mapped the spatial distribution of the probability-based completeness magnitude.In the EMR method,we employed a segmented model fitted to the observed data to determine the detection probability and completeness magnitude for every grid point in the study region.We discussed the sample dependency and low-magnitude failure phenomena of the PMC method,noting the potential overestimation of detection probabilities for lower magnitudes and the underestimation of MC in areas with weaker monitoring capabilities.The results obtained via the two methods support these hypotheses.The assessment results indicate better monitoring capabilities on the eastern side of the study area but worse on the northwest side.The spatial distribution of network monitoring capabilities is uneven,correlating with the distribution of stations and showing significant diff erences in detection capabilities among diff erent stations.The truncation eff ects of data and station selection aff ected the evaluation results at the edges of the study area.Overall,both methods yielded detailed descriptions of the earthquake catalog,but careful selection of calculation parameters or adjustments based on the strengths of diff erent methods is necessary to correct potential biases.
文摘The Ordos Basin in the western part of the North China Craton is commonly believed to be a multi-controlled oil- bearing basin. It is bounded by the Xing＇an--Mongolian Orogen to the north, the Qingling Orogen to the south, the Lüliang mountain to the east and the Helanshan--Liupanshan mountain belt to the west. The interpretation of geophysical data reveals a latitudinal （38°） fault belt in the centre of the Ordos Basin, which controls the hydrocarbon generation, migration and accumulation in the basin. This study attempts to investigate this belt from outcrops and indicates a structurally controlled system of migration fairway within the fault belt.
基金supported the National Natural Science Foundation of China(Nos.:U19B6003,U20B6001)the Class A Strategic Pilot Science and Technology Program of the Chinese Academy of Sciences(No.:XDA14000000)a project entitled Oil and Gas Enrichment Rules and Favorable Target Selection in the Iran-Iraq region initiated by the Sinopec Science and Technology Department.
文摘In recent years,the Fuxian area in the southeastern Ordos Basin has undergone significant exploration,with industrial gas flow tested in wells drilled into the Ordovician marine carbonates.Despite this,the gas accumulation patterns of this area are not fully understood,posing challenges for further exploration.Our analysis of geological conditions indicates that the Ordovician Majiagou Formation in this area hosts two gas plays:one found in weathering crusts and the other found in interior of the formation.We investigated various typical gas reservoirs in the area,focusing on differentiating the geological conditions and factors controlling gas accumulation in the weathering-crust and interior gas reservoirs.The results suggest three primary gas accumulation patterns in the Majiagou Formation in the Fuxian area:(1)upper gas accumulation in weathering crusts,present in the high parts of landforms such as residual paleo-hills or buried paleo-platform(Pattern I);(2)the stereoscopic pattern with gas accumulation in both weathering crusts and strata interior,arising in high parts of landforms such as residual paleo-hills or buried paleo-platforms(Pattern II);(3)lower gas accumulation in strata interior,occurring in the upper reaches and on both sides of paleo-trenches(Pattern III).This study will serve as a geological basis for future exploration deployment in the Fuxian area.
基金funded by National Natural Science Foundation of China(Grant No.42072231).
文摘A set of low-grade clastic metamorphic and carbonate rocks,and greenschists outcropping in the southwestern(SW)margin of the North China Craton(NCC),was originally classified as the Paleoproterozoic Xiong’er Group according to stratigraphic correlation.To verify the age,this paper carried out detrital zircon U–Pb LA-ICP-MS dating of low-grade clastic metamorphic rocks exposed in the Changqing area at the SW margin of the Ordos Block in the SW part of the NCC.Results from detrital zircon dating indicate that the metamorphic and carbonate rocks can be classified into the Neoproterozoic Nanhua System,which is the only Nanhua System stratum in this block so far,and it probably could provide new clues to Rodinia break-up and Snowball Earth of the NCC.The nine peak ages of the low-grade clastic metamorphic rocks reflected its relatively complex provenance,and almost all major geological events experienced by the NCC basement since the Neoarchean,but some age peaks were difficult to correspond to that of the NCC,indicating that the southwestern part of the Ordos Block was also affected by the Qinling and Qiliang orogenic belts during Nanhua System of Neoproterozoic.Combined with provenance analysis,it was revealed that the current southwest boundary of the Ordos Block was the previous southwest boundary of the Ordos Block during the Qingbaikou-Nanhua Period of the Neoproterozoic.
基金supported by the National Basic Research Program of China(Grant No.2009CB219602)
文摘In the eastern edge of the Ordos Basin,the coalbed methane(CBM)development has not made substantial progress in the past20 years,and the origin of gas can be used to guide the CBM block-selecting and development.Based on the 37 sets of carbon isotope data,the origin of the gas was determined and the origin mechanism was studied in this work.The 13CPDB of methane ranges from 70.5‰to 36.19‰in the eastern edge in the Ordos Basin and the value becomes heavier from the north to the south.The secondary biogenic gas and the thermogenic gas are mixed in the shallow area and the thermogenic gas occurs in the medium and deep levels.The phenomenon is controlled mainly by the distribution of coal rank and hydrodynamics.Firstly,based on the relationship between China coal rank and methane 13CPDB,the medium rank coal is dominant in the eastern edge of the Ordos Basin,and the mixture of the secondary biogenic gas and the thermogenic gas is formed in the coal of vitrinite reflectant ratio(Rmax)between 0.5%and 2.0%if there is appropriate hydrodynamics;at the same time,because of the shallow burial depth,and the well-developed coal outcrop,meteoric water and other surface water carrying bacteria recharge the coal reservoir,metabolize the organic compounds at a relatively low temperature,and generate methane and carbon dioxide.Wherever the trapping mechanisms occur in the coal,such as Liulin and Hancheng,modern gas content should be high.
基金supported by the Special Projects of Scientific Research of the Earthquake Industry (Grant No. 201408023)the National Natural Science Foundation of China (Grant Nos. 41474076 & 41474077)
文摘The transitional area between the northeastern margin of the Qinghai-Tibetan Plateau, Ordos Block and Alxa Block, also being the northern segment of the North-South Seismic Belt, is characterized by considerably high seismicity level and high risk of strong earthquakes. In view of the special tectonic environment and deep tectonic setting in this area, this study used two seismic wide-angle reflection/refraction cross profiles for double constraining, so as to more reliably obtain the fine-scale velocity structure characteristics in both the shallow and deep crust of individual blocks and their boundaries in the study area, and further discuss the seismogenic environment in seismic zones with strong historical earthquakes. In this paper, the P-wave data from the two profiles are processed and interpreted, and two-dimensional crustal velocity structure models along the two profiles are constructed by travel time forward modeling. The results show that there are great differences in velocity structure, shape of intra-crustal interfaces and crustal thickness among different blocks sampled by the two seismic profiles. The crustal thickness along the Lanzhou-Huianbu-Yulin seismic sounding profile （L1） increases from -43 km in the western margin of Ordos Block to -56 km in the Qilian Block to the west. In the Ordos Block, the velocity contours vary gently, and the average velocity of the crust is about 6.30 km s^-1; On the other hand, the velocity structures in the crust of the Qilian Block and the arc-like tectonic zone vary dramatically, and the average crustal velocities in these areas are about 0.10 km s^-1 lower than that of the Ordos Block. In addition, discontinuous low-velocity bodies （LVZ1 and LVZ2） are identified in the crust of the Qilian Block and the arc-like tectonic zone, the velocity of which is 0.10-0.20 krn s^-1 lower than that of the surroundings. The average crustal thickness of the Ordos Block is consistently estimated to be around 43 km along both Profile L2 （Tongchuan-Huianbu-Alashan left banner seismic sounding profile） and Profile L1. In contrast to the gently varying intra-crustal interfaces and velocity contours in the Ordos Block along Profile L 1, which is a typical structure characteristic of stable cratons, the crustal structure in the Ordos Block along Profile L2 exhibits rather complex variations. This indicates the presence of significant structural differences in the crust within the Ordos Block. The crustal structure of the Helan Mountain Qilian Block and the Yinchuan Basin is featured by ＂uplift and depression＂ undulations, showing the characteristics of localized compressional deformation. Moreover, there are low-velocity zones with altemative high and low velocities in the middle and lower crust beneath the Helan Mountain, where the velocity is about 0.15-0.25 km s^-1 lower than that of the surrounding areas. The crustal thickness of the Alxa Block is about 49 kin, and the velocity contours in the upper and middle-lower crust of the block vary significantly. The complex crustal velocity structure images along the two seismic sounding profiles L1 and L2 reveal considerable structural differences among different tectonic blocks, their coupling relationships and velocity structural features in the seismic zones where strong historical earthquakes occurred. The imaging result of this study provides fine-scale crustal structure information for further understanding the seismogenic environment and mechanism in the study area.
文摘The area along the Grear Wall in northern Shaanxi between the Loess Plateau and the Ordos Desert is one of the major agrarian-pastoral regions of northwest China. Historically, the land was fought over by the nomadic and the agrarian peoples of the region. The Yansui section of the Ming Great Wall and thirty-nine fortified encampments along it were built during fighting between the Mongols and the Han people. As all of them were located along communication lines vital to economic development, they played an important role in politics, the economy and transport over three hundred years of Ming and Qing rule. However, they fell into disuse in the late Qing and the Republican period and are now in ruins. The main reason underlying their decline was that the sites had been chosen for their defensive value, so the subsequent northern expansion of China’s borders and structural changes in the border economy hastened their decline. Thus the rise and fall of these fortified towns at the intersection of the agrarian and the pastoral regions was closely related to imperial political and military activity and was in line with ethnic and tribal movements and migrations and the evolution of civilizations. In sum, the reasons behind the demise of these fortified towns and camps were highly complex and usually involved multiple factors.
