| Ordos block is located in the tectonic transition zone in the east and west of China,and is a relatively complete and nearly rigid block in the northern part of the Chinese mainland.Seismic records show that earthquakes rarely occur inside the Ordos block,and the maximum magnitude of the earthquake has never exceeded 6.However,the peripheral boundary faults and tectonic belts around the Ordos block have intense seismic activity,and destructive earthquakes often occur.Therefore,it is of great significance to study the seismicity of the Ordos block and its adjacent areas for earthquake prevention and disaster mitigation and safety production in this area.Researches show that the seismicity in Ordos and its adjacent areas has strong spatial and temporal inhomogeneity.The seismicity of its east and west boundaries are much larger than that of the north and south boundaries and has obvious time alternation characteristics.After the Wenchuan earthquake,the southwestern margin of the Ordos block,the westernmost part of the Weihe graben,the eastern and western parts of the Hetao graben,and the central part of the Shanxi graben had a high probability of earthquakes.Most of these studies were only based on methods such as earthquake distribution and b value,and the reliability and resolution was still limited.It is necessary to further verify these speculations and conclusions and further study the seismicity of the Ordos area.According to the temporal and spatial aggregation degree and related characteristics of earthquakes,an earthquake sequence is defined as a group of earthquake events that occur in a continuous time and space range,which contains rich information about the preparation,development and occurrence of earthquakes.The type of earthquake sequence is closely related to the medium structure and stress distribution,and it provides important clue for the earthquake preparation processes,stress changes and fault slips.Therefore,the study of earthquake sequences can infer information such as regional stress changes and seismic activity trends.Earthquake sequences often overlap in time and space and are submerged in massive background seismic events,making it difficult to identify accurately and efficiently.To surpress the difficulties in identifying earthquake sequences,in this study we collected and relocated the earthquake event data in the Ordos area from 1990 to 2019,Then the multiple clustering method developed in this study was used to extract the earthquake sequence.Combined with the earthquake energy theory and the b-value method,the seismicity of the Ordos and its adjacent region,the characteristics of stress changes,energy release and earthquake sequence types are analyzed.The result shows:(1)The multiple spatiotemporal clustering algorithm developed in this paper not only solves the problem that single clustering is prone to events misclassification,but also avoids the difficulty of weighting parameter selection caused by using spatiotemporal coupling distances,and can also obtain key parameters of earthquake sequences,which provides a quantitative basis for revealing the seismic structure,the mechanism of the earthquake,and the seismicity.(2)The b value of the Shanxi faulted basin is about 0.85,and the seismic energy is mainly released in the western part of Datong Basin in the terrace position of Liulengshan Fault and Hengshan Piedmont Fault,the northern part of Taiyuan Basin and the northern part of Linfen Basin.The distribution of earthquake sequences can well describe the distribution direction of the Shanxi faulte basin,and to a large extent depict the distribution of several sub-basins.(3)The seismic activity in the Weihe faulted basin is generally in a stable stage,but the spatial distribution is uneven,and there is a phenomenon of earthquake aggregation in small areas.The three earthquake sequences are distributed on the fault zone,and the seismic energy is mainly released in the intersection area of the Huashan Piedmont Fault and the Weihe Fault in the west of the Weihe Basin and the central part of the Weihe Basin.(4)The seismic energy of the Haiyuan-Liupanshan fault is mainly distributed along the strike of the fault,and there are accumulations on the northeastern side of the southern section of the Liupanshan fault and the eastern end of the Haiyuan fault.There is a large area of high b value in the southeastern segment of the Liupanshan fault,where high-density earthquake sequences also appear.In the area of XiangshanTianjingshan fault and Yantongshan fault,there are earthquake swarms across faults,and it is speculated that there may be transform faults.(5)The seismic activity in the Yinchuan-Jilantai fault depression basin is mainly located in the southern Yinchuan Basin.Earthquake distribution and energy release are mainly in a band in the Yinchuan Basin along the strike of the the Yellow River-Lingwu fault.The seismic sequence also shows that the seismic activity in the whole region is mainly in the Yinchuan Basin,especially in the southern part of the Yinchuan Basin,where the earthquake swarms are very dense and all are close to the Yellow River fault.The low b-value area in the southern margin of the Yinchuan Basin may be the location of future major earthquakes.(6)The seismic activity of the Hetao fault depression basin is mainly located in the Baiyanhua Basin in the middle and the intersection area of the surrounding faults.An obvious low b value anomaly shows in the Linhe Basin,Located in the west of the Hetao Basin,and the distribution of earthquakes in this area is sparse,which is judged to be the future earthquake risk area of the Hetao Basin.The seismic activity in the Hubao Basin in the east of the Hetao Basin is mainly distributed on the fault zone.In conclusion,analysis of the seismic activity around the Ordos,using the multiple spatiotemporal clustering algorithm,b-value and energy release estimation.The result shows that in the past three decades,the seismic activity around Ordos is mainly concentrated in the Shanxi rifted basin,Haiyuan-Liu Panshan fault zone and the Yinchuan Basin,In the Weihe faulted basin and the Hetao faulted basin,the seismic activity is at a low level,and seismic accumulation only exists in some areas.The southern part of the Yinchuan Basin,the western part of the Hetao Basin,the eastern segment of the Haiyuan fault,and the southern segment of the Liupanshan fault are areas of strong earthquake risk that need attention in the future. |
