Study On The Tectonic Stress Field And Seismogenic Environment In Beijing Tianjin Hebei Region

The North China block is located in the intersection area of the three major tectonic domains inChina,and has experienced a complex process of tectonic evolution.Strong activities still occur under the action of the India Ocean plate,Eurasia plate and the Pacific plate.Beijing,Tianjin and Hebei region is a densely populated area with highly developed political,economic and cultural areas.The area is located at the intersection of the Zhangjiakou Bohai tectonic belt and the NE-NNE tectonic belt in the North China Plain,and is also the most intense area in the eastern part of China.In view of the special geological structure position and the background of the new tectonic activity in the Beijing Tianjin Hebei region,it is urgent and necessary to strengthen the research,evaluation and prediction of the modern tectonic stress field and the seismogenic environment in this area.In particular,the study of active faults is of great practical significance for engineering site selection and mitigation of natural disasters.According to the geological data collection and field geological survey,the development characteristics and seismic tectonic background of active faults in the area are analyzed;based on the results of the crustal crustal stress measurement data and the tectonic stress field,in accordance with the three dimensional numerical simulation method,the internal relationship between the spatial distribution pattern and the seismic activity in the tectonic movement process and how the active fault system promotes earthquake initiation,triggering and seismic activity on active faults and tectonic stress fields and their interaction relationship are analyzed.Firstly,according to the historical seismic data in Beijing Tianjin Hebei area,the temporal and spatial distribution characteristics of the earthquake are analyzed;secondly,the distribution characteristics of the present tectonic stress field in the shallow crust of the Beijing Tianjin Hebei region are studied.the fault strength of Beijing,Tianjin and Hebei region is studied by combining the Anderson fault criterion,the Kulun rupture criterion and the Byerlee law.the earthquake danger in the Beijing Tianjin Hebei region is discussed.Finally,the finite element numerical analysis is used with the GPS crustal deformation data and the results of geostress test as constraints,the control effects of the two groups of NW and NE fault zones on earthquake inoculation and triggering in the Beijing Tianjin Hebei region are analyzed.The seismic tectonic models of the Beijing Tianjin Hebei region are discussed from two aspects of the tectonic stress field and the dynamic model,The main results are as follows:(1)According to the seismic data obtained,it is found that the earthquakes in the scope of the Beijing Tianjin Hebei region are mainly distributed in the front of the mountain plain and the inter mountain basin,and their distribution forms constitute the linear combination of NE direction and NWW direction.The periods of quiet and active periods of strong earthquakes in Beijing Tianjin Hebei region are about 120～150a and 50 to 100a,respectively.According to the seismic source mechanism of Beijing Tianjin Hebei region,it is found that under the action of two groups of fracture zones in Beijing Tianjin Hebei region,under the action of NNE to the maximum principal pressure stress,the NWW direction fault zone is easy to move,thus forming the intensive active zone of the earthquake on the NWW fault zone,while the strong earthquake occurs on the conjugate NE fault zone.(2)The results of geostress measurements in each section of the fault zone indicate that the geostress gradient in the northwestern section of the Zhangjiakou Penglai fault zone and the middle section of the Taihang Mountain fault zone is higher,especially the fitting gradient of the Zhangbei borehole reaches 0.057MPa/m,which has certain directive significance for the earthquake prediction in this area.The ratio of the average horizontal stress to the vertical stress and the maximum horizontal stress to the vertical stress increases with the depth,which tends to a constant value,reflecting the dominant tectonic stress in the shallow crust,and the vertical stress increases with the increase of depth,and the tectonic stress gradually decreases.(3)From the two aspects of qualitative and quantitative analysis,we study the fault intensity in Beijing Tianjin Hebei area.The results show that the friction coefficient of the fault along the Zhangjiakou Penglai fault zone is 0.48,and the future seismic activity in this area is worth paying close attention to.The mean value of friction coefficient at each measuring point in the middle part of the fault zone is 0.30,indicating that the stress accumulation level is low and the possibility of earthquake activity is relatively small.In the southeast section of the Zhangjiakou Penglai fault zone,the average value of the friction coefficient calculated by the borehole data is 0.26,the level of stress accumulation in this section is low and the probability of seismic activity is small.For the Taihang Mountain piedmont fault zone,in the middle section of the fault zone,the friction coefficient of the fault is higher and close to the lower critical value of the fault activity of 0.6.Therefore,it is necessary to pay attention to the change trend of the stress state in this section.The change of friction coefficient in the direction of the vertical Zhangjiakou Penglai fault zone:the friction of the Li Siguang memorial hall near the fault is lower,while the other two boreholes far away from the fault are higher,which is the same as that measured by Zoback and so on in the vertical direction of the San Andreas fault.(4)Taking the two typical earthquake events of Tangshan and Japan in North China as time nodes,the research results of the changes of tectonic stress field before and after the Tangshan earthquake are summarized.Secondly,the changes of tectonic stress field before and after the great earthquake in Japan are studied.It is found that the tectonic stress field in the northern part of North China and the eastern part of the Tanlu fault zone did not change obviously before and after the Tangshan earthquake,and the tectonic stress field of the southern part of North China area changed greatly before and after the earthquake.The maximum horizontal principal stress orientation in the North China region before the great earthquake in Japan,tension effect period caused by the earthquake,the present tectonic stress field are NEE direction,NNW direction and NEE direction respectively,reflecting the evolution of tectonic stress field before and after the adjustment of the earthquake in North China.(5)Under the present tectonic setting,the distribution characteristics of the stress field and the relationship between the distribution of the seismicity and the initiation and triggering mechanism of the seismicity in the area of Beijing,Tianjin and Hebei are studied by numerical simulation under the control of NE and NWW fault zones.In the study area,the intersection area of NE and NWW two fault zones presents stress concentration and stress intensity,and is often the site of earthquake development.The gentle dip type positive fault structure in the Taihang Mountain Fault Zone obstructed the stress from the Shanxi graben system,which led to the low stress phenomenon in the North China depression and formed a certain "protection" role in the North China depression.The stress disturbance caused by the Xingtai earthquake is mainly concentrated in the area of two groups of NWW and NE fault confining limits in the North China depression,and the stress disturbance caused by the Xingtai earthquake still does not pass to the west side of the Taihang Mountain mountain front fault zone.The area affected by the Tangshan earthquake is mainly along the vicinity of the two fault zones and farthest to Zhangjiakou area.In the central part of North China,under the structural system of NE and NWW fault zones,the two groups of fault zones have joint control on the tectonic stress field of the whole region.The results of the above numerical simulation show that in several regional tectonic systems,the NE fault zone occupies the dominant position in the seismic activity.