Study On Three-dimensional Electrical Structure And Deep Seismic Structural Conditions In Zhenjiang Area

Zhenjiang is located in the densely populated and economically developed Jiangsu Province of China.According to tectonic units,it belongs to the lower Yangtze fault block and is located in the middle and lower Yangtze River and the Yellow Sea earthquake zone.Seismic activity data indicates that several destructive earthquakes have occurred in Zhenjiang and its adjacent areas in history.Accurately identifying the spatial location and development of faults and evaluating the seismic hazards of faults in Zhenjiang will provide a reliable geophysics evidence for earthquake prevention and mitigation.In order to study the development and spatial distribution of fault structures in Zhenjiang area,magnetotelluric observation was carried out in Zhenjiang City and its surrounding areas.68 observation points were arranged on 7 survey lines basically perpendicular to the structural trend,with an average distance of 2 km and an observation time of about 20 h.Due to the serious electromagnetic interference in the observation area,the acquired magnetotelluric data are processed by cross reference or far reference technology,and the data with a length of 320 Hz?108 s is obtained at each measuring point.In order to obtain the three-dimensional electrical structure of Zhenjiang area,the phase tensor analysis of the data is carried out.The results show that the reflection of the data on the characteristics of the underground electrical structure is related to the frequency.In the high frequency part,the data shows one-dimensional characteristics,mainly the reflection of thick sediments in the region;with the decrease of frequency,the three-dimensional characteristics gradually highlight and the direction of the ellipse axis is complex and changeable.According to this fact,three-dimensional regularization inversion processing technology is selected.The specific inversion parameters are as follows:?1?take the regularization factor as 10;?2?take the initial model resistivity as 100??m;?3?the error threshold of the main impedance is 5%*|Z_xy?Z_yx|^1/2;?4?the error threshold of the auxiliary impedance is 10%*|Z_xy?Z_yx|^1/2;?5?take the unstructured grid finite element method to realize the forward calculation.Numerical experiments show that the stable convergence results can be obtained after118 iterations.On the whole,the electrical property of the underground in the observation area is low resistance,only in the edge of the basin or the mountain outcropping area is high resistance.According to this feature,the electrical structure and spatial distribution of Shangdang Heyang fault,Dantu Jianshan fault and Maoshan north extension fault are analyzed under the constraint of other geophysical data in the working area.The specific conclusions are as follows:?1?Shangdang Heyang fault is a concealed fault in the region.Its strike is nearly east-west and inclines to north-south.It is located in the electrical gradient zone and the gravity anomaly gradient zone.It may intersect with Dantu Jianshan fault.?2?The landform of Dantu Jianshan fault is the boundary of topography,the southwest of which is Jurong basin and the northeast of which is the east section of Ningzhen mountain range.The strike of the fault is northwest and tends to southwest.The electrical structure and gravity anomaly show obvious gradient belt,which may cross the Yangtze River.?3?Maoshan fault is concealed under Jurong basin,its strike is ne,trend is se,it is in electric gradient zone and gravity gradient zone,and it has crossed the Yangtze River,it may intersect with Dantu Jianshan fault near the Yangtze River.On the basis of the above conclusions,according to the previous discussion of strong earthquakes and the data of medium strong earthquakes in the study area in recent years,the seismogenic structural conditions of medium strong earthquakes are analyzed,and the following general laws are obtained:?1?medium strong earthquakes have strong correlation with regional geological structures;?2?they are mostly occurred near the gravity,magnetic and electrical structural zones;?3?they are pregnant Low velocity anomalies are widespread in the earthquake site.Based on the data of gravity,magnetic force and velocity in Zhenjiang target area,combined with the electrical structure,the seismogenic environment is analyzed.The results show that:?1?Maoshan fault is the boundary of controlling basin and depression,which is located in velocity gradient zone,gravity gradient zone and electrical gradient zone.It has the seismogenic history of MS5.0 earthquake and above.Maoshan fault has medium strong earthquake environment.?2?The deep velocity structure,Poisson's ratio and gravity anomaly of the Mufushan Jiaoshan fault all meet the conditions for the preparation of medium strong earthquakes.Historically,there have been many earthquakes of MS 5.0 or above around the Mufushan Jiaoshan fault,which are distributed in clusters near the fault and have the conditions for the preparation of medium strong earthquakes.?3?Dantu Jianshan fault controls the northern edge of Dantu depression,which is located in the gradient zone of gravity and electrical structure,and passes through Hengshan Jianbi fault and Maoshan fault to control the low resistivity anomaly of A1section.Combined with the plane P-wave velocity map,the fault has obvious velocity anomaly at the intersection of Maoshan fault and Mufushan Jiaoshan fault.Therefore,the intersection of Dantu Jianshan fault,Maoshan fault and Mufushan Jiaoshan fault has the conditions for medium strong earthquakes.?4?Although Shangdang Heyang fault is basin controlling fault,its gravity gradient,magnetic anomaly gradient and electrical gradient scale are small,and according to the existing data,P-wave velocity has no obvious abnormality and Poisson's ratio is relatively low,so it does not have the conditions for preparing medium strong earthquakes.