Deep Electrical Structure And Seismogenic Mechanism In Seismic Active Area Of Songyuan

Songyuan city,located in the middle of Songliao Basin,is an active seismic area in Songliao Basin.Historically,an earthquake of magnitude 6₄³ occurred in 1119.In March 2006,during October to November 2013 and May 2018,there have occurred some earthquakes of magnitude more than 5 in Songyuan area,causing more than200,000 people to suffer disasters,more than 57,000 houses damaged,and the direct economic loss exceeded 2 billion RMB.At present,there are considerable divergences on the formation mechanism of earthquakes in Songyuan area.Therefore,it is of great significance to study the formation mechanism of earthquakes and to predict the seismic risk.Songliao Basin is a large Mesozoic-Cenozoic composite sedimentary basin with dual structure of fault depression;it experienced four stages of tectonic evolution,the basement has double structure of‘fault’and‘depression’.In order to find out the formation mechanism of earthquakes in Songyuan area,we carried out a wide-band magnetotelluric observation project in the central and southern part of Songliao Basin.94 observation stations were laid out with a distance of about 35 km,and the record time is about 20 hours.In the concentrated area the staions are encrepted with a distance of 10km.besides,we also out long-period magnetotelluric observation,a total of 12 observation stations,with 20 to 30 days observation time.The observe data are been carefully processed and analysis.The result of phase tensor analysis shows that the three-dimensional property is strong and the direction of ellipse axis is complex in low frequency.The main directions are NE and NW,and EW directions are also included.Therefore,three-dimensional inversion is necessary in this area.After lots of inversion tests of different parameters,including the data error level,the mesh size,the regularization factor,and the initial mode.Finally,the error threshold of the main impendence is set to be 5%*|Z_xy?Z_yx|^1/2,while that of the diagonal impedance is 10%*|Z_xy?Z_yx|^1/2;the mesh size of the horizeontal direction is 10km;the initial regularization is 10,and the initial model is a uniform half space with the resistivity of 100??m.moreover,we also test the resolution of the inversion,It is proved that the resolution of the observation system composed of measuring points and frequencies can meet the needs of this study,and the reliability of the inversion results is also indirectly proved.The electrical structure of the study area can be roughly divided into three electrical layers:the first layer is shallow low resistivity layer with resistivity value of about 10??m,which is widely found in the study area.The thickness of the central depression area is the thickest,with an average thickness of 3 km and a maximum thickness of 7 km.The second layer is a crustal high resistivity layer with an average thickness of 10 km in the depth of 5-20 km,it is presumed to be the metamorphic rock and granite.The high resistivity layer is discontinuously distributed in the upper crust and been cut off by low resistivity bodies.The third layer is the lithospheric high conductivity layer,which is located below the high resistivity layer of the crust.And it is widely developed in the study area.It is likely that the high conductivity anomalous bodies in the upper mantle surge through the weaker crust and connect with the shallow low resistivity bodies from the deeper part of the earth.The resistivity inversion model in Songyuan area shows that in laterally,the shallow central depression shows low resistivity,but the low resistivity anomalies are separated by high resistivity anomalies in the middle;the northwest and southeast of the study area show high resistance;at the upper mantle scale,there are large-scale low resistivity anomalies in the study area.Vertically,the electrical structure can be roughly divided into three layers:the first layer is shallow low resistivity layer;the second layer is discontinuous high resistivity layer in the crust;and the third layer is lithospheric high conductivity layer.Among them,the most prominent anomaly is that the seismic source in Songyuan seismic area is mostly located in the electrical gradient zone,and there are low resistivity anomalies from deep below.It is inferred that the occurrence of earthquakes is closely related to low resistivity anomalies.Combining the results of Geophysics and geochemistry,it is analyzed that the low resistivity anomaly is magma.In order to study the deep source of magma,the long-term magnetotelluric section passing through the epicenter is analyzed.It is found that the shallow section is in good agreement with the broadband magnetotelluric section.The low resistance anomaly in the broadband magnetotelluric results exists in the long-term section and extends to the deeper part.Rheological analysis of low resistance anomalous bodies on asthenosphere scale in long period magnetotelluric profiles shows that the proportion of partial melting of two deep low resistance anomalous bodies is about 2%-6%and 0.2%-6%,respectively.Therefore,it is inferred that the magma in the crust and mantle originates from the asthenospheric material which partially melts.In the study of seismic tomography,long-period magnetotelluric and geomagnetic sounding,there are obvious large-area high conductivity and low velocity anomalies in the depth of 250-400 km in Northeast China,which are considered as the horizontal Pacific plate and contain a large amount of water.The retreat of the Pacific plate resulted in partial melting of subduction and asthenosphere materials with high water content under the combined action of decompression and high temperature.Partially melted thermal material upwells through weak zones in the lithosphere to the crust,forming the low resistivity anomalies we found.By projecting the epicenter to the results of three-dimensional resistivity,it is found that the earthquakes are mainly concentrated in two areas,one is located in the southwest direction（E1）of Songyuan City and the other is located in the northeast direction（E2）of Songyuan City.From the slice maps of different sections,it is found that there are two low resistivity anomalous areas in the deep part.The epicenter is located on the contact zone of low resistivity and high resistivity anomaly,and is on the side of high resistivity.Low resistance anomalies are usually plastic in mechanical properties,while high resistance anomalies are brittle in mechanical properties.When the low resistivity abnormal upwelling occurs,the upward stress increases,and the stress accumulates in the high resistivity body above.When the stress is greater than the rock bearing degree,an earthquake occurs.It is inferred that the low resistivity anomalous body in the crust is caused by the subduction of the Pacific plate,the dehydration in the depth of the transition zone and the upwelling of some molten materials.After the weak part of the lithosphere intrudes upwards,it encounters the relatively brittle formation barrier of the upper crust with high strength.Some of the low resistivity anomalous bodies accumulate in the brittle-ductile transition zone,and some of them continue to upwelling.As the temperature decreases,the accumulated part condenses and releases water.The released water accumulates in the fracture zone,which makes the stress of the fracture zone unstable and leads to earthquakes.The other part of the upwelling makes the upward stress in the surrounding brittle strata accumulate continuously,and ultimately exceeds the stress that the rock can bear,causing earthquakes.Therefore,it is inferred that the Songyuan earthquake is essentially a fluid-induced earthquake.