The Structure Of The Pacific Stagnant Plate In The Mantle Of Eastern China Revealed By Three-dimensional Geomagnetic Depth Sounding Images And Its Significance

A mount of the present seismic tomographic results show that the Pacific plate subducted and stagnant in the mantle transition zone in eastern China,and some scholars think that there may be a material transport channel in the middle of the detained plate,and the latest research results show that the front end of the detained plate may have fallen down and entered the lower mantle.Different subduction patterns affect the interpretation of different geological structures,earthquakes and volcanic events,so it is of great significance to study the detention patterns of the Pacific plate.However,the Pacific plate carries a lot of water into the mantle of eastern China,which leads to the high conductivity of the remaining plate.Therefore,the shape of the remaining plate can be obtained by studying the conductivity characteristics of the region.On the contrary,the conductivity can be used to calculate the water content and melting characteristics of materials in different regions,thus providing a scientific explanation for the preparation of strong earthquakes in China and the widespread volcanic activities in eastern China.Therefore,it is of great scientific significance to study the electrical structure in the mantle of China.As a geophysical method to study the electrical structure of the deep underground by using the surface geomagnetic field signal,the effective detection depth can be increased to about 1600 km depending on the super long signal period(up to more than 100 days).Therefore,this method can be used as an effective means to obtain the mantle conduction model in eastern China.In order to obtain the high-resolution electrical structure of mantle transition zone and deep region in China,we collected the data of geomagnetic stations widely distributed in China.However,due to the interference of noise,there are many "outliers" in the calculated C-response.Therefore,we based on the spectral response of each component obtained by birrp software package,combined with regularization inversion theory and overall smoothness Constraint technology realizes a set of new data processing methods,which can get more smooth and stable C-response estimation.In addition,there are a large number of relative observation data in China,which do not contain baseline values.In order to make full use of these data,we propose and verify an approximate processing method,so that we can make use of these data.Finally,we obtain the C-response data of densely distributed geomagnetic stations in China and its surrounding areas,including 41 absolute observation stations,the period range is 1.33-113.78 days,19 stations are relatively observed,and the period range is 1.33-42.67 days.Later,we have carried out three-dimensional inversion of the above data.During the inversion,we use the model grid parameterization instead of the spherical harmonic sparse expansion parameterization method.The forward modeling uses the staggered grid finite difference method in spherical coordinate system,the inversion uses the finite memory quasi Newton method(L-BFGS),and the initial model is the global average conductivity model obtained by Kelbert(2008).After a large number of parameter debugging,we have obtained a set of stable three-dimensional electrical structure imaging results.After that,in order to verify the reliability of the inversion results,we first carried out the sensitivity analysis of different data sets.The results show that in the area where the dense stations are distributed,the geomagnetic sounding can effectively distinguish the small-scale electrical anomalies of 6 °× 6 ° in the depth range of 410-900 km.In addition,we simulate the subsidence of the detention plate,and verify the feasibility of the geomagnetic sounding for the exploration of the detention plate form through theoretical data inversion.Finally,through the analysis of the conductivity model of the mantle of Eastern China,it is found that there are there are obvious high conductivity anomalies in the transition zone area and the top of the lower mantle in eastern China,and there are great differences between the upper and lower abnormal forms in the transition zone.Trough petrophysical analysis,it is speculated that the high conductivity anomalies in the mantle transition zone and the top of the lower mantle in eastern China correspond to high water content(up to 1.7 wt%),which may be carried by subducted plates Combined with seismic wave velocity and other geophysical and geochemical evidences,it is speculated that most of the Pacific plate lies in the mantle transition zone of eastern China,and some of the plate has sunk into the lower mantle.