Study Of Correlation And Mechanism Between Lithospheric Magnetic Field And Continental Seismicity In China And Surroundings

The Lithospheric Magnetic Field(LMF)is one of the basic and important geophysical fields.Its spatial distribution is closely related to the magnetic and thermal structure of the underground,and thus of vital importance to study tectonics and seismicity.Since the beginning of the 21st century,the development of geomagnetic survey,especially satellite magnetic observation,has brought lots of opportunities to the modeling and application of LMF.Scholars have carried out relevant studies on the distribution characteristics of LMF and its correlation with seismic activities,and found that the vertical component(Bz)reflects the surficial tectonics well,and most earthquakes occurred in areas of negative magnetic anomalies or zero Bz belts.However,the studies of statistical characteristics and mechanism of seismicity on LMF are still rare.Therefore,here we compared Bz at 200km altitude calculated by the NGDC-720 LMF model with the tectonic and dynamic characteristics of Mainland China and surroundings,and analyzed the statistical characteristics of seismicity in different periods and regions on Bz by using methods such as histogram,boxplot and K-S test.According to the statistical results and the mechanism of the generation of continental LMF,we established a Two Equivalent Source Dipole(TESD)model to explain the correlation between LMF and seismicity,and discussed the influence of different parameters on the averaged Bz generated at the boundary of two blocks with different Curie depths.The main conclusions are as follows:1.There are good correspondences between Bz and tectonics:significant positive anomaly areas usually relate to stable sedimentary basins,where the GPS displacement is relatively small and the lithospheric strength is relatively high,while significant negative anomaly areas generally relate to plateaus and active orogenic belts,where the GPS displacement is relatively large and the lithospheric strength is small.Some faults are distributed at the edge of significant magnetic anomalies.2.Earthquakes above Ms5.0 tend to occur in areas with negative Bz and boundaries of significant anomalies,while earthquakes rarely occurred in areas with significant positive Bz.After statistical(K-S)test,we found that the distribution of Bz remarkably affects the seismicity.Other than previous conclusions that most earthquakes occurred in zero Bz belts,our histogram and boxplot results show that more than half(53.2%)of the earthquakes occurred in the Bz range between-5 and-3 nT during the same period of satellite data(2004-2007),and the seismic energy ratio in this range is 94.6%.Consistent statistical characteristics have been found for different periods,but the proportion of earthquakes in the Bz range of-5?-3 nT gradually decreased with the extension of the statistical period(up to 110 years).Statistics of the horizontal gradient of Bz showed that only a few earthquakes occurred in areas with large Bz gradient.3.With selected parameters,our TESD model showed that Bz at the boundary of the blocks(defined by different Curie depths)is close to or in the range of-5 to-3 nT,and the viscosity contrast at the bottom of the boundary caused by temperature difference may reach 50 times,which partly explained the concentration of seismicity in specific Bz ranges.We also divided the research area into six sub regions and performed the same magnetic-seismic statistical studies respectively,and different statistic results show,which could be explained by selecting different TESD model parameters.