Double-difference Tomography Of The Crustal P-wave And S-wave Velocity Structure In North China

North China is not only the political and cultural center of our country,but also the economically developed and densely populated area.Its historical disastrous earthquakes have caused serious casualties and property losses.These earthquakes are closely related to the complex geological movement in this area,such as the destruction of the North China Craton,which makes the lithosphere thin.In order to study the seismogenic environment,the mechanisms of earthquakes and the destruction of the craton,a lot of exploration work has been carried out on the deep part of North China,especially on the structure of the crust,and fruitful results have been achieved.With the development of seismic networks,the employment of temporary seismic arrays and the improvement of seismic observation instruments in North China,a lot of high-quality seismic records have been accumulated,which makes it possible to study in detail the deep structure of the area.With observed seismic data,we have many methods to obtain the deep structure of the earth.One of the most widely used imaging methods in recent years,is the one called double-difference tomography.This method uses absolute travel time and relative travel time data to invert the velocity structure and seismic source parameters at the same time.It has no the contradiction between the velocity model and source parameters,which usually occurs in the conventional inversion methods.Therefore,it can give more accurate velocity structure and source parametersIn this paper,using the travel time data of P-wave and S-wave(Pg,Pn,Sg,Sn)from June 2008 to June 2018,the velocity structure of three-dimensional P-wave and S-wave in the crust of North China(37 °-42 °N,113.5 °-118.5 °E)is established and the source parameters are determined again.The results show that the average of RMS of travel time residuals decreases from 0.265 s to 0.008 s;the relocated hypocenters are mainly located between 6 and 16 km in depth,and the corresponding epicenters are mostly distributed along faults.Most of the earthquakes occur between low-and high-velocity areas,and more approach the later.In Tang Shan and Xingtai,where great earthquakes took place,middle and down parts of the crust show low velocities,probably resulting from the upwelling of deep fluids and hot mantle material.The velocity distribution is very different in the upper and lower crust: In the upper part,it is mainly controlled by the tectonic structure and the faults;in the middle and lower parts,it is closely related to the crust-mantle interaction,the Moho uplift,and the upwelling of the asthenosphere material.Based on the tectonic pattern,evolution history and previous studies,we believe that the destruction of the North China Craton results in the thinning of the lithosphere and the upwelling of the asthenosphere material;hence,deep fluids and hot magma go up through faults and crannies and intrude the middle and lower crust;as a result,these crustal parts become weak and partly melt,which is favorable for generating earthquakes.