Study On The Crustal And Upper Mantle Velocity Structure Beneath The Northern Section Of The South-North Seismic Zone Using Surface Wave Imaging Method Based On Dense Array

The northern part of the north-south seismic belt is located in the northeastern margin of the Qinghai-Tibet Plateau,and contains several active blocks such as Alashan block,Ordos block,Qinghai-Tibet block and Sichuan basin.These blocks,especially their junctions,have a very complicated structure.In this paper,we first use the smaller array of Tianshui,the spatial autocorrelation of the data to extract the dispersion curve,and use the traditional imaging method to obtain the shallow velocity structure within 50 meters.Then,the background noise surface wave imaging method,background noise surface wave In recent years,with the deployment of more and more intensive arrays,some new methods based on dense arrays are extended,such as the process of equations Imaging and Helmholtz Imaging.In this way,the20s-70 s surface wave velocity and anisotropy information are obtained.The periodicity of the surface wave is longer in this earthquake,and the area is more complicated.It needs to restrain the shallow velocity.For the shallow information of10 km or more,Need to be explained by another feature of the surface wave.Through the calculation of the same period,the amplitude aspect ratio reflects the shallow depth of information,but too much intensive array data,nine component NCF calculation is greater,the use of cloud computing methods,significant savings in computing time.Main work in the paper is as following:(1)In order to select the array layout with high efficiency and convenience for surface wave exploration using natural sources,four tests of data acquisition with different layout are carried out in the same field.We perform comparative analysis of the dispersion curves extracted using SPAC and ESPAC for different array layout.The velocity structure of the shallow layer is obtained by inversion.Our study shows that the dispersion curves extracted from the four arrays are very similar;and spectrum energy in the frequency-velocity domain is more concentrated for the nested equilateral triangle and circular arrays than that for the linear and L-shape arrays.The dispersion curve extracted from the L-shape array is better than the linear array in thehigh frequency band(8-40Hz),while worse in the low frequency band(4-8Hz).Since worse dispersion is found at high frequencies for the linear array,joint exploration with both artificial and natural sources are proposed.The experiment proves that the joint method can not only improve the quality of dispersion curve in the high frequency band to ensure the reliability of the detection precision,but also achieve dispersion data in a much broader frequency range.Consequently both shallow and deeper structure can be obtained in the inversion.(2)The northern part of the north-south seismic belt is located in the northeastern part of the Tibet Plateau in the eastern part of the eastern Kunlun fault zone.It is an important area to study the interaction between the northeastern margin of the Tibet Plateau and the surrounding block field.In this paper,the data of Rayleigh wave is extracted by cross-correlation method from December 2013 to May 2015,and the Rayleigh wave velocity dispersion curve is measured by matched time-frequency analysis technique.The velocity and phase velocity distributions of the Rayleigh wave group are obtained from the 10s-22 s in the northern section of the north-south seismic zone by using the background noise surface wave imaging method.The results show that the boundary and fault direction of the fault are better.(3)At present,surface wave tomography is an important method to study the mantle structure in the crust.And some new methods of surface wave tomography based on dense array are presented with the layout of the dense array.In this paper,we get the 20 seconds to 70 seconds of Rayleigh wave phase velocity distribution images and anisotropy in different directions,by the method of ASWMS with the dense array data of north-south seismic belt.By combining the results of this paper with the results of previous studies,this paper has a deeper understanding of the distribution of low velocity layer,the boundary of the movable block and the deformation information of the northern section of the north-south seismic zone.By combining two kinds of imaging methods based on array,we can obtain a relatively complete structure of crust and mantle.Therefore,we can have a more in-depth understanding of the boundary and interaction of the activity blocks in this area.(4)The nine-component cross-correlation function(NCF)has been paid more and more attentions with the development of research based on ambient noise crosscorrelation.However,with the development of large aperture and dense arrays,it is challenging to quickly calculate the cross-correlation function from large-amount dataset in tradition high performance workstations,especially for the nine-component cross-correlation functions which will roughly take nine times longer compared to the vertical-vertical component alone.In present paper we propose one possible solution to speed the calculation of nine-component cross-correlation functions for large dataset using the cloud computing.The cloud computing can provide scalable computation power and storage which is suitable for data intensive computing tasks,while calculating NCFs from large amount data is exactly one data intensive computation.Based on the cloud services provided by Aliyun,we have developed one framework which could factorize the entire computation into small pieces and execute each piece in one single virtual server evoked at the cloud end.Since all those virtual servers can run simultaneously,the time cost to obtain NCFs from large dataset could be highly reduced,which is roughly inversely proportional to the number of evoked virtual servers.We apply this technique to obtain the nine-component NCFs based on the continuous three component records of China Array from 2014 to 2015,which consists of 674 broadband stations and covers a ten by ten degree area in northeast Tibet.Our results show that the entail computation can be finished in eleven hours,which is about 400 times faster compared to that on one single tradition server.We further validated the resulting NCFs by calculating the Rayleigh wave ZH ratios from both the stacked nine-component NCFs and earthquake results and the results suggest our computation method is fast and reliable.Seismology is developing in an era of big data and our study suggest by utilizing the techniques from computing science of mass data,we can benefit from the advances in observational capabilities.