Research On Site Classification Of Earthquake Early Warning Station Based On Microtremor

The "National Earthquake Intensity Quick Report and Early Warning Project" aims to establish a national earthquake intensity report and early warning observation network with about 15000 stations,many of which lack complete site information.Evaluate the site classification and dynamic characteristics of stations economically and efficiently,can improves the accuracy of early warning time,guarantee the accuracy earthquake magnitude estimation and provide a theoretical basis for the distribution of disasters and direction of rescue,addresses an important role in improving the information of early warning stations.The site classification method of the H/V spectral ratio of microtremor method,compared with the witch based on borehole shear-wave velocity data which requires plenty of human and material resources,is simple,economical,convenient,and effectively reflecting the dynamic characteristics of the site.As far the most early-warning stations without site classification and the variety of early-warning instruments,it's necessary to obtain the time-frequency characteristics of the microtremor observed by various instruments and the correlation between H/V spectral ratio and site classification.The site classification scheme based on the microtremor is given,and the applicability of the site classification scheme to the microtremor observation data from different sources is analyzed.The main work and research result completed of this dissertation is as follows:1.The statistical method is used to analysis the basic period and the predominant period of the microtremor's H/V spectrum ratio in different types of sites,and the site classification scheme based on the dominant period of the microtremor's H/V spectrum ratio corresponding to current seismic design code is proposed.By comparing the methods of numerical simulations,theoretical calculations,and background noise before earthquakes demonstrates the rationality of the scheme.2.Generating large number of shear-wave velocity structure models through stochastic simulation methods and the transfer function curve of different types of sites are calculated and summarized.Furthermore,a site classification method based on H/V spectrum ratio of microtremor,considering the H/V spectrum ratio of microtremor and the transfer function comprehensively,is proposed.3.The time domain analysis methods and frequency domain analysis methods are used in comparing and analyzing the microtremor observed by the force balance accelerometer,the MEMS accelerometer and the velocimeter with different sensitivities on the same site and station.The result shows that the overall amplitude level of the microtremor observed by the MEMS accelerometer is slightly higher and the waveform is poor,but it tends to be consistent with that of the force balance accelerometer with the increase of amplitude.The three-dimensional Fourier spectrum is similar to the force balance accelerometer,but the H/V spectrum ratio tends to be flat.The higher the sensitivity of the accelerometer,the easier it is to identify the dominant period of the microtremor H/V spectral ratio.The amplitude of the microtremor observed by the velocimeter is similar to that of the force balance accelerometer,and the Fourier spectrum in three directions is similar,but the stability of H/V spectrum ratio is slightly poor in low frequency band.4.Aiming at the shortcoming of low signal-to-noise ratio of the microtremor observed by the MEMS accelerometer,Observing the microtremor at 24 different sites by the MEMS accelerometer and the force balance accelerometer simultaneously,and comparing and analyzing the amplitude level and the difference of the Fourier spectrum of the microtremor between two instruments.Two correction methods are applied to correct the Fourier spectrum of microtremor recorded by the MEMS accelerometer.The H/V spectrum ratio of the MEMS accelerometer before and after the correction and of the force balance accelerometer are calculated and compared,and the empirical evaluation plan of the site classification based on the microtremor observed by the MEMS accelerometer is given.The result shows that with the increase of the amplitude,the Fourier spectrum value of the microtremor and the force balance accelerometer are closer,but the H/V spectrum ratio of the MEMS accelerometer still tends flatten;the method of adjusting the Fourier spectrum of the MEMS accelerometer by difference adjustment is more suitable for site classification.