Study On The Effectiveness And Correction Method Of Horizontal/Vertical Spectral Ratio Of Microtremor In Characterizing Seismic Site Effect

The site condition has significant influence on the intensity and spectrum characteristics of ground motion,and the research method is mainly based on multi-dimensional site model numerical simulation and strong motion record statistics.Microtremor data has also been used in the site effect study of ground motion,but it is still controversial.In order to verify the effectiveness of the ground motion horizontal/vertical spectrum ratio(MHVR)and ground motion horizontal/vertical spectrum ratio(EHVR)were obtained by statistical analysis of more than 210,000 sets of strong acceleration records from 403 stations in KIK-NET network.Peak clarity test and reliability test were carried out on MHVR spectrum ratio curves of each station to verify the reliability of extracting superior frequencies from stable MHVR curves.The comparative analysis of EHVR and MHVR shows that when MHVR has identifiable peak value,the EHVR of S-band,tail band and full band is basically the same,which is close to the MHVR spectral shape,but there is a systematic deviation with the MHVR amplitude.Based on this,an empirical correction model based on MHVR spectral shape classification is established,and the relevant influencing factors are analyzed.MHVR can be divided into clear single peak,obvious single peak,double peak,wide or multipeak,low frequency single peak and no obvious peak according to the spectrum shape.The discreteness of EMR in different peak types was statistically analyzed by using the ratio EMR of EHVR and MHVR as the index.The results show that the MHVR amplitude can be modified effectively by using the average EMR of each spectrum as the correction coefficient,which makes it closer to EHVR.When MHVR is a clear single peak,the correction effect is most significant.The empirical correction method is affected by the nonlinear response of soil layer.The analysis of the correction results of 403 stations under different ground motion intensities shows that the proposed method is suitable for lower ground motion intensities(40 cm/s2).The effects of site characteristic parameters,topographic and geomorphic conditions and site nonlinearity on MHVR spectrum of different sites were investigated.The results show that there is no significant correlation between the thickness of overburden layer,equivalent shear wave velocity,geomorphic classification and MHVR spectrum classification.The horizontal/vertical spectrum ratio method of ground motion is one of the hot issues in the site effect study of ground motion,because it can obtain the information of dominant frequency and the lower limit of amplification coefficient only by single platform pulse record,and it has a broad application prospect in areas lacking strong vibration record.Although the corrected MHVR as the actual site amplification coefficient still has a large error due to the large dispersion of statistical relationship and the difference between the horizontal/vertical spectrum ratio and the soil/bedrock spectrum ratio,the corrected method proposed in this paper can be used as a transition to predict the ground motion site amplification coefficient by using microtremor.How to find the relationship between MHVR,EHVR and actual site amplification and use MHVR to predict site amplification coefficient still needs further research.