Study On Estimating Near-surface Shear Wave Velocity Using Horizontal-to-vertical Spectral Ratio Method

Shear-wave velocity,the basic index to evaluate the dynamic characteristics of soil,is widely used in geotechnical engineering and geophysics,such as being an important parameter in site classification of earthquake resistant design and evaluating liquefaction.In this paper,the author use two different methods based on Horizontal-to-Vertical Spectral Ratio to evaluate near-surface shear-wave velocity by analysing data collected by KiK-net stations in Japan.The accuracy and applicability of the methods are discussed.The first method is to build an empirical relationship between equivalent shear wave velocity within different depth and fundamental site period evaluated by HVSR method.The second method is to observe shear-wave velocity structure from the inversion of mean HVSR curves of earthquake data based on diffused field assumption theory and Monte Carlo method.The following studies were conducted and conclusions are as follows:(1)fundamental site periods of 562 stations are estimated based on HVSR method.The results are compared with the results from existing research that these periods are in good agreement when their periods are less than 0.5s.The correlation coefficient is 98.8% and the standard deviation of error is 0.03 s.(2)An relationship between fundamental site period and equivalent shear wave velocity within different depth(10m,20 m,30m,60 m,100m,200m)is observed.The relationship shows that the correlation between them increases from 0.52 to 0.88 as depth increases.Since the correlation between period and the equivalent shear wave velocity within 10 m,20m and 30 m is poor,it is not recommend to use the first method in site classification.(3)The author estimate the shear-wave structures of 98 chosen KiK-net stations based on the Monte Carlo inversion of HVSR curves and diffused field assumption.Then these stations are classified according to the standard in NEHRP(National Earthquake Hazards Reduction Program)based on results in this paper.The classification of 87 stations are in agreement with velocity profiles,which consist of 89% of all chosen stations.Thus the results demonstrate the applicability to classify sites by using shear-wave velocity estimated by inversion from HVSR based on earthquake data.