| Ground motion is affected by the source(fault rupture),propagation medium(crustal structure)and site(soil structure,topography).After the 1985 Mexico City earthquake,scientists realized that sedimentary layers amplify the amplitude and duration of long-period ground motions.The simulation and prediction of strong ground motion will provide guiding opinions for urban earthquake emergency work,improve the city's ability to resist earthquakes and reduce disasters,reduce the number of casualties and reduce economic losses.Thus,the three-dimensional(3-D)velocity structures of the subsurface sedimentary layers must be estimated for the prediction of long-period ground motions.Taking the Yuxi basin as an example,we provide a modeling procedure,including data preprocessing,model building,model updating and model testing.Because there is a large deviation between the shear wave speeds obtained by drilling and seismic exploration for the same location,based on the credibility of various data,we propose a function that can eliminate the velocity deviation among various data.On the other hand,subsurface shallow velocity profiles can be determined from geotechnical or geophysical investigations.Compared with data obtained by drilling and seismic exploration,microtremor data are obtained by inexpensive techniques that do not require high logistical effort.Since Nogoshi proposed the ratio between the Fourier spectral amplitudes of the horizontal and vertical components of microtremor recordings(MHV)in 1971,the horizontal-to-vertical(H/V)curves of microtremors have been widely used to evaluate subsurface velocity structures.However,the key to estimating subsurface velocity structures by the inversion of MHV curves is the simulation of MHV curves.Subsurface velocity structures must be estimated to predict long-period ground motions and seismic hazards.Subsurface velocity structures can be constructed via an inversion of the horizontal-to-vertical(H/V)spectral ratio of microtremor(MHV)curves;thus,a method of simulating the MHV curves is the key.In this study,we use the H/V spectral ratio of the surface wave(SHV)based on the surface wave propagation theory in a layered half-space to simulate the MHV curves at sites A and B of the Yuxi basin.Then,we attempt to analyze the features of the SHV curves.We find the H/V ratio of the microtremor loading source to be independent of the peak frequency of the SHV curve,but it has some relation to the amplitude of the SHV curve.Moreover,to reduce the error in subsurface velocity structures obtained by the MHV curves,we suggest that the SHV curves at near-peak frequencies should not be considered in the inversion because the amplitude deviation is higher at the peak frequency of the MHV curve.In addition,the best frequency ranges for the inversion of the microtremor H/V spectrum are between the peak and trough frequencies of the microtremor H/V spectrum.In this paper,an improved model updating method is proposed in order to avoid the dependence on the seismic waveform data and the simulation of the microtremors H/V spectra.We apply the theory that the H/Vspectral shape of the fundamental-mode Rayleigh,including its peak and trough periods,is similar to that of the microtremors to update the 3-D velocity structure model.When the depth of each sedimentary layer in the velocity structure under a single microtremors measuring point is increased by about 15 m,the H/V spectral peak and trough periods of the fundamental-mode Rayleigh will increase about 0.1 s respectively.Moreover,the H/Vspectral peak and trough periods of the fundamental-mode Rayleigh are controlled by deep and shallow velocity structures of the sedimentary layers respectively.Because of the convenience obtainment of the microtremors data,the model updating method has extensive applicability.Simultaneously,the 3-D velocity structure model of the Yuxi basin modified by the model updating method has been confirmed a high accuracy after inspection. |
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