微动H/V谱比方法

A review on microtremor H/V spectral ratio method

微动H/V谱比,即地表记录的不同频率地震背景噪声的水平分量与垂直分量的比值.在工程地震领域,通常用V表示微动记录的垂直分量,用H表示微动记录的水平分量,测得作为频率函数的H/V谱比曲线后,依据一定的关系(通常是经验的),建立H/V谱比曲线的峰值与地层结构基阶共振频率之间的关系,从而估计沉积层厚度或场地放大因子,有时也称为HVSR(Horizontal-to-Vertical Spectral Ratio)或QTS(Quasi Transfer Spectrum)方法.由于微动中波型成分的物理来源模糊不清,其主导能量究竟是Rayleigh波、S波或者其它波型成分存在争议,因此,虽然在工程地震领域获得了广泛应用,微动H/V谱比法仍然缺乏严格的理论解释.这导致该方法趋于两个方向发展:一是从地震记录中,识别出Rayleigh波能量,计算Rayleigh波的ZH幅度比,又称Rayleigh波椭率(ellipticity).之所以称为Rayleigh波ZH幅度比,是因为在地震层析成像领域,V常用来表示Rayleigh波水平分量的特征函数,多用Z表示Rayleigh波的垂直分量.作为独立变量,Rayleigh波ZH幅度比对浅层速度结构更为敏感,在区域尺度地震层析成像领域获得广泛应用,用于弥补单独相(群)速度对浅层结构,尤其是沉积层结构约束不够的缺点.这种方法意味着H/V谱比曲线中的主要能量是Rayleigh波,除了在区域尺度与Rayleigh波的频散和(或)接收函数联合反演地球结构之外,在工程物探领域,也利用Rayleigh波椭率反演近地表S波速度结构.基于H/V谱比曲线的峰值推断场地响应的理论假设是SH波占据微动的主导能量,这与微动观测记录通常由Rayleigh波能量占据主导地位的情况不符,因此H/V谱比法的另一个研究方向是发展不同的背景噪声源模型,考虑可能贡献的背景噪声能量,解释H/V谱比曲线.这样就避免了微动记录的主导成分是面波还是体波的争论,发展更适合或接近实际记录的微动模型解释H/V谱比曲线,该方向的发展是伴随地震干涉理论的发展而逐步发展起来的.我们曾经对区域尺度的(地震事件或背景噪声)Rayleigh波ZH幅度比的研究和应用进行了评述.本文主要评述微动H/V谱比法在工程地震领域和近地表S波速度结构反演中的应用及相应的理论解释.包括基于SH波共振频率解释的微动H/V谱比法估计场地特征,基于Rayleigh波占据微动主导能量的Rayleigh波椭率在反演近地表速度结构中的应用,以及为解释实际微动H/V谱比曲线而发展的背景噪声源模型.

The microtremor H/V spectral ratio is the ratio of the horizontal component to the vertical component of the ambient seismic noise at different frequencies,recorded at the earth surface.In the field of engineering earthquake,V and H are usually used to denote the vertical and horizontal components of the microtremor,respectively.Based on the measured H/V curve,which is a function of frequency,the sedimentary thickness or site amplification factor can be estimated using the established relationship between the peak of the H/V curve and the fundamental resonance frequency of the formation structure.Although it has been widely used in the field of engineering earthquake,the theoretical explanation on the microtremor H/V method,which is also called HVSR(Horizontal-to-Vertical Spectral Ratio)or QTS(Quasi-Transfer Spectrum),is still under debate,since which is the dominant wave type of the microtremor(Rayleigh wave,S wave or the other types of waves)is unclear.This leads the micro-tremor H/V method to be developed in two directions.One is to identify the Rayleigh wave energy from the micro-tremor and calculate the ZH amplitude ratio of the Rayleigh wave(i.e.Rayleigh wave ellipticity),where Z repre-sents the vertical component of the Rayleigh waveand V represents the eigenvalue of the horizontal component.As an independent variable,the Rayleigh wave ZH ratio is sensitive to shallow structure and it has been widely used to constrain the shallow structure combined with the phase or/and group velocity in regional-scale seismic tomo-graphy.This process assumes that the dominant energy in the H/V curve is from Rayleigh wave.Apart from being used in the joint inversion of the large scale earth structure with the Rayleigh wave dispersion and/or receiver func-tion,Rayleigh wave ellipticity is also used to invert the near-surface S-wave velocity structure in engineering geo-physical prospecting on a small scale.The peak of the H/V curve can be used to figure out the site characteristics under the assumption of that SH wave dominates the microtremor.This assumption is in consistent with the obser-vation that the Rayleigh wave energy is usually dominated.Another direction of the microtremor H/V method de-velopment is to develop noise source model,with which all possible contributions of noise energy are considered.The H/V curve is then interpreted based on this model.The controversy on which type of wave dominates the mi-crotremor is avoided.The model has been proved to be suitable for explaining the real microtremor record and the shape of H/V curve.This direction is developed with the develovement of seismic interference theory.We have re-viewed the theory and application of Rayleigh wave ellipticity on the regional scale.This paper mainly reviews the theory and application of microtremor H/V method in the field of engineering earthquake and near-surface S-wave velocity structure inversions,including the theoretical interpretation of H/V curve based on SH wave resonance fre-quency,the application of microtremor H/V method in site estimation,and the noise source model developed to ex-plain the H/V curve.