A review on microtremor H/V spectral ratio method

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 microtremor H/V method to be developed in two directions. One is to identify the Rayleigh wave energy from the microtremor and calculate the ZH amplitude ratio of the Rayleigh wave (i.e.Rayleigh wave ellipticity), where Z represents 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 tomography. 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 function, Rayleigh wave ellipticity is also used to invert the near-surface S-wave velocity structure in engineering geophysical 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 observation that the Rayleigh wave energy is usually dominated. Another direction of the microtremor H/V method development 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 microtremor 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 reviewed 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 frequency, the application of microtremor H/V method in site estimation, and the noise source model developed to explain the H/V curve.