Inversion of the HZ ratio of microseisms forS-wave velocity in the crust

Abstract

S U M M A R YS-wave velocity in the shallow crust is an important controlling parameter for ground motion amplification. It is a key parameter for prediction of ground motion and thus for earthquake hazard mitigation in general. Shallow S-wave velocity structure has often been obtained from phase velocity of microtremors and microseisms. We present a new approach which directly inverts the ratio of horizontal to vertical amplitudes of microseisms which is referred to as the HZ ratio in this paper. Our approach consists of (i) isolation of Rayleigh-wave dominant portions in seismograms by using the 90-degree phase shift between horizontal and vertical component, (ii) measurement of the HZ ratios as a function of frequency and (iii) iterative inversion for S-wave velocity structure in the crust. Depth sensitivity kernels of the HZ ratios for density, P-wave velocity and S-wave velocity are derived by a numerical method. Examples of the application of this method to data in southern California show that the HZ ratio data require modification of the standard seismic velocity model in this region (Southern California Earthquake Center Community Velocity Model 3.0 or SCEC CVM) in the upper 10-15 km. The derived models show amplification of incident SH waves by factors of 2-8, relative to the prediction by the SCEC CVM. The proposed approach has promise for applications to other regions in the world because microseisms are observed everywhere in the world. The method may become even more powerful if it is combined with phase velocity data.

Fundamental and higher‐mode Rayleigh wave characteristics of ambient seismic noise in New Zealand

“…Two distinctive dispersive waves, namely fundamental and higher-mode Rayleigh waves, are observed in the beamformed data. This result differs from those of other studies of double-wave-frequency microseisms, which have shown the noise field to be dominated by a single mode, namely fundamental mode Rayleigh waves [Lacoss et al, 1969;Tanimoto and Alvizuri, 2006]. The dominant source regions of the two signals we observe are interpreted with reference to New Zealand's oceanographic conditions.…”

Application of Surface-Wave Methods for Seismic Site Characterization

“…Recently, Tanimoto and Alvizuri (2006) and Hobiger et al (2009) proposed to extract the ellipticity of Rayleigh waves from NHV curves by a random decrement technique. This might in turn be used as input for the inversion analyses to estimate the S-wave velocity profiles.…”

Stepwise joint inversion of surface wave dispersion, Rayleigh wave ZH ratio, and receiver function data for 1D crustal shear wave velocity structure

“…The subscripts sw, zh, and rf in all equations represent dispersion, ZH ratio, and RF data, respectively; r sw ; r zh ; r rf are the corresponding residual data vectors for different datasets; G sw ; G zh ; G rf are the corresponding partial derivative matrices, which can be calculated numerically through difference algorithms (e.g., Julià et al 2000;Tanimoto and Alvizuri 2006;Julià 2007). Instead of inverting three datasets at the same time, we propose a stepwise joint inversion strategy for better taking advantages of their complementary sensitivities to the v S structure.…”

“…Joint surface wave tomography with both earthquake surface waves and ambient noise cross-correlation can successfully characterize crustal and upper mantle structure (e.g., Yao et al 2008Yao et al , 2010. Studies have also shown that the vertical and horizontal components of fundamental mode Rayleigh waves have a 90°phase shift, and their amplitude ratio (ZH ratio or ellipticity) is frequency dependent and only controlled by the structure beneath the recorded seismic station (Tanimoto and Alvizuri 2006;Tanimoto and Rivera 2008) under the 1D layered model assumption. Rayleigh wave ZH ratio has higher sensitivity to shallower structures compared to Rayleigh wave dispersion at the same period .…”

“…Recent studies (Chong et al 2015(Chong et al , 2016) also indicate that Rayleigh wave ZH ratio is sensitive to the smooth velocity variation (long wavelength velocity contrast) in the depth range that it samples. Therefore, Rayleigh wave ZH ratio can also serve as another useful measurement for wave speed structure inversion (Tanimoto and Alvizuri 2006;Tanimoto and Rivera 2008;Yano et al 2009;Tanimoto et al 2013). Lin et al (2012Lin et al ( , 2014 successfully performed joint inversion of Rayleigh wave dispersion and ellipticity using a linearized inversion algorithm for the crustal and upper mantle shear wave velocity (v S ) structure of the Western United States.…”