Inversion of the HZ ratio of microseisms for S-wave velocity in the crust

S-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 SC EC CVM. The proposed approach has promise for applications to other regions in the world because microscisms are observed everywhere in the world. The method may become even more powerful if it is combined with phase velocity data.