Techniques to identify microtremor wave contributions and impact to seismic site characterization

Abstract

We investigate whether varying wavefield contributions are the likely cause to variation in microtremor horizontal-to-vertical spectral ratio (MHVSR) amplification shape between six sites in the relatively homogeneous geologic setting of Windsor, Ontario. We quantify the MHVSR shape in terms of peak broadness and its fitness using mathematical functions to identify potential wave type contributions. We develop a technique that uses particle motion plots of cross-correlated microtremor recordings to establish the dominant wave types contributing to the microtremor wavefield within three important frequency bandwidths (below, spanning, and above the fundamental peak frequency). We investigate the variability in the inverted shear-wave velocity (V_S) depth profile by performing 21 MHVSR inversions with varying Rayleigh, Love, and body wave contributions. The impact to seismic site characterization is that the depth and V_S of the resonator (half-space) layer are overestimated consistently by an average of 28% compared to the often-default-assumed Rayleigh ellipticity forward amplification model. Our study demonstrates the importance of correctly identifying wave type contributions of the microtremor wavefield for the proper estimation of V_S depth profiles, especially to obtain correct thickness of the sediment layer and resonator V_S and thereby the average V_S of the upper 30 m (V_S30).