Direct estimation of VS30 using spatial autocorrelation and centreless circular array coefficient curves obtained from microtremor array data

Abstract

The average S-wave velocity (VS) in the upper 30 m (VS30) is a proxy for seismic wave amplification. Microtremor array exploration is one of the available methods for site characterization, but the recorded data require complicated processing that can lead to different estimations of VS30 depending on the analyst and processing software. We propose a method of estimating VS30 by using derivatives obtained in the early stages of microtremor array data processing. Statistical analysis with 2376 virtually generated subsurface VS structure models revealed that the frequencies at which the spatial-autocorrelation (SPAC) coefficients and centreless circular array (CCA) coefficients take specific values strongly correlate with VS30, which we used to develop formulas for estimating VS30. Numerical validations using actual VS profiles at 616 sites in Japan showed that the proposed method could estimate VS30 with a root-mean-square deviation (RMSD) of 57–80 m/s with SPAC coefficients and 56m/s with CCA coefficients. Our proposed methods were applicable for 98–100 per cent of theVS profiles when we limited our estimation to sites with VS30 < 760 m/s. The results indicated that SPAC coefficients from arrays with radii of 8–20 m can be used for VS30 estimation and are less affected by incoherent noise. In contrast, CCA coefficients are much more sensitive to incoherent noise, which resulted in the overestimation of VS30. The estimated VS30 values from the recorded microtremor array data were in good agreement with the reference values from the actual VS profiles. The proposed method allows for robust and efficient VS30 estimation without relying on the analyst’s skills or software.