High-resolution spatial elastic wave velocity variation detected using multiple-linear-array microtremor measurements

We propose a method for capturing the near-surface elastic wave velocity variation using microtremors. The proposed method does not directly estimate the velocity of the medium; instead, it measures the spatial velocity change. We apply the stretching method, which is often used to detect the time variation of shear wave velocity based on the monitoring of seismic ambient noise, to microtremor recordings. Because spatially dense ambient noise records are unavailable, we performed microtremor measurements using multiple linear arrays with multiple sensors along the survey line to collect as many recordings as possible. The synchronised microtremor recordings of each array were used to compute the horizontal-to-vertical spectral ratio (H/V), cross-correlation (CC), autocorrelation (AC), and single-station cross-correlation (SC). The H/V ratio and the waveforms of these correlation functions showed significant spatial variation. The velocity variation was measured quantitatively using the stretching method, which correlates a compressed and stretched reference function with each function on the line. The velocity variation of the vertical and radial components of CC compares well with the variation of the H/V peak, indicating that the radial components of CC retrieved the Rayleigh-wave Green's function. The waveforms of AC and SC also showed spatial variation but the measured value of velocity change was smaller than that for CC. Further investigation is awaited to conclude if this is due to low-quality data or inappropriate measurement conditions. The reliability of the velocity change was confirmed by comparing the results to dispersion curves obtained using the conventional array method.