Despite the lack of a robust theoretical foundation, the horizontal-to-vertical spectral ratio (HVSR) method of microtremor Fourier amplitudes has become a widely used tool for estimating site parameters and analyzing seismic site effects. This study refines the microtremor HVSR approach and introduces a novel empirical method for estimating the surface/borehole acceleration spectral ratio (SBSRa) under ground motions. The proposed method facilitates the estimation of ground motion amplification factors using in situ microtremor observations, offering critical data for seismic site effect research. Drawing on 296 958 strong motion acceleration records from 699 stations in Japan's KiK-net network, stable microtremor HVSRs were extracted from pre-event microtremor data (prior to P-wave arrivals). Statistical analysis revealed a strong correlation between microtremor HVSR and SBSRa under weak ground motions, with their spectral shapes and predominant frequencies showing high consistency. Using a frequency normalization approach, empirical correction coefficients were derived for various types of microtremor HVSR, leading to the development of a practical method for predicting SBSRa based on microtremor HVSR. Reliability analyses demonstrated that the predicted SBSRa achieves engineering-level accuracy, with Jaccard similarity coefficients exceeding 0.58 for more than 75% of the stations. These results validate the effectiveness of microtremor HVSR as a reliable proxy for approximating SBSRa under weak ground motions.