Improved In Situ VP/VS Estimation with Applications to Dense Seismic Arrays

Abstract The compressional-to-shear-wave velocity ratio (VP/VS) provides critical insights into rock composition, fluid saturation, porosity, and fault-zone properties. High-resolution imaging is facilitated by in situ VP/VS estimation, which leverages closely located earthquake clusters. However, conventional in situ VP/VS estimation methods, which rely on demeaning differential travel times to mitigate origin time errors, tend to cluster data points near zero, weakening regression constraints and increasing estimation uncertainty. Here, we present an improved method through a two-step grid search that corrects origin time differences while preserving the true distribution of travel-time differences and simultaneously estimates VP/VS. Synthetic tests demonstrate that this approach enhances sensitivity to VP/VS and reduces uncertainty compared with conventional methods. We further validate our method by applying it to three event clusters from the 2021 Ms 6.4 Yangbi earthquake sequence, recorded by a dense seismic array, confirming the method’s robustness under realistic observational conditions. The results reveal spatially variable VP/VS along the fault zone. Our method, optimized for dense array observations, has potential applications in fault zone imaging, earthquake source studies, and seismic hazard assessments.