Improved in-situ Vp/Vs Estimation Using Dense Seismic Array with Application to the 2021 Ms 6.4 Yangbi Yunnan Aftershock Sequence

The ratio of compressional to shear wave velocity (Vp/Vs) provides critical insights into rock composition, fluid saturation, porosity, and fault characteristics. However, resolving fine-scale Vp/Vs structures is challenging with conventional tomographic imaging techniques due to uneven ray path coverage and regularization constraints in inversion. Alternatively, high-resolution Vp/Vs imaging can be achieved through in-situ Vp/Vs ratio estimation by analyzing differential P-wave and S-wave arrival times (Δt_p,Δt_s) from closely located earthquake clusters, effectively circumventing the limitations of traditional tomography. Conventional in-situ Vp/Vs estimation method mitigates origin time difference errors by subtracting the mean Δt_p and Δt_s value for each event pair. However, this approach clusters data points near the coordinate origin, increasing estimation uncertainty. In this study, we present an improved in-situ Vp/Vs ratio estimation technique that corrects the origin time difference errors for each event pair within a cluster using spatially dense seismic arrays. This correction ensures that P-wave and S-wave time differences precisely represent the actual travel time differences, thereby enhancing the reliability of Vp/Vs estimation. Synthetic tests confirm the robustness of the method and its ability to reduce uncertainty. Applying the improved method to the 2021 Ms 6.4 Yangbi Yunnan aftershock sequence, we identify significant spatial variation in Vp/Vs ratio along the fault zone, with an increase in Vp/Vs at greater depth and a strong correlation with fault structures and their geometries (figure 1). We further investigate how mineral composition, stress state, and fluid content influence Vp/Vs in the Yangbi region. Our method, suitable for dense array observations, demonstrates strong potential for application in other seismically active regions.       

Figure1. Spatial Variations and uncertainties of in-situ Vp/Vs in the 2021 Ms 6.4 Yangbi Yunnan aftershock sequence. The red star is Yangbi earthquake. (a)In-situ Vp/Vs ratio of the Yanbi aftershock sequence. The black line indicates the fault trace, where F1 represents the Weixi-Qiaohou-Weishan Fault. (b)Uncertainties of the Yanbi aftershock sequence. The blue line represents the fault surface, and the gray triangles denote seismic stations. (c)Vp/Vs variation with depth.