Changes in Vp/Vs with depth: Implications for appropriate velocity models, improved earthquake locations, and material properties of the upper crust

Properties associated with the arrival times of shallow microearthquakes are determined for three tectonically different sites: Soviet Central Asia; the Central United States; and southern California. Simple graphical presentations of the earthquake data, including Wadati and Riznichenko diagrams, are used to resolve the trade-off in origin time and focal depth. Estimates of the average half-space velocities for P and S waves, and of the travel-time ratio (t_s/t_p) are also obtained that are relatively free of any model constraints used to initially locate the earthquake. In all cases, P and S arrivals exhibit a systematic decrease in t_s/t_p with depth in the upper few kilometers. This decrease is significant and matches similar variations observed in situ for the velocity ratio (V_p/V_s). Based on laboratory and theoretical studies, such observations are consistent with the closing of saturated microcracks with increasing confining pressure. Independent determinations of velocity structure and rock properties from both refraction studies and borehole experiments show analogous results. By specifying separate velocity models for both P and S waves that reflect this variation, rather than assuming a constant value of V_p/V_s, we find improved stability in hypocenter determination and increased resolution of shallow seismicity patterns.