Testing the reliability of Vp/ Vs anomalies in traveltime tomography

SUMMARY In traveltime tomography, V,/V, models calculated as ratio between the resolved P and S models are affected by the different ray coverages and phase-onset reading accuracies of P and S waves. The result of this is that the computed V,/V, models can display large fluctuations which are unrelated to the true structure. Introduction of some P-S coupling (e.g. proportionality, correlation) as a priori data permits us to stabilize the resulting V,/V, model about a preselected average value and to minimize the insurgence of fictitious V,/V, anomalies. In general, addition of P-S coupling trades off with data misfit and adoption of this technique is suggested for robustness tests of resolved V,/V, features. The technique is applied to a synthetic data set generated using an idealized fault structure and the source-receiver geometry existing at Parkfield, California.