Accounting for lateral variations of the upper mantle gradient in Pn tomography studies

The effect of an upper mantle velocity gradient on regional arrival times has been approximated by a cubic distance term, which can be extended to two dimensions for use in tomographic studies. To demonstrate this, we add a laterally varying upper mantle gradient to the standard Pn time‐term tomography technique, and apply to a data set from Asia compiled using ground truth, event location criteria. We observe strong lateral variations in the gradient, ranging from −0.001 to 0.003 s−1, with high gradients associated with the Tethys convergence zone. The gradient patterns may reflect lateral variations in the thermal gradient of the mantle lid. Variance reduction is 63% with respect to Pn tomography without gradients. Adding gradients allows the use of longer path lengths, improving velocity image definition in high‐gradient regions with sparse station distribution, such as Tibet.