Differential travel time vs epicentral distance for three focal depths starting at an epicentral distance for which the time differential is 0.1 s. The differential travel time is defined as the flat-earth travel time minus the spherical-earth travel time for the P-wave based on the iasp91 velocity model. The curves are labeled by the focal depths in kilometers, For other crustal focal depths the curves are almost identical to that shown for a surface focus, as it is only in the head wave (flat earth) and Pn (spherical earth) parts of the ray path that the differences become non-negligible. For focal depths in the mantle, the differential travel times increase with focal depth. Click here for file listing the differential travel times for this plot.