Longitudinally averaged forward travel-time perturbations, farther averaged over the separation distances 6°–18° and over the time period from January 2011 to December 2014, as a function of latitude. They have been binned every 3.6° in latitude. The typical standard error of the mean, computed in each binning interval, is about 0.003 s, and is not shown here. The vertical gray shaded areas show the mean active latitudes.