The difference between arrival times from one seismic event to a pair of receivers is largely insensitive to source location error when certain geometric conditions are met. Using catalog data from a Chinese regional network, we illustrate the use of arrival time differences to control data quality and to derive a two-dimensional map of Pn velocity and relative site terms. The resulting velocity patterns follow regional geology closely, site terms reflect variations in crustal thickness, and both are consistent with previous work based on single ray methods. The model fits the time difference data to better than 1 s and we obtain a variance reduction of 75%. We believe that the continued development of time-difference techniques will lead to improved location accuracy and precision using regional network data.