The system for locating earthquake epicenters was developed by using seismograms from earthquakes whose epicenters could be easily pinpointed from physical evidence. From these seismograms, travel-time graphs were constructed (Figure 8.11) Using the sample seismogram for Nagpur, India, in Figure 8.10A and the travel-time curve in Figure 8.11, we can determine the distance separating the recording station from the earthquake in two steps: (1) Using the seismogram, determine the time interval between the arrival of the first P wave and the arrival of the first S wave, and (2) using the travel-time graph, find the P-S interval on the vertical axis and use that information to deter- mine the distance to the epicenter on the horizontal axis. Following this procedure, we can determine that the earthquake occurred 3,400 kilometers (2,100 miles) from the recording instrument in Nagpur, India.