During an earthquake, seismic waves are sent all over the globe. Though they may weaken with distance, seismographs are sensitive enough to still detect these waves. In order to determine the location of an earthquake epicenter, seismographs from at least three different places are needed for a particular event. In Figure 13.9, there is an example seismogram from a station that includes a minor earthquake.
Once three seismographs have been located, find the time interval between the arrival of the P-wave and the arrival of the S-wave. First, determine the P-wave arrival, and read down to the bottom of the seismogram to note at what time (usually marked in seconds) that the P-wave arrived. Then do the same for the S-wave. The arrival of seismic waves will be recognized by an increase in amplitude – look for a pattern change as lines get taller and more closely spaced (ex. Figure 13.10).
By looking at the time between the arrivals of the P- and S-waves, one can determine the distance to the earthquake from that station, with longer time intervals indicating longer distance. These distances are determined using a travel-time curve, which is a graph of Pand S-wave arrival times (see Figure 13.11).
Though the distance to the epicenter can be determined using a travel-time graph, the direction cannot be told. A circle with a radius of the distance to the quake can be drawn. The earthquake occurred somewhere along that circle. Triangulation is required to determine exactly where it happened. Three seismographs are needed. A circle is drawn from each of the three different seismograph locations, where the radius of each circle is equal to the distance from that station to the epicenter. The spot where those three circles intersect is the epicenter (Figure 13.12).