When seismologists analyze an earthquake, they first determine its epicenter, the point on Earth’s surface directly above the hypocenter, or focus ▼. One method used for locating an earthquake’s epicenter relies on the fact that P waves travel faster than S waves.

The traveling waves are analogous to two racing automobiles, one faster than the other. The first P wave, like the faster automobile, always wins the race, arriving ahead of the first S wave. The greater the length of the race, the greater the difference in their arrival times at the finish line (the seismic station). Therefore, the longer the interval between the arrival of the first P wave and the arrival of the first S wave, the greater the distance to the epicenter. The figure below ▼ shows three simplified seismograms for the same earthquake. Based on the P-S interval, which city—New York, Nome, or Mexico City—is farthest from the epicenter?

The system for locating earthquake epicenters was developed by using seismograms from earthquakes whose epicenters could be easily pinpointed based on physical evidence. From these seismograms, travel–time graphs were constructed ▼. Using the sample seismogram for New York in ▲ and the travel–time curve in the figure below ▼ , we can determine the distance separating the recording station from the earthquake in three steps:

1. Using the seismogram for New York, we determine that the time interval between the arrival of the first P wave and the arrival of the first S wave is 5 minutes.

2. Using the travel–time graph, we find the location where the vertical separation between P and S curves is equal to the P-S time interval (5 minutes in this example).

3. From the position in step 2, we draw a vertical line to the horizontal axis and read the distance to the epicenter.

Using these steps, we determine that the earthquake occurred 3700 kilometers (2300 miles) from the recording instrument in New York City.

Now we know the distance, but what about direction? The epicenter could be in any direction from the seismic station. Using a method called triangulation, we can determine the location of an epicenter if we know the distance to it from two or more additional seismic stations ▼. On a map or globe, we draw a circle around each seismic station with a radius equal to the distance from that station to the epicenter. The point where the three circles intersect is the approximate epicenter of the quake.

Modern seismographs are placed in a variety of locations, for both short-term and long-term monitoring of near and far earthquakes. For example, the Global Seismographic Network (GSN), consisting of 152 stations worldwide, produces near real-time seismic data freely available to anyone. These networks, along with computers and modeling, allow quick estimations of earthquake location, time, and duration, as well as characteristics of the fault slippage.

• The distance separating a recording station from an earthquake’s epicenter can be determined by using the difference in arrival times between P and S waves. When the distances are known from three or more seismic stations, the epicenter can be located using a method called triangulation.

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