The shaky ground people call an earthquake is caused by the sudden release of energy, deep within the Earth’s crust, when titanic blocks of rock jostle against each other. This energy acts much like the sound of a gunshot or other loud noise: just as sound waves travel through the air to reach our ears, the energy released by a quake travels through solid ground in the form of waves. Seismologists, scientists who study the Earth’s interior, identify several different kinds of waves of energy caused when the rocks shift. The three most important types of these seismic waves are identified by the letters P, S and L; which stand for primary, secondary and long waves.
Seismic waves caused by an earthquake or other event such as a volcanic eruption or nuclear test all start at the same time and place. That means that for nearby observers the different types of seismic waves all arrive at more or less the same time. The different wave types travel through the Earth at different speeds, however; so as the distance to the site of the earthquake increases, they gradually become widely separated events even as they become weaker. P waves, the fastest, always arrive first; L waves, the slowest, arrive last; and S waves arrive somewhere in the middle.
Seismic waves can be detected even thousands of miles from their source by delicate electromechanical instruments called seismographs or seismometers; which record the tiny ground movements caused by passage of the seismic waves. The record of an earthquake is the familiar wiggly line on a roll of paper, or a seismogram. By studying the shape of a line or group of lines, scientists can identify the exact times at which the different seismic waves arrived at their location.
Over decades of study with thousands of earthquakes each year, seismologists have determined the speeds at which the different wave types travel. Armed with that knowledge and the difference in times between wave arrivals, a little bit of arithmetic allows a scientist (or more likely a computer program) to calculate the distance the seismic waves have traveled to reach the seismometer. This can also be done with a standard time-distance chart. The next step is to pull out a globe or map and draw a circle whose radius is the travel distance, with its center at the location of the seismograph. The epicenter of the earthquake – the point on the surface immediately above the focus, or center of the energy release – lies somewhere along that circle.
Unfortunately, seismographs cannot determine the direction to an earthquake. To solve this problem, seismologists in several locations share their distance data. Two circles drawn on the map, recording the distances from two different seismic stations, will intersect at two points; either of which may be the epicenter. Combine this map with the record from a third station, drawing a third circle, and the correct intersection can be identified. In reality, there many more than three seismic stations: there are thousands of seismographs in use around the world. Combined with modern near-instantaneous communication, this network allows seismologists to identify the epicenter of earthquakes large and small in a matter of minutes.