6.6: How Are Earthquakes Located?
How Are Earthquakes Located? Links to an external site.
During an earthquake, seismic waves are propagated throughout the entire Earth. Though they may weaken with distance, seismographs are sensitive enough to detect these waves. In order to determine the location of an earthquake epicenter, seismographs from at least three different locations are needed. Once three seismographs have been located, the S-P interval must be measured (Figure 6.13). To do this, you must first determine when the P-wave arrived at the seismograph station. On the seismogram, find the arrival of the P-wave, which can be recognized by an increase in amplitude (wave height), and note the time of P-wave arrival. Repeat this procedure for the S-wave, then subtract the arrival time of the P-wave from the arrival time of the S-wave. In general, longer time S-P intervals indicate further station distance from an epicenter.
Figure 6.13: The P-waves are the first to arrive at each station, followed by the S-waves. The P-waves travel faster than S-waves the great distance between the two (S-P interval), the further away the earthquake epicenter (CC-BY-SA 4.0; Benjamin J. Burger Links to an external site., modified by Chloe Branciforte).
Once you have calculated the S-P interval, you will now be able to determine the distance to an earthquake epicenter from a specific seismic station. This is typically done using a travel-time curve Links to an external site., which is a graph of P- and S-wave arrival times (Figure 6.14).
Figure 6.14: A travel-time graph that includes the arrival of P-waves and S-waves. Note that these curves plot distance versus time and are calculated based on the spherical shape of Earth. Curves vary with the depth of earthquake because waves behave differently with depth and change in material. This curve is used for shallow earthquakes (<20 km deep) with stations within 800 km. The S-P curve refers to the difference in time between the arrival of the P-wave and S-wave. (CC-BY-SA 3.0; Randa Harris Links to an external site.).
Though distance to the epicenter can be determined using a travel-time graph, direction cannot. To pinpoint the location of the epicenter, we can use distance information from three seismograph stations and triangulate. To do this, draw a circle around one of the seismograph locations with a radius of the distance to the earthquake. We know that the earthquake occurred somewhere along this circle, but a circle contains an infinite number of possible epicenters; this is why it is necessary to have data from at least three seismic stations. Draw a circle around at least two more 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 6.15).
Figure 6.15: In order to locate this earthquake’s epicenter, seismograms from Portland, Salt Lake City, and Los Angeles were used. The time between P and S wave arrivals was calculated, and travel timetables gave a distance. Circles with each distance for their radii were drawn from each station. The one resulting overlap, at San Francisco, was the earthquake epicenter. (CC-BY-SA 3.0; Randa Harris Links to an external site.).