Main Concept

Seismology is the study of earthquakes, their effects, and seismic waves. The location of an earthquake's epicenter (point on the earth's surface directly above the location of rupture or faulting) can be determined using information about two of these seismic waves.

The first type of wave to consider when determining the epicenter of an earthquake is the P-wave. These waves are also referred to as primary or compressional waves. Important characteristics of P-waves are:

The second type of wave to consider when determining the epicenter of an earthquake is the S-wave. These waves are also referred to as secondary or shear waves. S-waves can also be characterized by some unique properties:

The following animation helps to understand the motion of each type of wave.

A seismogram is the graph output from a seismograph, which is used to determine the epicenter of an earthquake. When consulting the seismogram, P-waves always appear before S-waves, as they travel faster and can travel through three states of matter as opposed to one.

To determine the distance of an earthquake epicenter:

	Earthquake Time Travel Graph

Example

Adjust the sliders to change the arrival times of the P-wave and the S-wave.

Suppose the P-wave arrives at t = 1.0 min and the S-wave arrives at t = 6.0 min. Determine the distance of the earthquake epicenter given the following information.

Solution

To determine the distance of the earthquake epicenter, you can use the steps in the previous section.   1. Determine the arrival times of the  P-wave and the S-wave:   Arrival time of P-wave: 1.0 min Arrival time of S-wave: 6.0 min   2. Calculate the difference between the arrival time of the P-wave and the S-wave.   Time Difference = $6.0 -1.0\=5.0 \min$   3. Refer to the Earthquake Time Travel Graph. Determine the location on the graph where the two curves have a time difference equal to the time difference you previously calculated.   After looking at the Earthquake Time Travel Graph, it is clear that the two curves have a difference of 5 units on the time axis at x = 3.4. Therefore the earthquake has an epicenter distance of $3.4 \cdot {10}^3$ km, or 3,400 km.

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