Understanding the Basics
* P-waves (Primary waves): These are compressional waves, meaning they push and pull the rock they travel through. They are the fastest type of seismic wave.
* S-waves (Secondary waves): These are shear waves, meaning they shake the rock they travel through perpendicular to the direction of travel. They are slower than P-waves.
The Time Difference
* The key: P-waves travel faster than S-waves, so they reach a seismograph station first. The difference in arrival times between the P-wave and the S-wave (the "S-P interval") is directly related to the distance between the seismograph and the earthquake's epicenter.
* Greater distance, greater time difference: The further away an earthquake is, the longer the time difference between the P-wave and S-wave arrivals at the seismograph.
Determining Distance
1. Record the S-P interval: Seismographs record the arrival times of both P-waves and S-waves. By subtracting the P-wave arrival time from the S-wave arrival time, you get the S-P interval.
2. Use a travel-time graph: Scientists have created travel-time graphs or charts that show the relationship between S-P intervals and distances. These graphs are based on the known speeds of P-waves and S-waves through the Earth's crust.
3. Find the corresponding distance: By locating the S-P interval on the travel-time graph, you can determine the distance to the earthquake's epicenter.
Important Note:
* To pinpoint the exact location of the epicenter, data from at least three seismograph stations is required. This is because a single S-P interval only provides a distance, not a specific location. By triangulating the distances from multiple stations, the epicenter's coordinates can be determined.
