1. Understanding Seismic Waves
* P-waves (Primary Waves): These are compressional waves that travel fastest through the Earth. They cause a push-pull motion.
* S-waves (Secondary Waves): These are shear waves that travel slower than P-waves. They cause a side-to-side motion.
2. The Principle of Time Difference
* Time Lag: The key is that P-waves arrive at a seismic station before S-waves. The time difference between their arrivals (known as the "S-P interval") is directly related to the distance the waves have traveled.
3. The Travel-Time Curve
* Standard Curves: Geophysicists have developed standard travel-time curves that plot the relationship between S-P interval and distance for various depths within the Earth. These curves are based on our understanding of how seismic waves propagate through different layers of the Earth.
4. Using the Travel-Time Curve
* Measuring the S-P Interval: You measure the time difference between the arrival of the P-wave and the arrival of the S-wave on a seismogram.
* Finding the Distance: You then locate the measured S-P interval on the travel-time curve. The corresponding distance on the curve is the distance the waves have traveled from the earthquake's epicenter.
5. Example:
1. Imagine you record an earthquake on a seismograph.
2. The P-wave arrives at 10:00:00 AM, and the S-wave arrives at 10:00:15 AM. The S-P interval is 15 seconds.
3. You find the S-P interval of 15 seconds on the travel-time curve. Let's say it corresponds to a distance of 150 km.
4. Therefore, the earthquake's epicenter was approximately 150 km away from your seismic station.
Important Note:
* Multiple Seismic Stations: To pinpoint the earthquake's epicenter accurately, you need data from multiple seismic stations. By plotting circles on a map with radii equal to the distances calculated from each station, the point where the circles intersect is the earthquake's epicenter.
* Earth's Structure: The travel-time curves are based on assumptions about the Earth's structure. If there are significant variations in the Earth's layers beneath your station, the distance estimation might be slightly off.
