Overview
This lecture covers how to calculate and locate earthquake epicenters using the triangulation method, as well as the global distribution of earthquakes and its relation to plate tectonics.
Earthquakes and Seismic Waves
- Earthquakes are caused by rocks breaking under stress, releasing energy as seismic waves.
- The focus is the point underground where an earthquake originates; the epicenter is directly above the focus on the surface.
- Three types of seismic waves: P-waves (fastest, compression), S-waves (side-to-side, slower), and surface waves (most destructive, travel along the surface).
Seismograms and Lag Time Calculation
- Seismic waves are detected by a seismometer, producing a seismogram.
- The difference between the arrival times of S-waves and P-waves is called lag time.
- Lag time = S-wave arrival time − P-wave arrival time (e.g., 153s − 18s = 135s).
Determining Epicenter Distance
- Distance from station to epicenter is calculated using:
d = (lag time in seconds / 8) × 100 km
- Example: Lag time of 135 seconds gives a distance of 1,687.5 km.
Triangulation Method for Locating Epicenter
- Use lag times from three seismic stations to calculate each distance.
- Draw circles on a map for each station with the radius equal to the calculated distance.
- The intersection point of the three circles marks the epicenter.
Global Distribution of Earthquakes
- Earthquakes are mostly found along plate boundaries, supporting plate tectonic theory.
- Some earthquakes (intraplate earthquakes) occur inside plates due to internal stresses, not just at plate boundaries.
- Predicting the time and location of earthquakes remains impossible with current technology.
Key Terms & Definitions
- Epicenter — Point on Earth's surface directly above where an earthquake starts.
- Focus — Underground point where an earthquake originates.
- Seismogram — The visual record of seismic waves detected by a seismometer.
- Lag Time — The time difference between the arrival of the S-wave and the P-wave.
- Triangulation Method — Using distances from three stations to locate earthquake epicenter.
Action Items / Next Steps
- Complete Activity 1 in your module: compute lag times and epicenter distances for the given seismic station data.
- Review the example problems on epicenter triangulation in your module.