Overview
This lecture continues the topic of epicenter triangulation, focusing on problem-solving, calculations, and understanding its practical applications and limitations in earthquake detection.
Epicenter Triangulation Problem-Solving
- Three seismic stations (CMB, DOG, PIS) in North America detected an earthquake at 6:00 AM.
- P-wave and S-wave arrivals are recorded in hour, minute, and second format; conversion to seconds is required for calculations.
- CMB's P-wave: 0 sec, S-wave: 28 sec; DOG's P-wave: 5 sec, S-wave: 1 min 29 sec (89 sec); PIS's P-wave: 2 sec, S-wave: 1 min 6 sec (66 sec).
- Lag time is calculated using: Lag time = S-wave arrival time − P-wave arrival time.
- Lag times: CMB = 28 sec, DOG = 84 sec, PIS = 64 sec.
- Distance from epicenter is found by multiplying lag time by 12.5 km/sec.
- Distances: CMB = 350 km, DOG = 1050 km, PIS = 800 km.
- For map work, convert actual distance to scaled distance using 100 km/cm.
- Scaled radii: CMB = 3.5 cm, DOG = 10.5 cm, PIS = 8 cm.
Map Triangulation Procedure
- Draw circles on the map using the scaled radii from each station.
- The intersection of the three circles marks the earthquake's epicenter.
Analysis and Conceptual Questions
- The nearest station (CMB) detects seismic waves first and is most affected by the earthquake.
- The farthest station (DOG) detects seismic waves last and is least affected.
- Epicenter triangulation is not 100% accurate due to manual compass use and inherent limitations.
- Accuracy is sufficient for disaster risk reduction, early warning, and evacuation despite minor errors.
Cause-and-Effect Relationships
- The nearer a station is to the epicenter, the earlier it receives seismic waves and the greater its chance of being affected.
- The farther a station is from the epicenter, the later it receives seismic waves and the less it is affected.
Key Terms & Definitions
- Epicenter — the point on Earth's surface directly above the earthquake origin.
- Seismic Station — a facility that detects and records seismic waves.
- P-Wave (Primary Wave) — the fastest seismic wave, detected first.
- S-Wave (Secondary Wave) — a slower seismic wave, detected after the P-wave.
- Lag Time — the time difference between S-wave and P-wave arrivals.
- Epicenter Triangulation — method using distances from three stations to locate an earthquake’s epicenter.
Action Items / Next Steps
- Prepare for Performance Task 1.2 (group) and Long Test 1.2 (pair) next week.
- Review the process of converting time, calculating lag time, and drawing circles for triangulation.