Overview
This lecture explains how to determine the epicenter of an earthquake using data from three seismic stations by calculating lag times, distances, and plotting with a scaled map through triangulation.
Epicenter Location Using Seismic Data
- Seismic stations detect body waves (P waves and S waves), which arrive before surface waves.
- Arrival times for P and S waves are recorded at Cebu, Palawan, and Batangas.
- Lag time = S wave arrival time − P wave arrival time.
- Cebu: 96 s, Palawan: 60 s, Batangas: 40 s.
- Distance from epicenter = lag time × 12.5 km/s (given constant).
- Cebu: 1200 km, Palawan: 750 km, Batangas: 500 km.
- The closest station to the epicenter is Batangas and the farthest is Cebu.
Scaling and Mapping Distances
- To plot on paper, convert distances: scale = 100 km/cm.
- Cebu: 12 cm, Palawan: 7.5 cm, Batangas: 5 cm.
- Draw circles around each station on the map with radius equal to the scale distance.
- The intersection point of the three circles marks the earthquake’s epicenter.
Epicenter Triangulation Method
- Triangulation uses three stations to uniquely identify the epicenter.
- One or two stations are insufficient: one gives a circle, two give two possible points, three give a unique intersection.
- Multiple circles ensure triangulation only results in one epicenter.
Practical Challenges in Plotting
- Common difficulties include accurately measuring radii, reading rulers, and handling the compass.
- Some students' circles may not intersect exactly due to drawing inaccuracies, but close approximations are acceptable.
- Drawing skills and compass use improve with practice.
Exam and Discussion Questions
- Nearest station to the epicenter is the most affected (Batangas).
- Farthest station (Cebu) is the least affected by the earthquake.
- Three stations are necessary for precise triangulation.
- Main challenges are practical (drawing, measuring), not in the math calculations.
Key Terms & Definitions
- Epicenter — the point on Earth's surface directly above the earthquake origin.
- Body Waves — seismic waves (P and S waves) that travel through Earth’s interior.
- Lag Time — difference between S wave and P wave arrival times.
- Triangulation — method of determining a location using intersection of three circles.
- Scale Distance — conversion of real distance to map distance using a defined scale.
Action Items / Next Steps
- Practice plotting circles for epicenter triangulation.
- Prepare for part 2 by reviewing map drawing and compass skills.
- Complete any assigned problem sets involving epicenter location calculations.