Earthquake Epicenter Activity
Goal: Use S–P lag times from three seismograms to find how far away an earthquake was from each station, then locate its epicenter by triangulation.
What to do
- Open the Earthquake Epicenter Finder and choose Guided mode.
- On each seismogram, click where the first small wiggles start (the P wave), then where the big wiggles start (the S wave). Record both times and subtract: S − P = the S–P lag.
- Turn each lag into a distance: distance (km) = lag (s) × 8.4. Type it in to draw that station’s circle.
- Click where the three circles meet and press Reveal the epicenter. Record how far off your guess was.
Record your results
| Station | P arrives (s) | S arrives (s) | S–P lag (s) | Distance (km) |
|---|---|---|---|---|
| Ashford | ||||
| Bayview | ||||
| Cedar Ridge |
Questions
Which station was closest to the earthquake? How can you tell from the seismograms alone, before doing any math?
Show your calculation of the distance from Bayview to the earthquake.
P waves travel at about 6.0 km/s and S waves at about 3.5 km/s. Explain why the S–P lag gets longer the farther a station is from the earthquake.
With only one station, what do you know about where the earthquake happened? What about two stations? Why do we need three?
Your three circles may not meet at exactly one point. Give two reasons why.
Challenge: Cedar Ridge recorded a largest S-wave amplitude of 15.5 mm. Estimate the magnitude with M = log₁₀(A) + 3 × log₁₀(8 × lag) − 2.92.
Free from Earth Science Lab · earthsciencelab.org · MS-ESS3-2: Analyze and interpret data on natural hazards to forecast future catastrophic events and inform technologies to mitigate their effects.