The Seismograph Station
Three stations that felt the same quake disagree about when it arrived — and that disagreement is exactly how you find it.
Phase 0 · The phenomenon
Three clocks, one earthquake
Three seismogram traces stacked on a shared time axis. On each, a small wiggle marks the P-wave arrival and a larger burst marks the S-wave arrival. Station B's arrivals come first and closest together, Station C's next, and Station A's last with the widest gap between P and S.
At 2:14 in the afternoon, an earthquake shakes a region. Three seismograph stations record it. Station A logs the first shaking about 5 seconds after Station B. Station C is somewhere in between.
None of the stations saw where the earthquake happened. Each one only knows when two kinds of waves arrived at its own location.
Those disagreements are not errors. In this lab you will turn three sets of arrival times into a location, a size and a warning.
How can three stations that each only record WHEN shaking arrived work out WHERE an earthquake happened, how big it was, and how much warning a city can get?
What this lab works toward
HS-ESS2-1Model how Earth's internal and surface processes, working at different sizes and speeds, build continents and ocean-floor features.HS-ESS3-1Explain how natural resources, natural hazards and climate change have shaped human activity.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Wave RaceHow does the gap between the P and S waves reveal distance?
- TriangulationHow do three S−P times pin down one location?
- Magnitude ScaleHow does the size of the wiggle, corrected for distance, become a magnitude?
- Early WarningHow many seconds of warning can a city get, and who gets none?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- Waves travel straight through a uniform crust at fixed speeds (P 6.0 km/s, S 3.5 km/s). Real crust is layered, so waves bend and speeds change with depth.
- The earthquake is placed at the surface. Real earthquakes start kilometres down, which adds to every distance.
- Local magnitude uses the classic Richter nomogram formula, which is meant for nearby earthquakes on a particular kind of seismometer. Large earthquakes are measured with the moment magnitude scale instead.
- Early warning assumes instant detection at the nearest station plus a fixed processing time. Real systems need several stations and take longer for the largest earthquakes.